| In recent years,due to the rapid development of ultrafast laser technology,optoelectronic technology,and semiconductor crystal growth technology,terahertz(THz)science and related technologies have made significant advances.THz technologies provided powerful new research tools for many scientific fields,such as biology,physics,and chemistry,which have effectively promoted and widened the development of these research fields.Particularly in biomedical applications,THz wave energy matches the rotation and vibration energy of biomolecules(such as DNA,RNA and protein,etc.),making it possible to achieve resonance excitation and detection of biomolecule fingerprint peaks for obtaining biological information that is difficult to measure by other technologies.However,there still are numerous issues in the commercialization of THz biosensors,which limit their application in the field of biological detection and other non-professional laboratories in industry or medicine,for clinical applications,rapid screening,and trace detection.These issues are summarized as follows:(1)THz sources that can be used for biological detection have a problem with narrow bandwidth,which limits the diversity of biomolecules that can be measured;(2)the lack of reliable circularly polarized THz sources makes it difficult to intuitively detect the structural information of chiral biomolecules;and(3)due to the difficulty of sampling biological materials,the sample quantity is typically lesser,so the sensitivity of THz biosensor must be high enough.To address these issues,an efficient broadband THz source based on Bound state in continuum(BIC),a THz polarization modulator sweeping a full-Stokes space,and an ultra-highly sensitive biosensor based on singularity of non-Hermitian optical systems have been designed and fabricated in the thesis,which has provided the theoretical and experimental basis for the development of non-invasive,label-free,and ultrasensitive desktop-level THz biosensing systems.The main contents of the research are as follows:1.To achieve efficient broadband THz emission,a lithium niobite-silicon oxide composite structure based on quasi-bound states in the continuum(quasi-BICs)was designed and fabricated.Firstly,a quasi-BIC based on the interaction between the resonant modes in the silicon oxide metasurface and the lithium niobate film was realized.Secondly,using conventional micro-nanofabrication procedures,high-quality large-area silicon oxide metasurfaces were fabricated on thin-film lithium niobate substrates.Finally,the THz emission performance of the composite structure was characterized by THz time-domain spectroscopy.The results showed that the THz electric field strength of the composite structure was significantly enhanced in the range of 0-3 THz.2.Realizing active control of THz polarization in the full Stokes space,a metasurface composed of metal split resonators(SRRs)and Ge nanoblocks,based on the principle of polarization exceptional point(EP)was designed and fabricated.Firstly,the relationship between the loss of the optical system composed of metasurfaces and its eigenpolarization state was verified using the time-domain coupled mode theory.On this basis,the metasurface satisfying Parity-time(PT)symmetry and dynamically adjustable loss was designed,and the dynamic evolution law of the intrinsic polarization state in the process of the system changing from PT symmetry to PT symmetry breaking was determined.Secondly,the designed composite metasurface was fabricated using high-precision overlay techniques.Finally,polarization-resolved terahertz time-domain spectroscopy was used to measure the polarization control performance of the system.The results show that the dissipation loss of the metasurface system can be dynamically controlled by changing the energy density of the applied optical field.At this time,the eigenpolarization state of the non-Hermitian system will undergo a dynamic evolution process from the PT symmetry state to the PT symmetry breaking state,thus realizing the active regulation of THz polarization in the full Stokes space.3.A π-type plasmonic metal Al metasurface based on resonance singularity(EP)was designed to realize ultra-low concentration trace detection of embryonic stem cells.Firstly,using the time-domain coupled mode theory,the variation law of the coupling strength of the two resonances in the metasurface under the near-Coulomb interaction was investigated,and a straightforward approach for determining EP was proposed.Secondly,utilizing micro-nanofabrication technologies,a large-area metasurface supporting EP was created.Finally,the terahertz time-domain spectroscopy was used to measure the transmission spectra of the metasurface at various cell seeding concentrations.The results showed that the metasurface of this structure demonstrated ultrasensitive biosensing properties.Based on the above research,a compact,wideband,and sensitive THz biosensing system was conceived and constructed.The system comprises of a metasurface-based,all-dielectric,broadband,high-efficiency terahertz generator,a dynamic terahertz polarization modulator,and an ultrasensitive biosensor.The working bandwidth of the THz source is 0-3 THz,and its generation efficiency is comparable to that of Zn Te with a thickness of 100 μm.In conjunction with the biosensor,it is possible to identify embryonic stem cells at concentrations as low as 0.1%. |
