Lithium niobate (LN) has been widely used for second-harmonic generation (SHG) from bulk crystals. Recent studies have reported improved SHG efficiency in LN micro-ring resonators and hybrid waveguiding structures, as well as in LN nanostructures supporting anapole modes and plasmon-assisted dipole resonances. Here we numerically demonstrate that high
Q
-factor resonances associated with symmetry-protected bound states in the continuum can lead to highly efficient frequency doubling in LN metasurfaces. Simulations show that the radiative
Q
-factor and on-resonance field enhancement factor observed in the metasurface are closely dependent on the asymmetric parameter
α
of the system. Furthermore, high
Q
-factor resonances boost the SH conversion process in the LN nanostructures. In particular, for a LN metasurface with a
Q
-factor of
∼
8
×
10
4
, a 0.49% peak SH conversion efficiency is achieved at a pump intensity of
3.3
k
W
/
c
m
2
. This suggests that such high
Q
-factor LN metasurfaces may be good candidates for practical blue–ultraviolet light sources. Our work provides insight into the possible implementation of metadevices based on nanoengineering of conventional nonlinear crystals.
The Mie resonances of high-index nanostructures offer the possibility of manipulating light with extremely low loss. Enhanced optical magnetism, with a concomitant significant increase in the quality factor, can be achieved in high-index metasurfaces by adding a highly reflective backplane to the system. Here, we show that Mie-resonance-based hybrid metasurfaces consisting of an array of amorphous silicon nanodisks on a gold backplane can be used to manipulate light polarization upon reflection. Reflection matrix analysis reveals the nontrivial topological property associated with the Mie resonance of individual nanodisks. The topologically protected polarization conversion effect allows the generation of abundant and diverse polarization in the reflected waves by varying the incident wavevector. By presenting proof-of-concept demonstrations based on nonlinear modeling, we further show that the considered hybrid metasurfaces can serve as a platform for ultrafast all-optical polarization switching of near-infrared light. The topological nature of the metasurfaces' response offers great flexibility in polarization generation and dynamic modulation.
Artificial magnetism at optical frequencies is one of the most intriguing phenomena associated with metamaterials. The Mie resonance of high-index resonators provides an alternative approach to achieving optical magnetism with simple structures. Given the generally moderate refractive index exhibited by available materials at optical frequencies, Mie resonances usually suffer from coupling between the multipole modes and the corresponding response of the Mie metasurfaces can be analyzed based on the concept of "meta-optics". Here we show that the optical magnetism in high-index resonators can be significantly enhanced by adding a highly reflective back mirror to the system. To highlight the transformative ability of this approach for improving meta-optics in the linear and nonlinear regimes, two proof-ofconcept demonstrations are presented. Theoretical modeling reveals that low-pump power ultrafast nonlinear optics can be realized in periodic Si nanodisk arrays backed with a gold film, a system supporting guided resonance modes. Moreover, based on the enhanced magnetism of individual high-index resonators, a pair of the silicon cuboids is demonstrated as a magnetic antenna for directional excitation of surface plasmon waves. The interference-enhanced magnetism of high-index resonators provides a disruptive technology for enabling meta-optics comprising ultracompact, high-speed and power-efficient photonic devices.
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