Research On Terahertz Time-domain Spectroscopy And Novel Terahertz Thin-film Devices

At present, the research on terahertz time-domain spectroscopy and functional thin films and devices is a focus in the terahertz field. Firstly, the principle and application of terahertz spectroscopy are studied in dissertation. However, there is a problem in the reflection measurement of biological and liquid samples. Two novel methods are proposed to solve this problem. The interaction mechanism between liquid samples and terahertz wave is deeply analyzed. Secondly, a novel reflector of the transparent conductive film and antireflection coatings based on the conductive films are studied, and their applications in the terahertz systems are explored. The main contents and innovations in the dissertation are as follows:1. In the terahertz reflection measurement of biological and liquid samples, there is an important problem, which is that the reflection from the lower surface of the substrate significantly interferes with the sample reflection from the upper surface of the substrate. The problem influences the analysis and accuracy of extracted sample information. Therefore, two novel methods are proposed to solve this problem and improve the accuracy of extracted sample information effectively. Method one, A designed substrate is used as reference to acquire a reference reflection and eliminate the interference from the reflection measurement. Method two, the antireflection coating is used to eliminate the interference from the reflection measurement. The main advantage of the methods is its simplicity and accuracy and ease for application of the reflection systems with different incident angle.2. The propagation of terahertz wave in the reflection measurement is modeled using the Jones matrix theory. The optical parameters of the liquid samples in the terahertz region are accurately acquired using the model. The experimental results are analyzed by theoretical model, and the permittivity and relaxation time of the liquid samples are obtained. By analyzing these parameters, the interaction mechanism between the liquid sample and terahertz wave is further understood. This contributes to modeling and simulation of terahertz wave propagation in biological tissues.3. A special reflector based on Al-doped ZnO(AZO) thin film is presented in the dissertation. The AZO reflector possesses both high reflectance in terahertz frequency range and high transmittance in the visible range. The principle of the AZO reflector is analyzed by theory. The performance of the AZO reflector is verified by the experiment. In the terahertz system, the cost of Au reflector is expensive. In comparison with the Au reflector, the cost of the AZO reflector is inexpensive. The AZO reflector is suitable for large-scale application and important for application of multispectral terahertz systems. Further, the AZO reflector used as reference is well verified in terahertz reflection measurement.4. The antireflection coatings based on conductive films are studied in the terahertz region. The antireflection coatings based on AZO and metal films are prepared, and their performances in the terahertz region are verified by the experiments. The antireflection coatings are modeled using a wave-impedance matching approach. The experimental results demonstrated the excellent performance of the antireflection coatings in the terahertz frequency range. Further, a novel method using the antireflection coatings is firstly proposed to eliminate unwanted reflections, which interfere with the important reflection from the sample in terahertz reflection measurement. The proposed method significantly improves the accuracy of the extracted sample information in the reflection measurement. The antireflection coatings are suitable for large-scale application.