Theory of the interference tunability of second harmonic generation for two-dimensional materials in layered...

We theoretically study how the intensity of second harmonic generation (SHG) for a sheet of two-dimensional (2D) material is controlled by an underlying layered structure. By utilizing the transfer matrix method with the inclusion of a nonlinear sheet current to describe the response of the 2D material, an explicit expression for the intensity of upward propagating second harmonic (SH) light is obtained, and the effects of the layered structure can be identified by a structure factor β, defined as the ratio of SH intensity from such a structure to that from a freely suspended 2D material. Our results show that the influence of a layered structure on the SHG intensity arises from interference effects of both the fundamental light and the SH light; the value of the structure factor is 0 ≤ β ≤ 64. Furthermore, when the incident light is pulsed, the interference effects are partially canceled due to the existence of many wave vectors and frequencies, and the cancellation becomes severe for thick films, small beam spots, and short pulses. For a specific structure of 2D material/dielectric film/substrate, the thickness of the dielectric film can effectively tune the value of β in an interval [β_min, β_max], and detailed discussions are performed for the thicknesses when these two extreme values can be obtained. When there is optical loss or the substrate is not perfectly reflective, the extreme value of β_max or β_min cannot reach 64 or 0. A large β_max requires two conditions to be fulfilled: (1) the substrate should be highly reflective, and (2) the refractive indices of the dielectric film at the fundamental and the SH frequencies should differ. Our results indicate how practical substrate structures can be used to achieve high SH signals, and the simple expression we give for the SH enhancement will be useful in characterizing the nonlinear susceptibility of 2D materials.