Recent developments in millimeter and submillimeter waves

Millimeter and submillimeter-wave observations provide important information for the studies of atmospheric chemistry and astrochemistry (molecular clouds, stars formation, galactic study, comets and cosmology). But, these observations depend strongly on instrumentation techniques and on the site quality. New techniques or higher detector performances result in unprecedented observations and, sometimes, the observational needs drive developments of new detector technologies, for example, superconducting junctions (SIS mixers) because of their high sensitivity in heterodyne detection in the millimeter and submillimeter wave range (100–700 GHz), HEB (Hot Electron Bolometer) mixers which are being developed by several groups for application in THz observations. For the sub-millimetre wavelengths heterodyne receivers, the local oscillator (LO) is still a critical element. So far, solid state fundamental sources are often not powerful enough for most of the applications at millimetre or submillimetre wavelengths: large efforts using new planar components (HBV) and integrated circuits, or new technics (laser mixing) are now in progress, in a few groups.The new large projects as SOFIA, FIRST, ALMA, … for astronomy; SMILES, EOS-MLS, … for aeronomy and other projects for the planetary science (ROSETTA, Mars Explorer, …), will benefit of the new developments.     

Cosmic chemistry: Molecules in space

Molecular clouds contain most of the mass of gas in interstellar space and are the source of matter from which new stars are formed. The clouds can be studied by means of the radio emission in rotational transitions of the molecules that they contain. We describe the chemical routes by which these molecules are formed, and their ultimate incorporation into icy mantles frozen onto dust grains. The role that chemistry plays in tracing and controlling processes in star-forming regions and in near-stellar environments is briefly indicated.     

Comets at radio wavelengths

Comets are considered as the most primitive objects in the Solar System. Their composition provides information on the composition of the primitive solar nebula, 4.6 Gyr ago. The radio domain is a privileged tool to study the composition of cometary ices. Observations of the OH radical at 18 cm wavelength allow us to measure the water production rate. A wealth of molecules (and some of their isotopologues) coming from the sublimation of ices in the nucleus have been identified by observations in the millimetre and submillimetre domains. We present an historical review on radio observations of comets, focusing on the results from our group, and including recent observations with the Nançay radio telescope, the IRAM antennas, the Odin satellite, the Herschel space observatory, ALMA, and the MIRO instrument aboard the Rosetta space probe.     

Millimeter/submillimeter nonlinear spectroscopy

The techniques of coherent transient spectroscopy have been successfully applied to the microwave and optical regions of the spectrum, but lack of suitable sources has prevented convenient extension to the millimeter/submillimeter region where several strong molecular transitions occur. Analysis of the experimental requirements and theory for such experiments considering several molecules including H₂O and CH₃Cl indicates how such experiments can be performed employing recently developed oscillators with increased power.     

Prediction of the submillimeter spectrum of the cosmic backgroundradiation by a plasma model

A non-Big-Bang plasma model is used to predict the submillimeter spectrum of the cosmic background radiation (CBR). Early intermediate-mass stars heat dust which, in turn, warms electrons trapped in force-free filaments emitted by galactic nuclei. Current stars contribute further heating by the same synchrotron mechanism. The electrons emit a nonblackbody radiation which matches the observed CBR spectrum and isotropy. The model also predicts intergalactic absorption of radio radiation, as confirmed by observations of spiral galaxies, and accurately predicts the spectrum of radio-quiet sources     

Millimeter wave phase-locking in distributed Josephson arrays

Voltage tunable oscillators operating at millimeter and submillimeter wavelengths can be designed and fabricated using series arrays of Josephson junctions. The coherent radiation obtained from such oscillators results from the mutual phase-locking of the junction through the high-frequency Josephson current generated in the array. A 40-junction array of Josephson junctions distributed over many wavelengths has been designed, fabricated and tested for operation at 100 GHz. This paper presents the experimental results obtained for this prototype array. A review of the theory for optimizing array design is presented along the implications for power generation at submillimeter wavelengths in future arrays.     

