基于合金介电常数的可控特性增强光子自旋霍尔效应

Enhanced photonic spin Hall effect due to controllable permittivity of alloy film

基于平面角谱理论,系统研究了BK7玻璃-合金薄膜-空气结构中合金介电常数的变化对反射光自旋霍尔效应的调控规律.数值仿真结果表明,该结构发生表面等离激元共振的共振角主要受合金介电常数实部的影响,随介电常数实部的增加而增大,而虚部对共振角变化的影响相对较小.不同介电常数合金在其共振角处得到的较大光子自旋霍尔效应横移呈集中的带状分布,选取介电常数-2.8+1.6i的Ag-Ni合金时,光子自旋霍尔效应横移能达到1.2×10~5 nm.研究还发现将入射角固定为44.1°时,光子自旋霍尔效应横移随合金介电常数的变化呈轴对称分布,并以最大值为中心呈球面状辐射,离中心点越远光子自旋霍尔效应横移越小.选取介电常数-10.6+1.2i的Ag-Au合金时,光子自旋霍尔效应横移最大能达到8000 nm,相比于以往纯金属纳米结构BK7玻璃-金-空气中得到的最大光子自旋霍尔效应横移3000 nm有了较大的提高.该研究不仅能够有效增强光子自旋霍尔效应,还能为设计等离激元共振传感器等纳米光子器件提供理论依据.

Photonic spin Hall effect （SHE） is an interesting transport phenomenon, and has attracted growing attention. The spin-dependent splitting of photonic SHE as a weak effect is just tens of nanometers so that it can usually be detected indirectly with the weak measurement techniques. To detect it directly and use it properly, many efforts have been devoted to enhancing the photonic SHE. Recently, the surface plasmon resonance （SPR） excited by a pure nanometal structure is used to enhance the photonic SHE. However, the pure metal permittivities are limited, therefore the regulation of the photonic SHE is also restricted. It is worth mentioning that the alloy made from the pure metal with different composition proportions can achieve the artificial control of permittivity. More importantly, the alloy can also be used to manipulate the SPR. In this paper, we systematically investigate the photonic SHE in a nanoalloy structure composed of BK7 glass, alloy film and air in order to realize the enhancement of photonic SHE. First of all, the resonant angle of SPR varying with the permittivity of alloy is studied by using the angular spectrum theory of beam. It is found that the resonant angle of the SPR is mainly influenced by the real part of the permittivity of alloy, while the imaginary part has little influence on it. The resonant angle of SPR will increase with the increase of the real part of the permittivity. Secondly, the spin-dependent splitting is studied by changing the alloy permittivity when the incident angle is set to be a resonant angle. We find that the distribution of the larger spin-dependent splitting at the resonant angle is zonal. The optimal permittivity of alloy film is c2 = -2.8 ＋ 1.6i, and the alloy can be composed of Ag and Ni according to the Bruggerman theory. Under the condition of the optimal permittivity, the spin-dependent splitting reaches about 1.2×10^5 nm at a resonant angle of 51.5^, which is about 40 times larger than the previous result in a pure nanonmtal structure. Finally, when the incident angle is fixed at 44.1%, it is revealed that the spin-dependent splitting varying with the permittivity is axially symmetric and spherical radiation is centered at a maximum value. The farther away from the center, the smaller the corresponding beam shift is. The alloy permittivity in the spherical radiation center is c2 = -10.6 ＋ 1.2i, which can be composed of Au and Ag. The value of spin-dependent splitting reaches about 8000 nm, which is greatly improved when compared with the previous maximum value 3000 nm in a pure nanometal structure. These findings can effectively enhance the photonic SHE and provide theoretical basis for the research and development of nanophotonic devices such as the SPR-based sensor.