An ultra-wideband terahertz linear cross-polarization converter with integrated biosensing for multi-disease diagnosis

Abstract

Polarization converters are essential components in a wide range of terahertz applications including imaging, communication and sensing. However, existing polarization converters face significant challenges, such as limited operational bandwidth, low polarization conversion efficiency and the use of expensive or complex materials which hinder their practical implementation. To address these issues, this work presents a dual-functional terahertz metasurface that is capable of achieving ultra-wideband linear cross-polarization conversion along with high sensitivity biosensing. The design is constructed using aluminum and polyimide with a normalized unit cell size of 0.26λ_L × 0.26λ_L × 0.09λ_L (λ_L = wavelength at the lowest operating frequency). It achieves a polarization conversion ratio (PCR) above 90% across 4.442–8.444 THz, covering a bandwidth of 4.002 THz, an FWHM of 5.094 THz and an average PCR of 98.62%. It maintains a PCR above 98% over 3.66 THz and above 99% in two sub-bands totaling 2.598 THz. Moreover, the design sustains an average polarization conversion efficiency of 97.96% at a 20° oblique incidence. As a biosensor, it provides a maximum sensitivity of 1.50 THz per RIU, which enables precise detection of refractive index variations associated with various diseases including cervical and skin cancers, blood disorders and infections such as malaria. The design outperforms existing metasurfaces in terms of polarization conversion efficiency, operational bandwidth and biosensing sensitivity, filling a significant gap in the field of terahertz applications. It provides a cost-effective and fabrication-friendly solution that surpasses all existing designs and provides a reliable platform for multi-disease detection.