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Although previously reported terahertz absorbers can achieve high-sensitivity refractive index sensing, the resonant peak is too broad, which leads to a low figure of merit (FOM). Transmissive sensors based on bound states in the continuum (BIC) can achieve high FOM, but they have some limitations in high sensitivity. Herein, we propose a periodic triple parallel metal bars structure to obtain high quality, a strong field, and multiple hot spots by the Friedrich-Wintgen BIC. Numerical results show the sensitivity and FOM can reach 1877 GHz/RIU and 665, respectively. Compared to the previously reported transmissive sensors based on BIC, the sensitivity has been greatly improved.

Here we propose an ultra-wideband terahertz absorber (UWTA), which is composed of substrate, dielectric, and five-layer graphenes. By changing the chemical potential of graphene, the absorption coefficient of UWTA is larger than 0.9 from 1.51–8.25 THz. The absorption bandwidth is 6.74 THz. The relative bandwidth is 138.1%, and the modulation depth is 78.7%. Meanwhile, the number of each layer of graphene can be adjusted to achieve high absorption bandwidth, relative bandwidth, and modulation depth. Besides, due to the tunable chemical potential of graphene, the absorption coefficient can be adjusted to make up the manufacturing error.

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One-dimensional photonic crystal structures have been widely used to enhance fluorescence. However, its fluorescence enhancement is low-fold because of a weak excitation field region. In this paper, we used a genetic algorithm to assist in the design of two photonic crystals based on Al2O3 and TiO2 materials. One of them has a defect consisting of SiO2. The Fabry-Perot cavity (FP cavity) formed by the sandwiched photonic crystal achieves up to 14-fold enhancement of the excitation electric field. We modulate the electric field radiation distribution of the fluorescent material by using photonic forbidden bands. For a 3.18 μm thick layer of the fluorescent solution, the structure achieves up to 60-fold fluorescence enhancement. We also discussed that the reason for the different enhancement abilities in different places is the phase change caused by the optical path difference. This design is expected to have applications in display, imaging, etc.