Second-order nonlinear intersubband polaritonic metasurfaces
Frequency mixing is an essential nonlinear process with extensive applications in photonics, chemistry, biology, and energy sciences. Traditional nonlinear crystals have weak nonlinear responses and light beams need long propagation distances in the crystals to accumulate a significant wave mixing in practice. However, wave mixing in such bulky crystals results in stringent phase-matching requirements and bulk nonlinear crystals are not compatible with modern “flat” optics concept that enables complete control of the phase-front of the output beam but requires optical medium with subwavelength thickness. Fortunately, the emerging of metasurfaces has provided an efficient method to generate the large nonlinear response on nanoscale. The metasurfaces have enabled the development of “flat” optical elements with the intrinsic benefit of small thickness, intricate control of the optical wavefront, and, in case of nonlinear optical elements, relaxed phase-matching constraints. In my Ph.D. dissertation, I focus on the second-order intersubband polaritonic nonlinear metasurfaces. These structures combine enormous intersubband nonlinear response in III-V semiconductor heterostructures and field enhancement of plasmonic nano-resonators. Our earlier research has demonstrated giant nonlinear responses for the second harmonic generation in metasurfaces. In this dissertation, I propose several approaches to improve the performance of second harmonic generation metasurfaces and extend their functionality to difference-frequency and sum-frequency generation in the mid-infrared range. For the first part of this study, I have demonstrated new multiquantum-well designs for second harmonic generation with materials have much narrower linewidth compared with previous materials. This leads to a conversion efficiency of 1.2%. Second, I have demonstrated the mid-infrared difference-frequency generation in polaritonic nonlinear metasurface for the first time. The optimization of the metasurface, the theoretical investigation of the saturation effect, the fabrication of the metasurface, and the experimental characterization of the metasurface have been discussed. The effective nonlinear susceptibility is 340 nm/V and the differencefrequency generation conversion efficiency of this metasurface is 0.13%. I have also demonstrated the mid-infrared sum-frequency generation in a polaritonic nonlinear metasurface. Both the theoretical analysis of the saturation effect and the experimental characterization of the metasurface have been illustrated. The upconversion efficiency of this metasurface is 0.03% and the nonlinear susceptibility is 158 nm/V. In addition, as the prospect of the SFG metasurfaces, the performance of metasurfaces under extremely high pump intensity has been discussed and the metasurface designs for high conversion efficiency have been proposed. For the last part of this study, metasurfaces in the THz range have been explored. These metasurfaces are designed to generate 4~6 THz with a difference-frequency generation process from polaritonic metasurfaces at room temperature. The theoretical analysis, sample design, and preliminary experimental results have been discussed.