Layer- and frequency-dependent second harmonic generation in reflection from GaSe atomic crystals

We report optical second-harmonic generation (SHG) in reflection from GaSe crystals of 1 to more than 100 layers using a fundamental picosecond pulsed pump at 1.58 eV and a supercontinuum white light pulsed laser with energies ranging from 0.85 to 1.4 eV. The measured reflected SHG signal is maximal in samples of similar to 20 layers, decreasing in thicker samples as a result of interference. The thickness-and frequency-dependence of the SHG response of samples thicker than similar to 7 layers can be reproduced by a second-order optical susceptibility that is the same as in bulk samples. For samples similar to 7 layers, the second-order optical susceptibility is reduced compared to that in thicker samples, which is attributed to the expected band-gap increase in mono-and few-layer GaSe.