Polymers enabling independent dual-band quasi-bound states in the continuum with ultrahigh Q factors at high THz frequencies

Quasi-bound states in the continuum (QBIC) offering high Q factors possess great value in fields such as sensing, nonlinear optics, and imaging. However, limited by loss in materials and the demanding requirements for accuracy in fabrication, realizing high-quality QBIC devices is challenging in THz, especially at high THz frequencies where conventional materials become more lossy and the dimension of the device shrinks. Here, we report a polymeric dual-band QBIC device in THz. It exhibits an ultrahigh experimental Q factor of 427.1 above 4 THz. The device is composed of an asymmetric resonant waveguide grating structure made of low-loss styrene-butadiene-styrene polymer, obtained by a simple and precise version printing method suitable for mass production. Spectral shapes and Q factors of the two QBIC bands can be independently adjusted, demonstrating flexibility towards different potential applications. The presented strategy with the achieved device shall promote the application of THz technology in high-frequency THz bands.