Nonlinear photonic crystals (NPCs) are special materials in which the second-order susceptibility is spatially modulated, while the linear susceptibility remains constant. They were initially reported by Bloembergen as a means to increase the conversion efficiency in nonlinear frequency conversion process. Presently, NPCs play an important role in nonlinear optics because their ability to get the high-brightness coherent radiation. In addition, these special crystals exhibit certain fundamental phenomena of nonlinear optics, such as nonlinear Raman-Nath diffraction, nonlinear Cherenkov radiation, nonlinear Huygens-Fresnel principle, nonlinear Talbot effect, and nonlinear volume holography. Recently, NPCs exhibit interesting phenomena in not only classical optics but also quantum optics. These crystals can be used to produce high-brightness quantum light sources and manipulate quantum states. Furthermore, the future development of integrated photonics increases the scope of applications for specially designed NPCs.

In previous studies, the structure of NPCs was detected using a common optical microscope after etching with hydrofluoric acid, which is a common destructive detection method. To avoid destruction of the original structures, nondestructive methods were proposed for the detection of NPCs using nonlinear Cherenkov radiation, nonlinear Talbot effect, and nearly diffraction-free effect. However, these optical methods for the detection of NPCs are limited by the resolution of the optical system used.

In the present study, we reported the application of a moiré superlattice in second-order nonlinear optical parametric processes and demonstrated the super-resolution nondestructive detection of NPCs using the nonlinear moiré effect of the nonlinear moiré superlattice. The schematic of the formation of the nonlinear moiré effect in the nonlinear moiré superlattice is shown in Figure 1. Figure 2(a) shows the nonlinear domain structure image after etching with hydrofluoric acid. Figure 2(b) and 2(c) show the nonlinear SH image formed by the two PPLNs and captured using the nonlinear imaging system. When only one NPC was placed in the imaging system, the structures are indistinct owing to the low resolution of the imaging system. Figure 2(d) can clearly show the nonlinear moiré effect. Therefore, the structure information can be reconstructed using the nonlinear moiré effect. Thereafter, we calculated the detection range of this method using an optical microscope. The results revealed the successful detection of micro- and nanoscale structures of NPCs.

Figure 1 Schematic of the formation of the nonlinear moiré effect in the nonlinear moiré superlattice.

Figure 2 Observation of the nonlinear moiré effect. a) Nonlinear domain structure image after etching with hydrofluoric acid. b,c) The nonlinear SH image formed by the two PPLNs. The structures are indistinct owing to the low resolution of the imaging system. d) The clearly shown nonlinear moiré effect.

This research was published in “Haigang Liu and Xianfeng Chen, Nonlinear Moiré Superlattice for Super-Resolution Nondestructive Detection of Nonlinear Photonic Crystals, Laser & Photonics Reviews, 2000596 (2021)”.

Link: https://onlinelibrary.wiley.com/doi/full/10.1002/lpor.202000596