Theoretical design of the nonlinear Janus metastructure based on second harmonic generation with difference detection

A second harmonic wave (SHW) is a prominent nonlinear effect that has garnered widespread attention in the field of nonlinear superstructures. In this paper, a nonlinear Janus metastructure (NJM) based on lithium-tantalate-crystal modulation is proposed. The one-dimensional NJM is investigated using the quasi-phase matching method for second harmonic generation (SHG) in both forward and backward directions. Under the conditions of phase matching, a groundbreaking enhancement of forward and backward second harmonic waves within the NJM is observed. Detection within a small angle range is achieved by utilizing the difference in the peak frequency (PF) of the output SHW between the forward and backward scales. Additionally, the variation in the peak output (OP) of the SHW is employed for thickness sensing and detecting changes in the refractive index (RI) of the sample material. The results demonstrate the effectiveness of PF difference detection in an angle range of 5° to 9°. For OP detection, the thickness sensing range is from 160 nm to 180 nm. Furthermore, the RI of the sample material can be measured within the ranges of 1.55 to 1.65 and 1.70 to 1.80. This work provides valuable insights for the design of differential electromagnetic detection devices based on the NJM.