A Frequency-Tunable Terahertz Bandpass Filter for Future Communication Systems

The importance of the terahertz (THz) region in wireless communication is increasing. THz tunable filters are key components for next-generation communication systems. In this article, we have designed a THz narrowband bandpass filter based on a double-layer frequency selective surface (FSS), which is composed of two X-shaped FSSs cascaded together with different geometric parameters. The center frequency of the double-layer structure is located between the center frequencies of the two single-layer FSSs. Based on the principle of inverse design, we employed intelligent algorithms to automatically optimize the structural parameters of the single-layer FSS. We simulated the transmission spectra of both single-layer and double-layer FSSs using finite-difference time-domain. Subsequently, FSSs were fabricated using femtosecond laser micromachining and characterized using time-domain THz spectroscopy. The experimental results are highly consistent with simulations. The designed double-layer filter exhibits a high Q factor and tunability of the center frequency by changing the spacing between the two FSS layers. Finally, based on an equivalent circuit model, we analyzed the transmission phenomenon of the nonequivalent double-layer FSS structure and attributed it to the resonances of equivalent capacitance and inductance between the FSS layers at different spacings. This approach can be applied to design tunable filters for other frequency bands.