With the maturation of terahertz (THz) imaging technology, the spatial resolution, signal-to-noise ratio, imaging speed and ability acquiring information of the imaging system are gradually enhanced. Researchers have paid more attention to THz imaging applications in fundamental researches and industrial exploitation. In this paper, several recent studies of THz digital holography are reviewed, including performance demonstration of THz planar elements, function validation of optical tunable THz elements, observation of the longitudinal component in diffraction THz fields and analysis of THz surface waves on the metallic sub-wavelength devices. These research works are very valuable for the development of THz integration systems and THz imaging technology.

The complete vector information of converging terahertz (THz) beams with linear, circular, and cylindrical vortex polarization are precisely measured by using a THz digital holographic imaging system. The transverse (Ex, Ey) and longitudinal (Ez) polarization components of the THz fields around the focal point are separately obtained utilizing the detection crystals with different crystalline orientations. The measured results are in good agreement with the theoretical expectations. This imaging technique provides an effective way for revealing the vector diffraction properties of the THz electro-magnetic waves.

A virtual all-optical tunable terahertz Fresnel zone plate is achieved utilizing the localized distribution of the transient electron plasma on a silicon wafer. Its focusing and imaging performance are experimentally demonstrated. Experimental results show that the effect of the virtual zone plate is the same as an actual one. Adjusting the spatial pattern of the electron plasma, the central wavelength and the focal length of the virtual zone plate can be all-optically dynamically steered. The research is a significant step to the development of tunable optical imaging elements.

Three dimensional information of the Gouy phase shift in a converging spherical terahertz (THz) beam is directly observed by using a THz balanced electro-optic holographic imaging system. The major properties of the Gouy phase shift are presented, including the longitudinal and transverse distributions, relationships with the frequency and the f-number, influence on the THz polarization. The imaging technique supplies an accurate and comprehensive measurement method for observing and understanding the Gouy phase shift.

In this presentation, some technologies are proposed to improve the performances of the real time focal plane imaging system. Firstly, the quasi near field technology is proposed to effectively improve the resolution. The object is located as close as to the detection crystal, usually the distance between the object and detection crystal is 100μm. Thus the evanescent wave of the object can be detected and hence the resolution can be improved. The balanced detection method has also been adopted to improve the signal noise ratio (SNR). The balanced detection method has been widely used in TDS to improve the SNR, here two CCDs have been used and the probe beam has been separated into two beams. The substraction of two images can present better SNR, the improvement of 4 to 10 times have been achieved. The polarization information about the object is also quite important. An approach is proposed to get the polarization information. In this method, the polarization of the probe beam has been rotated to achieved the electric field distribution on the vertical direction.

Much attention has been paid to continuous-wave (CW) terahertz (THz) imaging recently because of its stability and simplicity [1,2]. However, most of CW THz imaging systems only acquire the intensity information, as described by Karpoicz and X.-C. Zhang [3]. For some samples, phase maps are more important to reveal the relative profile of sample's surface and interior [4]. In this paper, An imaging method is proposed to obtain the sample's phase information with continues-wave terahertz interferometer. In the experiment, a fixed-phase-shift method and a phase-unwrapping algorithm are used to provide accurate phases of samples respectively.

A terahertz(THz) quasi-near-field real-time imaging system was presented,with which three industrial flat cables with minute structures were nondestructively tested.In the obtained image at the peak of signal,data from metal,defect,and cladding material were extracted and Fourier transformed,and then comparisons and analyses had been performed in the time domain,frequency domain and phase domain. Furthermore,images for amplitude and phase were obtained by processing the original images,and it showed that the phase images reflect the defects more obviously than others.Testing results showed that THz quasi-near-field real-time imaging could be widely applied in quality monitoring and inspection of materials with minute structures.