Method for radar cross section measurements in millimeter and submillimeter wave regions

We present a new method for electromagnetic modeling radar scattering processes. In this method we use a quasi-optical waveguide of the type of "hollow dielectric waveguide" as a fundamental component of radar cross section (RCS) instrumentation systems. Such waveguide structure produces required target-illuminating quasiplane wave and suppresses unwanted waves as well as transmits the legitimate signal from test object, mounted inside this waveguide, to the reception zone. Presented method is especially effective one in millimeter and submillimeter wave regions, in particular, for investigation RCS of targets with small reflectivity.     

The T Tauri stars

The observational aspects of T Tau stars are described, treating data in the optical, infrared, radio, X-ray and ultraviolet regions of the spectrum. Erratic flare-like behaviour is very characteristic of these objects which are pre-main-sequence stars, mostly on their convective tracks, and represent early phases in the evolution of roughly solar-mass stars. Herbig-Haro objects are discussed-shocked nebulae found in association with either visible T Tau stars or infrared sources deeply embedded in the parental dust clouds. It is argued that highly anisotropic mass loss, probably in unpredictable short-lived episodes, is found in the stars that excite Herbig-Haro objects. These exciting stars could well be the precursor to T Tau stars and display a fascinating array of astonishingly well-collimated phenomena.     

Future solar systems

Widely regarded as pathological variable stars – with erratic photometric and spectroscopic behavior of unknown physical origin – 20 years ago, T Tauri stars (TTSs) turned out in the last decade to be promising laboratories for observing the formation of solar systems such as ours. This is because circumstellar, presumably protoplanetary disks were found to surround a large fraction of them. While evidence for disks was primarily indirect until 1995, recent high resolution imaging confirmed earlier claims that the infrared (IR) and ultraviolet (UV) excesses seen in the spectral energy distribution (SED) of these stars were due to disk emission. The activity displayed by young stellar objects at all wavelengths is due to the interaction between the circumstellar disk and the magnetized star and to non-stationary accretion/ejection phenomena. In this review, we briefly summarize properties of these young solar-type stars and describe their circumstellar disks in some detail, focusing on current optical, infrared and millimeter high angular resolution observations that now allow us to resolve the disks.     

On the anomalies and role of water clusters in atmospheric absorption of millimeter and submillimeter radio waves

Experimental data are presented which do not confirm the existence of anomalies observed by many investigators in the atmospheric absorption of millimeter and submillimeter radio waves and make questionable the hypothesis on the importance of large water clusters in absorption that is invoked to explain the anomalies.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 4, pp. 483–495, April, 1996.This work was supported in part by the Russian Foundation for Fundamental Research under Project No. 94-02-05407-a).     

紫外光子在星周天体化学中的作用

介于1到8个太阳质量之间的恒星在演化后期的渐近巨星分支(AGB)阶段,会以气体和尘埃的形式损失大量物质. AGB星提供了星际介质中高达35%的尘埃,其中包含形成太阳系所需要的物质. 此外,AGB星周包层是复杂有机化学所发生的重要场所之一,在其中已探测到的有机分子已超过80多个. 这里展示了AGB星自身或者其近距离的伴星所辐射的紫外光子会显著地改变星周包层的化学特性,尤其是具有团块结构的星周包层. 研究发现,在恒星存在伴星(例如白矮星)的情况下,高通量的紫外光子会破坏富碳AGB星周包层内部的H₂O分子,使其含量低于观测到的水平,并且在星周包层内部产生C⁺等物质,这与以往的星周化学模型的研究结果是不同的.     

一种有效提高太赫兹时域光谱装置成像空间分辨率的方法

太赫兹波成像技术一个最显著的制约因素是其有限的空间分辨率。提出通过在样品前加小孔的方法来提高传统太赫兹时域光谱装置成像的空间分辨率。采用在样品前约2 mm处加直径为0.5 mm小孔的方法使成像的空间分辨率从1.276 mm提高到0.774 mm,提高0.502 mm,约39％。通过这个简单的方法在传统的太赫兹时域光谱成像装置上实现了空间分辨率从毫米量级到亚毫米量级的提高。聚乙烯板上直径为1 mm的小孔被作为成像的研究对象,分别采用传统的太赫兹时域光谱装置对样品进行直接成像和在样品前约2 mm处加直径为0.5 mm的小孔后对样品成像两种方式,并采用损失成像中信噪比较好的能量损失成像,对比两种方式得到的样品的太赫兹像,结果显示聚乙烯板上小孔的边界加小孔后成像比不加小孔直接成像明显清晰。证实了在样品前加小孔可以有效的提高太赫兹成像系统的空间分辨率。从理论上对通过在样品前加小孔提高系统空间分辨率的方法进行了分析,指出小孔尺寸越小,系统的空间相干长度越大,空间分辨率越高,但同时太赫兹信号的强度会相应减小。该方法可以简单有效的提高太赫兹时域光谱装置成像的空间分辨率,从而进一步拓展太赫兹谱成像技术的应用领域。     

