Terahertz Bio-Sensing With Resonant Metasurfaces

Current mainstream biosensors generally face the problem of low efficiency in the situation where a rapid and accurate detection is necessary.The emerging metasurface biosensors are expected to solve this problem.The tunability of metasurfaces makes it a promising approach to label-free bio-sensing with EM waves.The Terahertz(THz)waveband containing the spectroscopic labels for many bio-samples.The combination of THz radiation with metasurfaces is a feasible approach to biological detection and sensing.However,the sensitivity for most THz metasurface based bio-detection methods is not high.To improve the sensitivity,two approaches are proposed in this project,which adopt the toroidal dipole resonance and the exceptional point in a non-Hermitian system on a graphene metasurface,respectively.Using a graphene metasurface increases the effective sensing area significantly while the two physical phenomenon strengthen the interaction between the sample and the THz radiation.Compared with the higher order toroidal multipoles,the toroidal dipole can provide a strong field concentration effect,thus enhancing the interactions between the samples and the THz radiation.Its combination with THz metasurfaces facilitates high sensitivity bio-sensing.In this project,the principle of toroidal dipole excitation is first theoretically analyzed,based on which a patterned graphene metasurface THz bio-sensor is designed.The electromagnetic response of the metasurface is first analyzed with COMSOL simulations,which confirms that the metasurface supports toroidal dipole resonances.It also shows that the metasurface has a high quality factor and is highly absorptive at the resonant frequency with an absorptivity of 80%.Its bio-detection sensitivity is also simulated by measuring the refractive index change induced frequency shift,being 42GHz/RIU.Exceptional points(EPs)are the status points in a non-Hermitian system where the eigenvalue and the eigenvector simultaneously coalesce.At these points,the system usually experiences some strange phenomena,one of which is that it is supersensitive to the external disturbances.Adopting this on a THz metasurface can greatly improve the bio-detection sensitivity of THz metasurfaces.This project exploits the high tunability of the system parameters for a non-PT(parity-time)-symmetric system and proposes a nonPT-symmetric non-Hermitian metasurface based on patterned graphene.The EP of the system is then found with the coupling mode theory and full-wave simulations.The sensitivity of the metasurface is also analyzed with the same method used for the toroidal metasurface.The results show that the microbiological detection sensitivity of the metasurface reaches a higher 71 GHz/RIU.The influence factors of the two metasurface sensitivity are further analyzed,revealing that the relaxation time has a large influence.Finally,the two proposed metasurfaces are fabricated with nanofabrication methods and their optical and electric properties are measured.The results further validate the actual performance is in good agreement with simulation results.The research results of this project provides a novel approach to high-sensitivity THz sensing with metasurfaces and will push it forward towards real applications.