Efficient second-harmonic generation in high Q-factor asymmetric lithium niobate metasurfaces

Abstract

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.

Giant Nonlinear Circular Dichroism from High Q‐Factor Asymmetric Lithium Niobate Metasurfaces

“…In particular, for LN, which is anisotropic, the electric field component along the optical axis contributes more to the nonlinear conversion. [31] Therefore, integrations of (E y /E 0 ) 2 in the whole unit cell were calculated for RCP and LCP, respectively, as shown in Figure 3d. We found that the E y 2 enhancement reaches only seven times at 1.334 μm for LCP.…”

“…Recently, LN metasurfaces with excellent photoelectric properties have been reported. Note that LN has a high second-order nonlinear polarization coefficient and low optical loss in the UV to mid-infrared spectral regions, [15,[30][31][32] which make it suitable for the study of the second harmonic. [33][34][35] Many methods have been employed to improve the second-harmonic generation (SHG) efficiency of LN, such as plasmonic resonance and anapole mode, [15,36] however, they still experience low efficiency.…”

“…[37] In practice, BIC can be realized as a quasi-BIC with finite bandwidths and field enhancements by breaking the in-plane inversion symmetry of the unit cell. Recently, it has been reported that quasi-BICs are also suitable for nonlinear LN metasurfaces, and the efficiency of SHG can be ≈0.49%, [31] which is significantly higher than the efficiency assisted by plasmonic. Therefore, quasi-BICs provide unique opportunities for improvements of nonlinear efficiency and realization of more nonlinear applications.…”

Excitations of Multiple Fano Resonances Based on Permittivity-Asymmetric Dielectric Meta-Surfaces for Nano-Sensors

“…Recently, high sensitivity dielectric meta-surfaces are developed [17]. Dielectric meta-surfaces can support both electric and magnetic dipolar Mie-type resonances [18] and avoid the Ohmic losses efficiently, generating much narrower Fano resonances compared with traditional plasmonic meta-surfaces [19].…”

Giant asymmetric second-harmonic generation in bianisotropic metasurfaces based on bound states in the continuum

“…Although ideally BICs necessitate an infinite Q-factor, one can observe a finite Q-factor by breaking the symmetry of the meta-atoms, so-called quasi-BICs [21]. To date, thanks to this concept, enhancement of the conversion efficiency of the second- [22][23][24][25], third- [26][27][28], and higher-order [29,30] harmonics in dielectric metasurfaces have been demonstrated theoretically and experimentally. The physical origin of these large nonlinear conversion efficiencies can be understood by the field enhancement [31].…”