Universality of optical absorptance quantization in two-dimensional group-IV, III-V, II-VI, and IV-VI semiconductors

The optical absorptance of a single graphene layer over a wide range of wavelengths is known to be remarkably constant at the universal value pi alpha where alpha is the fine structure constant. Using atomistic tight-binding calculations, we show that the absorptance spectra of nanometer-thin layers (quantum wells) of group-IV, III-V, II-VI, or IV-VI semiconductors are characterized by marked plateaus at integer values of pi alpha, in the absence of excitonic effects. In the case of InAs, the results obtained are in excellent agreement with the currently available experimental data. By revisiting experimental data on semiconductor superlattices, we show that pi alpha is also a metric of their absorption when normalized to a single period. We conclude that the pi alpha quantization is universal in semiconductor quantum wells provided that excitonic effects are weak and is therefore not specific to the zero-gap graphene case. The physical origin of this universality and its limits are discussed using analytical models that capture the main underlying physics of the lowest optical transitions in III-V and II-VI semiconductor quantum wells. These models show that the absorptance is ruled by pi alpha independent of the material characteristics because of the presence of a dominant term in the Hamiltonian, linear in the wave vector k (similar to V center dot k), which couples the conduction band to the valence bands. However, the prefactor in front of pi alpha is not unity as in graphene due to the different nature of the electronic states. In particular, the spin-orbit coupling plays an important role in bringing the absorptance plateaus closer to integer values of pi alpha. The case of IV-VI semiconductor (PbSe) quantum wells characterized by a rocksalt lattice and multivalley physics is very similar to that of graphene, with the specification that a "massful gap" is formed around the Dirac points.