Vol. 67, No. 2 (2018)
2018-01-20
GENERAL
2018, 67 (2): 020301.
doi:10.7498/aps.67.20171641
Abstract +
In the study of thermal entanglement of the Heisenberg spin chain model, one usually considers only the spin interaction between the nearest neighboring qubits. Actually, a generalized Heisenberg model, so-called J1-J2 Heisenberg model, which is constructed by considering the fact that not only the nearest neighboring but also the next nearest neighboring spin interaction also plays an important role. In J1-J2 Heisenberg model, due to the next nearest neighboring spin interaction, the frustration effect can occur and has an important influence on the magnetic properties of the model. In this paper we investigate the thermal entanglement of a five-qubit XXZ Heisenberg spin chain with the next nearest neighboring interaction in a magnetic field. Using the numerical method, we calculate the pairwise concurrences of the nearest neighbouring qubits and the next nearest neighboring qubits, abbreviated as C12 and C13 respectively. The numerical results show that the frustration parameter α has an important effect on the pairwise thermal entanglement. Moreover, C12 and C13 have different variations with the change of the frustration parameter α. Meanwhile, it is found that the temperature, magnetic field, Dzyaloshinkii-Moriya (DM) interaction and anisotropic parameter also have great effects on the thermal entanglement. The increasing of temperature can reduce the thermal entanglement. The magnetic field can enhance the thermal entanglement between both two nearest and next nearest neighboring qubits, but when the magnetic field becomes strong enough, only the thermal entanglement between the two nearest neighboring qubits is suppressed. A certain extent of DM interaction can enhance the thermal entanglement between the two nearest neighboring qubits. But for the next nearest neighboring qubits, without the magnetic field, the increasing of DM interaction mainly enlarge the entanglement vanishing area of frustration parameter α. When the system changes from anisotropic to isotropic state, the entanglement vanishing area also changes obviously for C12 and C13. Thus, we can choose appropriate magnetic field strength, temperature, frustration parameter, DM interaction parameter and anisotropic parameter to effectively control and enhance the thermal entanglement of the system.
2018, 67 (2): 020302.
doi:10.7498/aps.67.20172087
Abstract +
In the present work, we study the fidelity susceptibility and the entanglement entropy in an antiferromagnetic spin-1 chain with additional next-nearest neighbor interactions and three-site interactions, which are given by H=(J1SiSi+1+ J2SiSi+2)+[J3(SiSi+1)(Si+1Si+2)+ h.c.]. By using the density matrix renormalization group method, the ground-state properties of the system are calculated with very high accuracy. We investigate the effect of the three-site interaction J3 on the fidelity susceptibility numerically, and then analyze its relation with the quantum phase transition (QPT). The fidelity measures the similarity between two states, and the fidelity susceptibility describes the associated changing rate. The QPT is intuitively accompanied by an abrupt change in the structure of the ground-state wave function, so generally a peak of the fidelity susceptibility indicates a QPT and the location of the peak denotes the critical point. For the case of J2=0, a peak of the fidelity susceptibility is found by varying J3, and the height of the peak grows as the system size increases. The location of the peak shifts to a slightly lower J3 up to a particular value as the system size increases. Through a finite size scaling, the critical point J3c=0.111 of the QPT from the Haldane spin liquid to the dimerized phase is identified. We also study the effect of the three-site interaction on the entanglement entropy between the right half part and the rest. It is noted that the peak of the entanglement entropy does not coincide with the critical point. Instead, the critical point is determined by the position at which the first-order derivative of the entanglement entropy takes its minimum, since a second-order QPT is signaled by the first derivative of density matrix element. Moreover, the entanglement entropy disappears when J3=1/6, which corresponds to the size-independent Majumdar-Ghosh point. The positions of quantum critical points extracted from these two quantum information observables agree well with those obtained by the string order parameters, which characterizes the topological order in the Haldane phase. Secondly, we also study the case of J20, and obtain the critical points by both the fidelity susceptibility and the entanglement entropy. Finally we provide a ground-state phase diagram of the system. To sum up, the quantum information observables are effective tools for detecting diverse QPTs in spin-1 models.
2018, 67 (2): 020701.
doi:10.7498/aps.67.20171881
Abstract +
In a conventional system of quartz-enhanced photoacoustic spectroscopy (QEPAS), the size of block-like optical collimation focusing lens group is difficult to reduce, and the structural stability is poor, which makes it hard to adapt itself to some special conditions, such as narrow space and vibrating circumstance. Based on this situation, in this research the fiber evanescent wave technique is combined with QEPAS. Therefore, trace gas detection for acetylene (C2H2) based on an all-fiber structural QEPAS system is developed. To obtain the characteristics of fiber evanescent wave, the optical distribution of micro structural fiber is simulated and the evanescent wave power ratio is calculated based on the COMSOL Multiphysics software. In order to increase the QEPAS 2f signal amplitude, the optical path between fiber taper and quartz tuning fork (QTF) and the laser wavelength modulation depth are optimized. In addition, two kinds of QTFs with different resonant frequencies are optimized. Finally, a QTF with a lower resonant frequency of 30.720 kHz is adopted as the acoustic wave transducer, and a minimum detection limit (MDL) of 6.2510-4 (volume fraction) is obtained with a laser wavelength modulation depth of 0.24 cm-1. To investigate the evanescent wave power of micro structural fiber, the fiber taper diameter is measured by a scanning electron microscope. Subsequently, by combining the diameter of fiber taper with the theoretical calculation results, we determine an evanescent wave power of 455.9 W, and the normalization of noise equivalent absorption (NNEA) which indicates the sensor sensitivity is 4.1810-7 cm-1WHz-1/2.
2018, 67 (2): 020702.
doi:10.7498/aps.67.20171884
Abstract +
An ultra-low frequency vibrational noise isolation apparatus from external vibration can be a critical factor in many fields such as precision measurement, high-technology manufacturing, scientific instruments, and gravitational wave detection. To increase the accuracies of these experiments, well performed vibration isolation technology is required. Until recently the cold atom gravimeter has played a crucial role in measuring the acceleration due to gravity and earth gravity gradient. The vibration isolation is one of the key techniques in the cold atom gravimeter. To reduce the vibrational noise caused by the reflecting mirror of Raman beams in the cold atom gravimeter, a compact active low-frequency vibration isolation system based on sliding-mode robust control is designed and demonstrated. The sliding-mode robust control active vibration isolation method is used to solve the vibration problem of Raman mirror in the cold atomic gravimeter. The purpose of vibration control is that the controller enables the system to be at zero state as the system states are away from the equilibrium due to vibration disturbance. In this system, the mechanical setup is based on a commercial passive isolation platform which only plays a role at higher frequency. A sliding-mode robust control subsystem is used to process and feed back the vibration measured by a seismometer which can measure the velocity of the ground vibration. A voice coil actuator is used to control and cancel the motion of a passive vibration isolation platform. The simulation and experiment results of vibration isolation platform show, on the one hand, that the vibration noise power spectral density decreases by up to 99.9%, and that the phase noise in cold atom interferometry produced by vibration decreases by up to nearly 85.3% compared with the results of the passive vibration isolation platform. On the other hand, compared with the lead-lag control method, the vibration noise power spectral density decreases by up to 83.3% and the phase noise in cold atom interferometry produced by vibration decreases by nearly 40.2%. Therefore, the sliding-mode robust control has the advantages of less tuning parameters, strong anti-interference ability, and more obvious vibration isolating effect.
2018, 67 (2): 020601.
doi:10.7498/aps.67.20171837
Abstract +
With the advantages of high precision and great environmental adaptability, laser heterodyne interferometry has been successfullyused in some areas, such as measuring distance and angle and other point detection. The Hertz-level frequency-shifting technology greatly improves the accuracy and stability of surface measurement and extends its application to the areas of array detection, such as three-dimensional topography measurement, smooth surface measurement, digital holography, speckle measurement, etc. The frequency difference of heterodyne interferometry is realized by acousto-optic frequency shifter under the control of two radio frequency signals each with a fixed frequency value. However, a deviation of the real value from the design value of frequency always exists, which is referred to as frequency difference deviation. It causes the heterodyne frequency and the frame rate of the array detector to be unable to be strictly matched, thus affecting the improvement of measurement accuracy. According to the theory of full-field heterodyne measurement, we derive the relationship between frequency difference deviation and measurement accuracy of the heterodyne measurement instrument, and analyze the effects of relevant parameters including the value of frequency difference, frequency deviation, initial sampling time, initial phase, sampling frequency, and sampling cycles on measurement accuracy. A method of improving the measurement accuracy is proposed by reasonably selecting the sampling time and frame number. Analysis shows that the initial sampling time and initial phase have the same effect on the measurement accuracy. With the reasonable choosing of measurement parameters and processing methods, the measurement accuracy of the instrument could be greatly improved. In addition, the peak value of full-field heterodyne measurement error is linearly related to the frequency difference deviation. In the case of a certain frequency difference deviation, the instrument could achieve a higher measurement accuracy with greater frequency difference, but requires a higher frame rate of detector at the same time. As a result, designers should choose an appropriate value of frequency difference for measurement instrument. Furthermore, increasing the sampling frequency could also improve the measurement accuracy. Actually, if sampling frames are more than fifteen in a single cycle, the improvement of measurement accuracy would be limited. Multi-period sampling has little effect on measurement error caused by frequency difference deviation, and the measurement error is the limiting value of measurement accuracy that the instrument could reach. Therefore, this study could be used as a theoretical basis of the design and parameter selection and also the measurement accuracy analysis for full-field heterodyne measurement instrument development.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2018, 67 (2): 024202.
