PurposeTerahertz (THz) medical imaging is a promising noninvasive technique for monitoring the skin’s conditions, early detection of the human skin cancer, and recovery from burns and wounds. It can be applied for visualization of the healing process directly through clinical dressings and restorative ointments, minimizing the frequency of dressing changes. The THz imaging technique is cost effective, as compared to the magnetic resonance method. Our aim was to develop an approach capable of providing better image resolution than the commercially available THz imaging cameras.ApproachThe terahertz-to-infrared (THz-to-IR) converters can visualize the human skin cancer by converting the latter’s specific contrast patterns recognizable in THz radiation range into IR patterns, detectable by a standard IR imaging camera. At the core of suggested THz-to-IR converters are flat matrices transparent both in the THz range to be visualized and in the operating range of the IR camera, these matrices contain embedded metal nanoparticles (NPs), which, when irradiated with THz rays, convert the energy of THz photons into heat and become nanosources of IR radiation detectable by an IR camera.ResultsThe ways of creating the simplest converter, as well as a more complex converter with wider capabilities, are considered. The first converter is a gelatin matrix with gold 8.5-nm diameter NPs, and the second is a polystyrene matrix with 2-nm diameter NPs from copper–nickel MONEL® alloy 404.ConclusionsAn approach with a THz-to-IR converter equipped with an IR camera is promising in that it could provide a better image of oncological pathology than the commercially available THz imaging cameras do.
Prospects for the development of devices for visualizing terahertz (THz) radiation sources can be associated with the use of the results of old studies (1965–1978) on the absorption of THz radiation by metal nanoparticles. This “renaissance” demonstrates that metallic nanoparticles can be used as nanotransducers of invisible THz radiation to infrared (IR) radiation detectable by a commercial IR camera. The investigated THz-to-IR converters are matrices that are transparent both in the THz radiation range to be visualized and in the operating range of the IR camera; matrices contain embedded metal nanoparticles. The latter, when irradiated with THz rays, convert the energy of THz photons into heat and become nanosources of IR radiation for the IR camera. In metal nanoparticles, the mechanisms of absorption of THz radiation and its conversion into heat are realized through dissipation of the energy of THz photons due to multiple scattering of electrons, as well as because of excitation of two types of phonons (transverse and longitudinal ones). The conversion of THz energy into the energy of transverse phonons occurs directly, while dissipation and excitation of longitudinal phonons occurs indirectly, through the excitation of Fermi electrons. Polyvinylchloride (PVC) was chosen as the matrix material, and gold nanoparticles were chosen as nanoparticles-fillers.
The two-phonon scheme of generation of terahertz (THz) photons by gold nanobars (GNBs) is considered. It is shown that in GNBs, by choosing their sizes, it is possible to provide conditions for converting the energy of longitudinal phonons with THz frequencies into the energy of THz photons. The prospects of designing GNBsbased soft THz radiation sources (frequencies: 0.14; 0.24; 0.41 and 0.70 THz) with a large flow cross-section (diameter ∼ 40 cm) intended for detection of hidden objects under clothing to ensure security in public places (airports, railway stations, stadiums, etc.) are assessed. The choice of the above frequencies is a compromise between the requirements of low absorption of THz radiation by water vapor in air, good penetration through the fabric of clothing, favoring a sufficient resolution of the imaging system, and an abundance of corresponding longitudinal phonons, capable of exciting Fermi electrons in GNBs. Estimates of the characteristics of the terahertz-to-infrared converter based on gold nanospheres (GNSs), which could work in tandem with these sources of THz radiation – as a means of visualization of hidden objects – are also given.
In a new type of terahertz (THz)-to-infrared converter for the visualization of THz radiation sources, nanoparticles are embedded in a THz-transparent matrix material. The nanoparticles are made of a metal or an alloy having a partially filled peak of the electron density of states at the Fermi energy. The converter is designed to operate with the THz objective forming a source's image on the matrix, wherein the irradiated nanoparticles convert THz energy to thermal energy, whose spatial distribution is captured by an infrared camera. Nickel nanoparticles are very appropriate for high-frequency THz radiation and can find application in real-time passive imaging of biomedical objects at room temperature. Nanoparticles of compounds with heavy fermions are suitable for low-frequency THz radiation, which could be used to image concealed objects carried by a human being.
In photothermal cancer therapy, gold nanoparticles and silica-gold nanoshells of specific shapes are used in order to generate heat by excitation of plasmons. We have investigated a different mechanism for heating metal nanoparticles: the Joule effect, whereby the energy resonantly absorbed from laser radiation first dissipates in electron processes before being finally converted into lattice vibrations. For this mechanism to exist, the electronic structure of the nanoparticulate material must possess enhanced densities of states at the Fermi energy and within the energy range 1.3 to 1.9 eV above it, i.e., in the transparency window of the biological tissues. Numerical estimations indicate that Fe-, Mo-, and Ta-doped gold, and Fe-doped tantalum are plausible materials for non-plasmonic hyperthermia. No special conditions concerning the shapes or inner structure of nanoparticles are imposed, which is an advantage over existing techniques.
