Studies And Improvement Of Key Techniques In THz Real-Time Imaging

During the past decades, terahertz (THz) photonics has become a novel optics sub-ject with the development of the ultrafast laser technology. In this region, THz pulsereal-time imaging technique is an important research branch. On one hand, it retainsthe advantages of THz sensing and raster scan imaging techniques, which can exactlyacquire two-dimensional optical information of substances. On the other band, with tech-nical features of real-time imaging, it greatly reduces the experimental time and enhancesthe practicability of THz imaging. Because THz radiation possesses many distinctive op-tical features, the THz real-time imaging has large application potentials in fundamentalresearch, industry, military fields, biology, medicine, and other fields.In this thesis, key techniques in THz real-time imaging were chosen as researchobjects, which are the re(?)ection measurement, quasi-near-field measurement, balancedelectro-optic (EO) detection and polarization detection. From four aspects, includingthe measurement method, improvement of the resolution, optimization of the signal tonoise ratio and acquirement of the polarization information, we studied how to improvethe imaging system in detail so as to enhance its performance. The main works are asfollows:A THz pulse re(?)ective real-time imaging technique was studied and the three-dimensional terahertz information of a hidden sample was measured. By using theTHz pulse re(?)ective real-time imaging system, the tomographic measurement of a metal-lic cross hidden behind a high resistivity silicon wafer is accomplished. Making use ofre(?)ective THz pulse sequences from the sample's interior, the morphology on each layerof the sample was acquired. Taking advantage of the experimental dataset, the spatial-temporal distribution image was obtained and the distances between adjacent interfacesof the sample were calculated. In addition, a THz pulse normal re(?)ective spectrometerwas built to supply an experimental fundament for further optimizing the imaging system.A THz pulse quasi-near-field real-time imaging technique was studied and itsimaging properties was analyzed. The spatial resolution of the imaging system wasobtained by measuring a razor blade edge and the imaging result with the sub-wavelengthresolution was discussed in detail. By analyzing these resolution curves with di(?)erentsingle frequency component, the in(?)uence of the light frequency to the resolution was investigated. By measuring metallic plates with di(?)erent thicknesses, the in(?)uence of thesample's thickness to the resolution was analyzed. By using the imaging system, threemetallic sub-wavelength hole arrays were measured to verify the imaging ability of thissystem.The balanced EO detection method was applied into THz real-time imagingand a THz pulse balanced real-time imaging technique was studied. The principleand advantages of the imaging system were discussed in detail. A digital image process-ing algorithm was proposed to accomplish the auto-match of two images. The spatialresolution of this system was obtained by measuring a razor blade edge and the exper-imental result demonstrates that the application of the balanced EO detection does notin(?)uence the imaging quality. THz images of a wooden embossment and a powder sam-ple including two chemicals were acquired by using this system and were compared withthat of the traditional detection method. The in(?)uences of the balanced EO detection tothe enhancement of the signal to noise ratio and the spectrum measurement accuracy wereinvestigated.An alternative THz polarization measurement method was proposed and a THzpolarization real-time imaging technique was studied. By improving the THz pulsebalanced real-time imaging system, the measurement of the THz polarization was accom-plished by adjusting the probe polarization with a half wave plate. The principle of themethod was explained and checked theoretically and experimentally. The polarizationof modulated THz waves by a wire grating polarizer was measured by using this systemand the polarization measurement sensitivity of this system was verified. By using thissystem, three samples (quartz glass, common glass, and quartz crystal) were measuredand the in(?)uence on the THz polarization due to the birefringence of the quartz crystalwas identified. By using this system, two pieces of quartz crystals with partly overlapwere measured and in(?)uences of quartz crystals with di(?)erent thicknesses on the THzpolarization were observed.