doi:10.7498/aps.67.20170782
Abstract +
Insufficient anti-counterfeiting performance and difficulties in manufacturing lead to performance degradation of the subwavelength rectangular structure grating, when it is applied to the field of optical anti-counterfeiting. To solve the problem, an embedded subwavelength one-dimensional simple periodic sinusoidal grating structure is proposed in this paper to replace the previous structure with a rectangular structure. By using equivalent medium theory, we find that the rectangular structure whose duty ration is 0.5 has the same effective refractive index as the sinusoidal structure. Then equivalent structure parameters of a sinusoidal structure are obtained based on a rectangular structure tricolor-shifting device, and the characteristics of the reflection peak are analyzed. The result shows that the sinusoidal structure gating can realize the same tricolor-shifting properties with a higher reflective efficiency as the rectangular structure gating. When the incidence angle of natural light is 45 for TE and TM polarization, the highest reflectivity values of 90%, 89% and 100% in blue, green and red bands can be obtained at the azimuths of 24, 63 and 90, respectively. Then the azimuth-induced color shifts of blue, green and red are realized. Physical mechanism of the equivalent rectangular structure to sinusoidal structure is explained in non-resonance and resonance conditions. Under the non-resonance condition, both of them can be regarded as a layer of completely equivalent optical film, possessing exactly the same optical properties. Under the resonance condition, they can be regarded as a slab waveguide. So when their effective refractive indexes, periods, film thicknesses and depths are equal, they have the same optical characteristic matrixes, supported guided modes, and resonant peak positions. In addition, we investigate the influences of the deviations of key parameters, including grating period, grating depth, coating film thickness, and incidence angle, and propose the rigorous redundancy of these parameters. When the values of period, depth, thickness, and incidence angle are kept within the ranges of 430-455 nm, 88-160 nm, 10-40 nm, and 40-50, respectively, the device can well keep the color-shifting effects of blue, green and red light. A model structure of the sinusoidal grating is fabricated by two-beam laser interference lithography experimentally. The tricolor-shifting device based on the sinusoidal structure presented in this paper can realize high diffraction efficiency azimuth-induced color shifts of blue, green and red light when natural light is incident, which breaks through the limit of bi-color shifting technology and lowers the difficulties in manufacturing, and may have great applications in the field of the optically variable image security.
2018, 67 (2): 024204.
doi:10.7498/aps.67.20171902
Abstract +
Physical random number, which is non-reproducible and non-periodical, has attracted much attention due to its potential applications in various fields such as secure communication, statistical analysis, and numerical simulation. Recently, fast physical random number generators based on optical chaotic entropy sources have been demonstrated to reach a rate of up to several hundreds of Gbit/s. Although many efforts have been made to optimize the schemeis of chaotic-based random number generation, most of them are based on distributed feedback semiconductor lasers and can only generate single-channel physical random number. After taking into account the costs and technological applications, the multi-channel physical random number generation technique needs developing. On the other hand, vertical-cavity surface-emitting lasers (VCSELs) can simultaneously emit two orthogonally polarized components under appropriate parameter conditions, and then each polarized component can be used as an entropy source for generating random number. As a result, VCSEL-based chaotic entropy sources may be suitable for multi-channel random number generation. In this work, a scheme for achieving multi-channel physical random number is proposed. Also the influence of the coupling parameters on the performance of the randomness of final bit sequences is investigated. For such a scheme, two orthogonally mutually coupled VCSELs are used to supply four-channel chaotic signals with a comparable output power and weak time-delay signature (TDS). The four-channel chaotic signals, which serve as chaotic entropy, are quantized by 8-bit analog-to-digital converters (ADCs) with 20 GHz sampling rate, and then the m least significant bit (m-LSB) post-processing method is adopted for generating final four-channel random bit sequences. Firstly, based on the spin-flip mode of VCSELs, the influences of coupling strength and frequency detuning on the dynamics of two orthogonally mutually coupled 1550 nm VCSELs are analyzed. Next, the optimized parameter regions for generating four-channel chaotic signals with comparable output power and weak TDS are preliminarily determined. For a given optimized value of coupling strength and different frequency detunings within the optimized parameter regions, the generated four-channel chaotic signals are taken as the entropy sources for obtaining final bit sequence by quantizing the 8-bit ADC and m-LSB post-processing. Finally, the randomness of the four final bit sequences is tested by NIST SP 800-22 statistical test suite, and the regions of preferred coupling parameters for simultaneously generating four-channel random numbers are determined.
2018, 67 (2): 024208.
doi:10.7498/aps.67.20171828
Abstract +
The initial decomposition micro-mechanism of energetic materials has attracted much attention because it is a critical factor for the safe use of energetic materials. The thermally triggered chemical reactions are usually related to the vibrational properties of molecules. A time-resolved coherent anti-Stokes Raman scattering (CARS) spectrum system is constructed to study the molecular coherent vibrational dynamics of nitromethane at a microscopic level for clarifying the relation of molecular vibration to initial chemical reaction. In this experiment, the ultra-continuous white light is used as Stokes light, and the CARS spectra of different vibrational modes can be obtained by adjusting the time delay of the Stokes light. The vibrational dephasing time of different chemical bonds in nitromethane is provided by fitting the vibrational relaxation curves. The dephasing time of the CH stretching vibration located at 3000 cm-1 is shown to be 0.18 ps, which is far less than the dephasing time 6.2 ps of the CN stretching vibration located at 917 cm-1. The vibrational dephasing time is closely related to thermal collision for liquid nitromethane system without intermolecular hydrogen bond, that is, the scattering of thermal phonons causes the dephasing of coherent vibration. Therefore, the stretching vibration of the CH bond is more easily affected by the thermal phonon than the stretching vibration of the CN bond. The CH bond of nitromethane molecule is expected to be excited first, causing an initial chemical reaction under thermal loading.
Surface structure for manipulating the near-field spectral radiative transfer of thermophotovoltaics
2018, 67 (2): 024209.
doi:10.7498/aps.67.20171531
Abstract +
To improve the efficiency and output power of the nano-gap thermophotovoltaic (TPV) power generation system, surface rectangular grating structures are added to the top surface of the group Ⅲ-V semiconductor cell to control the spectrum of near-field radiative transfer. Doped zinc oxide that supports surface waves at near-infrared wavelengths is selected as the TPV emitter. When paired with GaSb grating structures, the surface plasmon polariton excited by the emitter and the light trapping effect by the grating tunnels will be coupled, which results in a significantly and selectively enhanced near-field radiative heat flux within a narrow spectral region above the cell bandgap, thereby fulfilling the design purpose. This physical mechanism is explained by a direct finite-difference time-domain (FDTD) simulation based on the Langevin approach. The material volume meshgrids filled with random dipole sources can act as the thermal emission source and the radiative heat flux is calculated by solving the Maxwell equations numerically. The spectral results show that adding rectangular grating structures to GaSb not only increases radiative transfer in the expected wavelength region over the unstructured case, resulting in a heat flux surpassing that of a far-field blackbody source at the same temperature, but also suppresses the unwanted long-wavelength heat flux that causes radiative loss and cell heating. With a vacuum gap of 200 nm between the emitter and the cell, using a bulk GaSb cell with rectangular gratings can double the spectral flux of the blackbody emitter case, and using an ultrathin GaSb cell with surface structures and back reflectors further increases this ratio to 2.84 due to the total internal reflection controlled by the cell thickness. The amplitude and wavelength of the spectral peak are controlled by the grating size parameters. Low filling ratio gratings with lower-aspect-ratio grating channels generally have sharper enhancement peaks but lower total radiative heat flux, while high filling ratio structures with higher-aspect-ratio channels have better heat flux improvement but might also result in lower conversion efficiency due to the broader spectrum. The rigorous approach reveals the detailed physical mechanism that is otherwise unseen with effective medium approaches for inhomogeneous structures or the Derjaguin proximity approximation. Overall the results of this study enable an enhancement of near-field radiative heat flux limited within a narrow wavelength range shorter than the cell bandgap, offering practical benefit to the application of TPV power generation with higher feasible power and conversion efficiency.
