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本文主要研究铯原子Λ型三能级原子的部分PT对称和相变, 利用铯原子基态$\left| {6{{\mathrm{S}}_{1/2}}, F = 3} \right\rangle $、$| 6{{\mathrm{S}}_{1/2}}, $$ F = 4 \rangle $和激发态$\left| {6{{\mathrm{P}}_{3/2}}, F' = 4} \right\rangle $组成Λ型三能级原子系统, 由失谐Δ3 = 607 MHz的探测光与耦合光形成双光子拉曼吸收, 构成损耗通道. 增加了共振作用于能级$\left| {6{{\mathrm{S}}_{1/2}}, F = 3} \right\rangle $与$\left| {6{{\mathrm{P}}_{3/2}}, F' = 4} \right\rangle $跃迁的泵浦光改变两个基态能级的布居, 从而使Λ型三能级系统的吸收减小, 在一定条件下形成原子系统的增益通道, 从而构成部分PT对称的原子系统. 实验中通过改变耦合光和探测光的腰斑比σ, 观察到部分PT对称系统中由对称向破缺相的转变. 此外研究了探测光束强度分布的不对称程度Dasym, 精确地测量了部分PT对称的破缺点, 理论计算与实验测量结果相符. 本文所报道的部分PT对称性及其相变的结果, 为主动操纵非厄米系统中的多维激光束开辟了一条途径, 并在设计激光不同部分的光放大和衰减光学器件方面具有潜在的应用价值.
Parity-time (PT) in atomic systems is of great significance for exploring exotic phenomena in non-Hermitian physics and non-Hermitian systems. It has been found that if PT symmetry is satisfied only in a certain spatial direction, then the Hamiltonian of the system still has a spectrum with eigenvalues of real numbers, which is called partial PT symmetry. In this paper, we use a Λ-type three-level atomic system, which is composed of two ground states $\left| {6{{\mathrm{S}}_{1/2}}, F = 3} \right\rangle $,$\left| {6{{\mathrm{P}}_{3/2}}, F' = 4} \right\rangle $and an excited state $\left| {6{{\mathrm{P}}_{3/2}}, F' = 4} \right\rangle $of cesium atom, to investigate the partial PT symmetry. A probe laser with the detuning of Δ3 = 607 MHz and a coupling laser satisfy the condition of two-photon Raman absorption of cesium atom, forming a loss channel. In order to construct the gain channel, we add the repumping laser that resonates during the transition of $\left| {6{{\mathrm{S}}_{1/2}}, F = 3} \right\rangle $to $\left| {6{{\mathrm{P}}_{3/2}}, F' = 4} \right\rangle $, changing the population of the two ground state energy levels, thus reducing the absorption of the Λ level system and forming the gain channel of the atomic system under certain conditions. In order to obtain the equilibrium condition of the partial PT-symmetric system, firstly, the light spot of the repumping laser in the experiment is covered by the probe laser, and then the repumping laser is moved to overlap with half of the probe laser of the detection light. When the gain and loss are balanced, the partial PT-symmetric system is in equilibrium. By changing the beam-waist ratio σ of the coupling laser to the probe laser, the transition from symmetry to broken phase is observed in partial PT-symmetric systems. By measuring the asymmetry of the detection-beam intensity distribution Dasym, we can accurately determine the partial PT symmetry breaking point, and the breaking point is located at $\sigma = {\sigma _{{\mathrm{cr}}}} \approx 3.8$. The theoretical calculations are in good agreement with the experimental measurements. The results of partial PT symmetry and its phase transition, reported in this study, open up a way to actively manipulate multidimensional laser beams in non-Hermitian systems and have potential applications in the design of optical devices for laser amplification and attenuation in different parts of the laser. -
Keywords:
- partial PT symmetry /
- gain and loss /
- PT symmetry broken
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