Second harmonic generation in air-exposed few-layer black phosphorus

Abstract

Black phosphorus (BP) shows significant saturable absorption and other unique properties in nonlinear optical effects, and it can be used for nonlinear optical elements and other photonic devices. However, the second harmonic generation (SHG) emission is prohibited due to its inversion symmetry. Here we report emerging SHG emission in BP by exposing BP to oxygen so as to break the inversion symmetry. The intensity of SHG signal shows a dependence on thickness and exposure time. It is interesting to find that the thinner BP flakes have faster SHG intensity change rate. Our results demonstrate that the intensity of SHG signal is attributed to oxidation degree. Air exposure also causes a blue shift of photoluminescence (PL) peaks, which is consistent with our calculation results. Our study demonstrates a feasible approach to modulate the SHG emission of BP, which may provide insights to other inversion symmetric crystals like BP. • We report emerging SHG emission in BP by exposing BP to oxygen so as to break the inversion symmetry. • It is founded that the intensity of SHG signal shows a dependence on thickness and exposure time. • It is founded that air exposure causes a blue shift of PL peaks, which is consistent with our calculation results. The results demonstrate that the intensity of SHG signal is attributed to oxidation degree. • To the best of our knowledge, there is no report on the SHG in BP before. This work demonstrates a feasible approach to modulate the SHG emission of BP, which may provide insights to other inversion symmetric crystals like BP.