2018, 67 (2): 024210.
doi:10.7498/aps.67.20171880
Abstract +
In cavity quantum electrodynamics (cQED), how an atom behaves in a cavity is what people care about. The coupling strength (g) between cavity field and atoms plays a fundamental role in various QED effects including Rabi splitting. In the solid-state case, when an atomic-like two-level system such as a single quantum dot (QD) is placed into a cavity, Rabi splitting would occur if g is strong enough. In the classical limit, when a QD in a cavity changes into a classical oscillator, the normal-mode splitting would also take place. It is known that g relies on the local fields at the places of the QDs or classical oscillators inside the cavity. However, for both cases, the traditional cavity modes involved are all in the form of standing waves and the localized fields are position-dependent. To ensure strong coupling between QDs or classical oscillators and photons, they should be placed right at the place where the cavity field is maximum, which is very challenging. How is the positional uncertainty overcome? Recently, the peculiar behaviors of electromagnetic (EM) fields inside zero-index metamaterial (ZIM) in which permittivity and/or permeability are zero have aroused considerable interest. In ZIMs the propagating phase everywhere is the same and the effective wavelength is infinite, which strongly changes the scattering and mode properties of the EM waves. In addition to the above characteristics, the fields in ZIM could be homogeneous as required by Maxwell equations. While the special properties of ZIMs are investigated, the fabrication of ZIMs is widely studied. It is found that a two dimensional (2D) photonic crystal consisting of a square lattice of dielectric rods with accidental degeneracy can behave as a loss-free ZIM at Dirac point. To overcome the positional uncertainty, in this paper we propose a cavity filled with effective zero-index metamaterial (ZIM). When the ZIM is embedded in a cavity, the enhanced homogeneous fields can occur under the resonance condition. Finally, experimental verification in microwave regime is conducted. In the experiments, we utilize a composite right/left-handed transmission line with deep subwavelength unit cell to mimic a ZIM and use a metallic split ring resonator (SRR) as a magnetic resonator whose resonance frequency is determined by structural parameters. The experimental results that in general agree well with the simulations demonstrate nearly position-independent normal-mode splitting.
2018, 67 (2): 024211.
doi:10.7498/aps.67.20171440
Abstract +
Optical fiber vibration sensing technology is based on the phase modulation of the transmitted light caused by the external vibration to achieve vibration measurement. A lot of researches have reported a variety of schemes for optical fiber vibration sensing and set up several application systems, achieving the functions of oil field safety monitoring, perimeter security, pipeline safety monitoring, etc. The phase modulation caused by raindrops is mixed with the sensing signal, however, when the sensing fiber cable is exposed to the atmospheric environment and the rainfall directly acts on the sensing cable. It is difficult to distinguish the valid signal which can cause the false alarms, which thereby seriously affects the normal operation of the sensing system. To our knowledge, to date, there has been no report on the phase modulation of the transmitted light in the optical fiber caused by raindrops. Based on the theory of cloud dynamics, the transformations of refractive index and shape in the core of optical fiber and the phase modulation of light caused by raindrop collision with optical fiber cable are analyzed. The model of optical phase modulation caused by raindrop collision with optical fiber cable is established, and the relationship between phase modulation and rainfall intensity is obtained. With the increase of rainfall intensity, the phase modulation increases. When the length of the optical fiber cable is fixed, the larger the cable diameter, the larger the phase modulation is. The larger the length of the cable, the greater the phase modulation is, with the cable diameter fixed. The phase modulation caused by raindrops has a positive correlation with the cable diameter and the cable length, which is related to the rainfall intensity received on the cable surface, and increases monotonically with the rainfall intensity. A laboratory verification system for phase modulation caused by raindrop collision with optical fiber cable is established, and the relationship between the phase modulation caused by raindrops and the output signal is obtained. The experimental results are compared with the simulation results at the rainfall intensities of 3, 5, 7, 10, 15, 18, 22, and 30 mm/h. The experimental and simulated results are consistent with each other under different rainfall intensities and the error is less than 9%. The results show that the model can be used to simulate the phase modulation caused by rainfall under different rainfall intensities. It provides a theoretical basis for studying the effect of rainfall on the vibration sensing system, based on which the application system can be optimized and the feasible rainfall compensation scheme can be found.
2018, 67 (2): 024201.
doi:10.7498/aps.67.20171877
Abstract +
Optical path design of high power laser facilities should consider several optimization measures such as those that are related to image transmission, ghost avoidance, and stray light management. According to the diffraction optical propagation theory, we study the the influences of wavefront characteristics of large aperture optical components on optimizing the design parameters of optical path in view of increasing the output load. The results show that the arrangement interval of the last stage optical drive can be very useful in improving the output load of the laser facilities if it is controlled to be over 6 m long. In general, a large aperture optical element with a phase error peak value of 0.34 can reduce the near field beam quality of a high-power laser by about 10% and give rise to a maximum decrease of about 21% when the phase error reaches 1.36. Superposition of multiple optical elements with different phase error distribution characteristics can reduce the negative effect of the mid frequency phase error. However, the nonlinear effect of large aperture optical components can aggravate the influence of the intermediate frequency phase error on the damage resistance capacity of the device. Under the premise that the damage threshold of the large caliber optical element is limited to 20 J/cm2, the using of a laser facility with a compact optical path, with an input laser energy density controlled to be below 16.8 J/cm2, will avoid damaging the optical components efficiently. A relatively flexible optical layout can further increase the average energy density of the final output laser and is very beneficial to the enhancing of the output load capacity of the laser facility.
2018, 67 (2): 024203.
doi:10.7498/aps.67.20171542
Abstract +
Continuous variable (CV) quantum entanglement is an essential resource for quantum computation and communication protocols. The use of CV quantum entanglement at a telecommunication wavelength of 1.5m in combination with existing fiber telecommunication networks offers the possibility to implement long-distance quantum communication protocols like quantum key distribution (QKD) and applications such as quantum repeaters, quantum teleportation in the future. In spite of the fact that the optical power attenuation of light in a standard telecommunication fiber is lowest at a wavelength of 1.5m, the entangled states will interact with fiber channels and the disentanglement will occur. It is one of the important factors restricting the development of long distance quantum information. In this paper, CV entangled state at 1.5m telecommunication band is obtained by using a type-II periodically poled KTP (PPKTP) crystal inside a nondegenerate optical parametric amplifier (NOPA). A wedged PPKTP is used for implementing frequency-down-conversion of the pump field to generate the optically entangled state and achieving the dispersion compensation between the pump and the subharmonic waves. By controlling the temperature and the length of the PPKTP crystal, a triply resonant optical parametric oscillator with a threshold of 80 mW is realized. Einstein-PodolskyRosen (EPR)-entangled beams with quantum correlation of 8.3 dB for both the amplitude and phase quadratures are experimentally generated by using a single NOPA at a pump power of 40 mW and an injected signal power of 10 mW when the relative phase between the pump and injected signal is locked to . The generated entangled state is coupled into a single-mode optical fiber, and the transmission characteristics of the generated EPR entangled beams through standard single-mode fibers are investigated experimentally and theoretically. A fiber polarization controller is used to compensate for the polarization state variation induced by random fluctuations of birefringence of the single mode fiber when the light propagates along the fiber, and to keep the polarization of light linear at the fiber output. A 0.21 dB quantum entanglement could still be observed for the EPR-entangled beams transmitted through a 50-km-long single-mode fiber. The theoretical prediction considering the excess noise in fiber channel is in good agreement with the experimental result. The generated CV quantum entanglement is highly suitable for the required experiments, such as CV measurement-device-independence QKD based on standard fibers, owing to the fact that the tolerance of the excess noise in the quantum channel can be enhanced significantly with respect to a coherent state if EPR-entangled beams are used.