In satellite-borne particle characterization instruments, for example, in the solar wind charge-energy-mass spectrometers and ion traps, there is a need to suppress the effect of the 121.6 nm Lyman-alpha line of the hydrogen spectrum, the most intensive line of the solar UV radiation resulting in high level of the detector’s noise. To reduce this effect, the electrodes of instruments are usually covered with electroconductive light-absorptive coatings having low reflectivity at the wavelength of 121.6 nm. In this paper the physical mechanisms are considered applicable to reduce the reflectivity of the A1-based coatings to be applied on electrodes of the particle analyzing instruments. Particular emphasis has been given to the role of three phenomena: (i) multiple light scattering light traps of the rough surface, (ii) diffraction of the incident light at the rough surface of the coating, and (iii) electron scattering in a skin layer. It is presented the behavior of reflectivity of the A1-based coating in the course of mechanical and environmental tests simulating standard shipping, storage, launching, flight, and operating conditions of the space equipment. The noise measurements of the Faraday cups used on board the INTERBALL-1 mission are also given. As the hydrogen is the most prevalent substance in the Universe, perhaps, the 121.6 nm stray light problem is the common one not only for the solar wind missions and solar astrophysics telescopes, but also for the Far UV astronomy and future UV space astrophysics missions.
This paper concerns a behavior of the total hemispherical reflectance (THR) of the Al-N coating in the course of mechanical and environmental tests. The Al-N coating has been designed to reduce the stray sunlight background in the satellite-borne optical instruments and charge-particles-analyzing apparatus operating in open space under intensive solar radiation. Usually, this problem arises when a density of instruments installed on the satellite is high and it is difficult to avoid getting to instrument the light reflected by neighboring devices. Resolution of this problem is also important in connection with development of the extra-atmosphere Far UV astronomy. The THR measurement results are presented for 10 wavelengths wihtin a range from 400 to 927 nm, and also at 121.6 nm, the most intensive line of the solar UV spectrum able to result in considerable contribution to the detector noise in space devices. The samples of the Al-N coating were exposed to standard mechanical loads including the vibratory loads, linear overloads, and impacts, to which the space equipment may be subjected when shipping to the space-vehicle launching site and also when lauching. The samples were also exposed to environmental tests simulating the vacuum, humidity, and cyclic temperature conditions, which may influence the space instruments while shipping, storing, launching, in flight, and under operating conditions. The THR measurements of the samples were made following exposure to each test. The THRs of tested samples at the wavelength of 121.6 nm were as low as 1.5-2%.
Results of measurements of the total hemisphere reflectance (THR), emissivity ε and solar radiation absorption factor αs of the Al-Cr stray-light-absorptive coating are reported. This coating was designed to reduce the stray sunlight background in aerospace instruments. Generally, this problem arises when a density of instruments installed on the satellite is high and it is difficult to avoid getting to instrument the light reflected by neighboring devices. The THR, ε, and αs measurement results are presented for 10 wavelengths within a range from 400 to 927 nm, and also at 121.6 nm, the most intensive line of the solar UV spectrum, which is able to create considerable contribution to the detector noise in space devices. The goal of present experiments was to test an idea that by providing the electron scattering in the coating skin layer, it is possible to reduce the THR. To have the samples with increased electron scattering intensity, the samples of the Al-Cr coating were manufactured. As in teh Al-Cr alloy the d-band of chromium electrons is located in the vicinity of the Fermi energy of Al, in this alloy a considerable electron scattering takes place. Therefore, within the skin layer of such coating, the electrons excited by photons transfer their energies and momenta to the lattice and other electrons instead of releasing the secondary photons. Hence, the reflectivity of the Al-Cr coating might be reduced. The prepared samples and performed experiments confirmed this assumption. According to the measurement results, the chromium appears to be an acceptable additional admixture to provide a further reflectivity reduction in the previously sutdied promising Al-N coating.
This report deals with comparison of the total hemispherical reflectances (THR), emissivities, and solar radiation absorption factors of coatings designed to reduce a stray light background and provide the thermal control both in space-based instruments and in land-based equipment. The THR measurement results are presented that have been carried out at 10 values of wavelengths within visible, near- ultraviolet, and near-infrared ranges (400 to 927 nm). The coatings being compared are as follows: chemically oxidized copper, coating on the basis of Al-N system, black-chromium coating, black bituminous paint, commercial black enamel used in solar converters, and plasmatron sputtered nichrome and constantan alloys. The chemically oxidized copper has been chosen as a material possessing high solar radiation absorption factor and low emissivity. Black-chromium coating is a widespread material for thermal control in space instruments. The black bituminous paint has been chosen as a possible cheap alternative solar absorber. Commercial black enamel served as a standard conventional coating used in some inexpensive solar converters. Nichrome and constantan have been also selected for investigations as the alloys having high specific resistivities, which promised that intensive electron scattering in skin layers of these alloys could take place, i.e., intensive transfer of energy and momentum received from the light wave to the lattice.
