The precise calculation of atmospheric absorption in a microwave band is highly important for atmospheric remote-sensing with ground-based and satellite-borne radiometers, as it is a key element in ...procedures for temperature, humidity or trace gas concentration retrieval. The accuracy of the absorption model directly affects the accuracy of the retrieved information and reliability of the resulting forecasts. In this study, we analyze the difference between observed and simulated microwave spectra obtained from more than four years of microwave and radiosonde observations over Nizhny Novgorod (56.2°N, 44°E). We focus on zenith-measured microwave data in the 20–60 GHz frequency range in clear-sky conditions. The use of a conventional absorption model in simulations leads to a significant difference in frequency channels within the 51–54 GHz range, while calculations employing a more accurate model based on the Energy Corrected Sudden (ECS) formalism for molecular oxygen absorption reduces the difference several-fold.
We study variations in the dielectric properties of a semi-insulating Gallium arsenide (GaAs) wafer under millisecond pulses of extreme subterahertz power density of up to 180 kW/mm2 at 263 GHz. ...Increasing the duration and power of the pulse, we have obtained sequential down- and upshifts within the range of more than two orders in the effective loss tangent of the wafer in experiments. We have observed the existence of an optimal regime of subterahertz irradiation, in which the accurate, simple and selective annealing of the wafer from the 300 nanometer-thick surface layer of oxides is achieved, even in plain air, without a damage to pure GaAs. An finite-difference time-domain-based numerical simulation explains such selectivity with a difference in tangent losses of about 25 times between pure GaAs and its impurities in the subterahertz band.
We present the results of theoretical and experimental studies of the reflectivity of metals at cryogenic temperatures in the millimeter wavelength range. High-Q Fabry-Perot resonators operated at a ...temperature of 4-300 K in the frequency range 150-250 GHz are used for the experimental study. Silver, copper, gold, aluminum, and beryllium reflectors with different structures are examined. It has been shown that the reflection loss at cryogenic temperatures varies considerably (several times) depending on the structure of the sample surface and the presence of impurities. The obtained data make it possible to calculate the thermal noise of cooled reflectors of the antenna systems in millimeter and submillimeter telescopes.
Ground-based microwave radiometers are increasingly used in operational meteorology and nowcasting. These instruments continuously measure the spectra of downwelling atmospheric radiation in the ...range 20–60 GHz used for the retrieval of tropospheric temperature and water vapor profiles. Spectroscopic uncertainty is an important part of the retrieval error budget, as it leads to systematic bias. In this study, we analyze the difference between observed and simulated microwave spectra obtained from more than four years of microwave and radiosonde observations over Nizhny Novgorod (56.2° N, 44° E). We focus on zenith-measured and elevation-scanning data in clear-sky conditions. The simulated spectra are calculated by a radiative transfer model with the use of radiosonde profiles and different absorption models, corresponding to the latest spectroscopy research. In the case of zenith-measurements, we found a systematic bias (up to ~2 K) of simulated spectra at 51–54 GHz. The sign of bias depends on the absorption model. A thorough investigation of the error budget points to a spectroscopic nature of the observed differences. The dependence of the results on the elevation angle and absorption model can be explained by the basic properties of radiative transfer and by cloud contamination at elevation angles.
On the base of the high quality Fabry-Perot resonator a method for measuring dielectric parameters, namely, the refractive index ( n ) and tanδ values of powder materials, which does not require the ...measurement of the poured layer thickness, has been developed. The results of measuring the Al 2 O 3 , TiO 2 , ZnO, SnO, and WO 3 metal oxide powders in the frequency range 160-253 GHz and in the temperature range 20−360 °C are presented. The method was developed and tested for a detailed understanding of the processes of powders heating and evaporation under the radiation influence of high-power terahertz (submillimeter) gyrotrons with subsequent condensation in the form of highly pure nanopowders.