2018, 67 (2): 024205.
doi:10.7498/aps.67.20171676
Abstract +
In recent years, high power fiber laser has received great attention, leading to wide applications in numerous fields such as industry, biology and relevant research. Nevertheless, the output power of typical diode pumped fiber laser is limited by the thermal effect and brightness of pump source. Owing to the low quantum deficit, the tandem pumping employing ytterbium-doped fiber lasers (YDFLs) as the pumping source can effectively reduce the thermal issue and achieve high power output. With the much lower absorption coefficient at 1018 nm than at 976 nm, longer gain fiber is necessary in tandem pumped configuration to sufficiently absorb pump light, which in turn induces a more severe nonlinear effect such as the stimulated Raman scattering, bringing in more challenges in laser configuration design. In this paper, we demonstrate an all-fiber laser under master oscillator power amplifier configuration based on tandem pumping with domestic gain fiber produced by China Electronics Technology Group Corporation No. 46 Research Institute. The diameters of the core and inner cladding of the Yb3+ doped double cladding fiber are 25 m and 250 m, respectively. The modified chemical vapor deposition method with gas-solution co-doping method is adopted so that the fiber has a more uniform distribution of Yb ions, larger absorption cross section and higher absorption coefficient (0.41 dB/m@1018 nm). In the amplifier stage, a 40-m-long gain fiber is pumped by fourteen 1018 nm fiber lasers with a maximum total output power of 3511 W. A 67.8 W 1080 nm seed is amplified to 3079 W with a corresponding slope efficiency of 85.9%. The beam quality factor M2 is measured to be 2.14. In addition, no stimulated Raman scattering is found in output spectrum and the 3 dB band width of output laser is measured to be 1.4 nm. To the best of our knowledge, this marks the highest result ever reported for tandem pumping based on domestic gain fiber. Taking stimulated Raman scattering into account, the rate equations are built to calculate the properties and power evolution in the fiber amplifier. The numerical results are in good agreement with the experiment results. Besides, based on heat conduction equation, heat power density in the fiber core is analyzed, showing that the tandem pumping has great advantages in heat management and a huge potential to reach a higher power compared with the method of direct pumping. The theoretical and experimental results show that with ever-maturing fiber manufacturing technology, domestic fiber is capable of withstanding laser power as high as 3 kilowatts. Meanwhile, domestic fiber may achieve a higher output power by increasing the pump power, optimizing the gain fiber length and improving the cooling condition.
2018, 67 (2): 024206.
doi:10.7498/aps.67.20171848
Abstract +
In this paper, the composite Nd:YVO4 laser crystal is in-band pumped by a wavelength-locked laser diode at 878.9 nm, with the purpose of reducing thermal effects and improving pump absorption simultaneously. By using the YVO4 and BaWO4 crystals as Raman media, the influences of crystal properties, resonator structure and stability on the performance of continuous-wave intracavity Raman laser are investigated experimentally and theoretically. The results show that the resonator stability greatly affects laser performance due to the long cavity length of intracavity Raman laser. By choosing the Raman medium with high Raman gain, we can not only obtain higher Raman conversion efficiency, but also reduce the thermal effect to a certain extent. Furthermore, the smaller the curvature radius of the output mirror in the plano-concave cavity structure, the greater the power density of the fundamental laser in the Raman crystal is and the wider the dynamic stability region of the resonator, and hence the higher output power of the Raman laser can be achieved. Finally, by using 30-mm BaWO4 crystal as Raman medium, a highest Raman output of 3.02 W is obtained at a pump power of 25.1 W, corresponding to a diode-to-Stokes optical conversion efficiency of 12%.
2018, 67 (2): 024207.
doi:10.7498/aps.67.20171767
Abstract +
Squeezed states are important sources in quantum physics, which have potential applications in fields such as quantum teleportation, quantum information networks, quantum memory, and quantum metrology and precise measurements. For our interest, the squeezed vacuum will be used in the quantum-enhanced optical atomic magnetometers, filling the vacuum port of the probe beam to improve measurement sensitivity. Based on the sub-threshold optical parametric oscillator (OPO) with PPKTP crystal, the squeezed vacuum at rubidium D1 line of 795 nm is obtained. In our work, we investigate the noise sources in an OPO system. By carefully controlling the classical noise source, the squeezing band extends to the analysis frequency of 2.6 kHz. The flat squeezing trace is 2.8 dB below the shot noise limit. In our work, we focus on the difference between the squeezing results at the analysis frequency of kilohertz regime at two different wavelengths, 1064 nm and 795 nm. The difference mainly comes from the absorption of 795 nm laser and its second harmonic at 397.5 nm in crystal (397.5 nm laser is at the edge of transparent window of PPKTP crystal that has an absorption index much higher than at other wavelength). The absorption induced nonlinear loss and thermal instability greatly affect the squeezing results, which is discussed in our work. Squeezing level at 795 nm is worse than at 1064 nm due to the above-mentioned factors. Noise coupling to the detection system limits the squeezing band. In the audio frequency band, squeezing is easily submerged in roll-up noises and the measured squeezing level is limited. Two factors limit the obtained squeezing:the technical noise induced in the detection and the squeezing degradation by the noise coupling of the control beams. In experiment, we carefully control the classical noise at analytical frequency of kilohertz by means of a vacuum-injected OPO, a counter-propagating cavity locking beam with orthogonal polarization, low noise homodyne detector, stable experimental system and quantum noise locking method for squeezing phase locking. Firstly, to preclude the classical noise from coupling the laser source, we use the vacuum injected OPO. A signal beam helps optimize the parametric gain and is blocked in the squeezing measurement process. In order to maintain the OPO, a counter-propagating beam with orthogonal polarization is used for locking the cavity. Then, a low noise balanced homodyne detector with a common-mode rejection ratio of 45 dB helps improve the audio frequency detection. Finally, the quantum noise locking provides a method to lock the relative phase between the coherent beam and the squeezed vacuum field. With the combination of these technical improvements, a squeezed vacuum of 2.8 dB is obtained at the analysis frequency of 2.6-100 kHz. The obtained squeezing level is mainly limited by the relatively large loss in OPO, which is induced by ultra-violet absorption in PPKTP crystal. The generated squeezed field is used to reduce the polarization noise of probe beam in an optical magnetometer, in order to increase detection sensitivity.
2018, 67 (2): 024212.
doi:10.7498/aps.67.20171677
Abstract +
A simple and high sensitivity optical fiber relative humidity (RH) sensor based on Mach-Zehnder interferometer (MZI) is proposed and demonstrated in this paper. A single-mode fiber and a graded-index multimode fiber are connected by a fiber taper to form a section. Then an uncoated dispersion compensation fiber is sandwiched between two short sections of the graded-index multimode fiber.Therefore, a sensing structure is set up as a single-mode fiber-taper fiber-graded-index multimode fiber-dispersion compensation fiber-graded-index multimode fiber-taper laser-single-mode fiber. The taper fiber is used to augment the energy of the cladding mode. The two nodes of the graded-index multimode fiber can be looked as a mode coupler. Thus an MZI is constructed. Since the external RH change can make the transmission spectrum energy changed, we can obtain the RH by detecting the peak energy variation of the interference pattern induced by the evanescent-field interaction. The experimental results show that the peak energy changes linearly with surrounding relative humidity. Under the condition of 35%Rh-85%RH, the sensitivity of the sensor with a 20 mm dispersion compensation fiber is -0.0668 dB/%RH and the linearity is 0.995. Moreover, temperature response characteristics are investigated. Experimental results suggest that the transmission spectrum energy of the sensor is insensitive to temperature. With temperature increasing, the transmission spectrum presents obviously a red-shift, yet the peak energy of the monitoring point barely moves, which demonstrates its potential for measuring simultaneously RH and temperature. The proposed sensor has a small size and simple manufacturing process, which can make it widely used to measure RH.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2018, 67 (2): 025201.
doi:10.7498/aps.67.20170845
Abstract +
When hypersonic vehicle flies in the atmosphere at a high altitude with a high speed, plasma sheath is generated around the vehicle, and thus attenuating the electromagnetic wave signals and even interrupting the communication. Therefore the control, guidance, and navigation of hypersonic vehicle can be affected seriously by the plasma sheath. It is necessary to study this problem in reasonable ground experiment. The inductively coupled plasma (ICP) wind tunnel is an ideal equipment for studying electromagnetic transmission characteristics in plasma because it can produce uncontaminated plasma and the electrode cannot be ablated in the process of plasma production. We carry out the experiment in ICP wind tunnel. A thin slice of plasma jet is generated by a rectangular nozzle with an outlet size of m 50 mm250 mm. Plasma jets with different parameters are obtained by adjusting the operating power and inlet flow of the wind tunnel. Four kinds of states are provided with the electron densities of 7.01010, 5.01011, 3.51012 and 1.01013/cm3, and the collision frequencies of 1.5109, 1.6109, 2.0109 and 9.0109 Hz, respectively. The amplitude attenuations and phase changes of the electromagnetic waves are measured with microwave diagnostics system consisting of a vector network analyzer and high gain antennas. And electron density and collision frequency of plasma are obtained according to the transmission characteristics of electromagnetic waves in plasma. The attenuations of the electromagnetic wave in plasmas of different states are measured via microwave transmission system which is composed of a vector network analyzer and pairs of horn antennas covering a frequency range of 2.6-40 GHz. The results show that both the amplitude attenuation and attenuation band increase with the increase of electron density. The classical theory and thin layer theory are used to simulate the transmission attenuation. The results are compared with the experimental ones. The results in this paper provide basic data for further theoretical and numerical study of electromagnetic wave transmission characteristics in plasma.
