Recently, a new type of 2-D material, free standing InAs nanomembranes (thickness of 3 - 19 nm), as a representative of III-V semiconductors, was realized by layer transfer and this enables optical studies of 2-D InAs which were previously inaccessible, by decoupling those ultra-thin layers from original growth substrates to any optically transparent substrates. By using Fourier Transform Infrared (FTIR) spectroscopy, we directly observe the optical transitions from 2-D subbands, with energy spacing in line with the particle in the box model. Furthermore, it is found that the individual absorption steps from interband transitions plateau at ~1.6% for all samples, despite that the thickness is being changed by ~6x. The electron-photon interaction is also studied through Fermi’s Golden rule, we found that all the materials parameters such as carrier effective masses and bandgap cancel out, leading to a nearly material-independent absorptance of A_Q ≈ 8πα/(3nr) for each optical transition step, where nr is between 3 and 4 for most semiconductors in the wavelength range of interest. The work here presents a universal law of absorption for 2-D semiconductors. Besides its significance in the basic understanding of electron-photon interactions in quantum confined semiconductors, this result provides a new insight toward the use of 2-D semiconductors for novel photonic and optoelectronic devices.
Project end date: 08/15/12