Terahertz Sensing Based On Subwavelength Metal Microstructures

Terahertz wave has the characteristics of low photon energy,rich spectrum information,strong penetration,which makes terahertz technology has a broad application prospects in the field of sensing.In recent years,due to the powerful ability of controlling electromagnetic wave,the sub-wavelength metal microstructure that supporting the Spoof Surface Plasmon Polaritons(SSPP)is highly concerned.Especially,the sub-wavelength metal microstructure-based metamaterials and plasmonic terahertz sensing technology is becoming an important direction for the development of biosensing technology because of its advantages such as mark-free and high sensitivity.Based on the study of SSPP effect,this dissertation explores the application of sub-wavelength metal microstructure in the novel and highly sensitive terahertz sensing devices.The specific research includes:For the highly sensitive terahert sensing,combinating the surface plasmon resonance(SPR)and terahertz wave,a terahertz SPR refractive index sensing scheme based on sub-wavelength periodically grooved metal surface was proposed.The dispersion relation of SSPP wave on the grooved metal surface was deduced by using the mode expansion method.The sensing performance was analyzed through the SSPP dispersion curve,showing that the small incident angle and high-order mode SSPP could significantly improve the sensing performance,and the sensitivity of terahertz SPR sensing based on first-order high mode was 2.27 THz/RIU,the sensing quality factor FOM was 262 which were better than those for fundamental mode,1.28 THz/RIU and 148.In addition to the refractive index sensing,the simulation results for absorbing sample showed that the proposed terahertz SPR sensing device could also achieve simultaneous detection of the terahertz absorption spectrum for the filling analytes.In order to suppress background scattering in the detection of terahertz near field sensing,a method of using sub-wavelength grooved metal wire as the waveguide probe was proposed.The dispersion equation of the SSPP wave on the grooved metal wire was deduced theoretically.The simulation results showed that the terahertz wave could propagate on the surface of the probe by means of SSPP mode and the terahertz electric field got a more than 20 times enhancement at probe tip.Further study of the helically grooved metal wire showed that the degenerate HE1 mode of the SSPP wave on the helically grooved metal wire was decomposed into two chirality-opposite spiral modes,HE-1 and HE + 1 modes due to the symmetry-break;and using this chiral SSPP the sub-wavelength focused and circularly polarized terahertz electric field could be obtained.In view of the large thickness,high refractive index of the substrate will lower sensing performance of terahertz metamaterial sensor,an ultra-thin and flexible terahertz planar metamaterial sensor consisting of sub-wavelength U-split ring resonators(U-SRR)and metal wire(MW)structure was proposed and designed.It was theoretically clarified that the low frequency Fano peak and high frequency electromagnetic induced transparent EIT sensing peaks in the transmission spectrum were generated from the coupling of LC resonance,quadrupole resonance of U-SRR and dipole resonance of MW,respectively.The femtosecond laser micromachining system and the terahertz time domain spectroscopy system were built and used to test the proposed metamaterial sensor.Experimental results showed that when the sensor surface was loaded with different solution samples,the peaks of sensing signal appeared different degrees of frequency-shift;and with the thickness of loaded film sample increased the sensing signal peaks shifted towards low frequency with a gradual saturation,that was basically consistent with the theoretical simulation results.In this dissertation,the characteristics of sub-wavelength metal structure in terahertz band are studied theoretically,and its applications in several different types of terahertz sensing devices are discussed in detail,the obtained results can provide theoretical support and experimental basis for the design of terahertz sensing devices with high performance.