Submillimeter Wave Range Research Articles

Modern Resonator Spectroscopy at Submillimeter Wavelengths

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.

Observation of Thermocapillary Effect in Gas -Assisted CO2 Laser Cutting of Steel

Investigations of melt’s dynamics have been performed with pyrometer in the gasjet-assisted CO2 laser cutting. Pyrometer consists of 4 sensors and local luminosity of metal melt has been measured precisely on different depth along front of cutting with long dimension resolution about 0.1 mm and with time resolution as short as 0.06 ms. The results of data processing of brightness temperature are reported for laser cutting of mild-steel plate 3 mm, 6 mm and 10 mm thick, that allows obtaining spectra of melt’s surface deformation for different values of cutting velocity and assisted gas pressure. The generation of capillary-wave turbulence following thermo-capillary effect can be observed in the locations, where intensity of radiation of CO2 laser on melt’s surface exceeds 1 MW/cm2. The R-M-S of temperature fluctuations is greater than 10 K in the range of sub-millimeter capillary waves. Thus, an additional mechanism of the anomalous absorption on the front of cutting can compensate the low absorption of the metal in case of 10.6 μm laser in comparison with the absorption of the metal in the near infrared range.

Analysis of the terahertz rotational spectrum of the three mono-13C ethyl cyanides (13C–CH3CH2CN)

Context. Millimeter- and submillimeter-wave spectra of regions such as the Orion molecular cloud show many rotational-torsional lines that are caused by the emission of complex organic molecules (COM). Previous laboratory investigations have been conducted for three isotopologues of ethyl cyanide up to 360 GHz, and subsequently, several hundred lines of the three isotopologues have been detected in Orion IRc2 using the IRAM 30 m radiotelescope. Aims: In this survey we present the analysis based on a Watson Hamiltonian for an asymmetric one-top rotor of the 13C-substituted ethyl cyanide 13CH3CH2CN, CH313CH2CN and CH3CH213CN in the frequency range 480-650 GHz and 780-990 GHz. Methods: The rotational spectra of the three species were measured with a submillimeter spectrometer (50-990 GHz) using solid-state sources. Results: A new set of spectroscopic parameters was determined from a least-squares fit procedure for each isotopologue. These parameters permit a new accurate prediction of rotational lines suitable for an astrophysical detection in the submillimeter wave range. Full Tables B.1-B.3 are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/543/A135

The complex refractive indices of the liquid crystal mixture E7 in the terahertz frequency range

We have used terahertz time-domain spectroscopy to investigate the complex optical constants and birefringence of a widely used liquid crystal mixture E7 in both nematic and isotropic phases (26°C–70°C). The extinction coefficient of E7 at room temperature is less than 0.035 and without sharp absorption features in the frequency range of 0.2–2.0 THz. The extraordinary (ne) and ordinary (no) indices of refraction at 26°C are 1.690–1.704 and 1.557–1.581, respectively, giving rise to a birefringence of 0.130–0.148 in this frequency range. The temperature-dependent (26°C–70°C) order parameter extracted from the birefringence data agrees with that in the visible region quite well. Further, the temperature gradients of the terahertz optical constants of E7 are also determined. The optical constants of E7 in the terahertz or sub-millimeter wave range are found to deviate significantly from values predicated by the usual extended Cauchy equations used in the visible and near-infrared.

Estimation of the Effective Collective Thomson Scattering Volume Using a Gaussian Beam in the LHD ECRH System

Collective Thomson scattering (CTS) is a promising candidate for direct measurement of the velocity distribution function of ions in fusion devices. The relevant frequency for a CTS probing beam source is in the millimeter- to submillimeter-wave range. The scattering volume of CTS is an important parameter for determining the intensity of the scattered power and the spatial resolution of the measurement. The conventional method of estimating the scattering volume is to simply calculate the geometrical volume of the probing and receiving beams. This calculation is valid only when the beams are diffraction free. The effective scattering volume is defined and applied to a CTS system using strongly focused beams and its validity is examined experimentally.

