Through the combination of nanoscale Mie-resonance and photothermal/thermo-optical effect, plus a nanosecond excitation source that matches the thermal relaxation time of a silicon nanostructure, we demonstrated an ultra-large nonlinear index n2 = 1 um^2/mW, six-orders larger than the value in bulk. Under a confocal laser scanning scheme, unexpected sharp transition of scattering intensity is unveiled, suggesting a rapid temperature transient. The super-continuum wavelength tunability offers high-efficiency excitation among nano-silicon with various sizes. This robust and ultra-large nonlinearity shall be useful in optical switching and super-resolution mapping of semiconductor nanophotonic structures.
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