2018, 67 (2): 025202.
doi:10.7498/aps.67.20170521
Abstract +
A new type of laser fusion indirect drive octahedral spherical hohlraum has been built up by Chinese researchers in recent years. The hohlraum with 6 laser entrance holes (LEHs) has superiority over other hohlraum configurations in both robust inherent high symmetry and high coupling energy efficiency from laser to hotspot for inertial confinement fusion study. Recently, an experimental investigation on radiation emission from the spherical hohlraum with two LEHs has been performed on the SGⅢ laser facility. In this experiment, 32 laser beams (24 beams from the top, 8 beams from the bottom) are injected into the hohlraum within 3 ns, and the total laser energy is 86.4 kJ. The hohlraum radius is 1.8 mm, and the radius of laser entrance hole is 0.6 mm. The experiments are conducted under two conditions:one is that a 0.48-radius capsule is located at the center of the hohlraum, and the other is that nothing is located in the hohlraum. Some flat response X-ray detectors (FXRDs) are installed at different angles on the target wall to collect the radiation energy. We carry out three-dimensional (3D) simulations of the experiment by using our 3D radiation implicit Monte Carlo code IMC3D. This code was developed in recent years based on fleck and Cumming's ideas. The hydrodynamics is not taken into consideration in the simulations, so we deduct 30% laser energy lost to hohlraum wall movements and back scattered by laser plasma instabilities. Based on the approximation, the simulation results are reasonable in principle. As a result, the radiation temperature of the hohlraum with capsule is 230 eV, and the radiation temperature of the hohlraum without capsule is 238 eV. At the end of laser injection, the capsule reflection ratio is 0.83. Compared with the experimental data, most of the simulation data agree well with the detector observations, except the data at 0 angle. The possible reasons for the difference are analyzed. The flux at 0 angle is more sensitive to the wall plasma movements than at the other angles. So if we ignore this phenomenon, then the witch will occur both in experiment and in simulation, yielding obvious differences for those quantities which strongly relate to the hydrodynamics of wall plasma. Finally, the methods of eliminating the difference are proposed and the prospect of IMC3D is presented.
2018, 67 (2): 025203.
doi:10.7498/aps.67.20171640
Abstract +
The dynamic hohlraum is a possible approach to driving inertial confinement fusion.Recently, dynamic hohlraum experiments on the primary test stand (PTS) facility were conducted, and preliminary results show that a dynamic hohlraum is formed, which can be used for driving target implosion.In this paper, the implosion dynamics of Z-pinch dynamic hohlraum driven target implosion with the drive current of PTS facility is numerically investigated.A physical model is established, in which a dynamic hohlraum is composed of a cylindrical tungsten wire-array and a CHO foam converter, and the target is composed of a high density CH ablator and low density DT fuel.The drive current is calculated by an equivalent circuit model, and the integrated simulations in (r, Z) plane by using a two-dimensional radiation magneto-hydrodynamics code are performed to describe the overall implosion dynamics.It is shown that the wire-array plasma is accelerated in the run-in stage, and in this stage the target keeps almost immobile.As the accelerated wire-array plasma impacts onto the low-density foam converter, a local region with high temperature and high pressure is generated near the W/CHO boundary due to energy thermalization, and this thermalization process will last several nanoseconds.This high temperature region will launch a strongly radiating shock.At the same time, high temperature radiation also appears and transfer to the target faster than the shock.When the high temperature radiation transfers to the surface of the target, the ablator is heated and the ablated plasma will expand outward, and a high-density flying layer will also be generated and propagate inward.After the high-density layer propagates to the ablator/fuel boundary, the DT fuel will be compressed to a high-density and high-temperature state finally.At the same time, the cylindrical shock, which is generated from the impact of the wire-array plasma on the foam converter, will gradually propagate to the ablator plasma.After it propagates over the converter/ablator boundary, it will be decelerated by the ablation pressure, which is beneficial to isolating the fuel compression from the direct cylindrical shock.It is shown that though the trajectories of the outer boundaries of the ablator at the equator and at the poles are completely different due to shock interaction at the equator, the fuel compression is nearly uniform due to radiation compression. It is shown that the asymmetry of fuel compression is mainly caused by the non-uniformity of the hohlraum radiation at the equator and at the poles.Generally, there are two differences between the radiation temperatures at the equator and at the poles, namely the time difference due to the finite velocity of radiation transfer, and the peak temperature difference due to energy coupling.If the target is small, the peak radiation temperature at the equator is almost the same as at the pole.The fuel at the equator is first compressed just because the radiation first transfers to the target equator.As the size of the target is increased, the difference in peak radiation temperature will be more serious, thus causing weaker fuel compression at the equator than at the poles.Certainly, if the target size is too large, the cylindrical shock will directly interact on the target at the equator, resulting in complete asymmetry at the equator with respect to the shock at the poles, which should be avoided.Furthermore, it is shown that as the target size is increased, the final neutron yield will first increase and then decrease, which means that there is a relatively optimal size selection for target implosion.
CONDENSED MATTER:STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2018, 67 (2): 026501.
doi:10.7498/aps.67.20171198
Abstract +
Bulk polythiophene material is usually regarded as thermal insulator because it has low thermal conductivity (less than 1 Wm-1K-1). However, the report demonstrates that along the amorphous polythiophene nanofiber axis, the pure polythiophene nanofibers have high thermal conductivity (more than 4.4 Wm-1K-1), which is obviously higher than that of the bulk polythiophene material. In order to throw light on this situation, molecular dynamics (MD) method is used to detect the high thermal conductivity of a polythiophene chain. However, the MD method is highly sensitive to the choice of empirical potential function or simulation method. Even if the same potential function (ReaxFF potential function) is adopted, the thermal conductivity of a polythiophene chain could also have obviously different results. To overcome the instability of MD method, we use the first-principles to calculate the force constant tensor. In such a case the properties of quantum mechanics in a polythiophene chain can be reflected. In our algorithm, several disadvantages of MD that different potential functions or different simulation methods probably lead to very different thermal conductivities for the same transport system are avoided. Based on the density functional theory (DFT), the central insertion scheme (CIS) method and nonequilibrium Green's function (NEGF) approach are used to evaluate the isotope effect on thermal transport in a polythiophene chain, which includes 448 atoms in a scattering region and has a length of 25.107 nm. It is found that the thermal conductivity of a 32-nm-long pure polythiophene chain reaches 30.2 Wm-1K-1, which is close to the thermal conductivity of lead at room temperature. The reduction of average thermal conductance caused by C atom impurity is more remarkable than by S for a pure polythiophene chain when the mixing ratios of 13C to 12C and 36S to 32S are equal. The most outstanding isotope effect on quantum thermal transport appears when the mixing ratio of 13C to 12C is 1:1. It will cause the average thermal conductance to decrease by at least 30% in the polythiophene chain at room temperature. Moreover, we find that the thermal conductance of a pure polythiophene chain is inversely proportional to the atomic weight of carbon, and increases nonlinearly with the increasing atomic weight of sulfur. It is of significance to optimize the thermal conductance properties of polythiophene function material.
CONDENSED MATTER:ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2018, 67 (2): 027101.
doi:10.7498/aps.67.20171969
Abstract +
Optoelectronic integration technology which utilizes CMOS process to achieve the integration of photonic devices has the advantages of high integration, high speed and low power consumption. The Ge1-xSnx alloys have been widely used in photodetectors, light-emitting diodes, lasers and other optoelectronic integration areas because they can be converted into direct bandgap semiconductors as the Sn component increases. However, the solid solubility of Sn in Ge as well as the large lattice mismatch between Ge and Sn resulting from the Sn composition cannot be increased arbitrarily:it is limited, thereby bringing a lot of challenges to the preparation and application of direct bandgap Ge1-xSnx. Strain engineering can also modulate the band structure to convert Ge from an indirect bandgap into a direct bandgap, where the required stress is minimal under biaxial tensile strain on the (001) plane. Moreover, the carrier mobility, especially the hole mobility, is significantly enhanced. Therefore, considering the combined effect of alloying and biaxial strain on Ge, it is possible not only to reduce the required Sn composition or stress for direct bandgap transition, but also to further enhance the optical and electrical properties of Ge1-xSnx alloys. The energy band structure is the theoretical basis for studying the optical and electrical properties of strained Ge1-xSnx alloys. In this paper, according to the theory of deformation potential, the relationship between Sn component and stress at the critical point of bandgap transition is given by analyzing the bandgap transition condition of biaxial tensile strained Ge1-xSnx on the (001) plane. The energy band structure of strained Ge1-xSnx with direct bandgap at the critical state is obtained through diagonalizing an 8-level kp Hamiltonian matrix which includes the spin-orbit coupling interaction and strain effect. According to the energy band structure and scattering model, the effective mass and mobility of carriers are quantitatively calculated. The calculation results indicate that the combination of lower Sn component and stress can also obtain the direct bandgap Ge1-xSnx, and its bandgap width decreases with the increase of stress. The strained Ge1-xSnx with direct bandgap has a very high electron mobility due to the small electron effective mass, and the hole mobility is significantly improved under the effect of stress. Considering both the process realization and the material properties, a combination of 4% Sn component and 1.2 GPa stress or 3% Sn component and 1.5 GPa stress can be selected for designing the high speed devices and optoelectronic devices.