The goal of the present study was the elucidation of the level, down to which it is possible to reduce a reflectivity of aluminium in visible, near infrared, and vacuum ultraviolet wavelengths ranges by introduction of nitrogen atoms into the skin layer of aluminium. The used method consists in coprecipitating the aluminium and nitrogen ions, such that the nitrogen atoms introduced into the lattice of aluminium cause infringements of periodicity of the aluminium crystal lattice field owing to local deformations when forming strong and spatially-oriented covalence bonds with surrounding aluminium atoms. In such deformed Al-N system, namely, on borders of areas with broken periodicity of the Al lattice field, there will take place a scattering of free electrons. An increase in electron scattering intensity within the skin layer results in reduction of probability of release of secondary photons by free electrons excited by light wave in the skin layer. The latter will cause reduction of reflectivity of the surface of deformed aluminium lattice. Reflectivity measurements of samples of Al-N were carried out at ten wavelengths within a range from 400 up to 927 nm, and also at 121.6 nm. It turned out, evaporation of aluminium in argon plasma containing the nitrogen is really capable to lower reflectivity of such coating to values acceptable for practical applications, just down to a level of 4% in a visible range, 5% in near infrared, and 2% at 121.6 nm. So low levels of reflectivity (especially, at 121.6nm - the most intensive line of the short-wave part of ultraviolet solar spectrum) allow to apply this coating for reduction of solar stray light in instruments and to decrease the detectors noise of space equipment operating under conditions of open space and intensive solar ultraviolet illumination.
This paper concerns the light-absorptive coating intended for reduction of stray sunlight in the satellite-borne optical instrumentation and particle-analyzing apparatus operating in open space under intensive solar illumination. Through the inner surfaces are the passive elements in optical instruments, nonetheless the condition and reflectivity of their surfaces can have noticeable effect on the signal-to-noise ratio of their detectors. In order to reduce an influence of the stray light on sensitivity of devices, the reflectivities of inner surfaces are strived to keep minimum in those wavelengths ranges, where the detectors spectral responses are high. We measured reflectivities of several types of traditional light-absorptive coating and also tested our own approach to development of low reflectivity materials. The reflectivities were measured in visible and near-infrared wavelengths ranges (400 to 927 nm), as well as at the hydrogen Lyman-α line (121.6 nm), the most intensive line of the solar UV radiation. Also, the values of the solar absorptance αs and the emissivity ε were measured. They made it possible to classify the investigated coating with their respects to a possible use in converters of solar radiation into the heat. Conclusions are given on application of diverse coatings depending on the instrument's detector type, operational wavelengths range, and operating conditions.
This paper concerns the light-absorptive black-chromium (BC) coating intended for reduction of stray light in the satellite-borne optical instruments and particle-analyzing apparatus operating in open space under intensive solar UV illumination. Resolution of this problem is also important in connection with development of the extra-atmospheric Far UV astronomy. The total hemispherical reflectance (THR) measurements of the BC coating samples were carried out at a wavelength of 121.6 nm -- the most intensive line in the short-wave part of the solar UV spectrum which creates the main contribution to the background noise of detectors in space apparatus. THR of the samples tested at a wavelength of 121.6 nm was as low as 2%. The samples of the BC coating were subjected to standard mechanical and environmental tests simulating the shipping, storage, launching, flight, and operating conditions of the space equipment. THR measurements of the samples were made following exposure to each test. As a result of mechanical and environmental testing the following features of the BC coating were discovered: (1) the coating stands up well to all types of mechanical effects (vibratory loads, linear overloads, and impacts) to which the space equipment may be subjected both when shipping to the space- vehicle launching site and also when launching; (2) the most undesirable effect for the undamaged surface of the coating is humidity -- this causes an increase of reflectivity with a factor of 1.2 on average; (3) the most dangerous outcome for the BC coating is the effect of rubbing -- the reflectivity is increased about two times at the place of contact. Advantages of the BC coating consists in its low reflectivity both in the vacuum UV and within the visible ranges, as well as in the short-wave infrared range. The BC coatings is also assumed to possess following valuable attributes: (1) at grazing incidence its reflectivity might be less than that of other types of coatings; (2) low fraction of the incident radiation reflected into the specular peak relative to the THR.
We have developed absorptive Mg coatings for the vacuum UV (VUV) wavelength range. The total hemispherical reflectivity at normal incidence was measured at 121.6 nm. The reflectivity of the Mg coating produced by magnetron sputtering (presumably from the vapor phase) is close to 1%, comparable to the best known coatings. In contrast to the latter they are mechanically robust. After storage in air for 2 years the reflectivity increased by a factor 2. The reflectivity of Mg and Ag coatings produced by thermal evaporation was observed to be about 2 - 3%. The reflectivity of the Ag coatings proved to be stable over a period of 58 months in air.
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