Giant two-photon absorption response from a silicon quasi-BIC metasurface

We propose an amorphous silicon (a-Si) metasurface based on a quasi-bound state in the continuum (q-BIC) with a theoretical Q-factor up to 930, which consists of Π-like nanostructures with symmetrical defects. The influences of slot offset parameter and the size of added symmetric defect on the near- and far-field characteristics of the metasurface have been analyzed. Due to local near-field enhancement by the tailored high-Q quasi-BIC resonance, our designed metasurface exhibits significantly enhanced two-photon absorption (TPA) response in the experiment, i.e., the TPA coefficient β0 = 173.2 cm/MW (3–4 orders of magnitude larger than a-Si film) and the TPA saturation intensity Isat = 93.0 MW/cm2. Our work offers a promising alternative scheme to improve nonlinear optical effects at the nanoscale.