Quantum of optical absorption in two-dimensional semiconductors

The optical absorption properties of free-standing InAs nanomembranes of thicknesses ranging from 3 nm to 19 nm are investigated by Fourier transform infrared spectroscopy. Stepwise absorption at room temperature is observed, arising from the interband transitions between the subbands of 2D InAs nanomembranes. Interestingly, the absorptance associated with each step is measured to be similar to 1.6%, independent of thickness of the membranes. The experimental results are consistent with the theoretically predicted absorptance quantum, A(Q) = pi alpha/n(c) for each set of interband transitions in a 2D semiconductor, where alpha is the fine structure constant and n(c) is an optical local field correction factor. Absorptance quantization appears to be universal in 2D systems including III-V quantum wells and graphene.