来自扭曲光的二维激子和光子轨道angular momentum的命运
Two-dimensional excitons from twisted light and the fate of the photon's orbital angular momentum
摘要：
作为两个带相反电荷的粒子的束缚态，激子作为氢原子的电子类似物从光激发的半导体中出现。在二维（2D）情况下，在量子阱系统或真正的2D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$n$和用于沿着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，只有激子的$j=0$态是光学活性的。在这里，我们考虑的情况下，空间结构光源，特别是扭曲光束与非零轨道角动量每光子。在所谓的偶极近似下，光源的空间变化发生在远大于半导体晶胞尺寸的长度尺度上，我们表明光子（线性和/或角）动量耦合到激子的质心（线性和/或角）动量。我们的研究建立了激子的内部状态的选择规则，从而激子光谱，是独立于光源的空间结构。
英文摘要：
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.
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