Bidirectional UV metasurface: Achieving switchable perfect absorption for gas/biosensing and optical shielding

UV photonic devices with tailored absorption characteristics are highly desirable for applications ranging from environmental monitoring to biomedical sensing, while achieving simultaneous broadband and narrowband perfect absorption in a single compact structure remains challenging. This study presents an innovative UV absorber capable of bidirectional perfect absorption with distinct spectral characteristics determined by illumination direction. When illuminated from the bottom (-z-axis), across the 185–400 nm band, the structure maintains 98.22 % average absorptivity, with polarization-insensitive and wide-angle absorption. It exhibits an optical shielding efficiency exceeding 100 dB throughout this spectral range, consistently achieved foe both TE and TM polarization over a wide range of incident angles, demonstrating excellent potential for applications in UV protection. Conversely, the top illumination (+z-axis) generates dual ultra-narrowband absorption peaks with near-perfect absorption (99.96 % and 99.76 %). The corresponding full width at half maximum (FWHM) values are 1.5 and 2.7 nm, respectively. Moreover, the absorber demonstrates exceptional sensing performance to be a gas sensor and a biosensor. As a gas sensor, the peak sensitivities are 109.6 nm/RIU and 32 nm/RIU. The corresponding figures of merit reach 109.6 RIU⁻¹ and 33.68 RIU⁻¹, and quality factors are 185.92 and 104.63. The proposed absorber promises potentially valuable applications for ultraviolet protection, sensors, and stealth technology.