Aparajita Bandyopadhyay

THz reflection spectroscopy of C-4 explosive and its detection through interferometric imaging

In recent times, Terahertz (1 THz = 1012 cycles/sec and 300 μm in wavelength) spectroscopy has become a promising technique for spectroscopic identification of different materials having contemporary interest. In this study we report a direct measurement of reflection spectra of the explosive C-4, which shows significant absorption around 0.8 THz, using THz time domain spectroscopic techniques. A contrast in reflection of around 8% has also been observed between the neighboring frequencies of 0.7 THz and 0.9 THz. The spectral data have been used to create realistic synthetic images for use in simulations of interferometric detection in a stand-off THz imaging system. The results obtained are analyzed using Artificial Neural Networks for positive identification of the agents with an interferometric array of few linear detectors in near field mode.

Terahertz interferometric imaging of a concealed object

Experimental results of two-dimensional homodyne terahertz interferometric imaging are presented. The performance of an N element detector array is imitated by only one detector placed at N positions. Continuous waves at 0.25-0.3 THz are used to detect concealed objects: a metal object and an RDX sample. The terahertz interferometric imaging method can be used in defense and security applications to detect concealed weapons, explosives as well as chemical and biological agents.

Grain size dependent scattering studies of common materials using THz time domain techniques

In recent times, the far infrared or the terahertz (1 THz = 1012 cycles/sec and 300μm in wavelength) region of electromagnetic spectrum has become a promising radiation for spectroscopic identification of different types of biomaterials. The present work investigates the effect of grain size on the THz spectra of chalk, salt, sugar and flour using THz time-domain spectroscopy. It has been observed that at lower frequencies, solids of small grain sizes of nonabsorbing materials show rising trends in their extinction spectra. Here, we obtain extinction spectra of granular salt, chalk, sugar and flour between 0.2 to 1.2 THz and show that the experimentally obtained extinction can be predicted on the basis of the Mie Scattering model for small grain sizes. The current study is an attempt to understand the absorption spectrum of a few such materials having no significant intrinsic absorption in the THz region by separating the independent contributions of true absorption of the material and scattering losses due to its morphology in the extinction of the material. This would help in distinguishing these materials based on their rising trend of the extinction spectra at lower frequencies.

Square-shaped metal screens in the infrared to terahertz spectral region : Resonance frequency, band gap, and bandpass filter characteristics

Resonance frequency of freestanding, square-shaped thick metal screens have been studied here in the wavelength range of infrared (IR) to mm (20 to 0.2 THz). It was found that their peak transmission has a linear relationship to the screen’s pitch. An experimental spectral feature, unaccounted for in typical simulations with plane parallel incident beams, was observed in the transmittance envelope for measurements in focused beams. In the past, this spectral feature was assigned to Wood’s anomaly. Yet, unlike the latter, the observed spectral feature appears here in the long wavelength regime as well. We investigated this phenomenon for a large frequency range and assigned the spectral feature to the formation of a photonic band gap at oblique incidence. Many IR Fourier transform spectrometers use a noncollimated incident beam and such spectral features will appear whenever the local state of polarization includes components which are parallel to the plane of incidence.

Terahertz interferometric and synthetic aperture imaging

Experimental results of homodyne terahertz interferometric 1-D and 2-D imaging are presented. Continuous waves at 0.25-0.3 THz are used to detect a metal object behind a barrier. The performance of an N element detector array is imitated by only one detector placed at N positions. The reconstructed images are in a good agreement with theoretical predictions. The terahertz interferometric imaging method can be used in defense and security applications to detect concealed weapons, explosives as well as chemical and biological agents.

TERAHERTZ INTERFEROMETRIC AND SYNTHETIC APERTURE IMAGING

Experimental results of homodyne terahertz interferometric 1-D and 2-D imaging are presented. The reconstructed images of a point source are in a good agreement with theoretical predictions. The performance of an N element detector array is imitated by only one detector placed at N positions. Continuous waves at 0.25-0.3 THz are used to detect a metal object behind a barrier. 1-D images of a C-4 sample have been obtained at several terahertz frequencies. Focusing issues of 2-D imaging have been demonstrated. The terahertz interferometric imaging method can be used in defense and security applications to detect concealed weapons, explosives as well as chemical and biological agents.

Interferometric terahertz imaging for detection of lethal agents using artificial neural network analyses

Interferometric imaging is a non-invasive, non- contact method to detect concealed lethal agents employing spectral imaging in the terahertz (THz) range. Parallel to the experimental testing over short range, extensive modeling simulates reconstructed images of lethal agents at different frequencies applying interferometric techniques. Near-field correction to such imaging has been accounted for and edge probe algorithm and consequent artificial neural network analyses identify the agents under consideration. This work addresses the issues related to THz imaging for rapid and successful recognition of lethal agents in security screening.

Study of Morphological Effects on Terahertz Spectra Using Ammonium Nitrate

The effect of morphology on Terahertz spectra in the region from 0.2 to 1.2 THz was studied using Ammonium Nitrate of different grain sizes. The results agree with Mie scattering theory for small grain sizes.

A non contact characterization technique of the defect states of high k dielectrics using THz radiation

The effect of high-k dielectric Hafnium dioxide films on 200 mm diameter p-type silicon substrates is investigated and compared with conventional dielectric material, Silicon dioxide, using terahertz (THz) time- resolved spectroscopy and visible pump/THz probe spectroscopy. The interfacial defect density at the Hf/p+ interface was estimated to be 60-100 times larger than that of Si/p+ interface.

Application of THz Imaging in Security Screening

Non contact detection of concealed lethal agents using Terahertz radiation is described. Applying interferometric techniques, synthetic images are reconstructed at different frequencies. Aspects affecting image identification are considered for successful design of the imaging system.