When sunlight enters the earth through the atmosphere, the light in the solar-blind ultraviolet (UV) band (200-280 nm) will be absorbed by the ozone layer. As a result, solar-blind UV detection has the advantages of little background interference, low false alarm rate, and high resolution. Solar-blind UV detection can be widely used in UV communication, high-voltage discharge detection, missile warning, search, rescue, navigation, and positioning. The solar-blind UV filter is an important component of a UV detection system. The solar-blind band-pass filter should achieve high transmittance in the range of 200-280 nm while maintaining deep cut-off in other bands. There are two kinds of solar-blind UV filters: absorption filter and interference filter. At present, although band-pass in the solar-blind band can be realized by a variety of existing optical medium structures, some problems remain to be solved. For example, the transmittance of the solar-blind UV band and the cut-off steepness on both sides of the band-pass are not high, and the uniformity of the band-pass is poor. In this paper, a band-pass filtering method is proposed by weakening photon localization and multi-defect coupling effect. The defect peak of the photonic crystal is amplified, and its shape is adjusted. The amplified and adjusted defect peak can effectively improve the transmittance of the solar-blind UV filter and have better cut-off steepness.

This paper proposes a high-transmittance band-pass filtering scheme based on the coupling of multiple photonic crystal defects. The optical transmission characteristics of a solar-blind UV band-pass filter are studied by the transmission matrix method. Firstly, by reducing the photonic crystal periods on both sides of the defect layer and weakening photon localization effect, the defect peak can be widened in the transmission spectrum. Secondly, by adjusting defect coupling, the paper studies the influence of the number of defect modes on the transmission spectrum shape, the average transmittance of the band-pass, and the cut-off steepness. Then, it investigates the influence of medium thickness on the position of the transmission spectrum and optimizes the structural thickness parameters. Finally, this paper analyzes the degree of band-pass blue shift and the filtering performance of photonic crystal at different incident angles in TE and TM modes and studies the sensitivity of the structure to incident angles.

By adjusting the structure of the defect layer, photon localization effect can be weakened, and the bandwidth of the band-pass can be effectively widened. When N₁ is equal to 1, the band-pass has the maximum bandwidth (Fig. 3). By analyzing the defect peak characteristics formed by multi-defect coupling, it is found that with the increase in N₂, the central position of the band-pass does not move, but the shape of the photonic crystal transmission peak keeps changing (Fig. 4). The cut-off steepness of the photonic crystal band-pass is getting better and better. The band-pass transmittance first increases and then decreases with the increase in the number of defects. When N₂ is 4, the average transmittance of the band-pass has a maximum value of 91.01%. As the thickness of the dielectric layer increases, the band-pass of the photonic crystal shifts in the long wavelength direction (Fig. 5). The optimization of the thickness parameters of three media yields the band-pass range of 238.8-280.3 nm for the photonic crystal, the bandwidth of 41.5 nm, and the average band-pass transmittance of 90.71%. The average transmittance of the band-stop in the range of 290-360 nm is 1.47% (Fig. 6). With the increase in angle, the transmission spectra of the TE mode and the TM mode are separated at the same time as blue shift (Fig. 7). When the incident angle is less than 15°, the band-pass bandwidth is still about 40 nm, and average transmittance is close to 90% in both the TE mode and the TM mode (Table 3 and Fig. 8).

In light of the defect coupling principle, this paper designs a one-dimensional photonic crystal band-pass filter with high transmission at 239-280 nm and deep cut-off on both sides of the band-pass. Firstly, a multi-defect photonic crystal model with structure [(H/L)N1A(H/L)N1]N2 is proposed. By weakening photon localization effect, the transmission peak is widened. When N₁=1, the band-pass reaches a maximum bandwidth of 42.2 nm. Secondly, the band-pass shape of the photonic crystal is adjusted by coupling different numbers of defects to improve the cut-off steepness and increase the average band-pass transmittance. It is found that when N₂=4, the band-pass transmittance is the highest, and the cut-off steepness is better. Then, the influence of medium thickness on the location of the band-pass is analyzed, and the thickness parameters of the structure are optimized. Finally, the blue shift of the transmission spectra under different incident angles is studied. The filtering effect is still ideal when the incident angle is less than 15°. A solar-blind UV band-pass filter is obtained with the structure of [(Si₃N₄/SiO₂)¹Air(Si₃N₄/SiO₂)¹]⁴ and a medium thickness of dSi3N4=30 nm, dSiO2=40 nm, and d_Air=67 nm. The average transmittance of the filter band-pass (239-280 nm) is 90.71%, which can achieve high transmittance in the solar-blind UV region. The average transmittance of the band-stop (290-360 nm) is 1.47%, which can achieve deep cut-off. This paper employs the method of widening the transmission region of the band-pass and adjusting the shape of the band by weakening photon localization and multi-defect coupling, which can provide a new idea for the design of high-transmission band-pass filters.