On Estimating and Canceling Parasitic Capacitance in Submillimeter-Wave Planar Schottky Diodes

The equivalent circuit model of a submillimeter planar diode is investigated. In particular, the parasitic finger to pad capacitance that shunts the diode junction severely degrades the performance of frequency multiplier. Therefore, precise estimation of its value is crucial to circuit design in the submillimeter wave range. In this letter, a parallel diode structure is analyzed, and symmetry is utilized to remove parasitic elements external to the finger loop. This analytical approach is verified through measurement on a 200 GHz planar diode circuit. To increase the diode junction capacitance modulation ratio, one possible solution is also suggested.

150 GHz Band-Pass Filter Using LTCC Technology

A low temperature co-fired ceramic 150 GHz bandpass filter is, for the first time, designed and manufactured. Experimental electric performances are encouraging and validate the use of LTCC technology at submillimeter wave range. A Chebyshev 2-pole bandpass filter is achieved, with 1.6% bandwidth and 4.6 dB insertion loss. The unloaded quality factor is found to be 290.

The electromagnetic response of human skin in the millimetre and submillimetre wave range

Recent studies of the minute morphology of the skin by optical coherence tomography revealed that the sweat ducts in human skin are helically shaped tubes, filled with a conductive aqueous solution. This, together with the fact that the dielectric permittivity of the dermis is higher than that of the epidermis, brings forward the supposition that as electromagnetic entities, the sweat ducts could be regarded as low Q helical antennas. The implications of this statement were further investigated by electromagnetic simulation and experiment of the in vivo reflectivity of the skin of subjects under varying physiological conditions (Feldman et al 2008 Phys. Rev. Lett. 100 128102). The simulation and experimental results are in a good agreement and both demonstrate that sweat ducts in the skin could indeed behave as low Q antennas. Thus, the skin spectral response in the sub-Terahertz region is governed by the level of activity of the perspiration system and shows the minimum of reflectivity at some frequencies in the frequency band of 75–110 GHz. It is also correlated to physiological stress as manifested by the pulse rate and the systolic blood pressure. As such, it has the potential to become the underlying principle for remote sensing of the physiological parameters and the mental state of the examined subject.

Scattering database in the millimeter and submillimeter wave range of 100–1000 GHz for nonspherical ice particles

The inference of ice cloud properties from spaceborne sensors is sensitive to the retrieval algorithms and satellite sensors used. To approach a better understanding of ice cloud properties, it is necessary to combine satellite measurements from multiple platforms and sensors operating in visible, infrared, and millimeter and submillimeter‐wave regions of the electromagnetic spectrum. The single‐scattering properties of ice particles with consistent ice particle models are the basis for estimating the optical and microphysical properties of ice clouds from multiple satellite sensors. In this study, the single‐scattering properties (extinction efficiency, absorption efficiency, single‐scattering albedo, asymmetry factor, and scattering phase matrix) of nonspherical ice particles, assumed to be hexagonal solid and hollow columns, hexagonal plates, 3D bullet rosettes, aggregates, and droxtals, are computed from the discrete dipole approximation method for 21 millimeter and submillimeter‐wave frequencies ranging from 100 to 1000 GHz. A database of the single‐scattering properties of nonspherical ice particles are developed for 38 particle sizes ranging from 2 to 2000 μm in terms of particle maximum dimension. The bulk scattering properties of ice clouds consisting of various ice particles, which are the fundamental to the radiative transfer in ice clouds, are developed on the basis of a set of 1119 particle size distributions obtained from various field campaigns.

THE MILLIMETER- AND SUBMILLIMETER-WAVE SPECTRUM OF THETRANSANDGAUCHECONFORMERS OF ETHYL FORMATE

Since methyl formate (HCOOCH3) is found to have a high abundance in hot molecular cores and other types of clouds in the galactic center, it is reasonable to search among such sources for detectable abundances of the more complex analog ethyl formate (HCOOC2H5). Following a previous study of the millimeter-wave spectrum of ethyl formate, we have extended the analysis of the vibrational ground state of the trans and gauche conformers of ethyl formate into the submillimeter-wave range. Over 2200 new spectral lines have been measured and analyzed at frequencies up to 380 GHz. Fitting the data for each conformer to a Watson A-reduced asymmetric-top Hamiltonian has allowed us to predict the frequencies and intensities of many more transitions through 380 GHz.