A terahertz (THz) polarization real-time imaging system that can effectively reduce experimental time consumption for acquiring a sample's polarization information is achieved. An alternative THz polarization measurement method is proposed. In this method, a <110> zinc-blende crystal is used as the sensor, and the probe polarization is adjusted to detect THz electric fields on the two orthogonal polarization components. The relative sensitivity of the imaging system to the THz polarization angle is estimated to be less than 0.5°. To illustrate the ability of the system, two samples are designed and measured by using the system. From their THz polarization real-time images, each region of these samples can be precisely presented. Experimental results clearly show the special influences of different materials on the THz polarization. This work effectively extends the information content obtained by THz real-time imaging and improves the feasibility of the imaging technique.

The polarization information of various ores and gems in the terahertz (THz) frequency region has been investigated. The images of the samples for different polarization are captured by a balanced polarization imaging system. The system employs a balanced electro-optic (EO) detection method for two-dimensional THz real-time imaging, which systemically integrates the THz balanced EO sampling technique and the dynamic subtraction technique. The proposed method can effectively improve the signal to noise ratio and the spectrum measurement accuracy of the imaging system. The imaging speed is dramatically reduced by using two high speed CCDs which are used to detect two orthogonal polarization components of probe beam. The subtraction of two corresponding images presents the balanced image of the sample. By rotating the polarization of the probe beam by 45 degree, another polarization component which is orthogonal to the present one can be measured. The experiment results demonstrate the advantage of the method. The spectra of the samples on each point can be extracted from a series of images, thus the refractive indices, absorption coefficients, and polarization rotation coefficient can be drawn. The information can be used to identify samples.

Continues wave terahertz imaging has been used widely in the field of security inspection and nondestructive examination because of its simplicity and stability. In this presentation, we proposed an image method with continues wave terahertz interferometry to obtain the sample's phase information. In the experiment, a reference terahertz beam was added into the exiting continues wave terahertz imaging system to be a Michelson interferometer. With three maps obtained at three fixed phase-shift positions, a phase image is obtained by using a phase shift algorithm. Afterwards, this phase image is unwrapped to get its accurate phase profile. By this method, a bulk of foam with two height steps structure is imaged. The result shows that the inner structures of samples can be identified clearly and the relative optical depth profiles of samples can be obtained.

利用亚毫米波准近场实时成像系统对三种常用的微小结构的工业电子排线进行检测,直接获得了样品在信号峰值处的强度图像。分别对样品中的金属、缺陷及包覆材料连接处抽取数据,并对其进行频谱及相位处理,在时域、频域及相位图中做了对比与分析。通过对原始图像进行图像处理后,分别得到振幅和相位图像,其中相位图像能更清晰地反映排线中隐藏的缺陷。结果证明,亚毫米波准近场成像系统可以用于具有细微结构材料的无损检测。

We propose a method for two-dimensional terahertz (THz) real-time imaging which systemically integrates the THz balanced electro-optic sampling technique and the dynamic subtraction technique. The proposed method can effectively improve the signal to noise ratio and the spectrum measurement accuracy of the imaging system. Experiment results demonstrate the advantage of the method.

Terahertz radiation is a special electromagnetic spectrum range sandwiched between the infrared ray and microwave. With developing of super fast and super power laser technology, this special range has been explored extensively. Among all terahertz technologies, terahertz imaging may be the first one can be used in the practical applications. In this presentation, a novel terahertz multi-spectral focal plane imaging method is proposed. Using a CCD camera instead of the single detector, this method can capture the time domain waveform of an object quickly. Using the fast Fourier transform, both the amplitude and phase of the THz spectrum can be achieved. Based on this information, the layer structure inside the object can be presented. The multi-spectral phase imaging technology has also been employed to get high signal noise ration.

A terahertz (THz) quasi-near field real-time imaging system is proposed. Using this approach, not only the consumption of experimental time can be dramatically reduced, but also the resolution of the imaging system is improved to the magnitude of sub-wavelength of THz waves. A metallic plate with sub-wavelength air hole arrays is imaged by using the system and the microstructure of the sample can be clearly obtained in its THz image. Due to the filtering characteristic of the sample, variances of its THz images can be distinctly observed at different THz frequencies. It is believed that the THz quasi-near field focal plane imaging system should have tremendous potential applications in THz microscopy field.

