Mid-infrared and THz nonlinear metasurfaces for efficient harmonic generation

Abstract

Metasurfaces presenting nonlinear optical responses have significantly eased the phase matching conditions and enabled localized control at subwavelength scales. This breakthrough has introduced a fresh dimension in research, facilitating advancements in various areas including new frequency generation, engineering of nonlinear phase fronts, generation of nonlinear orbital angular momentum, and nonlinear applications in holography and imaging. Recently, there are growing interests in developing nonlinear flat optics based on gradient metasurface concept. Nonlinear polaritonic metasurfaces based on the coupling of giant nonlinear optical response of MQW layer with plasmonic resonators for efficient second harmonic generation (SHG) in mid- infrared region have been proposed and experimentally demonstrated. However, the harmonic conversion efficiency of nonlinear polaritonic metasurfaces is limited by the intensity saturation effect due to carrier depletion in the conduction band MQW layer at high pump intensity. Furthermore, electric fields induced in the MQW layer by plasmonic resonators are confined in a narrow region in the unit cell, limiting the active MQW layer thickness. Therefore, the intensity saturation of intersubband transitions occurs rapidly even at low pump intensity. In this thesis, I proposed and experimentally demonstrated lattice-resonance-coupled dielectric nonlinear polaritonic metasurface to produce efficient second-harmonic generation with greatly relaxed intensity saturation effect. Electric field that induced in the MQW layer by the lattice resonance are confined in a wide region in a unit cell. Furthermore, owing to the lattice resonance with large modes volume, much thicker active MQW layer can be used leading to further boosted conversion efficiency. In this work, I achieved greatly relaxed intensity saturation effect and record-high conversion efficiency of SHG (metasurface based). Experimentally, 0.37% of nonlinear conversion efficiency and 21 mW of SHG power were achieved at input pump wavelength of 10.55 μm and a pump intensity of about 1 MW/cm2. Electromagnetic waves in the terahertz (THz) region are expected to be applicable to various fields such as biomedical imaging, spectroscopy, and next-generation wireless communication. Therefore, much interest and research are being conducted on the development of electromagnetic wave sources in the THz region. Recently, since the advent of graphene, several theoretical proposals have predicted efficient THz frequency multiplication or harmonic generation in single- layer graphene in realistic technologically relevant scenarios, including room-temperature operation. Currently, THz high-harmonic generation in single layer graphene was demonstrated under realistic technological conditions. However, such high harmonic signals (odd-order harmonics) generated from a single layer graphene are propagating with the same channel (direction). In this thesis, I demonstrate a multi-frequency beam forming metasurface for de- multiplexing harmonic signals generated from graphene nonlinear devices. Recently based on the chiroptical response, new degrees of freedom in phase front engineering using metasurface can be given. Recently, giant nonlinear chirality has been demonstrated from metasurfaces consisting of meta-atom with three- (C3) and four-fold (C4) rotational symmetries. However, in order to apply such nonlinear chiral metasurfaces to practical applications, both giant chirality and efficient conversion efficiency must be met simultaneously. In this thesis, I proposed and experimentally demonstrated nonlinear chiral metasurfaces producing giant nonlinear circular dichroisms for SHG and THG in one-chip system based on combining the giant 2nd and 3rd order nonlinearities of MQW structure with C3 and C4 chiral meta-atoms for the SHG and THG, respectively. The large SHG and THG signal differences were obtained for the two circular polarized input pump beam for the wavelength range above 9.4 μm and the signal difference was maximized around the optimized design wavelength of 10.3 μm. The SHG and THG conversion efficiency at the wavelength of 10.3 μm were 4.78 × 10−4% and 6.89 × 10−5%, respectively for an input intensity of 60 kW 𝑐𝑚−2. The maximum nonlinear CD value 0.91 and 0.98 for SHG and THG respectively, was achieved and over 0.5 of CD values were obtained from both structures in broad wavelength range from 9.7 μm to 10.9 μm. In this thesis, I experimentally demonstrated over 90% of nonlinear circular dichroisms for both SHG and THG were achieved around 10μm of input pump wavelength and also demonstrated spin selective beam steering nonlinear measureface.