2018, 67 (2): 027301.
doi:10.7498/aps.67.20172055
Abstract +
Uranium film is a main functional component to realize the high efficiency conversion of laser to X-ray in the study on laser inertial confinement fusion. It also has important applications in relevant physics experiments. Due to its active chemical properties, the metal uranium film is extremely difficult to preserve in the atmosphere. A variety of methods may help to avoid the oxidation of uranium film, such as coating protective layer, vacuum or inert atmosphere protection. But under these conditions, the oxidation property of uranium film still needs experimental investigation. In this paper, the oxidation processes of uranium films under different atmospheres are studied by X-ray photoelectron spectroscopy (XPS) and depth profiling. Firstly, uranium films and gold-uranium multilayer films are prepared by ultra-high vacuum magnetron sputtering deposition, and then they are exposed to atmosphere, high purity argon and ultrahigh vacuum for a period of time. Finally, the distributions and valence states of oxygen and uranium elements are investigated by XPS depth profiling. The oxidation mechanism is analyzed according to the oxidation products and the microstructure characteristics of samples. The results show that the oxygen element is undetectable in the initial films. In the Au-U multilayer film, the protective ability of Au layer is greatly weakened by the micro-defects. The defect is not only the micro channel of oxygen entering into the sample directly, but also the origin of the interlaminar cracks at the Au/U interface. In about three weeks, the uranium layer is severely oxidized and large area lamination occurs. The oxidation products consist of a dense oxide thin film on uranium surface and corrosion pitting around the defects, which are mainly UO2. For the pure uranium film, the surface of the film is completely oxidized when it is exposed to high purity argon only for 6 h. The UO2 layers with different thickness values are formed on their surface, which is due to the rapid diffusion of oxygen atoms at the columnar grain boundaries of the film. After the sample is exposed to the ultra-high vacuum for 12 h, UO2 layer with a thickness of less than 1 nm is generated on the surface of the pure uranium film. In the etching of oxide by argon ion beams, the preferential sputtering effect of O is produced, and UO2 is reduced into non-stoichiometric UO2-x. The effect of preferential sputtering is weakened with the decrease of oxide content. When the oxide content is less than 10%, the reduction of UO2 can hardly be detected.
2018, 67 (2): 027303.
doi:10.7498/aps.67.20171827
Abstract +
The variational method has been widely used to study the electronic structures of heterostructure materials in spite of this method being less accurate than the numerical method, because analytical formulas for some electrical parameters can be derived using this method. However, effects of surface states on the two-dimensional electron gas (2DEG) have not been taken into account in the variational studies of GaN-based heterostructures. In the present study, analytical formulas for the electron wave function and ground state energy level of the 2DEG in InAlN/GaN heterostructures are derived using the variational method, and the influences of structural parameters of InAlN/GaN heterostructures on the electrical properties are discussed. In the theoretical model, evenly distributed surface states below the conduction band are assumed to be the origin of the 2DEG, and the polarization charges at the InAlN surface and the InAlN/GaN interface due to spontaneous and piezoelectric polarization effects in InAlN/GaN heterostructures are taken into account. A trial envelope wave function with two variational parameters is used to derive the expectation value of the total energy per electron. The variational parameters are determined by minimizing the expectation value. The model predicts a linear conduction band profile in InAlN barrier layer and an approximately triangular-shaped potential well on the GaN side of the InAlN/GaN interface. Electrons released from the surface states are confined in the potential well, forming the 2DEG. The 2DEG sheet density for the lattice-matched InAlN/GaN heterostructure with a 15 nm InAlN layer is 1.961013 cm-2, and the average distance from the InAlN/GaN interface of electrons is 2.23 nm. The 2DEG sheet density increases rapidly with InAlN thickness increasing when the InAlN layer exceeds the critical thickness, and starts to be saturated above 15 nm. The dependence of the calculated 2DEG sheet density on the InAlN thickness quantitatively agrees with recently reported experimental data. The increasing 2DEG sheet density results in increasing the ground state energy level and Fermi energy, and the energy spacing between the two also increases for containing more electrons. The polarization discontinuity at the InAlN/GaN interface decreases with increasing In mole fraction, causing the 2DEG sheet density to decrease, and thus the ground state energy level and the Fermi energy to decrease. This model is conducive to understanding the electrical behaviors of InAlN/GaN heterostructures and providing readily applicable formulas for studying the electron transport and optical transitions.
2018, 67 (2): 027801.
doi:10.7498/aps.67.20171730
Abstract +
Graphene exhibits excellent electronic and optical properties, which has been proposed as an advanced material for new generation of electronic and optical devices. We develop a detailed theoretical mode to investigate the optical properties of graphene-wafer systems. The photon-excited carriers and emission are obtained based on the mass-balance equation and the charge number conservation equation, which are derived from Boltzmann equation. The analytical results of photon excited carrier density and photon emission coefficient are achieved self-consistently in terahertz radiation fields. It is found that the photon excited carrier density increases with doped electron density or temperature decreasing. The higher the doped electron density and the lower the temperature, the larger the photon emission coefficient is. The optical emission increases with doped electron density increasing, and the optical emission increases with temperature decreasing. It shows that photon-excited carriers and emission of graphene can be effectively tuned by gate voltage. These theoretical results can be used to understand the relevant experimental findings. This theoretical study can benefit the applications in advanced optoelectronic devices based on graphene, especially terahertz devices.
2018, 67 (2): 027304.
doi:10.7498/aps.67.20171604
Abstract +
As a basic functional unit of molecular electronics, the structure of single molecule sandwiched between nano-electrodes has attracted a lot of interest in molecular science, in particular, its current-voltage (I-V) characteristic induced by an external field. Aiming at the molecular nano-junction which is composed of lead/molecule/lead, we use the method of extended master equation to compute the steady and transient current in the molecular nano-junction under the action of an externally applied electric field. The current can be adjusted by the external field, the relaxation in the molecule, the intra-molecular vibrational energy redistribution, etc. Owing to the strong electronic-vibrational coupling, the I-V curve has an inelastic characteristic in the molecular nano-junction and the stable current increases stepwise with the applied bias voltage increasing. The Franck-Condon blockage can be effectively removed by the external field. The molecular nano-junction being excited by different-width Gaussian pulses, the currents in the molecular nano-junction take different times to reach their steady state. The pulse width has a strong effect on the transient current enhancement. The transient current appears obviously for the 1 ps width pulse excitation. In this case the molecule is at a non-equilibrium state and the currents at both ends of the molecule are different. With the pulse width and the applied voltage increasing, the current through the molecular nano-junction tends to be balanced.
EDITOR'S SUGGESTION
2018, 67 (2): 027501.
doi:10.7498/aps.67.20172080
Abstract +
We describe the modeling of magnetoelectric (ME) effect in the plate-type Terfenol-D/PZT laminate composite by introducing a newly proposed interface coupling factor into the equivalent circuit model, aiming at providing a guidance for designing, fabricating and using the ME laminate composite based devices, such as current sensor, magnetic sensor, energy harvester, and wireless energy transfer system. Considering that the strains of the magnetostrictive and piezoelectric layers are not equal in actual operation due to the epoxy resin adhesive bonding condition, the equivalent circuit models of magnetostrictive and piezoelectric layers are created based on the constitutive equation and the equation of motion, respectively. An interface coupling factor kc is introduced which physically reflects the strain transfer condition between the magnetostrictive and piezoelectric phases. Specifically, the respective equivalent circuit models of magnetostrictive and piezoelectric layers are combined with an ideal transformer whose turn-ratio is just the interface coupling factor. Furthermore, the theoretical expressions containing kc for the longitudinal ME voltage coefficient v and the optimum thickness ratio noptim to which the maximum ME voltage coefficient corresponds are derived from the modified equivalent circuit model of ME laminate, where the interface coupling factor acts as an ideal transformer. To explore the influence of mechanical load on the interface coupling factor kc, two sets of weights, i.e., 100 g and 500 g, are placed on the top of the ME laminates, each with the same thickness ratio n in the sample fabrication for comparison. A total of 12 L-T mode plate-type ME laminate samples with different-thickness configurations are fabricated. The interface coupling factors determined from the measured v and the DC bias magnetic field Hbias are 0.15 for 500 g pre-mechanical load and 0.10 for 100 g pre-mechanical load, respectively. Furthermore, the measured optimum thickness ratios are 0.57 for kc=0.15 and 0.50 for kc=0.10, respectively. Both the measured ME voltage coefficient v and optimum thickness ratio containing kc agree well with the corresponding theoretical predictions. The relationship between the optimum thickness ratios under two different mechanical loads remains unchanged, i.e., the measured optimum thickness ratio for kc=0.15 is larger than for kc=0.10. The experimental results verify the reasonability and correctness of the introduction of kc in the modified equivalent circuit model. The possible reasons for different interface coupling factors under different loads are also qualitatively discussed in this paper.
