Wavefront Control and Intensity Modulation of Third Harmonic Generation in Nonlocal Metasurfaces

TL;DR

This work tackles the challenge of achieving high nonlinear conversion efficiency together with flexible wavefront control in metasurfaces. It introduces a nonlocal phase-gradient metasurface (NPGM) that leverages a quasi-bound state in the continuum (q-BIC) to boost third-harmonic generation (THG) and employs a nonlocal nonlinear geometric phase to impart polarization-dependent THG wavefronts, routing TH light into the $\pm$2nd and $\pm$4th diffraction orders with distinct polarizations. It further demonstrates interference-based all-optical intensity modulation of THG by introducing a secondary fundamental beam, achieving THG efficiency tuning from $3.9\times 10^{-9}$ to $5.5\times 10^{-3}$ with near-unity modulation depth by adjusting the relative phase, intensity, and polarization. Collectively, these results show a path toward multifunctional on-chip nonlinear photonic devices by combining high efficiency, wavefront control, and all-optical modulation in a single NPGM.

Abstract

Metasurfaces have emerged as a promising platform for integrated nonlinear optics. Nonlocal metasurfaces enable high nonlinear conversion efficiency, while the local ones can offer versatile wavefront control, yet achieving both within a single metasurface remains challenging. Here, using a nonlocal phase gradient metasurface, we firstly demonstrate efficient third harmonic generation (THG) with polarization-dependent wavefront control. Leveraging nonlocal nonlinear geometric phase existing at resonance, the third harmonic light with distinct polarizations is deflected into $\pm$ 2nd and $\pm$ 4th diffraction orders, simultaneously achieving a conversion efficiency up to $1.45\times 10^{-4}$ under a pump intensity of $1 GW/cm^{2}$. Then, by introducing a secondary fundamental beam, whose generated third harmonic light overlaps with that of the first beam, the intensity modulation of THG is obtained. The THG efficiency can be tuned from $3.9 \times 10^{-9}$ to $5.5 \times 10^{-3}$ by varying the relative phase, polarization and intensity of two fundamental beams. Through utilizing the advantages of both local and nonlocal metasurfaces, our results effectively pave the way to on-chip nonlinear photonic devices and signal processing.

Figures (5)

• Figure 1: Schematic of wavefront control and intensity modulation of THG on a single NPGM. (a, b) A fundamental beam of (a) RCP light incident from substrate side or (b) LCP light incident from air side is incident into the NPGM. The generated TH light in the air side is deflected into $+$ 2nd and $+$ 4th diffraction orders. (c) Intensity modulation of THG on the same NPGM when Beam 1 and Beam 2 are incident simultaneously. Their generated TH light overlaps, leading to the modulation of THG efficiency with a near-unity depth.
• Figure 2: Enhanced THG within the unit cell of NPGM (a) The schematic of THG process in the periodic unit cell. (b) The transmission/reflection spectra of unit cell with its electric field distribution in the xy-plane at q-BIC. (c) The THG efficiency of unit cell as a function of TH wavelength under a pump intensity of $1\,\mathrm{GW/cm^2}$, with the electric field distribution in the xy-plane at the wavelength of 511.3 nm. (d) The relation of THG efficiency with the pump intensity. Here, a fundamental beam of RCP light is incident into the unit cell normally from substrate.
• Figure 3: (a) The geometric parameters of unit cells in the NPGM. The major and minor axes of elliptical holes are adjusted with rotation angles to maintain the same q-BIC wavelength and similar THG efficiencies. (b) The THG efficiency and (c) nonlinear geometric phase of periodic unit cells with varying rotation angles. Here, the fundamental beam is RCP light with an intensity of $1\,\mathrm{GW/cm^2}$ to generate TH light of 511.3 nm.
• Figure 4: (a) The transmission/reflection spectra of NPGM with electric field distribution in the xy-plane at q-BIC. (b) The THG efficiency of NPGM as a function of TH wavelength. (c) The schematic of output angle $\alpha$ of TH light. (d) The distribution of normalized THG intensity with RCP and LCP fundamental beams incidence from substrate side normally.
• Figure 5: The intensity modulation of THG efficiency through varying (a) relative phase $\Delta \varphi$, (b) pump intensity of Beam 2, and polarization of Beam 2 with (c) $\Delta \varphi=0^\circ$ and (d) $\Delta \varphi=180^\circ$. The insets in (a) are the electric field distribution of fundamental beam in the xy-plane at $\Delta \varphi=180^\circ$ (left) and $\Delta \varphi=0^\circ$ (right), which exhibits large near-field enhancement under constructive interference, while approaches zero under destructive interference. The inset in (d) is the physical meanings of orientation angle $\psi$ and ellipticity angle $\chi$ on the Poincaré sphere.