| Terahertz waves have significant potential in material detection and sensing,security inspection,military,communication and other fields due to the characteristics of strong penetration,fingerprint spectrum,low photon energy,easy absorption by water,coherence and wide spectrum.The traditional terahertz time domain spectroscopy(THz-TDS)system often faces some difficulties in realizing micro and high resolution in sensing applications due to the limitation of the wavelength of THz waves.Moreover,the strong absorption of the water has greatly strict the engineering applications of terahertz sensing technologies in sensitive solution sensing.In order to solve the above challenges and provide the technical support of THz sensing technology in engineering applications,this thesis focuses on ultrasensitive THz metasurface sensing and aims at improving the sensitivity to meet the requirements of the ultrasensitive THz sensing in engineering applications.Based on the research on the technology of THz solution sensing,fingerprint spectrum identification of trace amuount of samples,and resonant structure with high Q factor and its applications in ultra-thin samples sensing,the ultrasensitive sensing technology based on THz metasurface and the sensing applications have been studied.The sensing performances have been verified by the self-built THz-TDS system.The innovation and main work of this thesis include the following three parts:1.Aimed at solving impact of water absorption on the sensitivities of terahertz solution sensing,a metasurface sensor for ultrasensitive sensing of aqueous solution has been proposed.Firstly,the theoretical model and the experimental method for the optical constants of pure water based on THz-TDS system have been established.The optical constants of pure water have been measured and the sensing experiment for sucrose solution have been further performed using the self-built THz-TDS system and an attenuated total reflection(ATR)prism made of high-resistivity silicon.In addition,the sensitivity of metasurface with different types of metal split rings and ATR prism have been analyzed and compared,and the structure parameters of the metasurface-enhanced ATR have been optimized,followed by the fabrication of the metasurface-enhanced ATR sensor by lithography technology.Finally,the resonant characteristics of the sensor have been verified by the reflective THz-TDS system.The results show that the detection limit of the metasurface-enhanced ATR for sucrose detection is 0.03125 mol/L,and its sensitivity is 4 times higher than that of the traditional ATR.This study is an important reference and exhibits great potential in THz solution sensing field,especially provides an effective way for applying THz metasurface in aqueous biosensor applications.2.In order to solve the problem that the traditional THz-TDS is difficult to realize THz fingerprint spectrum recognition for trace amounts of solid samples in engineering,a THz specific sensor based on all-dielectric metasurface has beenproposed.Firstly,the theoretical model of optical constant calculation has been established,and the absorption spectrum of the sample has been measured and calculated by using the transmitted THz-TDS system based on the conventional sample in shape of disk.Then,an ultrasensitive specific sensor based on all-dielectric metasurfaces with frequency scanning in the THz range has been designed with the help of the newly developed angle-multiplexed technology.It is found that when the incident angles of the THz waves are scanned in two different polarization modes,the resonance frequency will form a wide reflection spectrum to cover the absorption peak of the analyte.The recognition of absorption peaks for trace amounts of tyrosine and santonin has been successfully realized by establishing a physical model and solving the reflection spectrum envelope.Finally,by comparing the reflection spectrum with different thicknesses of analytes covered on the metasurfaces,we have successfully identified the tyrosine and the santonin with the detection limits of 6.7μg/cm~2 and 59.35μg/cm~2,respectively.The results of this study have important applications in the target analytes detection in THz band.3.A terahertz all-silicon metasurface with sharp resonant has been proposed and its engineering applications in both refractive index sensing and graphene thin film sensing have been studied.Firstly,the mechanism of the sharp resonant in the all-silicon metasurface has been analyzed,and the structure parameters of the metasurface have been further optimized.The Q factor,refractive index sensing sensitivity and FOM index can reach 39587 nm/RIU,16,042 nm/RIU and 533 nm,respectively.Then,the physical model of the metasurface for graphene film sensing has been established,and the effects of monolayer graphene and multilayer graphene on Fano resonance have been calculated and analyzed.Consequently,the sensing characteristics of the all-silicon metasurface for the graphene film can be obtained.Finally,the all-silicon metasurface has been fabricated by deep silicon etching process,and the transmission of the metasurface and the change for the graphene films have been measured by the transmitted THz-TDS system.The results show that when the metasurface sensor is covered by the graphene films,the variation trend of its transmission and the Q factor are consistent with the simulation results,which verifies the validity of the design and processing.The results of this study can provide a promising approach for the research of sensing ultra-thin materials or biomolecules with the high conductivity.The research works and the achievements in this thesis are helpful for solving the problems in engineering applications of terahertz sensing technology,and can provide important technical support for engineering applications such as ultrasensitive solution sensing,fingerprint spectrum identification for trace amounts of solid samples and thin-film sensing. |
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