We propose an angular-dependent polarization-insensitive filter in the terahertz (THz) region, based on the guided-mode resonance of one-dimensional zero-contrast grating architectural design. Particle swarm optimization combined with the rigorous coupled-wave analysis method is used to design the filter and investigate the influences of the planes of incidence on the characteristics of the proposed all-dielectric THz filter. With the planes of incidence set at 0°, 30°, 45°, and 60°, the polarization-independent resonances occur at 0.458 THz, 0.459 THz, 0.461 THz, and 0.465 THz under oblique incidences of 9.3°, 10.8°, 13.3°, and 19.2°, respectively, which means the oblique incident angle of the polarization-independent THz filter increases with the rotation of the planes of incidence from classic mounting to fully conical mounting. In addition, for the fully conical mounting case, the resonance has high angular stability and is no longer split, compared with classic incidence; meanwhile, there is only a tiny blue shift in resonance of less than 3 GHz when changing the incident angle from 0° to 10°. The physical mechanism of the spectral characteristics is also analyzed in detail. The spectral properties proposed herein enable significant potential application in the fields of spectroscopy, image sensors, communication, etc., in the THz region.

The existing polarization-insensitive filters based on two dimensional (2-D) grating are mostly under normal incidence. However, in filtering optical devices, the normal incidence regime is usually avoided because it requires the use of beam separators which yields a loss of efficiency. Herein, we present a compact method to realize polarizationindependent filter under oblique incidence, which is implemented on 2-D crossed grating in the telecommunication region. Plane of incidence is set to be in xz plane and three different polarization angles (0°, 45°, and 90°) are used to demonstrate the polarization-insensitivity. Realization of the polarization-insensitivity of the 2-D filter is based on the split feature under oblique incidence, according to the analysis of the physical mechanism. The location of the polarization-independent resonance is mainly determined by the grating period along x-axis and will right shift with increasing the period. Results show that the polarization-insensitive resonances occur at 1356.9 nm, 1372.6 nm and 1398.5 nm when the grating periods along x-axis are 830 nm, 850 nm and 870 nm, respectively. Moreover, the results and methods provided herein can be applied to search for the polarization-insensitive resonances of the 2-D grating with other planes of incidence.

All-dielectric metasurfaces, which are periodic 2D gratings on slab waveguides, are designed to obtain angulardependent polarization-insensitive guided-mode resonance filters operating in the terahertz region. The effect of the metasurface grating periods along the x-and y-axis on the filter's performance is systematically studied via the rigorous coupled-wave analysis method. When the grating periods along the x-axis are 380 µm, 400 µm, and 420 µm, the polarization-independent resonances of the split bands occur at 0.517 THz, 0.498 THz, and 0.481 THz, respectively. When the grating periods along the y-axis are adjusted to the above values, such polarization-independent resonances show good robustness, which means the polarizationindependent resonance frequency of the split bands is mainly determined by the grating periods along the x-axis and is in an inverse proportion. Meanwhile, there are additional polarizationindependent resonances formed by the intersection of the split and degenerate bands for the two unequal grating periods. This kind of component provides potential applications for narrowband filtering in emerging terahertz devices and systems for communication or sensing.