Second harmonic generation in two-dimensional Janus monolayers: A first-principles investigation

Abstract

Two-dimensional Janus structures have been regarded as promising candidates for nanoscale optical devices due to the unique optical responses resulting from vertical atomic asymmetry within a monolayer. In this paper, we systematically investigate the second harmonic generation (SHG) in 54 Janus MXY (M = Cr, Hf, Mo, Nb, Ta, Ti, V, W, Zr, and X/Y = O, S, Se, Te, and X ≠ Y) monolayers and calculate the second-order nonlinear susceptibilities of each material at six incident laser wavelengths of 405 nm, 532 nm, 800 nm, 1030 nm, 1064 nm and 1550 nm through first-principles calculations. The X-M-Y asymmetry results in non-zero components in the vertical direction, in contrast to non-Janus structures. Focusing on the SHG induced by incident light at wavelengths of 800 nm, 1064 nm, and 1550 nm, polarization-dependent responses of three Janus CrXY (X/Y = S, Se, Te, and X ≠ Y) monolayers are demonstrated. S-polarization exhibits six-fold rotational symmetry, whereas p-polarization exhibits triple rotational symmetry. We expect these results to provide theoretical support and guidance for further screening and designing new nonlinear optical materials.