Resonantly enhanced second-harmonic generation using III-V semiconductor all-dielectric metasurfaces (1608.02570v2)

Abstract: Nonlinear optical phenomena in nanostructured materials have been challenging our perceptions of nonlinear optical processes that have been explored since the invention of lasers. For example, the ability to control optical field confinement, enhancement, and scattering almost independently, allows nonlinear frequency conversion efficiencies to be enhanced by many orders of magnitude compared to bulk materials. Also, the subwavelength length scale renders phase matching issues irrelevant. Compared with plasmonic nanostructures, dielectric resonator metamaterials show great promise for enhanced nonlinear optical processes due to their larger mode volumes. Here, we present, for the first time, resonantly enhanced second-harmonic generation (SHG) using Gallium Arsenide (GaAs) based dielectric metasurfaces. Using arrays of cylindrical resonators we observe SHG enhancement factors as large as 104 relative to unpatterned GaAs. At the magnetic dipole resonance we measure an absolute nonlinear conversion efficiency of ~2X10^-5 with ~3.4 GW/cm2 pump intensity. The polarization properties of the SHG reveal that both bulk and surface nonlinearities play important roles in the observed nonlinear process.

• The paper demonstrates a 100× SHG enhancement in GaAs metasurfaces at magnetic dipole resonance, achieving a nonlinear conversion efficiency of ~2×10^-5.
• The study employs GaAs nanodisk resonators fabricated via electron-beam lithography to confine Mie resonances and bypass phase-matching constraints.
• Experimental and simulation results validate that both bulk and surface nonlinearities contribute to robust SHG, paving the way for advanced nonlinear optics devices.

Resonantly Enhanced Second-Harmonic Generation in III-V Semiconductor Dielectric Metasurfaces

The paper under consideration explores the resonantly enhanced second-harmonic generation (SHG) achieved through the use of dielectric metasurfaces composed of Gallium Arsenide (GaAs). This work offers significant insights into nonlinear optical processes by addressing how phase-matching constraints can be bypassed, thus achieving high nonlinear optical conversion efficiencies using nanostructured materials. SHG efficiencies are shown to be markedly improved compared with those obtained from bulk GaAs. The research presents a detailed analysis of GaAs-based cylindrical resonator arrays and demonstrates SHG enhancement factors of up to 100 times compared to unpatterned GaAs, achieving an absolute nonlinear conversion efficiency of approximately 2 × 10^-5 at magnetic dipole resonance.

Design and Experimental Approach

The authors designed GaAs resonators with deliberate Mie magnetic and electric dipole resonances extending beyond the GaAs bandgap to avoid absorption and operate within the spectral range of a femtosecond Ti:sapphire laser. They produced a metasurface array with dense spacing to minimize inter-resonator interactions. Electron-beam lithography techniques were employed to fabricate these GaAs nanodisk resonators with dimensions of ~250 nm in diameter and 300 nm in height. This meticulous fabrication enabled the definition of Mie modes with tightly confined electromagnetic fields, critical for effective nonlinear optical interactions.

The experimental procedures involved measuring SHG in a reflection geometry to circumvent the absorption concerns of SHG wavelengths beyond the GaAs bandgap. Polarization characteristics of SHG were systematically recorded, revealing that both bulk and surface nonlinearities significantly contribute to the observed SHG.

Results and Analysis

The paper reports a substantial SHG enhancement when tuned to the resonance of magnetic dipole modes, attributed to the increased electromagnetic field intensities and a larger mode volume within dielectric resonators. At magnetic dipole resonance, the SHG conversion efficiency is observed to be ~100 times greater than that at electric dipole resonance—indicative of differing efficiencies derived from bulk and surface nonlinearity contributions. The authors have described the power dependence of the SHG signal, affirming the quadratic relationship at lower pump powers, which deviate as resonator damage occurs at peak intensities (~1.5 GW/cm^2).

In terms of electromagnetic enhancement, the effective nonlinear susceptibility is introduced as a defining factor in SHG power proportionality to intensified incident pump power. Simulations corroborate the experimental findings, aligning the SHG intensity peaks with theoretical predictions of magnetic and electric dipole resonance involvement. The paper suggests the enhancement of SHG efficiency through modifications in resonator design, such as using larger dimensions and materials like AlGaAs to mitigate absorption impacts.

Implications and Future Directions

This research has robust implications for advancing the application of dielectric metasurfaces in nonlinear optics. The observed second-order nonlinear phenomena facilitate the development of phase-matching-independent nonlinear optical devices, potentially transforming methods for frequency conversion, all-optical signal processing, and quantum photonic applications. The paper underscores opportunities for further research into surface-induced nonlinearities and the optimization of structural parameters for improved SHG efficiency. Subsequent investigations could delve into materials with varying nonlinear susceptibilities and design geometries that accentuate modal overlaps between fundamental and SH wavelengths.

In conclusion, the work presents significant advancements in the understanding and enhancement of SHG using dielectric metasurfaces. By leveraging the properties of GaAs and the strategic design of resonators, this paper lays groundwork for optimized nonlinear frequency up- and down-conversion technologies, heralding a new frontier for efficient optical materials that circumvent traditional phase-matching requirements.