Transient Super‐/Sub‐Linear Nonlinearities in Silicon Nanostructures

Abstract

By virtue of its unique advantages such as natural abundance and mature fabrication engineering, silicon (Si) is widely utilized in the electronic industries. However, in the field of photonics, the indirect bandgap nature of Si prohibits emissionrelated applications. Despite this limitation, Si exhibits a relatively high refractive index that allows efficient light confinement, especially in nanostructures. [1][2][3] The strong confinement and accompanying field localization offer substantial enhancement of the intrinsically weak optical nonlinearity in Si, including twophoton absorption, [4,5] harmonic generation, [6] and photothermal effects, [7,8] leading to the emerging field of nonlinear Si nano-photonics [3] with applications covering all-optical switching, wavelength conversion, and superresolution imaging.Conventionally, optical nonlinearity is characterized via intensity-scan methods such as z-scan, [9,10] which is suitable to investigate thin-film samples. In the z-scan method, a thin sample moves along the propagation direction (z-axis) of a focused laser beam, and z-position dependent transmittance or divergence Nonlinear silicon nano-photonics has recently attracted significant attention due to the plethora of electric and magnetic Mie resonances that offer substantial enhancement of optical nonlinearities. Conventionally, the characterization of nonlinearity and its transient nature rely on intensity-scan methods (z-scan) in the spatial domain and pump-probe techniques in the temporal domain. However, most studied ultrafast nonlinear effects are instantaneous, that is, strongest at zero pump-probe delay, and have a solitary nonlinear power dependency (square, cubic, etc.). Here the authors found that when relaxation lifetime is dependent on pump fluence, transient nonlinearity appears. The effect is exemplified via Auger-based nonlinear carrier dynamics of a nano-silicon Mie-resonator. The Auger-induced transient nonlinearity not only locates at the time delay of several tens of picoseconds, but also displays diverse nonlinearities, including sub-linear, super-linear, and surprisingly full saturation, which features a "crossing point" where the probe scattering is pump-fluence independent. The crossing point exists when the relaxation lifetime is inversely dependent to second power of carrier density. Combining confocal intensity-scan (x-scan) and pump-probe temporal scan, the authors demonstrate that sub-linearity and super-linearity lead to swelled and reduced full-width-at-half-maximum (FWHM) of single-nanostructure images, further confirming the nonlinearity as well as the potential of sub-diffraction microscopy. The results open up a new avenue in nonlinear silicon nano-photonics by adding new degrees of freedom in temporally tuning the types of transient nonlinearities, which are valuable in all-optical signal processing and nano-imaging.

Large Optical Modulation of Dielectric Huygens’ Metasurface Absorber

“…The carrier recombination time of amorphous SiNBs in this work is on the order of a few picoseconds, which is much shorter than that of crystalline SiNBs on the order of several tens of picoseconds. [62] The variation of carrier lifetime was also observed in crystalline and amorphous silicon thin films. [60,61] From Figure 3a,b, the modulation depths of sample 1 and sample 2 at 19 ps, where they are already dominated by thermal modulation, are 9% and 0.2%, respectively.…”

Roadmap on Label‐Free Super‐Resolution Imaging

“…For instance, the ultrafast temporal dynamics of the nonlinear Mie-scattering has not yet been thoroughly scrutinized. [538]…”

Ultrasensitive Photothermal Switching with Resonant Silicon Metasurfaces at Visible Bands

“…Note that the heating/cooling effect of silicon nanostructures occurs typically on the order of the nanosecond scale, according to previous research on photothermal tuning. 32,38 Therefore, the modulation frequency of photothermal switching of the q-BIC state can be anticipated to be up to GHz. Considering that such a nanoscale thermal relaxation time is much faster than the dwell time (10 μs) in our experiments, we measure the reflection intensity after the nanostructures reach thermal equilibrium at each scanning pixel.…”