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.
Currently, one of the most important application of flow cytometry is the real-time analysis of aerosols, in particular, to ensure biosafety. In most cases, such analysis is aimed at detecting fluorescent signals from aerosol particles corresponding to the light emission of tryptophan and nicotinamide adenine dinucleotide (NADH). Further development of the method is largely related to the improvement of the light detecting systems for recording and processing of fluorescence and scattered light signals. In this work, a comparative analysis of flow cytometers for bioaerosols detection based on photo-multiplier tubes (PMT) and avalanche photodiodes (APD) operating in analog and photon-counting modes was carried out. The limit of detection (LOD) of bioaerosols, response time and ability to detect particles with low scattering and fluorescence cross section were calculated and examined. The calculations were carried out for the well-known optical scheme of fluorescence detection based on discrete photodetectors and dichroic mirrors combined with an air flow chamber equipped with elliptical and spherical mirrors. An ultraviolet light emission diode (LED) was used as a model source of exciting radiation. To estimate the optical properties of aerosol particles, experimental results obtained for a model bovine serum albumin bioaerosol and published data on various other bioaerosols were used. The calculation of the total number of fluorescent photons, emitted by particles of various sizes while passing the flow chamber was carried out. The obtained data were compared with parameters of photodetectors operating in analog and photon-counting modes. The critical particle size was determined for the effective registration in a photon-counting mode. Considering the size distribution of aerosol particles, it was concluded that application of the photon-counting mode will reduce the LOD of bioaerosols by more than an order of magnitude.
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.
It is known that luminescent optical sensors are perspective for detection towards nitroaromatic compounds that are a basis of a many kind of explosives. Operation of these sensors is based on quenching luminescence, which is caused by photo-induced electron transfer from a luminophore (donor) to a nitroaromatic molecule (acceptor). The conjugated polymers, small molecule dyes and metal-organic frameworks are used as a sensitive luminophores currently. One of the methods to improve these sensors is embedding the luminophore into porous matrix with properties of photonic crystal, which may be a porous silicon (pSi) Bragg mirror or a microcavity (MC). The PPV derivatives polymers are usually used as the sensitive luminophores for embedding into pSi matrix. However, there is a task to find an optimal set of luminophores to develop a highly sensitive and selective sensor. In this work we investigate embedding of 5- triphenylamino-4-(triphenylaminothiophen-2-yl)-pyrimidine (HEM-461) into pSi MC and examine the sensitivity of obtained structures. The pSi MC were fabricated using a standard electrochemical etching process. The eigenmode of the pSi MC had a width of 4-6 nm. The samples were oxidized to stabilize the surface chemical properties and to prevent quenching of luminescence of the embedded luminophores after fabrication. The embedding of the dye into the pSi MC was performed at excess pressure. Well known conjugated polymer MDMO-PPV was used as a reference. In this work, we compared the photophysical properties of MDMO-PPV and HEM-461 in solution and into pSi MC. The luminescence parameters and resistance to heat have been studied. Comparative studies of sensitivity of MDMO-PPV and HEM-461 to trinitrotoluene in liquid and gaseous phases have been carried out. It was concluded that pSi MC with embedded HEM-461 is a promising structure for developing sensors of nitroaromatic compounds.
It is known that development of optical sensors for explosives detection is currently of great interest. Among others sensors based on the luminescence quenching of conjugated polymers caused by photoinduced electron transfer have attracted considerable attention. Embedding such polymers into porous silicon (pSi) microcavity (MC) allows modify its luminescence spectrum and increase specific surface area and sensitivity of sensor. At the same time optimization of pSi MC structure and its mode of operation are important aspects of sensors design. This study presents the results of the structure and temperature optimization of pSi MC with embedded PPV derivatives polymers. The pSi MCs were fabricated using a standard electrochemical etching process. The luminescence spectra of polymers were drastically narrowed after embedding in pSi MC. It was experimentally found that optimal thickness of the front mirror is from 4 to 5 pairs of low and high porosity layers. The optimal thickness of the rear mirror is about 15 pairs of low and high porosity layers. We also discovered that temperature of pSi MC strongly influences on the rate of the polymer luminescence quenching under exposure to TNT vapors. In particular, it was shown that a decrease of MC temperature to 5° C leads to more than three times drop of quenching time. The obtained results can be applied for the design of optical sensors of explosives based on pSi MC.
We demonstrate a new way for detection ultralow concentration of explosives in this study. It combines an ion mobility spectrometry (IMS) and a promising method of laser desorption/ionization on silicon (DIOS). The DIOS is widely used in mass spectrometry due to the possibility of small molecule detection and high sensitivity. It is known that IMS based on laser ion source is a power method for the fast detection of ultralow concentration of organic molecules. However requirement of using high energy pulse ultraviolet laser increases weight and size of the device. The use of DIOS in an ion source of IMS could decrease energy pulse requirements and allows one to construct both compact and high sensitive device for analyzing gas and liquid probes. On the other hand mechanisms of DIOS in gas media is poorly studied, especially in case of nitroaromatic compounds. The investigation of the desorption/ionization on porous silicon (pSi) surface of nitroaromatic compounds has been carried out for 2,4,6-trinitrotoluene (TNT) using IMS and mass spectrometry (MS). It has been demonstrated that TNT ion formation in a gas medium is a complicated process and includes both an electron emission from the pSi surface with subsequent ion-molecular reactions in a gas phase and a proton transfer between pSi surface and TNT molecule.
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