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[Display omitted] • A novel asymmetric permittivity all-dielectric membrane metasurface with the simplest centrosymmetric square lattice, where dual-band polarization-insensitive ε - q BICs induced by permittivity asymmetry is first designed and simulated. • The dual quasi-BIC modes are dominated by the coupling between the magnetic dipole and electric quadrupole and the coupling between the toroidal dipole and magnetic quadrupole. • The ε - q BICs modes are robust against the shape of the guest medium, giving a wide tolerance for precision in nanofabrication. It is generally accepted that the symmetry-protected quasi-bound states in the continuum (qBICs) are induced by breaking geometry symmetry in the regular quasi-planer nanostructure. However, it is very challenging to allow precise control of the asymmetry parameter in the process of fabrication and realize a tiny tailoring geometry symmetry. Here, a novel permittivity-asymmetric membrane metasurface, which can also induce polarization-insensitive qBICs by a symmetry breaking in the permittivity of the comprising host and guest medium is proposed and investigated. Simulation results show the line width and spectral position of resonances can be controlled by increasing or decreasing the refractive index of the guest medium. The far-field multiple decompositions and the near-field distributions reveal the physical mechanism of dual qBIC modes, which are dominated by the magnetic dipole and toroidal dipole and display opposite optical properties. Furthermore, it is found that such BICs are robust against the shape of the guest medium. Eventually, the sensing performance of the proposed structure is evaluated. This study provides a simple and promising structure for the excitation of qBICs and conceives a feasible approach to achieving a high Q factor. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ascribing to the high refractive index of PDMS, the tapered SMF covered with PDMS can work at turning around point with larger taper diameter, which effectively enhance the structural strength and stability. • The porous character of PDMS film makes the interferometer have ultrahigh gas pressure sensitivity. • By using spin-processing method, the thickness of the PDMS film covering the fiber taper can reach only the order of micron, so that the pressure recovery time of the sensor is as short as 20 s. An ultra-sensitive gas pressure sensor based on tapered single-mode fiber (SMF) coated with thin polydimethylsiloxane (PDMS) film which works at turning around point (TAP) is proposed and demonstrated. Ascribing to the high refractive index of PDMS, the tapered SMF covered with PDMS can work at TAP with larger taper diameter, which effectively enhance the structural strength and stability. At the same time, the porous character of PDMS film makes the interferometer have ultrahigh gas pressure sensitivity. By using spin-processing method, the thickness of the PDMS film covering the fiber taper can reach only the order of micron, so that the pressure recovery time of the sensor is as short as 20 s. Experimental results show that the gas pressure sensitivity of the proposed sensor can be as high as −159.80 nm/MPa in the range from 1 KPa to 100 KPa, and another dip away from the TAP obtains the sensitivity of −69.61 nm/MPa. [ABSTRACT FROM AUTHOR]