Silicon photonics have attracted significant interest because of their potential in integrated photonics components and all-dielectric meta-optics elements. One major challenge is to achieve active control via strong photon–photon interactions, i.e. optical nonlinearity, which is intrinsically weak in silicon. To boost the nonlinear response, practical applications rely on resonant structures such as microring resonators or photonic crystals. Nevertheless, their typical footprints are larger than 10 μm. Here, we show that 100 nm silicon nano-resonators exhibit a giant photothermal nonlinearity, yielding 90% reversible and repeatable modulation from linear scattering response at low excitation intensities. The equivalent nonlinear index is five-orders larger compared with bulk, based on Mie resonance enhanced absorption and high-efficiency heating in thermally isolated nanostructures. Furthermore, the nanoscale thermal relaxation time reaches nanosecond. This large and fast nonlinearity leads to potential applications for GHz all-optical control at the nanoscale and super-resolution imaging of silicon.
In this paper, we theoretically and experimentally demonstrated photothermal nonlinearities of both forward and backward scattering intensities from quasi-perfect absorbing silicon-based metasurface with only λ/7 thickness. The metasurface is efficiently heated up by photothermal effect under laser irradiation, which in turn modulates the scattering spectra via thermo-optical effect. Under a few milliwatt continuous-wave excitation at the resonance wavelength of the metasurface, backward scattering cross-section doubles, and forward scattering cross-section reduces to half. Our study opens up the all-optical dynamical control of the scattering directionality, which would be applicable to silicon photonic devices.
Nonlinear optical interactions are of fundamental significance for advanced photonic applications, but usually the nonlinearity magnitude is insufficient. Here we review recent progresses to boost the optical nonlinearity of metal or semiconductor nanostructures via the combination of Mie resonance and coupled photothermal/thermo-optical effects. In plasmonic and silicon nanoparticles, the effective photothermal nonlinear index n2 is enhanced by 103 and 105 times over that of bulk, respectively. The large nonlinearities enable applications of not only all-optical switch, but also super-resolution imaging based on suppression of scattering, saturation (sub-linearity) and reverse saturation (super-linearity).
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