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Register a new userAnisotropic Second Harmonic Generation From Monocrystalline Gold Flakes
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Noble metals with well-defined crystallographic orientation constitute an appealing class of materials for controlling light-matter interactions on the nanoscale. Nonlinear optical processes, being particularly sensitive to anisotropy, are a natural and versatile probe of crystallinity in nano-optical devices. Here we study the nonlinear optical response of monocrystalline gold flakes, revealing a polarization dependence in second-harmonic generation from the {111} surface that is markedly absent in polycrystalline films. Apart from suggesting an approach for directional enhancement of nonlinear response in plasmonic systems, we anticipate that our findings can be used as a rapid and non-destructive method for characterization of crystal quality and orientation that may be of significant importance in future applications.
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We report second-harmonic generation (SHG) from thick hexagonal boron nitride (hBN) flakes with approximately 109-111 layers. The resulting effective second-order susceptibility is similar to previously reported few-layer experiments. This confirms that thick hBN flakes can serve as a platform for nonlinear optics, which is useful because thick flakes are easy to exfoliate while retaining a large flake size. We also show spatial second-harmonic maps revealing that SHG remains a useful tool for the characterization of the layer structure even in the case of a large number of layers.
We report the observation of optical second harmonic generation (SHG) in single-layer indium selenide (InSe). We measure a second harmonic signal of $>10^3$ $textrm{cts/s}$ under nonresonant excitation using a home-built confocal microscope and a standard pulsed pico-second laser. We demonstrate that polarization-resolved SHG serves as a fast, non-invasive tool to determine the crystal axes in single-layer InSe and to relate the sharp edges of the flake to the armchair and zigzag edges of the crystal structure. Our experiment determines these angles to an accuracy better than $pm$ $0.2^{circ}$. Treating the two-dimensional material as a nonlinear polarizable sheet, we determine a second-order nonlinear sheet polarizability $| chi_{textrm{sheet}}^{(2)}|=(17.9 pm 11.0)times 10^{-20}$ $textrm{m}^2 textrm{V}^{-1}$ for single-layer InSe, corresponding to an effective nonlinear susceptibility value of $| chi_textrm{eff}^{(2)}| approx (223 pm 138) times 10^{-12}$ $textrm{m} textrm{V}^{-1}$ accounting for the sheet thickness ($textrm{d} approx 0.8$ $textrm{nm}$). We demonstrate that the SHG technique can also be applied to encapsulated samples to probe their crystal orientations. The method is therefore suitable for creating high quality van der Waals heterostructures with control over the crystal directions.
We study second harmonic generation in nonlinear, GaAs gratings. We find large enhancement of conversion efficiency when the pump field excites the guided mode resonances of the grating. Under these circumstances the spectrum near the pump wavelength displays sharp resonances characterized by dramatic enhancements of local fields and favorable conditions for second harmonic generation, even in regimes of strong linear absorption at the harmonic wavelength. In particular, in a GaAs grating pumped at 1064nm, we predict second harmonic conversion efficiencies approximately five orders of magnitude larger than conversion rates achievable in either bulk or etalon structures of the same material.
Dielectric metasurfaces have shown prominent applications in nonlinear optics due to strong field enhancement and low dissipation losses at the nanoscale. Chalcogenide glasses are one of the promising materials for the observation of nonlinear effects due to their high intrinsic nonlinearities. Here, we demonstrate, experimentally and theoretically, that significant second harmonic generation can be obtained within amorphous chalcogenide based metasurfaces by relying on the coupling between lattice and particle resonances. We further show that the high quality factor resonance at the origin of the second harmonic generation can be tuned over a wide wavelength range using a simple and versatile fabrication approach. The measured second harmonic intensity is orders of magnitude higher than that from a deposited chalcogenide film, and more than three orders of magnitude higher than conventional plasmonic and Silicon-based structures. Fabricated via a simple and scalable technique, these all-dielectric architectures are ideal candidates for the design of flat non-linear optical components on flexible substrates.
Second-order nonlinear effects, such as second-harmonic generation, can be strongly enhanced in nanofabricated photonic materials when both fundamental and harmonic frequencies are spatially and temporally confined. Practically designing low-volume and doubly resonant nanoresonators in conventional semiconductor compounds is challenging owing to their intrinsic refractive index dispersion. In this work we review a recently developed strategy to design doubly resonant nanocavities with low mode volume and large quality factor by localized defects in a photonic crystal structure. We build on this approach by applying an evolutionary optimisation algorithm in connection with Maxwell equations solvers, showing that the proposed design recipe can be applied to any material platform. We explicitly calculate the second-harmonic generation efficiency for doubly resonant photonic crystal cavity designs in typical III-V semiconductor materials, such as GaN and AlGaAs, targeting a fundamental harmonic at telecom wavelengths, and fully accounting for the tensor nature of the respective nonlinear susceptibilities. These results may stimulate the realisation of small footprint photonic nanostructures in leading semiconductor material platforms to achieve unprecedented nonlinear efficiencies.
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