In this work, we compare the values of 15 convective indices obtained from radiosonde and microwave temperature and water vapor profiles simultaneously measured over Nizhny Novgorod (56.2°N, 44°E) ...during 5 convective seasons of 2014–2018. A good or moderate correlation (with coefficients of ~0.7–0.85) is found for most indices. We assess the thunderstorm prediction skills with a lead time of 12 h for each radiosonde and microwave index. It is revealed that the effectiveness of thunderstorm prediction by microwave indices is much better than by radiosonde ones. Moreover, a good correlation between radiosonde and microwave values of a certain index does not necessarily correspond to similar prediction skills. Eight indices (Showalter Index, Maximum Unstable Convective Available Potential Energy (CAPE), Total Totals index, TQ index, Jefferson Index, S index, K index, and Thompson index) are regarded to be the best predictors from both the true skill statistics (TSS) maximum and Heidke skill score (HSS) maximum points of view. In the case of radiosonde data, the best indices are the Jefferson Index, K index, S index, and Thompson index. Only TSS and HSS maxima for these indices are close to the microwave ones, whereas the prediction skills of other radiosonde indices are essentially worse than in the case of microwave data. The analysis suggests that the main possible reason of this discrepancy is an unexpectedly low quality of radiosonde data.
Modern space-based telescopes operating in the terahertz (THz) spectral range require highly reflecting and cryogenically cooled optics ensuring extremely low noise. The idea of using ...high-temperature superconductor YBa 2 Cu 3 O 7-δ as a telescope's mirror coating for providing low dissipation and high reflectivity of the surface for the subTHz frequency band has been explored. We are presenting here some results of measurements of the reflection losses in YBa 2 Cu 3 O 7-δ at cryogenic and room temperatures (4.2-300 K) in the frequency band 0.166-0.23 THz. High-accuracy measurements of the absolute value of reflectivity have been performed with the help of the resonance technique. The measured reflection losses in the given frequency band were below 1%. For the same frequency range, these values are well above the reflection losses in normal metals such as copper or aluminum, which are normally used to cover mirrors of telescopes. We, therefore, conclude that the use of high-T c superconductors YBa 2 Cu 3 O 7-δ as a highly reflecting coating for the THz telescope's mirror is not advisable.
New Frontiers in Modern Resonator Spectroscopy Koshelev, Maksim A.; Leonov, Igor I.; Serov, Evgeny A. ...
IEEE transactions on terahertz science and technology,
11/2018, Letnik:
8, Številka:
6
Journal Article
Recenzirano
New opportunities due to the advancements in the techniques and methods of broadband resonator spectroscopy in the millimeter and submillimeter range (also known as the terahertz region) are ...considered in this paper. An upgraded variant of the spectrometer produced at the Institute of Applied Physics, Russian Academy of Sciences (Nizhny Novgorod, Russia) possessing a sensitivity to the absorption coefficient of approximately 4 × 10 −9 cm −1 is presented. The spectrometer allows the continuous recording of gas spectra in the 45-500 GHz frequency range (six waveguide subbands) at pressures of 10-1000 torr. A fully automated spectrum recording within the waveguide subband reduces the overall time required for the experiment by almost an order of magnitude. Examples of recording the spectra of oxygen and water vapor resonance lines, as well as of continuum absorption, including the rotationally resolved water dimer spectrum, are presented. It is demonstrated that the uncertainty of measuring the broadening parameter of diagnostic atmospheric lines may be reduced to 0.1%, which meets the stringent present-day requirements of remote sensing of the atmosphere. The physically attainable ultimate sensitivity of broadband microwave resonator spectrometers of various types is discussed. Prospects for further development of the spectrometer and its application for solving fundamental and applied problems of spectroscopy are analyzed.
Classical resonator spectroscopy methods have been realized for the first time in the submillimeter wave range. A resonator spectrometer was developed earlier at the IAP RAS on the basis of an open ...Fabry-Perot resonator excited by the radiation of a backward wave oscillator whose frequency is stabilized by the phase lock loop system. This spectrometer is successfully used for high-precision measurements of the dielectric properties of solid, liquid, and gaseous dielectrics as well as for the metal and coating reflection measurements in the 36-370 GHz range. In this paper, we report an extension of the upper limit of the spectrometer operation frequency to 520 GHz. Features of operation of the main spectrometer systems in the extended frequency range are analyzed. The broadband measurements of absorption in modern MPCVD diamonds are presented. A continuous record of the absorption spectrum of the laboratory atmosphere in the 350-500 GHz range obtained for the first time by the high-sensitivity microwave method is demonstrated. Further prospects for extension of the spectrometer range to terahertz frequencies are discussed.
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