Two-dimensional excitons from twisted light and the fate of the photon's orbital angular momentum
Two-dimensional excitons from twisted light and the fate of the photon's orbital angular momentum
复制标题
DOI：
10.1103/physrevb.105.205202
复制
发表时间：
2022-01
影响因子：
3.7
通讯作者：
T. Grass;U. Bhattacharya;J. Sell;M. Hafezi
中科院分区：
文献类型：
--
作者：
T. Grass;U. Bhattacharya;J. Sell;M. Hafezi
作为两个带相反电荷的粒子的束缚态，激子从光激发半导体中出现，作为氢原子的电子类似物。在二维 (2D) 情况下，无论是在量子阱系统还是真正的二维材料（例如过渡金属二硫属化物）中实现，激子的相对运动由两个量子数描述：主量子数 $n$ 和沿垂直轴的角动量的量子数 $j$。角动量守恒要求在平面波照射的系统中只有$j=0$状态的激子具有光学活性。在这里，我们考虑空间结构光源的情况，特别是每个光子具有非零轨道角动量的扭曲光束。在所谓的偶极近似下，光源的空间变化发生在比半导体晶胞尺寸大得多的长度尺度上，我们表明光子（线性和/或角）动量与激子的质心（线性和/或角）动量耦合。我们的研究表明，激子内部状态的选择规则以及激子光谱与光源的空间结构无关。
As the bound state of two oppositely charged particles, excitons emerge from optically excited semiconductors as the electronic analogue of a hydrogen atom. In the two-dimensional (2D) case, realized either in quantum well systems or truly 2D materials such as transition metal dichalcogenides, the relative motion of an exciton is described by two quantum numbers: the principal quantum number $n$, and a quantum number $j$ for the angular momentum along the perpendicular axis. Conservation of angular momentum demands that only the $j=0$ states of the excitons are optically active in a system illuminated by plane waves. Here we consider the case for spatially structured light sources, specifically for twisted light beams with non-zero orbital angular momentum per photon. Under the so-called dipole approximation where the spatial variations of the light source occur on length scales much larger than the size of the semiconductor's unit cell, we show that the photon (linear and/or angular) momentum is coupled to the center-of-mass (linear and/or angular) momentum of the exciton. Our study establishes that the selection rule for the internal states of the exciton, and thus the exciton spectrum, is independent from the spatial structure of the light source.