Nonlinear mid-infrared meta-membranes

Abstract

Nanophotonic structures have shown promising routes to controlling and enhancing nonlinear optical processes at the nanoscale. However, most nonlinear nanostructures require a handling substrate, reducing their application scope. Due to the underwhelming heat dissipation, it has been a challenge to evaluate the nonlinear optical properties of free-standing nanostructures. Here, we overcome this challenge by performing shot-controlled fifth harmonic generation (FHG) measurements on a SiC meta-membrane - a free-standing transmission metasurface with pronounced optical resonances in the mid-infrared (λ _res ≈ 4,000 nm). Back focal plane imaging of the FHG diffraction orders and rigorous finite-difference time-domain simulations reveal at least two orders of magnitude enhancement of the FHG from the meta-membrane, compared to the unstructured SiC film of the same thickness. Single-shot measurements of the meta-membrane with varying resonance positions reveal an unusual spectral behavior that we explain with Kerr-driven intensity-dependent resonance dynamics. This work paves the way for novel substrate-less nanophotonic architectures.

Keywords: metasurfaces; mid-infrared; nonlinear optics; silicon carbide.

Figure 1:

Sample and setup. (a) SEM micrograph (left) and schematic (right) of the sample. The arrays are fabricated out of a continuous SiC membrane of a varying thickness. (b) An experimental mid-IR transmittance spectrum of the sample for the design polarization (solid line) and the corresponding FDTD calculation (dashed line). The spectrum of the pump laser is superimposed (gray shaded area). The calculated local field distribution at a wavelength of 4,000 nm in the middle section of the meta-membrane is shown to provide a substantial field enhancement at the pump wavelength. (c) Schematic of the experimental FHG BFP imaging setup.

Figure 2:

Single-shot FHG from the free-standing SiC meta-membrane. (a) BFP of the FHG emission from the metasurface for the vertical (V, design) polarization. (b) Simulated BFP image of FHG from a resonantly pumped metasurface (pump polarization V). The yellow circle denotes the numerical aperture of the collection system used in the experiment. (c) Calculated zeroth-order FHG intensity as a function of pump irradiance. An average enhancement of about 10³ is observed before saturation starts at an irradiance of around 1 TW/cm². (d, e) Log-log-plotted intensity-dependent FHG for V and H polarizations, respectively, showing the data points and exponential fits y = ax ^b . The fitted exponents are b _V = 5.0 ± 0.4 and b _H = 4.9 ± 0.7, respectively, closely matching the expected power relationship $I_{\text{FHG}}\propto I_{\text{pump}}^5$ in the perturbative regime.

Figure 3:

Shot-by-shot performance and damage. (a) FHG as a function of the number of consecutive pulses sent to the sample for vertically (blue circles) and horizontally (red crosses) polarized pump. Fit curves are exponentials with decay constants N _V = 7 pulses (R ² = 0.98) and N _H = 49 pulses (R ² = 0.96) for vertically and horizontally polarized pump, respectively. The decay is much faster for the design polarization (V) due to the mechanical structure of the metasurface unit cell. (b) The result of the metasurface irradiation by 100 V-polarized pulses. (c) The result of the metasurface irradiation of 100 H-polarized pulses. The white arrows indicate typical multi-shot damage propagating perpendicular to the polarization of the pump [46].

Figure 4:

Kerr-assisted FHG. (a) Experimental single-shot FHG as a function of the meta-membrane’s resonance (blue dots). The resonance wavelength is derived from the sample thickness map by using FTIR microscopy and numerical simulations. Data are fitted with a Gaussian (solid line), yielding peak FHG generation at 3,850 nm. The laser carrier wavelength is shown with a vertical dashed line, indicating an unusual behavior where the maximum FHG signal occurs when λ _res ≠ λ _pump. A Kerr-nonlinearity-driven model describes this behavior, producing the dotted curve; see text for details. (b) Zeroth-order FHG as a function of SiC membrane thickness at a field strength of 2 GV/m. FHG peaks at a thickness of h = 525 nm, at which the low-field resonance is blue-shifted. The inset shows the linear transmittance spectrum of the meta-membrane calculated at different thicknesses. Arrows indicate the cases presented in panels (c) and (d). (c) The transmittance of the meta-membrane at a thickness of h = 525 nm as a function of the pump field strength. The resonance is red-shifted and overlaps better with the pump, generating a high level of FHG. (d) Same for h = 575 nm, where the resonance is departing away from the pump at high field values. Larger-than-unity transmission in panels (c) and (d) indicates frequency mixing in the pump beams under strong-field excitation.