This work examines the possibility of real-time detecting of single nitro compounds microcrystals [cyclotrimethylenetrinitramine (RDX) and pentaerythritol tetranitrate (PETN)] with sizes of ∼130 to 600 μm by the terahertz (THz) imaging. A THz video camera based on a microbolometer matrix was used to record images in transmission and reflection optical schemes. A photoconductive antenna was used as a THz source. It was experimentally demonstrated that the spectral selectivity of identifying of nitro compounds microcrystals in the THz range strongly depends on their size, and the results of mathematical modeling based on the Mie scattering theory showed that this effect is due to the complex dependence of the extinction cross section of the microcrystals on their size. The results of the work can be used in the development of real-time THz visualization systems.
The THz reflection spectra of optically thin hexogen (RDX) samples were studied by terahertz imaging with spectral resolution. A photoconductive antenna excited by femtosecond laser radiation was used as a source of broadband THz radiation. The presence of a Fourier spectrometer (as well as band-pass THz filters) and a microbolometric THz video camera in the experimental setup made it possible to use the terahertz imaging method to study reflection spectra taking into account scattering in the range of 0.6 to 1 THz. The influence of the optical characteristics of the substrate on the THz reflection spectra was studied. In particular, the conditions for observing the effect of anomalous dispersion for RDX samples with different dispersion in the frequency region of the RDX absorption band 0.8 THz were studied. The obtained results demonstrate the application of the method of terahertz imaging with spectral resolution based on THz video camera for the identification of explosives with concentrations 0.75 to 50 mg / cm2 on various surfaces.
We exploit micro-nano structuration to achieve multifunctional windows offering outstanding optical and fluidic properties to enhance the operation of surveillance or detection devices under rainy conditions. These windows are based on synthesis of an artificial index gradient for antireflection properties and improvement of their water repellency property thanks to their structuration at a subwavelength scale with controlled conical geometries. We demonstrate the realization of multifunctional germanium windows for LWIR camera, using two approaches: nanoimprint lithography, well-known for its very high resolution enabling applications from visible to thermal infrared domain, followed by etching techniques, and 3D direct laser writing based on Two-Photon Polymerization (TPP), which is of interest thanks to its ability to manufacture complex 3D structuration directly. Optical characterization shows the ability of such windows to improve optical transmission within 8-14μm spectral range, as compared to non-structured window. In terms of water repellency, the structured windows enable an increase of the contact angle up to 160° with a very low hysteresis. To evaluate the advantage of the multifunctional windows for imaging devices, the windows are integrated in front of a thermal infrared camera and images analysis shows that the camera sensitivity is increased for the nanoimprint window thanks to the multifunctional window and high water repellency in presence of water.
The presence of peculiarities in terahertz spectra of many organic compounds allows the use of THz imaging and spectroscopy for the detection of various hazardous and explosive substances. This work is devoted to the study of the detection of trace amounts of 1,3,5-Trinitro-1,3,5-triazinane (RDX) in the form of particles localized in millimeter and submillimeter sizes using THz imaging with spectral resolution. As a result of the work, images of trace amounts of RDX in reflected THz radiation were obtained. The contrast in these images made it possible to detect single particles of the powdery substance. The difference in contrast for RDX and polyethylene (PE) in the obtained terahertz images makes it possible to use THz imaging with spectral resolution not only for detection, but also for the identification of chemical compounds.
The presence of characteristic peaks in the terahertz (THz) absorption spectra of many organic substances and the active development of the technology for manufacturing video cameras based on microbolometric matrices create an increased interest in methods for identifying explosive compounds using THz imaging with simultaneous recording of spectral information. The results of relevant studies can be used in the development of security systems. THz images (taking into account the spectral information in each pixel) were obtained in this work by passing radiation through a sample based on hexogen (RDX) microcrystals deposited on a polyethylene (PE) film. A photoconductive antenna was used as a source of broadband radiation in the range from 0.5 THz to 2.5 THz. Spectral resolution was provided using a Fourier spectrometer based on a Michelson interferometer. The images were recorded using a THz video camera based on a microbolometric matrix. The possibility of identifying RDX microcrystals using color visualization of spectral information in the frequency range of one of the characteristic RDX peaks (~0.8 THz) has been demonstrated.
The work is devoted to the influence of scattering of terahertz (THz) radiation by hexogen particles (RDX) in powdery samples on their transmission and reflection spectra. A terahertz radio-vision installation with spectral resolution was used to determine experimentally THz spectra of RDX. For samples with small RDX particles (the typical particle size is 100 μm), characteristic peaks at 0.8 THz and 1.06 THz are observed in absorption spectra despite scattering, that can be used to identify this substance. For large hexogen particles (a typical particle size is 450 μm), experiments and numerical simulation showed that even the most intense peak at 0.8 THz is not observed in absorption spectra, and the spectra are mainly due to the scattering effect and its depending on the wavelength of radiation. The reflection spectra of RDX layers (particle size is about 100 μm) qualitatively differ from the reflection spectra of RDX crystals and are formed as a result of absorption during propagation of THz radiation in the particle layer. Thus, the substance can be identified by absorption spectra in a reflection scheme.
The paper presents the results of application of terahertz radiation for detection of traces of explosives on surfaces of objects in reflected light. The process of detection and identification of explosives is based on a recording of interferograms of reflected radiation in spectral range of 0.5 -2.5 THz with help of a Michelson interferometer. The reverse Fourier processing lets to obtain reflection or transmittance spectra and images of objects. Spectral ranges for imaging are chosen by an operator. An installation elaborated for this purpose is described. Specific features of reflection spectra of some organic substances are determined.
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