Nonparaxial magnetron injection gun for a high-power pulsed submillimeter-wave gyrotron

We determine distinctive features of the systems forming helical electron beams (HEBs) for high-power pulsed gyrotrons operated in the submillimeter-wave range. It is shown that they are characterized by a nonparaxial magnetic field in the emitter region, short distances between the cathode and the cavity, and the necessity of supplementing the magnetic system with a cathode coil placed behind the emitter. In a diode-type magnetron injection gun, which forms a boundary beam with a power of up to 4 MW, one can obtain the HEB parameters which are acceptable for the gyrotron operation.

Collisional broadening and shifting of OCS rotational spectrum lines

Broadening and shifting of carbonyl sulfide (OCS) rotational spectrum lines by pressure of N 2, O 2 and OCS were accurately studied in the frequency range 24–850 GHz at room temperature using a spectrometer with radio-acoustic detection of absorption. Rotational dependences of collisional widths of OCS spectrum lines were determined by a simple empirical polynomial fit of experimental data. Experimental uncertainties were analyzed. Results of supplementary test measurements of self-broadening of rotational OCS lines in the ν 2 excited vibrational state and carbon monoxide (CO) lines in the ground vibrational state are presented. Comparison of the obtained results with previously known measurements and theoretical calculations is given. The performed work allows for the first time development of accurate gaseous etalon of absorption for atmospheric applications and laboratory use, covering continuously the whole millimeter- and submillimeter-wave range.

Dielectric-Parameter Measurements of SiC at Millimeter and Submillimeter Wavelengths

High-precision continuous spectra of the absorption coefficient, refractive index, complex dielectric permittivity, and loss tangent for several silicon carbide (SiC) specimens are reported in this paper over a broad millimeter- and submillimeter-wave range for the first time. Measurements have been successfully performed using three different types of specially constructed spectrometer systems: a dispersive Fourier transform spectroscopy, an automated 60-GHz open resonator, and a free-space quasi-optical millimeter-wave spectrometer equipped with high-power backward-wave oscillator sources and their associated state-of-the-art dielectric-measurement techniques. Data are presented as continuous functions of frequency from 30 to 600 GHz. The employment of various measurement systems and techniques ensured the measurement of polycrystalline SiC specimens with various degrees of absorption and dispersion characteristics over an extended frequency range with high precision. Results presented here provide comprehensive information of SiC on its optical and dielectric behavior as a function of frequency and purity for its potential application in semiconductor and radio-frequency devices and circuits. An error analysis of measured dielectric-parameter results is also provided.

Human Skin as Arrays of Helical Antennas in the Millimeter and Submillimeter Wave Range

Recent studies of the minute morphology of the skin by optical coherence tomography showed that the sweat ducts in human skin are helically shaped tubes, filled with a conductive aqueous solution. A computer simulation study of these structures in millimeter and submillimeter wave bands show that the human skin functions as an array of low-Q helical antennas. Experimental evidence is presented that the spectral response in the sub-Terahertz region is governed by the level of activity of the perspiration system. It is also correlated to physiological stress as manifested by the pulse rate and the systolic blood pressure.

Millimeter and submillimeter EPR spectroscopy

For a long time, the electron paramagnetic resonance (EPR) spectrometers have been operated in X and Q bands with wavelengths of microwave radiation about 3 cm and 8 mm. Increasing the operating frequency improves the basic parameters of an EPR spectrometer. In view of this, there has recently been rapid development of high-frequency EPR spectroscopy, including the submillimeter-wave range, related to significant progress in the millimeter-and submillimeterwave technique. This paper discusses characteristic features, application areas, and the state of the art of the experimental technique of EPR spectroscopy in the millimeter-wave range and the short-wavelength region of the millimeter-wave range. The design features of the high-frequency EPR spectrometer operated in the frequency range 65–1500 GHz, which was created at the E. K. Zavoisky Physico-Technical Institute of the Kazan’ Scientific Center of the Russian Academy of Sciences, are presented. The results of studying the structure of the paramagnetic centers formed by impurity Ho3+ ions in synthetic forsterite (Mg2SiO4), obtained by the method of tunable high-frequency EPR spectroscopy, are reported.

