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激子极化激元是在半导体微腔中在一定条件下出现的一种准粒子。这种准粒子是激子(强耦合作用下的电子-空穴对)与光子的耦合。它的粒子自旋符合玻色-爱因斯坦统计。在量子涡旋陀螺的概念下,激子极化激元被囚禁在一个具有圆周对称性的半导体微腔中。微腔的上下表面为分布式布拉格反射镜(Distributed Bragg Reflector,DBR)。这样,一定波长的泵浦光就可以在微腔内来回振荡,形成平板形法布里-珀罗腔。光子在微腔中来回振荡的传播模式称为腔模。这样可以在微腔中形成激子-光子强耦合,使它们在极化作用下形成“激子极化激元”。当泵浦光超过一定阈值时,这些激子由于其强烈的非线性相互作用,能量会逐渐减低到基态,形成BEC。通过携带OAM的涡旋光对这种凝聚态进行操纵,可以得到量子化的叠加态涡旋,这种量子化的叠加态涡旋可以表征BEC体系在惯性系下显示出的陀螺效应。
如图1所示,考虑陀螺仪的旋转特性,笔者研究了光驱动下圆周对称平面半导体微腔中激子极化激元凝聚的演化过程。激子极化激元场分布可以看做是激子场和光子场相互耦合形成的,具有典型的双分量结构。但是,它同时又是独立的,因此,根据研究对象的不同,研究激子极化激元的激发特性可以从两个方面着手,即双分量模型和单分量模型。
Figure 1. Exciton polariton system in microcavity of quantum vortex gyroscope. (a) Flat microcavity structure driven by pump light; (b) System of exciton polariton condensates on the rotational state; (c) Formation process of exciton polariton under light excitation[15]
这里用色散关系来描述微腔中激子和光子之间的能量耦合。色散关系由下列数学表达式给出:
半导体微腔中的激子极化激元场可以用激子场
$ {\overset{\lower0.5 em\hbox{$\smash{\scriptscriptstyle\frown}$}}{\psi } _X}\left( {r,t} \right) $ 与光子场$ {\overset{\lower0.5 em\hbox{$\smash{\scriptscriptstyle\frown}$}}{\psi } _C}\left( {r,t} \right) $ 的耦合来描述。这是一个写在极坐标空间中的双分量模型哈密顿量。$ i,j \in \left\{ {X,C} \right\} $ 表示空间坐标,分别表示激子场和光子场的分量。这些分量满足玻色交换关系,即:单个粒子的哈密顿量h为:
此时,激子与光子之间的耦合可以用矩阵中的
${\varOmega _R}$ 项来描述,称为拉比分裂。光子场的色散关系由以下公式给出:激子极化激元的色散关系由对角化的单粒子哈密顿量h给出,即:
以上是激子极化激元相位刻印的理论基础和实现原理。其本质在于光子与激子的耦合,而耦合过程就是能量与相位信息传递的过程。
量子涡旋陀螺仪体系的数值模拟,主要研究对象是半导体微腔中受激电子-空穴对的叠加态和演化特征,不包括光子成分。基于此,考虑选择一种单分量模型[10]:
这也称为Gross-Pitaevskii (GP)方程。用波函数
$\psi \left( r \right)$ 来描述激子极化激元场,其中${V_{ext}}\left( r \right)$ 为激子场感受到的结构势垒;$P\left( r \right)$ 为泵浦项;$g$ 表示激子极化激元之间的非线性相互作用;$\gamma $ 为系统损耗项;$\eta $ 为激励饱和参数。为了便于计算各系统参数对陀螺效应的影响,建立了GP方程的数值模型,并根据该模型计算和研究了不同初始值和边界条件下激子极化激元的演化过程。数值模型构建方法如图2所示。该GP方程的数值模型由主程序和若干子程序组成。其中主程序为系统各变量赋值并定义初始解,然后在for循环中实现激子极化激元叠加态涡旋随时间演化的过程的计算,得到
$\psi \left( r \right)$ 的概率密度${\left| {\psi \left( r \right)} \right|^2}$ 和相位分布随时间演化的结果。每个子程序都可以被主程序调用。差分方法如图3所示。子程序主要包括系统几何参数的定义、仿真变量的定义、势函数
${V_{ext}}\left( r \right)$ 的定义、四阶Runge-Kutta法差分、时域FDTD、柱坐标下的拉普拉斯算子,以及泵浦光和信号光的数值模型。通过系统变量的定义,将所有变量定义在一个字典中,实现子程序之间的相互调用。数值模型的变量如表1所示,均进行了无量纲处理。为了保证变量具有正确的物理意义,并使计算过程正确,对这些变量的取值范围和取值类型进行了指定,具体规定将在下文体现。通过调整系统差分步长和系统仿真总时间来控制计算精度和迭代速度。通过调整激子极化激元系统特性变量可以研究失谐、非线性相关因子、驰豫与饱和因子等重要系统特性对系统演化过程的影响,并以此进一步求出系统稳定或失稳的边界值。此外,通过调节泵浦光的参数,可以研究不同结构和不同强度的泵浦光对系统稳定性的重要影响。Important parameters Parameter meaning Important parameters Parameter meaning t0 Time step nt Calculate the total time t_order Time difference order IOAM Topological charge number NDIM Radial lattice number NANGLE Angular lattice number r1 Microcavity inner diameter r2 Microcavity outer diameter DR Radial lattice length DA Angular lattice point length Meff Effective mass g Nonlinear interaction γ Inherent loss of system η Saturation compensation term Pw Pump light size Pamplitude Pump light intensity r0 Pump optical center rw Light center nrhalfwave Radial half wavelength number φ0 Initial wave function Table 1. Key parameters of the numerical model and the corresponding parameter meaning
Simulation analysis of some key parameters of quantum vortex gyroscope (Invited)
doi: 10.3788/IRLA20220004
- Received Date: 2021-12-31
- Rev Recd Date: 2022-03-05
- Publish Date: 2022-05-06
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Key words:
- quantum vortex gyroscope /
- exciton polaritons /
- orbital angular momentum /
- vortex superposition state /
- time-space evolution characteristics
Abstract: The exciton polaritons in the semiconductor microcavity driven by light is a hot research field in physics and optics in recent years, and the superposition quantized vortex of the Bose-Einstein Condensates (BEC) driven by light in the microcavity has subversive potential application value in the field of quantum sensing. An accurate mathematical model via Runge-Kutta Difference and FDTD finite element method was constructed to characterize the time-space evolution of the quantum vortex gyrotron polariton system. On this basis, the influence of some key parameters related to pump light, signal light and semiconductor microcavity materials on the evolution characteristics of the quantum vortex gyroscope exciton polariton condensate was studied. For the pump light and signal light, the light intensity and geometric size of the annular spot were considered. Meanwhile, the effect of the microcavity material on the exciton polariton system was converted into the effect of the effective mass on the BEC system through mathematical transformation. By scanning a lot of parameters, some key factors affecting the performance of the quantum vortex gyroscope were obtained, including the geometric parameters and intensity of the pump light, the related influence of the pump light and the signal light, and the material properties of the semiconductor microcavity. The relationship between material properties and superposition state evolution of quantum vortex gyroscope was calculated by characterizing the relationship between effective mass and properties of different microcavity materials, and the range of reasonable values for effective mass was found to be narrow. These works provided an important reference for the engineering prototype development of the quantum vortex gyroscope.