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.
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.