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The ability to focus light on a subwavelength scale is essential in modern photonics. Optical microfiber-based sub-wavelength focusing will allow a miniaturized, flexible and versatile tool for many applications such as biomedical imaging and optomechanics. For a separate mode exited from an optical micro-/nanofiber endface, the photons will experience significant diffraction into the free space. This situation can be changed by incorporating two-mode interference along with the specific spatial distributions of both E-field amplitude and phase. Herein we report a novel approach to realizing sub-wavelength focusing based on the two-mode interference exited from an optical microfiber endface. By utilizing specific distributions of E-field amplitude and phase of two interacting optical modes, interference field patterns with a single focus (e.g., via a two-mode set of HE 11and HE 12) or multiple foci (e.g., via a two-mode set of HE 11and HE 31) can be obtained. Then, it is proved that the constructed foci will readily facilitate and selective trapping of nanoparticles. Circular polarization of optical mode is utilized in order to bring in angular symmetry of sub-wavelength focusing patterns compared with linear polarized optical modes. Our simulation results show that the smallest focal spot produced from the EH 11and HE 12mode interference has a full width at half-maximum (FWHM) of ~ 348 nm (i.e. 0.65 λ). Such a subwavelength focusing field is applied to the optical trapping of an 85 nm-diameter polystyrene nanosphere. Further calculation reveals that the stable trapping can be fulfilled with axial and transverse trap stiffness of 11.48 pN/(μm·W) and 64.98 pN/(μm·W), as well as axial and transverse potential well of 101 k BT/W and 641 k BT/W via two-mode interference of HE 11and HE 12. These values demonstrate the great improvement over conventional tapered fibers. Further investigations show that different foci, via a two-mode set of HE 11and HE 31, exhibit unlike trap stiffness and potential wells, justifying the potential for nanoparticle size sorting. Based on the flexible all-fiber device, this subwavelength focusing strategy by two-mode interference may find promising applications in optical manipulation, superresolution optical imaging, data storage and nanolithography.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] -
模式组合 模式1的分布特点 模式2的分布特点 聚焦光斑的分布特点 |E| Re(Ex) |E| Re(Ex) |E| 最小半高宽 工作距离/μm HE11+EH11 类高斯 类高斯 类8字型圆环 略相连的四瓣区域 椭圆形 0.83λ 0.38 HE11+HE31 类高斯 类高斯 空心圆环 独立的四瓣区域 双焦点 0.97λ 0.67 EH11+HE31 单一圆环 略相连的四瓣区域 空心圆环 独立的四瓣区域 四焦点 0.78λ 0.10 HE11+HE12 类高斯 类高斯 同心双环 同心双环 类高斯 1.21λ 0.35 EH11+ HE12 单一圆环 略相连的四瓣区域 同心双环 同心双环 三明治形 0.65λ 0.3 圆偏振态 HE11+EH11 — — — — 类高斯 1.77λ 0.40 模式组合 颗粒横向位置(x,y)/μm 平衡位置/μm Ux/y/(kBT·W–1) Uz/(kBT·W–1) κx/y/(pN·μm–1·W–1) κz/(pN·μm–1·W–1) HE11+EH11 (0, 0) 0.58 159 80 2.22/24.66 3.57 HE11+HE31 (±0.41, 0) 0.77 99 25 11.53 4.0 EH11+HE31 (0, ±0.5) 0.34 179 26 14.64 1.34 HE11+HE12 (0, 0) 0.43 641 101 64.98 11.48 EH11+ HE12 (0, 0) 0.30 288 38 88.07 10.11 圆偏振态 HE11+EH11 (0, 0) 0.43 185 79 17.6 4.1 a)捕获颗粒为PS小球, 直径85 nm;
b)Ux/y和Uz分别为横向和纵向的势阱深度;κx/y和κz分别为横向和纵向的捕获刚度.结构 尺寸/nm Ux/y/kBT/W Uz/(kBT·W–1) κx/y/(pN·μm–1·W–1) κz/(pN·μm–1·W–1) 文献 光纤锥 85 150 70 22 4 [27] 光纤锥微透镜 85 500 240 310 60 [27] 普通单模光纤超透镜 2000 — — 90 8 [37] 普通多模光纤微透镜 200 4.6×103 6.3×103 720 250 [36] 本工作 85 641 101 64.98 11.48 — a)文献[37]捕获的颗粒为SiO2球, 其余文献的捕获颗粒为PS小球;
b)Ux/y和Uz分别为横向和纵向的势阱深度;κx/y和κz分别为横向和纵向的捕获刚度. -
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