REVIEW
2018, 67 (2): 027302.
doi:10.7498/aps.67.20171917
Abstract +
The multiple exciton generation (MEG), a process in which two or even more electron-hole pairs are created in nanostructured semiconductors by absorbing a single high-energy photon, is fundamentally important in many fields of physics, e.g., nanotechnology and optoelectronic devices. Many high-performance optoelectronic devices can be achieved with MEG where quite an amount of the energy of an absorbed photon in excess of the band gap is used to generate morei additional electron-hole pairs instead of rapidly lost heat. In this review, we present a survey on both the research context and the recent progress in the understanding of MEG. This phenomenon has been experimentally observed in the 0D nanocrystals, such as PbX (X=Se, S, and Te), InX (X=As and P), CdX (X=Se and Te), Si, Ge, and semi-metal quantum dots, which produce the differential quantum efficiency as high as 90%10%. Even more remarkably, experiment advances have made it possible to realize MEG in the one-dimensional (1D) semiconductor nanorods and the two-dimensional (2D) nano-thin films. Theoretically, three different approaches, i.e., the virtual exciton generation approach, the coherent multiexciton mode, and the impact ionization mechanism, have been proposed to explain the MEG effect in semiconductor nanostructures. Experimentally, the MEG has been measured by the ultrafast transient spectroscopy, such as the ultrafast transient absorption, the terahertz ultrafast transient absorption, the transient photoluminescence, and the transient grating technique. It is shown that the properties of nanostructured semiconductors, e.g., the composition, structure and surface of the material, have dramatic effects on the occurrence of MEG. As a matter of fact, it is somewhat hard to experimentally confirm the signature of MEG in nanostructured semiconductors due to two aspects:i) the time scale of the MEG process is very short; ii) the excitation fluence should be extremely low to prevent the multi-excitons from being generated by multiphoton absorption. There are still some controversies with respect to the MEG effect due to the challenge in both the experimental measurement and the explanation of signal data. The successful applications of MEG in practical devices, of which each is composed of the material with lower MEG threshold and higher efficiency, require the extraction of multiple charge carriers before their ultrafast annihilation. Such an extraction can be realized by the ultrafast electron transfer from nanostructured semiconductors to molecular and semiconductor electron acceptors. More recently, an experiment with PbSe quantum dot photoconductor has demonstrated that the multiple charge extraction is even as high as 210%. It is proved that MEG is of applicable significance in optoelectronic devices and in ultra-efficient photovoltaic devices. Although there are still some challenges, the dramatic enhancement of the efficiency of novel optoelectronic devices by the application of MEG can be hopefully realized with the rapid improvement of nanotechnology.
2018, 67 (2): 028801.
doi:10.7498/aps.67.20171956
Abstract +
The conversion efficiencies of perovskite solar cells based on organic-inorganic hybrid metal halide materials have broken through 22% in just a few years, which provides a ray of hope in solving the future energy problem, and receives great attention and research enthusiasm from the academic circle. However, what is followed is commercialization and industrialization process, which will greatly enhance the importance and urgency of the research and development of the green, non-toxic, highly-efficient, and lead-free perovskite solar cells. In order to speed up the development of these environment-friendly perovskite solar cells, we summarize the recent research progress in the perovskite solar cells from the two categories of Pb-free and less-Pb materials. In the Pb-free aspect Sn-based perovskite solar cells are emphatically introduced. A maximum efficiency of 8.12% is obtained for the solar cells based on FA0.75MA0.25SnI3, but it lags far behind the Pb-based competitors. This may be caused mainly by the oxidation of Sn2+ ions and the band mismatch with carrier transport materials, etc. So, for further improving the efficiency, it is very important to optimize the device structure and material properties, and understand the role played by Sn4+ ions in films. In addition, more attention should be paid to the inorganic halide double perovskite materials as potential solutions for the toxicity and stability issues. In the less-Pb part, Sn-doping contributes to a large reduction of lead content in the film, and a maximum efficiency of 17.6% for the (FASnI3)0.6(MAPbI3)0.4 perovskite solar cells is achieved with good long-term stability. What is even more interesting is that it can be utilized to construct tandem cells through the bandgap regulation after doping. However, it is very difficult to determine the optimum Sn-doping ratio. More systematic, rigorous and normative experiments are extremely necessary to reveal the interaction mechanism between Pb2+ and Sn2+. For other doped elements, the effects of their concentrations on the properties of thin films and the performance of solar cells are also emphatically discussed, and it is very urgent to have a further understanding of the working principles of devices and the fundamental functions of substitution elements. Thus, this review highlights the recent research efforts in the development of Pb-free and less-Pb perovskite solar cells and also provides a perspective of future development of new environment-friendly and high performance perovskite solar cells.
EDITOR'S SUGGESTION
2018, 67 (2): 024213.
doi:10.7498/aps.67.20172026
Abstract +
Three-dimentional (3D) display is one of the effective ways to obtain visual information feeling like actual environment. Since holographic technique can provide full depth information for human eyes, it is considered to be an ideal 3D display technique. However, it is limited by the features of display elements and devices, such as the time-space (time and space) bandwidth product, massive data processing speed and low image quality and so on. To improve the display quality, expand the time-space bandwidth product, improve the performance of the system, and overcome the limitation, optical elements and devices made from novel materials are introduced, such as metamaterials, metasurfaces and two-dimensional (2D) materials, and thus bringing new challenges and opportunities to holographic display. Meta-atom structure whose unit size is much less than wavelength is designed and fabricated specially, and it can realize the isotropical or anisotropical manipulation of the amplitude and phase of the light wave. By encoding the meta-atom structures into the hologram, the 2D or 3D images can be achieved. The development of a refreshable metamaterials and their applications in dynamic holographic display will be one of the most important topics in the future. Though the 2D or 3D holographic displays based on the elements and devices made from novel materials still have some basic problems, it is expected that they would bring new impetus and promising perspective for the future display market.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2018, 67 (2): 028802.
doi:10.7498/aps.67.20171689
Abstract +
To further improve the absorption of thin-film silicon solar cells (TFSSCs), it is essential to understand what kind of texture morphology could present the best light trapping effect, or rather, which structural parameter plays the most important role, and offers the required lateral feature size, height or others. In this paper, the influences of structural parameters of conical two-dimensional photonic crystal (2D PC) on each-layer absorption of the microcrystalline silicon thin film solar cells are numerically studied by using the finite-difference time-domain method when 2D PC is introduced into the intrinsic layer. The results show that both the intrinsic absorption and parasitic absorption are significantly enhanced via introduction of 2D PC into the intrinsic layer. The parasitic absorption is mainly caused by the ITO layer, and the intrinsic absorption shows a sinusoidal fluctuation with the increase of period. It is found that the aspect ratio (height/period) of the 2D PC has a decisive influence on the cell intrinsic absorption. When the period of the 2D PC is less than 1m, the intrinsic absorption first increases and then decreases with the increase of the aspect ratio, and reaches a maximum value with an aspect ratio of 1. For the case of period larger than 1m, the aspect ratio needed to obtain the maximum result is smaller than 1. What is more, the larger the period, the smaller the aspect ratio for maximizing the intrinsic absorption will be. The peak intrinsic absorption can be obtained when a 2D PC with a period of 0.5m and an aspect ratio of 1 is introduced. Compared with that of the flat cell, the short-circuited current density of the above optimized 2D PC cell can be significantly enhanced by 5.8 mA/cm2(from 21.9 to 27.8 mA/cm2), corresponding to a relative enhancement of 27%. In order to improve antireflection performance, it is critical to adopt a textured front-surface morphology where the aspect ratio is higher than 1/2. In addition, the intrinsic absorption increases with the increasing fill factor, and reaches a maximum value when the fill factor of the 2D PC is close to 0.9. The research results of this paper break through the traditional viewpoint of light trapping mechanism which points out that the light trapping effect is mainly dependent on the lateral feature size of the texture, and provide an important guide for obtaining optimized random or periodic texture via experiment.
