Amorphous silicon metasurface enhanced third harmonic generation

In the past few decades, nonlinear optics has been an active research field, providing a basis for studying physical phenomena such as high-order harmonic generation, spontaneous parametric transformation, stimulated Raman scattering, and nonlinear Kerr effect. In recent years, the use of metasurfaces to enhance the generation of nonlinear light at sub-wavelength scales has attracted a lot of attention due to its ultra-thin and compact form factor and advanced features related to modern integrated photonics.

Recently, David Hähnel, a scientist from the University of Paderborn in Germany, and others proposed an amorphous silicon metasurface composed of elliptical nanoresonators to enhance third harmonic generation (THG), proving that this enhancement comes from a new multimodal Fano mechanism. These super-Fano resonances were studied in detail by full-wave simulation, and the metasurface behavior predicted by theory was experimentally verified by linear and nonlinear transmission spectroscopy. At a peak power intensity of 1.2 GW cm-2, the absolute conversion efficiency is up to ηmax ≈ 2.8 × 10-7, and the magnification factor is up to ~900 compared to patterned silicon films of the same thickness, paving the way for the development of potential applications of metasurface fano type multimode coupling with high THG.

Research background

Resonant plasmon metasurfaces have been widely used to enhance the generation of higher-order harmonics. However, they typically have relatively low nonlinear surface sensitivity, high absorption loss due to metal ohmic dissipation, and low damage thresholds. As an alternative, light-matter interactions are significantly increased by customizing optically induced Mie-type resonances in nanoparticles made of high refractive index materials. By varying the size and shape of the nanoparticles, a wide variety of electrical and magnetic multipole resonances can be spectrally customized, such as excitation Fano resonances, non-radiative modes, and resonances associated with bound states (BICs) in the continuum with ultra-high quality factors. Thus, the high-quality modes supported by the dielectric metasurface, combined with negligible ohmic losses, low heat, and high excitable mode capacity compared to plasmoplasmic components, make it an emerging application for efficient nonlinear light generation. At present, silicon nanoresonators based on the BIC principle or metasurfaces of materials with high nonlinear magnetic susceptibility, such as germanium, gallium arsenide, aluminum gallium arsenide, zinc oxide, etc., have been applied to harmonic generation processes with significant enhancement.

Research innovation

Figure 1 Schematic diagram of the total dielectric metasurface and its parameters for enhanced THG. Prepared SEM image of a silicon antenna.

The metasurface consists of an amorphous silicon elliptical cylindrical arrangement on a glass substrate, as shown in Figure 1. The lattice constant P = 916.67 nm, the height h = 590 nm, dx and dy vary in the range of 350 ~ 850 nm.

Figure 2 (a) Unit simulation model. (b) Transmittance. (c) Third harmonic intensity. (d,e) Diagram of the electric near-field mode of the different resonators calculated, in which (d) pump (1560 nm) and (e) third harmonic (520 nm) wavelengths are drawn. The maximum value of the third harmonic resulting from the period p = 916.67 nm, the cavity height h = 590 nm, the diameter dx = 740 nm and dy = 550 nm is expressed as “Super-Fano”.

Figure 3 (a) Illustration of the enhanced third harmonic generation of the Super-Fano resonance mechanism. (b) A single resonance formed by the transmission spectrum associated with the corresponding mode. (c) The resulting transmission spectrum from the coupling of the three modes with the intensity κ forms a sharp asymmetric resonance profile like Fano. (d) Corresponding high-intensity third harmonic spectrum.

The linear transmittance numerical calculation results are shown in Figure 2b. The plot reveals that there are multiple Mie resonance modes that influence each other when they are close together within the parameter space. Figure 2d shows the electric field amplitude plot that resonates at y=0 E(ω)|。 The four regions in the figure are magnetic dipole (MD), electric dipole (ED), magnetic quadrupole (MQ), and electric quadrupole (EQ) mode. The magnetic quadrupole mode (blue) interacts with the almost parallel electric dipole mode (green) to form a classical asymmetric Fano resonance feature, represented by MQ/Fano tags. MQ/Fano resonance is closely linked to the electric quadrupole mode (magenta) resonance, where the sharpest Fano resonance signature and strongest THG emission are obtained, labeled as the Super-Fano resonance point in Figure 2b.

The formation of this unique Fano resonance is shown in Figure 3. The excited resonance and coupling intensity κ interference form a typical asymmetric resonance curve in the resulting transmission spectrum (Figure 3c), which in turn produces a strong third harmonic emission (Figure 3D). The width and spectral position of the individual resonances as well as Fano resonances can be adjusted by changing the elliptical diameter dx and dy.

The calculated intensity of the third harmonic generation signal and the contour line overlay of the mode in the transmittance plot in Figure 2 are shown in Figure 2c. The third harmonic (TH) enhancement features are found to strictly follow the MQ/Fano mode path, indicating that they are the main influence of generating higher harmonic radiation. In addition, the electric field amplitude plot shown at the third harmonic shows that the relatively outward position of the THG hotspot supports enhanced TH emission compared to other hots that are more inward at the resonance. TH intensity spans 12 orders of magnitude, and the highest intensity is found near the intersection of MQ/Fano with ED and EQ modes in this region, where Super-Fano mode is present. Here dx = 740 nm and dy = 550 nm.

Figure 4 Third harmonic power as a function of pump power.

Figure 4 shows the third harmonic as the pump power function curve. The red and blue circles correspond to the gradually increasing or decreasing pump power, respectively. The gray line represents the law of cubs. The illustration in the lower right corner shows the conversion efficiency as a function of pump power. The illustration in the upper left corner shows the third harmonic green emitted by the sample at the pump wavelength of 1560 nm.

The results show that elliptical nanoresonator arrays made of amorphous silicon are periodically arranged and exhibit strong light-matter interactions due to Fano resonances. Theoretical calculations show that these Fano resonances are the result of coupling between magnetic quadrupoles and electric dipoles. As predicted by the theory, strong light-matter interactions can be used to greatly enhance the generation of third harmonics, especially the Super-Fano mode caused by the interaction with the electric quadrupole mode.

The article was published in the journal Light: Science & Applications under the title “A Multi-Mode Super-Fano Mechanism for Enhanced Third Harmonic Generation in Silicon Metasurfaces.” David Hähnel is the first author and corresponding author of this article, and Christian Golla is the co-corresponding author of this article. (Source: LightScience Applications WeChat public account)

Related paper information:‍-023-0‍1134-1

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