Advanced trapping of light in resonant dielectric metastructures for nonlinear optics

Abstract

In the past two decades new frontiers emerged in the rapidly expanding field of nanophotonics and the remarkable progress in engineering of efficient nanostructured devices for functional flat optics and nonlinear photonics was achieved by using resonant dielectric metastructures operating through the excitation of Mie resonances and their collective configurations. Further progress in subwavelength localization of light in Mie-resonant nanostructures and enhancement of their optical nonlinearities remained hampered by the leaky nature of optical modes. The last decade marked a series of intense studies of optical resonances with a giant quality factor, bound states in the continuum (BICs), aimed to resolve this issue. The unique electromagnetic properties of BICs were examined as a versatile tool to tailor optical response of photonic nanostructures, yet their physical nature and feasibility of realization in form of high-quality quasi-BIC resonances in planar and compact metadevices remains largely unexplored. Moreover, it remains unknown in many aspects how BICs can be utilized for engineering of resonant nonlinear metasurfaces and nanoantennas for efficient frequency conversion and observation of strong nonlinearities. In this thesis, we are focused on comprehensive analysis of fundamental physical properties of optical quasi-BICs in resonant dielectric metastructures and exploration of their practical feasibility for strong light confinement and nonlinear applications. We outline the general framework for design and optimization of nanostructured devices supporting quasi-BICs in the visible and infrared range for maximization of the local fields and associated enhancement of optical nonlinearities. More specifically, we focus on planar metasurfaces with broken-symmetry meta-atoms, and individual subwavelength resonators with compact footprint. Ultimately, we target the challenge of engineering of nonlinear dielectric metastructures with outstanding nonlinear performances, which may lead to new breakthroughs in realization of efficient nonlinear frequency converters, low-threshold nanolasers, and compact quantum sources. In Chapter 2 we propose a concept of light localization in dielectric metasurfaces composed of meta-atoms with broken in-plane inversion symmetry by using quasi-BICs resonances. We propose a universal framework for designing dielectric metasurfaces supporting sharp resonances with a specific operating wavelength and linewidth on demand. Chapter 3 is focused on analysis and experimental demonstration of harmonic generation and self-action effects in broken-symmetry Si metasurfaces supporting quasi-BICs in the near-IR and mid-IR. We demonstrate enhancement of optical nonlinearities of two-dimensional Van der Waals materials integrated with resonant Si metasurfaces. For strong field excitation, we demonstrate that Si metasurfaces generate high-harmonic signal and we demonstrate pronounced self-action effects. In Chapter 4 we propose a new mechanism of light trapping in isolated subwavelength dielectric resonators by formation of quasi-BICs due to destructive interference of several Mie modes in the far field. We explore the physical properties of quasi-BICs and show that the quasi-BICs can realized in subwavelength dielectric resonators with refractive index more than 2 in various spectral ranges from the visible to microwaves. The findings are verified in proof-of-principle experiments in the near-IR and radiofrequency range. In Chapter 5 we examine the efficiency of harmonic generation from individual dielectric nanoresonators supporting quasi-BICs and outline the criteria for maximization of second-harmonic conversion efficiency by optimizing the mode structure, the pump spatial and temporal profile and the environment design. We verify the developed model experimentally and show that the record-high measured conversion efficiency of the optimized nonlinear nanoantenna.