Contemporary Millimeter- and Submillimeter-Wave Astronomy

We discuss the scientific importance of the millimeter- and submillimeter-wave astronomy. The directions of research and the main achievements in cosmology, extragalactic astronomy, and studies of the Galaxy structure, star-formation regions, and Solar-System objects are described. Topical problems are formulated. Existing and planned instruments for the millimeter/submillimeter astronomy are briefly described.     

Subwavelength grating Fourier‐transform interferometer array in silicon‐on‐insulator

A planar waveguide Fourier‐transform spectrometer with densely arrayed Mach‐Zehnder interferometers is demonstrated. Subwavelength gratings are used to produce an optical path difference without waveguide bends. The fabricated device comprises of an array of 32 Mach‐Zehnder interferometers, which produce a spatial interferogram without any moving parts, yielding a spectral resolution of 50 pm and a free‐spectral range of 0.78 nm. As a result of similar propagation losses in subwavelength grating waveguides and conventional strip waveguides, loss imbalance is minimized and high interferometic extinction ratio of −25 to −30 dB is obtained. Furthermore, phase and amplitude errors arising from normal fabrication variation are compensated by the spectral retrieval process using calibration measurements.     

Laboratory Microwave Spectroscopy of Interstellar Molecules

Abstract

Modern radio astronomy has revealed that various kinds of molecules exist in ther very cold region between stars: interstellar space. An electromagnetic wave with a wavelength of a few centimeters to 1 mm corresponds in energy to a few to ten degrees of Kelvin. A molecule having its spectrum in this energy region provides us with valuable information about the low-energy environment where the molecule exists. New information provided by radio astronomy has rarely been     

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.     

Modeling of radar scattering conditions inside a circular hollow dielectric waveguide

A new ray representation of electromagnetic field inside a quasi — optical structure in the form of a circular hollow dielectric waveguide is suggested and confirms the possibility for the unusual application of such a waveguide as an off-beat microanechoic chamber for electromagnetic modeling in the near millimeter and submillimeter wave regions.     

Broadband Orotron Operation at Millimeter and Submillimeter Waves

Due to high selectivity of open cavity, wider electron beams with higher current can be used in orotron as compared with other Cherenkov devices. It provides important advantages at short millimeter and submillimeter waves. Theory and preliminary experiments in the short part of millimeter and long part of submillimeter wavelength range show that orotron with operating voltage of few kV can generate significantly higher output power than existing BWOs. Combined electronic and mechanical frequency tuning in the band of about of octave has been obtained.     

Measurement of the power density of electromagnetic radiation by the method of microwave nonstationary spectroscopy

We present a new method for measurement of the microwave power based on the analysis of transient signals of gas molecules. These signals arise due to the phase switch of microwave radiation, which interacts with molecular rotational transition with frequency equal to that of the radiation. The method allows one to measure the amplitude of radiation via determination of the Rabi frequency. This approach is especially preferable in the submillimeter wavelength range.     

Physical aspects of the model of propagation of centimeter and millimeter radio waves along inclined paths in the earth’s atmosphere

We present the physical aspects of constructing a computer model of propagation of centimeter and millimeter radio waves along inclined paths in the earth’s atmosphere, which allows us to calculate the integral attenuation in the frequency spectrum from 3 to 100 GHz, the time of group delay of the signal, refraction of radio waves, and the brightness temperature of the atmosphere for certain regions on the basis of standard atmospheric models and optimal statistical extrapolation of surface meteorological elements. The results of this paper were presented in part at the XVIIIth All-Russia Conference on Propagation of Radio Waves (St. Petersburg, September 17–19, 1996 [1] Radiophysical Research institute, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, No. 12, pp. 1463–1478, December, 1997.