Superconducting integrated on-board spectrometer of the submillimeter-wave range for atmospheric research

We present the latest results of developing and implementing a fully superconducting integrated receiver of the submillimeter-wave range on the basis of the superconductor-insulator-superconductor tunnel junction. Characteristics of a new-generation unique device operated in the frequency range 500–600 GHz and intended for atmospheric research onboard a high-altitude balloon within the framework of the international TELIS project are given. This frequency range comprises spectral lines of many chlorine-containing compounds, the registration of which is a major problem of the environmental monitoring of the atmosphere. In the process of creating the spectrometer, we developed the technology for manufacturing Nb/AlN/NbN junctions, which made it possible to reduce the emission linewidth of the superconducting local oscillator based on the long Josephson junction and decrease the noise temperature of the receiver. At the same time, the possibility to continuously tune the local-oscillator frequency in the entire operating range was realized.

Finite Element Analysis of Millimeter and Sub-Millimeter Wave Gyroelectric Waveguide Components

Gallium arsenide (GaAs) and indium antimonide (InSb) are two candidate semiconductor materials for supporting nonreciprocal behavior at millimeter wave and sub-millimeter wave frequencies. Each material is characterized as a function of bias field/frequency and different operating regions are identified either side of the extraordinary wave resonance. An eigenvalue finite element formulation is developed to calculate the complex propagation constant for general gyroelectric waveguide structures. The solver is used to investigate nonreciprocal phase shift and attenuation in a transversely magnetized, semiconductor slab loaded waveguide in the various operating regions. Several potential regions of isolation and differential phase shift have been identified for exploitation in the millimeter wave range for GaAs and in the sub-millimeter wave range for InSb.

Extension of the range of resonator scanning spectrometer into submillimeter band and some perspectives of its further developments

Extension of the working frequency of modern resonator spectrometers into submillimeter wave range is described. Experimental record of atmospheric absorption spectrum covering 45–370 GHz range is demonstrated for the first time, and measured water vapor J′ ka′, kc′ – J ka, kc = 5 1,5–4 2,2 at 325 GHz line parameters are presented. For the first time pressure lineshift for the 325-GHz water vapor line is measured. Further extension of working range is discussed. New estimations of physical limits of time needed for measurements of absorption in the whole Backward Wave Oscillator (BWO) range are given for phase continuous synthesizer regime. Basic schemes of fast broadband continuous phase synthesized sources are discussed. Verification of the previous measurements of water vapor 3 1,3–2 2,0 at 183 GHz line parameters is presented. Comparisons with ringdown resonator spectrometers are given.

Chemical analysis in the submillimetre spectral region with a compact solid state system

A new analytical system that uses the rotational signatures of gas phase molecules is described and demonstrated. It uses a solid state source to probe molecular systems in the millimetre and submillimetre wave range, the only region of the electromagnetic spectrum not yet used extensively for analytical purposes. It employs the FAst Scan Submillimetre Spectroscopy Technique (FASSST), which leads to an especially simple system architecture. Among the attributes of the system are generality, sensitivity, 'absolute' specificity, small size, simplicity, and the potential for very low cost. Applications to problems of analytical interest are also discussed.

Three-temperature model for hot electron superconducting bolometers based on high-T/sub c/ superconductor for terahertz applications

Nowadays, low temperature superconducting hot electron bolometers (LTSC HEBs) are performing really well in the submillimeter wave range. Planar technologies are feasible that can be used for combining bolometer and antenna structures. High temperature superconducting (HTSC) films could also become the next to be used, although it seems that HTSC HEBs cannot reach their LTSC counterparts because of different phonon dynamics. In some previous models, it has been shown, however, that HTSC HEBs should be able to compete with traditional photoconductive detectors in terms of sensitivity. As the substrate temperature was assumed to be constant in these models, we wish to introduce in this paper a more detailed formulation where an extra cooling process (through the substrate) is taken into account. In a first part, simple heat propagation through the substrate is simulated to confirm the theory. The whole detector performance is analyzed in a second part, through a three-temperature model, for different device critical dimensions and thermal parameters. The influence of these characteristics on the lower cutoff frequency and on the maximal responsivity is then discussed. Different ways to improve these detectors are also given.