Giant nonlinear response at the nanoscale driven by bound states in the continuum

Being motivated by the recent prediction of high-$Q$ supercavity modes in subwavelength dielectric resonators, we study the second-harmonic generation from isolated subwavelength AlGaAs nanoantennas pumped by a structured light. We reveal that nonlinear effects at the nanoscale can be enhanced dramatically provided the resonator parameters are tuned to the regime of the bound state in the continuum. We predict a record-high conversion efficiency for nanoscale resonators that exceeds by two orders of magnitude the conversion efficiency observed at the conditions of magnetic dipole Mie resonance, thus opening the way for highly-efficient nonlinear metadevices.Comment: 7 pages, 4 figures, 1 tabl

Advanced trapping of light in resonant dielectric metastructures for nonlinear optics

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

Nonradiating Photonics with Resonant Dielectric Nanostructures

Nonradiating sources of energy have traditionally been studied in quantum mechanics and astrophysics, while receiving a very little attention in the photonics community. This situation has changed recently due to a number of pioneering theoretical studies and remarkable experimental demonstrations of the exotic states of light in dielectric resonant photonic structures and metasurfaces, with the possibility to localize efficiently the electromagnetic fields of high intensities within small volumes of matter. These recent advances underpin novel concepts in nanophotonics, and provide a promising pathway to overcome the problem of losses usually associated with metals and plasmonic materials for the efficient control of the light-matter interaction at the nanoscale. This review paper provides the general background and several snapshots of the recent results in this young yet prominent research field, focusing on two types of nonradiating states of light that both have been recently at the center of many studies in all-dielectric resonant meta-optics and metasurfaces: optical {\em anapoles} and photonic {\em bound states in the continuum}. We discuss a brief history of these states in optics, their underlying physics and manifestations, and also emphasize their differences and similarities. We also review some applications of such novel photonic states in both linear and nonlinear optics for the nanoscale field enhancement, a design of novel dielectric structures with high-$Q$ resonances, nonlinear wave mixing and enhanced harmonic generation, as well as advanced concepts for lasing and optical neural networks.Comment: 22 pages, 9 figures, review articl

Nonlinear chiral metaphotonics

We review the physics and some applications of photonic structures designed for the realisation of strong $\textit{nonlinear chiroptical response}$. We pay much attention to the recent strategy of utilizing different types of $\textit{optical resonances}$ in metallic and dielectric subwavelength structures and metasurfaces, including surface plasmon resonances, Mie resonances, lattice guided modes, and bound states in the continuum. We summarize earlier results and discuss more recent developments for achieving large circular dichroism combined with the high efficiency of nonlinear harmonic generation.Comment: perspective, 35 pages, 5 figure