Nonlinear Light-Matter Interactions Enabled by Subwavelength Nanostructures

Abstract

Nonlinear light-matter interactions have ushered in pivotal advancements in areas like nonlinear imaging, all-optical switching, and supercontinuum generation. Chalcogenide glasses (ChGs) are especially noted for their large nonlinear susceptibility and excellent figure of merit, making them ideal for nonlinear optical applications. Historically, their optical applications have been limited to the near- and mid-infrared wavelength range. However, with the increasing demand for ultraviolet (UV) light sources in the fields of underwater communications, biological imaging, and photolithography, there has been a significant interest in exploring the possibilities of using these highly nonlinear glasses in the shorter wavelength range. Even under high absorption conditions, nonlinear wavelength conversion may result in a measurable, attributed to the inhomogeneous solution of the wave equation and a phenomenon known as phase-locking (PL). The PL has previously been both theoretically and experimentally demonstrated in semiconductors and negative index media. While for efficient nonlinear optical processes in bulk materials, perfect phase-matching conditions are usually desired, ultra-thin dielectric metasurfaces, enabling strong localization of the electromagnetic fields, offer a compelling avenue for fundamental studies and applications of nonlinear optics.In this dissertation, I introduce a new approach to the enhancement of third harmonic generation (THG) in the UV range. I demonstrate that nonlinear interactions in the arsenic trisulfide (As2S3) glass-based metasurface outperform unpatterned thin films by five times, thanks to the strong field localization at the Mie resonances of judiciously designed meta-atoms and a relatively strong nonlinear response of As2S3. Furthermore, combining the resonant response of the individual meta-atoms and the PL mechanism with the photonic bandgap-induced field localization in stacked As2S3 metasurfaces, demonstrates two orders of magnitude increase in third harmonic (TH) conversion efficiency, even when tuned into the 285 nm absorption range. In addition, I explored various approaches to achieving the femtosecond-speed tunable THG in the UV range using Mie and quasi-bound states in the continuum (qBICs) resonances in As2S3 metasurfaces. The excitation of qBICs results in a remarkable pump frequency field enhancement resulting in amplified phase-locked TH, enabling tunable nonlinear wavelength conversion even in the high-absorption wavelength range. Finally, I asked the following question: does the phenomenon of PL hold for the complex wavefronts of structured light beams such as optical vortices and knots? In other words, will a harmonic field have the same wavefront, topological charge, and number of crossings as the pump field? To answer this question I explore the phase information of the inhomogeneous component of the TH generated in a metasurface that was designed to have a resonant response at the fundamental wavelength only. I demonstrate the generation and conversion of structured light beams in metasurfaces transparent at the fundamental frequency (FF) and opaque at the TH frequency. I show that the inhomogeneous component of the TH possesses the topological properties of the original beam at the FF providing the first experimental demonstration of the PL effect for structured light beams. In summary, I experimentally investigated a new regime of nonlinear light-matter interactions enabled by the PL mechanism that allowed to significantly extend the operating range of nonlinear optical materials from the infrared to ultraviolet. I demonstrated that PL offers the possibility to imprint the topological properties of the fundamental frequency beam onto its harmonics which has not been discovered until now. The findings hold the potential to revolutionize metasurface photonics, heralding advances in fields ranging from higher-harmonic generation, optical manipulation, and advanced imaging to quantum computing and multichannel communication.