Bi-controlled toroidal dipole induced dual qBICs on flexible substrate for ultrahigh sensitive optical switch and gas sensor

We present a dielectric metasurface comprising four pairs of asymmetric silicon (Si) nanorod dimers (ASDs) arranged in a double mirror-symmetric supercell on a polydimethylsiloxane (PDMS) substrate. Unlike previously reported single quasi-bound states in the continuum (qBICs) based on electric or magnetic dipole modes in translationally symmetric ASD arrays, our design supports dual toroidal dipole qBICs with ultrahigh quality factors of 1.1 × 105 and 5.7 × 105. The resonant position shift ratio of the dual BICs can reach 6.3% and 3.4% when the stretching ratio (S) of PDMS is 10.0% along both x and y directions, respectively. A modulation depth of 99.90% and an ON/OFF ratio exceeding 24 dB are obtained with temperature varying (ΔT) within 0.2 K. Remarkably, for the dual thermo-optic and stretching control, the ΔT required to achieve the same spectral shift decreases from 0.2 to 0.1 K with only S of 0.02%. Finally, for real gas sensing, the high sensitivities of 384 and 255 nm/RIU as well as ultrahigh figures of merit of 1.2 × 106 and 1.8 × 105 RIU−1 are achieved. These findings highlight the potential of the dielectric metasurface for applications in optical switching and gas sensing.