Cascaded Optical Nonlinearities in Dielectric Metasurfaces

Abstract

Since the discovery of the laser, optical nonlinearities have been at the core of efficient light conversion sources. Typically, thick transparent crystals or quasi-phase matched waveguides are utilized in conjunction with phase-matching techniques to select a single parametric process. In recent years, due to the rapid developments in artificially structured materials, optical frequency mixing has been achieved at the nanoscale in subwavelength resonators arrayed as metasurfaces. Phase matching becomes relaxed for these wavelength-scale structures, and all allowed nonlinear processes can, in principle, occur on an equal footing. This could promote harmonic generation via a cascaded (consisting of several frequency mixing steps) process. However, so far, all reported work on dielectric metasurfaces have assumed frequency mixing from a direct (single step) nonlinear process. In this work, we prove the existence of cascaded second-order optical nonlinearities by analyzing the second- and third-wave mixing from a highly nonlinear metasurface in conjunction with polarization selection rules and crystal symmetries. We find that the third-wave mixing signal from a cascaded process can be of comparable strength to that from conventional third-harmonic generation and that surface nonlinearities are the dominant mechanism that contributes to cascaded second-order nonlinearities in our metasurface.

Second Harmonic Emission From Dielectric Nanoresonators in the Absorption Regime

“…This limited reduction in the SHG efficiency is mainly due to the higher intensity enhancement of the MQ resonance with respect to the MD. Finally, let us underline that no significant differences are observed in the simulated SH power when the surface nonlinearities are also considered [34].…”

Terahertz Pulse Generation from GaAs Metasurfaces

“… 27 − 30 Although surface nonlinearity has not been identified as a THz generation mechanism to date, it has previously been shown to enhance second harmonic generation (SHG) in all-dielectric metasurfaces. 31 − 34 The strong enhancement of these processes in all-dielectric metasurfaces therefore suggests that surface nonlinearities could also play a role in THz generation because of the high surface area to volume ratio.…”

“…Fortunately, semiconductor metasurfaces can take advantage of a broad range of THz generation mechanisms. Among them is optical nonlinearity, which is primarily caused by shift currents for above-the-band-gap excitation. − The shift currents arise from a shift of charge distribution within the crystal lattice as a result of electron excitation from the valence band to the conduction band. , This process should occur both in the material volume and at the surface, where the material discontinuity and surface states can alter the nonlinear characteristics. − Although surface nonlinearity has not been identified as a THz generation mechanism to date, it has previously been shown to enhance second harmonic generation (SHG) in all-dielectric metasurfaces. − The strong enhancement of these processes in all-dielectric metasurfaces therefore suggests that surface nonlinearities could also play a role in THz generation because of the high surface area to volume ratio.…”

Coherent Control of the Nonlinear Emission of Single Plasmonic Nanoantennas by Dual‐Beam Pumping

“…Note that this cascaded THG pathway does not require an external beam at 2ω. [17,29] The polarization state of the cascaded THG emission of non-centrosymmetric gold nanoantennas directly reflects the asymmetric distribution of second-harmonic fields within the structure, resulting in a tilted polarization axis of the THG radiation with respect to the linear polarization of the pump beam at ω. [17] Controlling the currents oscillating within the antenna at 2ω with an external beam is therefore expected to provide additional control also of the polarization of the 3ω signal, besides its intensity.…”