Kaijun Mu

Optical property and spectroscopy studies on the explosive 2,4,6-trinitro-1,3,5-trihydroxybenzene in the terahertz range

Using the terahertz time-domain spectroscopy, the complex refraction index and dielectric function of the pure 2,4,6-trinitro-1,3,5-trihydroxybenzene (TNPG) are calculated with the obtained parameters of the pure polyethylene (PE) and a mixture of TNPG and PE based on the Bruggeman effective medium theory. The theoretical investigation consistent with the experimental data in the vibration spectra of TNPG is presented in the range of 0.2–2.5THz. The results reveal that the two absorption features identified as the fingerprint of TNPG in our studied range are mainly dominated by the intramolecular collective vibration modes.

Identification of explosives and drugs and inspection of material defects with THz radiation

We report the sensing of explosive materials and illicit drugs by using terahertz time-domain spectroscopy (THz-TDS) and imaging. Several explosive materials, such as γ-HNIW, RDX, 2,4-DNT, TNT, Nitro-aniline, and illicit drugs, such as methamphetamine (MA) etc were researched here. Non-destructive testing, as one of the major applications of THz imaging, can be applied to an area of critical need: the testing of aerospace materials. Composite materials such as carbon fiber are widely used in this industry. The nature of their use requires technologies that are able to differentiate between safe and unsafe materials, due to either manufacturing tolerance or damage acquired while in use. In this paper, we discuss the applicability of terahertz (THz) imaging systems to this purpose, focusing on graphite fiber composite materials, carbon silicon composite materials and so on. We applied THz imaging technology to evaluate the fire damage to a variety of carbon fiber composite samples. Major carbon fiber materials have polarization-dependent reflectivity in THz frequency range, and we show how the polarization dependence changes versus the burned damage level. Additionally, time domain information acquired through a THz time-domain spectroscopy (TDS) system provides further information with which to characterize the damage. We also detect fuel tank insulation foam panel defects with pulse and continuous-wave (CW) terahertz system.

Intense thermal terahertz-to-infrared emission from random metallic nanostructures under femtosecond laser irradiation

We report intense (~10 mW), ultra-broadband (~150 THz wide), terahertz-to-infrared, Gaussian-wavefront emission from nanopore-structured metallic thin films under femtosecond laser pulse irradiation. The proposed underlying mechanism is thermal radiation. The nanostructures of the metal film are produced by random holes in the substrate. Under pulse-train femtosecond laser irradiation, we found dramatically enhanced optical absorption, with an absorptivity that was equal to as much as 95% of the metallic surface nanostructure, due to both an antireflection mechanism and dissipation of excited surface plasmon polaritons into the metal surface.

Phase characterization in broadband THz wave detection through field-induced second harmonic generation

We present a theoretical and experimental investigation of the THz pulse phase measured by a broadband heterodyne detection method via field-induced second-harmonic generation in ambient air. The dependence of the detected THz phase spectra on the positions of the wire shaped electrodes scanning along the detection plasma is discussed. An additional phase shift around the beam focus is observed. Theoretical deductions reveal that it is caused by the Gouy shift of the optical probe beam and THz beam during the heterodyne detection process.

High-power THz to IR emission by femtosecond laser irradiation of random 2D metallic nanostructures

Terahertz (THz) spectroscopic sensing and imaging has identified its potentials in a number of areas such as standoff security screening at portals, explosive detection at battle fields, bio-medical research, and so on. With these needs, the development of an intense and broadband THz source has been a focus of THz research. In this work, we report an intense (~10u2009mW) and ultra-broadband (~150u2009THz) THz to infrared (IR) source with a Gaussian wavefront, emitted from nano-pore-structured metallic thin films with femtosecond laser pulse excitation. The underlying mechanism has been proposed as thermal radiation. In addition, an intense coherent THz signal was generated through the optical rectification process simultaneously with the strong thermal signal. This unique feature opens up new avenues in biomedical research.

