Free-space high-Q nanophotonics

Abstract

High-Q nanophotonic devices hold great importance in both fundamental research and engineering applications. Their ability to provide high spectral resolution and enhanced light-matter interactions makes them promising in various fields such as sensing, filters, lasing, nonlinear optics, photodetection, coherent thermal emission, and laser stealth. While Q-factors as large as 10⁹ have been achieved experimentally in on-chip microresonators, these modes are excited through near-field coupling of optical fibers. Exciting high-Q modes via free-space light presents a significant challenge primarily due to the larger fabrication area and more lossy channels associated with free-space nanophotonic devices. This Review provides a comprehensive overview of the methods employed to achieve high-Q modes, highlights recent research progress and applications, and discusses the existing challenges as well as the prospects in the field of free-space high-Q nanophotonics.

Fig. 1

Schematic of free-space high-Q nanophotonic devices and their relevant applications including sensing, filters, lasing, nonlinear optics, photodetection, coherent thermal emission and laser stealth. Panels adapted from: AAAS^,,; ACS; Optica; Springer Nature; Elsevier

Fig. 2. Analytical models of optical resonators.

a One-mode resonators with the same radiative decay rates with respect to two ports (${\gamma }_{\mathrm{r}1}={\gamma }_{\mathrm{r}2}$). The maximum absorptance is 50%, and the reflection or transmission can have Fano line shape due to the interference between the mode and the background. b One-mode resonators with different radiative decay rates with respect to two ports (${\gamma }_{\mathrm{r}1}\ne {\gamma }_{\mathrm{r}2}$) due to structure asymmetry. The absorption of light incident from Port 1 (solid line) and Port 2 (dashed line) are asymmetric. c One-mode resonators with a reflective background that prohibits the transmission of light (${\gamma }_{\mathrm{r}2}=0$). Perfect absorption can be achieved when light is incident from Port 1. d Two-mode resonators where a bright mode with a large radiative decay rate couples with a dark mode with a negligible radiative decay rate. EIT can happen due to the mode coupling. e Two-mode resonators where two modes with different symmetry properties reside in the same resonator. Super absorption with the maximum absorptance larger than 50% can be achieved. For convenience, in (d) and (e), resonators with mirror symmetry with respect to the middle plane are chosen (${\gamma }_{\mathrm{r}1}={\gamma }_{\mathrm{r}2}$)

Fig. 3. Methods of achieving high-Q modes.

a Bound states in the continuum. b Guided mode resonances. c Surface lattice resonances. RA represents Rayleigh anomalies. d High-order resonances. e Tamm plasmon polaritons. f Fabry-Perot resonances. g Dispersion-assisted high-Q modes. h Material-assisted high-Q modes such as superconductor or gain materials

Fig. 4. High-Q non-absorbing nanophotonic devices.

a Various nanostructures supporting qBICs^,,–. b PhCs with multiple BICs merging at the same point in the momentum space. c GMRs based on PhCs. d GMRs based on periodic nanoantenna arrays. e GMRs combined with phase-gradient metausrfaces. f SLRs based on metallic metasurfaces. g SLRs based on dielectric nanostructures. Panels adapted from: AAAS; ACS^,,,,; Springer Nature^,,,–; APS^,; Wiley; De Gruyter

Fig. 5. High-Q absorbing/ thermal emitting nanophotonic devices based on BICs, GMRs, and Tamm plasmon polaritons.

a Plasmonic BICs based on MIM structures. b Plasmonic BICs based on anisotropic metasurfaces. c High-Q absorbers based on dielectric metamirror with dispersive reflection. d High-Q thermal emission based on 1D GMRs. e High-Q absorption based on 2D GMRs. f High-Q absorption based on Tamm plasmon polaritons. g High-Q thermal emission based on Tamm plasmon polaritons. Panels adapted from: ACS^,; Wiley^,,; AAAS; APS

Fig. 6. High-Q absorbing/ thermal emitting nanophotonic devices based on SLRs, PhCs, high-index dielectric metasurfaces, and MIM structures.

a SLRs composed of TiO2 metasurfaces. b PhCs combined with multiple quantum wells. c SiC metasurfaces with a high refractive-index near the SPhPs resonance. d MIM absorbing metasurfaces. Panels adapted from: Wiley^,,; Springer Nature

Fig. 7. Applications of free-space high-Q nanophotonic devices.

a High-Q metasurfaces for biosensing. b High-Q optical filters based on FP resonances. c High-Q metasurfaces for lasing. d High-Q nonlinear metasurfaces for generating complex quantum states. e High-Q Ge absorbers for photodetection. f High-Q SiC gratings for narrowband thermal emission. g High-Q gratings for laser stealth. h High-Q GMRs for super resolution imaging. Panels adapted from: AAAS^,,; ACS^,; Elsevier; Springer Nature^,

Fig. 8

Challenges on increasing the Q-factors of free-space high-Q nanophotonic devices