An ultra-wideband and ultra-thin reflective cross polarization converter is presented in this paper for detection of chlorophyll. The metasurface consists of two T-shaped resonators and circular-ring resonator interconnected by narrow metallic strips. The proposed polarization converter provides a high cross polarization conversion ratio (PCR) of \(\ge\) 94% from 1.96–4.79 THz. The ultra-wide bandwidth response is achieved by the combination of five distinct resonant modes occurring at 2.11 THz, 2.56 THz, 3.53 THz, 4.44 THz and 4.74 THz. The reflective polarization converter provides stable response for an angle of incidence of up to 40°. The metasurface is integrated with silicon in the gap between the resonators to achieve switchable characteristics. The frequencies within the operational bandwidth of the proposed metasurface matches very well with the THz chlorophyll modes and it can be used for label-free, non-destructive detection of chlorophyll. The refractive index variations caused by the presence of chlorophyll are sensitively captured through polarization conversion and the sensor demonstrates a high sensitivity of 480 GHz/RIU, making it ideal for precise leaf health monitoring.