Using a new terahertz reflective-transimission design suite (TDS) system, various agricultural chemical solution were measured. A series of reflectivity spectra of agricultural chemical solution were obtained. The relation between reflection ratio and concentration of the solution was drawn. A new method for measuring the concentration of agricultural chemical solution with terahertz reflective-TDS was proposed. This method is expected to be used in the environmental pollution monitoring and manufacture quality controlling.

A terahertz (THz) quasi-near-field real-time imaging system is presented. Not only the consumption of experimental time is dramatically reduced, but also the resolution of the imaging system is improved to the magnitude of sub-wavelength of THz waves. THz images of a razor blade edge are obtained and the spatial resolution of the imaging system is discussed in detail. For checking the imaging capability of this system, three metallic plates with different sub-wavelength air hole arrays are imaged and the microstructure of these samples can be clearly observed in their THz images. It is believed that the THz quasi-near-field real-time imaging system should have tremendous applications in the THz microscopic field.

The terahertz spectra of pure melamine and two kinds of its mixtures that mix with polyethylene and milk powder were measured using the terahertz time-domain-spectroscopy (THz-TDS). It was found that there are two absorption peaks at 1.99THz and 2.29THz in all three spectra. The absorption coefficient of the mixture varies with the proportion of melamine in the mixture. Increasing the percentage of melamine in the mixture, the absorption peaks in spectrum get more obvious and sharper. According to the Lambert-Beer law, the absorption coefficient and the proportion of the melamine should followed linear relationship. The experimental data accord to this theory well. Using density functional theory, the vibration of melamine crystal was calculated, which accords to experimental data well. This work provides a method of detecting melamine in milk powders. It is expected that the terahertz spectroscopy technology can be used in food safety and other applications.

According to the principle of optical heterodyne measurement,the interferometry is suitable for the signal whose intensity is weaker than reference signal,while the non-interference survey(direct measurement)is suitable for the signal whose intensity is stronger than reference signal.By using our self-made all optical fiber Mach-Zehnder interferometer,the displacement signal is measured.The experiment results show that the sensitivity of interference measurement is respectively 62.068 μW/mm and 9.90 mV/mm,and the sensitivity of non-interference measurement is 4.30 μW/mm and 0.35 mV/mm when the amount of displacement is lower.When the displacement is larger,the sensitivity of interference measurement is respectively 2.643 mV/mm and 0.055 mV/mm and the sensitivity of non-interference measurement is 12.326 mV/mm and 4.194 mV/mm.The experiment results are in keeping with the conclusion of our analysis.This work makes reference for the application of the intensity modulation measurement of all optical fiber interferometer.

A method to analyze the variety of the polarization state of terahertz (THz) wave by using a typical electro-optic sampling setup with <110> zinc-blende (ZnTe) crystal as a sensor is presented. To illustrate the knowledge of polarization of the THz pulse, the function of THz detect in ZnTe crystal is interpreted. Two kinds of Jones matrixes, birefringence device and polarizer device, are used to analyses the polarization change of the THz electric field vector caused by the sample. It was found that the THz polarization imaging is sensitive to the edge of the samples.

In this presentation, a system for THz en-face tomography imaging is proposed. The THz radiation reflected by the object is imaged on the ZnTe crystal, and then a CCD is used to detect the two dimensional modulated probe light. By adjusting the delay line, the information about the object can be achieved layer by layer. Expanded terahertz pulses are reflected from the different medium interfaces inside a sample, the amplitudes and time decays of these pulses can be used to determine the thickness of each medium inside the sample. In this way, the structure image of the sample can be reconstructed. The longitudinal resolution of the system is discussed by using the timing information of the reflection terahertz pulses.