2018, 67 (2): 028101.
doi:10.7498/aps.67.20171950
Abstract +
Micro-interconnection copper pillar bumps are being widely used in the packaging areas of memory chip and high performance computer due to their high density, good conductivity and low noise. Studying the interfacial behavior of copper pillar bump is of great significance for understanding its failure mechanism and microstructure evolution in order to improve the reliability of flip chip package. The thermoelectric stress test, in-situ monitor, infrared thermography test, and microstructure analysis method are employed to study the interfacial reaction, life distribution, failure mechanism and their effect factors of Cu/Ni/SnAg1.8/Cu flip chip copper pillar interconnects under 9 groups of thermoelectric stresses including 2104-3104 A/cm2 and 100-150℃. Under thermoelectric stresses, the interfacial reaction of Cu pillar can be divided into three stages:Cu6Sn5 growth and Sn solder exhaustion; the Cu6Sn5 phase transformation, exhaustion and the Cu3Sn phase growth; voids formation and crack propagation. The rate of Cu6Sn5 phase transforming into Cu3Sn phase is positively correlated with the current density. There are four kinds of failure modes including Cu pad consumption, solder complete consumption and transformation into Cu3Sn, Ni plating layer erosion and strip voids. An obvious polar effect is observed during the dissolution of Cu pads on the substrate side and the Ni layer on the Cu pillar side. When Cu pad is located at the cathode, the direction of electron flow is the same as that of the heat flow, and it can accelerate the consumption of Cu pad and the growth of Cu3Sn. When Ni layer serves as the cathode, the electron flow can enhance the consumption of Ni layer. Under 150℃ and 2.5104 A/cm2, the local Ni barrier layer is eroded after 2.5 h, which results in the transformation of Cu pillar on the Ni side into (Cux, Niy)6Sn5 and Cu3Sn alloy. The life of Cu pillar interconnection complies well to the 2-parameter Weibull distribution with a shape parameter of 7.78, which is a typical characteristic of cumulative wear-out failure. The results show that the intermitallic growth behavior and failure mechanism at Cu pillar interconnects are significantly accelerated and changed under thermoelectric stresses compared with the scenario under the single high temperature stress.
EDITOR'S SUGGESTION
2018, 67 (2): 028201.
doi:10.7498/aps.67.20171855
Abstract +
Nowadays flexible solid-state supercapacitors (FSCs) have received more and more attention than conventional capacitors due to the good operability and flexible fabrication process as well as high specific/volumetric energy density. In general, carbon based materials including amorphous carbon, carbon nanotube, grapheme, etc. can be used to fabricate electrolytic double-layer capacitance (EDLC)-type FSCs due to its extraordinary cyclic stability at high current density. Aligned carbon nanotube (ACNT) arrays are one of the ideal electrode candidates for energy storage due to their good capacity, highly efficient charge transfer rate, excellent rate performance and long cycle life compared with those of other carbon-based materials carbon nanotubes. However, the low density and the weak interaction between the carbon tubes cause the CNT arrays to tend to easily collapse during processing and transferring. Thus pure carbon nanotube arrays are unable to be directly used to assemble flexible electronic devices. In this paper, we use ethyl alcohol to shrink the CNT array to increase the density and mechanical strength. At the same time we embed the conductive polyvingle alcohol (PVA) gel into the carbon nanotube array to fabricate a flexible solid supercapacitor. Hydrogel-based solid electrolytes have been long considered to be used to prepare FSCs, because this method possesses obvious advantages including low cost, good environmental compatibility and simple manufacturing process. The ACNT/PVA complex can maintain good mechanical stability and flexibility during its folding and bending, and can also keep the high orientation of carbon nanotubes. The maximum capacitance of the hybrid flexible device can reach 458 mFcm-3 at a current density of 10 mAcm-3, which is much higher than the capacitance reported in the literature. After 5000 charging-discharging cycles, a capacity still keeps nearly 100%. The maximum energy density of CNTs/gel composite device can reach 0.04 mWhcm-3 with an average power density of 3.7 mWcm-3. The capacitance can be further increased to 618 mFcm-3 by a simple in-situ electrochemical oxidation treatment. The energy density can be further increased to 0.07 mWhcm-3 by the electro-oxidation treatment. The electrochemical performance of the device is far superior to that of EDLC-typed FSC reported in the literature. Additionally the equivalent series resistance (RESR) of the devices decreases from 120 to 30 and also the charge transfer resistance declines from 90 to 10 . This is mainly due to the effect of pseudo capacitance and electro-wetting effect caused by electro-oxidation. This easy-to-assemble hybrid devices thus potentially pave the way for manufacturing wearable devices and implantable medical devices.
2018, 67 (2): 028401.
doi:10.7498/aps.67.20171246
Abstract +
Orthogonal frequency-division multiple passive optical network (OFDM-PON) has emerged as one of the most promising solutions to meet the requirements for the next-generation wide-band optical access network with high capacity, strong fiber dispersion tolerance, and flexible resource allocation. However, like other optical access network in which the downstream signal is broadcasted to all the optical network units (ONUs), OFDM-PON is vulnerable to being eavesdropped. Thus the security of OFDM-PON should be taken seriously into consideration. Recently, some chaos based encryption methods, including chaotic scrambling and permutation, hyper-chaotic system and fractional Fourier transformation, chaos based IQ encryption method and chaos based two-dimensional scrambling, have been proposed to enhance the physical layer security of OFDM-PON system. Owing to the special chaos-related characteristics, such as ergodicity, pseudo randomness, and high sensitivity to the initial values, etc., these encryption methods are of high physical layer security. However, in most of these schemes, key distribution is not considered. In this paper, we propose a three-dimensional encryption OFDM-PON based on dynamic chaos-iteration. The key distribution is implemented through the dynamic chaos synchronization between the transmitter and receiver. The receiver tries to synchronize his chaos system with the transmitters' by calculating the correlation index of the synchronization sequence, which comes from the transmitter and is controlled by dynamic parameters in the parameter sets. The calculation is not very complex because the transmitter and receiver are acquainted with the parameter sets. The synchronized chaos system is used to generate keys for both encryption and decryption. In the proposed encryption scheme, one ONU is connected with four users, and the message is irrelevant to the users. Quadrature amplitude modulation (QAM) symbols from the users are mapped randomly onto the subcarriers in a flame based on the chaotic matrix M1. For the M1 is changeable, the number and position of subcarriers for different users are dynamically varying. Then the matrix M2 generated from chaos system is utilized to mask all QAM symbols. Finally the QAM symbol matrix is multiplied by an invertible chaotic matrix M3 to realize subcarrier perturbation. These three key matrixes are generated from the two-dimension logistic iteration chaos system, to which the initial sensitivity increases up to 10-15. The output sequence of the chaos system after quantification process is of good self-correlation and cross-correlation characteristic and can pass all NIST SP800-22 randomness tests. The key space of the encryption scheme is over 1086, which would be against exhaustive attack effectively. Specifically, a proof-of-principle experiment is conducted to demonstrate the aforementioned proposed scheme. In the experiment, a 13.3 Gb/s encrypted 64QAM OFDM signal transmits over 25 km standard single mode fiber in an OFDM-PON and successfully decrypts at the legal receiver. For an eavesdropper lacking correct keys, the received QAM constellation is totally in disorder and the bit error rate increases up to 0.46, which indicates that not any useful message is eavesdropped. The proposed scheme provides a promising candidate for the next-generation secure optical access networks.
NUCLEAR PHYSICS
2018, 67 (2): 022901.
doi:10.7498/aps.67.20171507
Abstract +
Thrust density distribution of ion thruster is an important factor that affects the orbit correction and station keeping of the spacecraft. Current empirical models mainly concern themselves with the overall thrust of the ion thruster, yet the thrust density distribution has not been fully understood. Hence it is necessary to investigate the thrust density characteristics of the ion thruster to devise the approach to optimizing the thruster performances. In this study, the thrust density characteristics of the ion thruster is analyzed and discussed by combining the empirical and theoretical methods. An ion thruster utilizes biased grids to extract ions from discharge chamber and accelerate them to high velocities, thereby forming a beam and generating thrust. In this paper, we analyze the working process of the ion thruster. The thrust expression as a function of beam micro-particle parameters is presented. Meanwhile the transport process of the plasma in the beam stream is simulated by the particle in cell-Monte Carlo (PIC-MCC) method for two-grid optics. The motion behavior of ions is modeled by the PIC method, while the collisions of particles are modeled by the MCC method. In the simulation, the particle trajectories are traced and the micro information about ejected charged ions is recorded with respect to singly charged ion, doubly charged ion and charge exchanged (CEX) ion. By analyzing the density and axial velocity of the charged particles in the beam stream, the thrust of the beam from a single grid hole can be calculated, based on which the thrust distribution of the thruster can be inferred by considering the distribution of plasma density at the exit of discharge chamber. Moreover, the above theoretical analysis of the thrust density is tested experimentally. The studies show that the thrust contribution percentages of the singly charged ion, doubly charged ion and CEX ion in the beam current are 84.63%, 15.35%, and 1.82%, respectively. Apparently, the main contributions to the thrust are made by the singly charged ions and doubly charged ions in the beam plasma, while the CEX ions have a trivial effect on the variation of the thrust. The distribution of the thrust density shows good symmetry along the central axis and it levels off after a fast decline in the radial direction. Comparisons of empirical and numerical results with the experimental results show that the empirical results have an error of about 4.1% and the numerical results have an error of about 2.8%. This indicates that the computational accuracy of our numerical model is better than that of the empirical model This work provides a reference for optimizing the thrust density uniformity of an ion thruster.