Terahertz spectroscopic investigations of explosives and the related compounds

THz spectra of two pure materials and some mixed explosive materials and the theoretical analysis results are introduced. The refractive index and absorption coefficient of the samples are measured in the region of 0.2-2.5 terahertz by time-domain spectroscopy (THz-TDS). The absorption spectra are obtained correspondingly. The obtained results indicate the THz-TDS technology can identify the different explosives and roughly analysis the main ingredients of the mixed samples.

Studies on the detection and identification of the explosives in the terahertz range

The sensing of the explosives and the related compounds is very important for homeland security and defense. Based on the non-invasive terahertz (THz) technology, we have studied some pure and mixed explosives by using the THz time-domain spectroscopy and have obtained the absorption spectra of those samples. The obtained results show that those explosives can be identified due to their different characterized finger-prints in the terahertz frequency region of 0.2-2.5 THz. Furthermore, the spectra analyses indicate that the shape and peak positions of the spectra for these mixed explosive are mainly determined by their explosive components. In order to identify those different kinds of explosives, we have applied the artificial neural network, which is a mathematical device for modeling complex and non-linear functionalities, to our present work. After the repetitive modeling and adequate training with the known input-output data, the identification of the explosive is realized roughly on a multi-hidden-layers model. It is shown that the neural network analyses of the THz spectra would positively identify the explosives and reduce false alarm rates.

Terahertz Electromagnetic Response for Metamaterial

Recently, more and more groups are concentrating on learning surface plasma on metal split with sub-wavelength of terahertz. There is a new material which is made of two materials named metamaterial. It is combined with semiconductor and metal materials. Metal parts are made with subwavelength microstructure. Abnormal phenomenon occurred when it is excited by an intense light, showing an enhancement transmission of terahertz wave. In this paper, we design a planar structure composed of gallium arsenide (substrate) and copper (metal). Copper array is plating on a GaAs wafer on order of micron with photoetching technology. The thickness of GaAs and copper layer are 650 and 0.5 micros individually. THz incidence has an enhanced transmission like surface plasmonic resonance with terahertz time-domain system when another exciting lighted on, which made current carriers in photoconductor. This resonance arises from an inductor-capacitor circuit resonance. Comparison is performed between results with and without excitation. The angle of sample plane with terahertz polarization and incidence can affect this electromagnetic response in some degree. Its transmittance with back-incidence is much stronger than that with front-incidence at certain frequencies. This kind of artificial structure has potential abilities in terahertz devices in the future. It is helpful to develop terahertz filter, polarizing film, beam reflector, phase retarder and so on.

Vibrational spectrum of PETN investigated in terahertz range

Although the experimental solid-state terahertz (THz) spectrum (0.2-2.5THz) of the high explosive petaerythritol tetranitrate (PETN, C5H8N4O12) has been presented previously, till now, the theoretical analysis of its THz spectrum is few reported before. In this paper, we used solid-state density functional theory (DFT) calculation to simulate the vibrational spectrum of PETN in THz range. The DFT calculations were performed using DMol3 (version 4.0). Solid-state DFT, employing the BP density functional, can be able to reproduce the experimentally observed solid-state structure and low-frequency vibrational motions, and final simulated spectrum has a good-to-excellent agreement with experimental THz spectrum. In order to observe more THz spectrum details of PETN, we increased the frequency-domain resolution. Furthermore, we also have obtained the refractive index, absorption coefficient of this sample by THz time-domain spectroscopy.

Nine wave-length THz spectrum for identification using backward wave oscillator

The sensing of the explosive is very important for homeland security and defense. We present a nine-wavelength continuous wave (CW) Terahertz (THz) spectroscopy for identification of explosive compounds (2,4-DNT, RDX and TNT) using three Backward Wave Oscillator (BWO) sources, which emit radiations from 0.2 THz to 0.38THz, 0.18THz to 0.26THz and 0.6THz to 0.7THz, respectively. To identify the target materials, only the transmitted THz power through the explosive pellets are measured at the nine discrete wavelengths. A hole, which is the same size as these pellets, is used as references to normalize the transmitted THz power. The measured discrete spectra was successfully identified and classified by using self-organizing map (SOM). These results prove that the backward wave oscillator is a convenient and powerful solution in future development of a standoff THz sensing and identification unit.