Sub-mm Wavebands Research Articles

Channel Sounding and Ray Tracing for Intrawagon Scenario at mmWave and Sub-mmWave Bands

In order to realize the vision of “smart rail mobility,” a seamless high-data rate wireless connectivity with up to dozens of gigahertz bandwidth will be required. This forms a strong motivation for exploring the underutilized millimeter wave (mmWave) and sub-mmWave bands. In this article, the wireless channel in one “smart rail mobility” scenario- the intrawagon scenario-is characterized through ultrawideband (UWB) channel sounding and ray tracing (RT) at mmWave and sub-mmWave bands. To begin with, a series of horizontal directional scan-sounding measurements are performed inside a real high-speed train wagon at 60 and 300 GHz frequency bands with a bandwidth of 8 GHz. Correspondingly, the channel characteristics such as Rician K-factor, root-mean-square (rms) delay spread (DS), azimuth spread of arrival, and azimuth spread of departure are extracted and analyzed. Based on the measurements, a self-developed RT simulator is validated through the reconstruction of the three-dimensional wagon model and the calibration of the electromagnetic (EM) properties of the main materials. This gives the chance to physically interpret the measurement results and characterize the intrawagon mmWave and sub-mmWave channels more comprehensively through extensive RT simulations.

H-ATLAS/GAMA: the nature and characteristics of optically red galaxies detected at submillimetre wavelengths

We combine Herschel/SPIRE sub-millimeter (submm) observations with existing multi-wavelength data to investigate the characteristics of low redshift, optically red galaxies detected in submm bands. We select a sample of galaxies in the redshift range 0.01$\leq$z$\leq$0.2, having >5$\sigma$ detections in the SPIRE 250 micron submm waveband. Sources are then divided into two sub-samples of $red$ and $blue$ galaxies, based on their UV-optical colours. Galaxies in the $red$ sample account for $\approx$4.2 per cent of the total number of sources with stellar masses M$_{*}\gtrsim$10$^{10}$ Solar-mass. Following visual classification of the $red$ galaxies, we find that $\gtrsim$30 per cent of them are early-type galaxies and $\gtrsim$40 per cent are spirals. The colour of the $red$-spiral galaxies could be the result of their highly inclined orientation and/or a strong contribution of the old stellar population. It is found that irrespective of their morphological types, $red$ and $blue$ sources occupy environments with more or less similar densities (i.e., the $\Sigma_5$ parameter). From the analysis of the spectral energy distributions (SEDs) of galaxies in our samples based on MAGPHYS, we find that galaxies in the $red$ sample (of any morphological type) have dust masses similar to those in the $blue$ sample (i.e. normal spiral/star-forming systems). However, in comparison to the $red$-spirals and in particular $blue$ systems, $red$-ellipticals have lower mean dust-to-stellar mass ratios. Besides galaxies in the $red$-elliptical sample have much lower mean star-formation/specific-star-formation rates in contrast to their counterparts in the $blue$ sample. Our results support a scenario where dust in early-type systems is likely to be of an external origin.

Клинотрон – умножитель частоты в субмиллиметровом диапазоне волн (l = 0,93 мм)

Клинотрон – умножитель частоты в субмиллиметровом диапазоне волн (l = 0,93 мм)

The concept of a Space-Space interferometer for observations in mm and sub-mm wavebands

At present, space radio astronomers and engineers study the prospects of design of the second-generation ground-space interferometers for astrophysical research with the microsecond angular resolution of sources. The implemented Japanese VSOP project (1998–2003) and the Russian Radioastron project (under preparation for space flight) are related to the first generation. In this paper, the ideology and configuration of the Space-Space interferometer are considered. It would allow one to obtain principally new capabilities: to exclude the Earth’s atmosphere influence, to realize a quasi-phase-stable interferometer, and to remove the problems of electromagnetic compatibility with other services. Moreover, a capability will appear to carry out preliminary correlation processing onboard the spacecraft due to achievement of small residual uncertainties in signal delay and frequency and, owing to this, to realize onboard data compression in order to transmit data to the Earth by usual space communication channel.

An IR and Submillimeter Telescope for Dome C

The Antarctic Plateau provides the best terrestrial astronomical observing conditions across the Mid-InfraRed (MIR) and submillimeter wavebands due to its remarkable atmospheric transparency and stability. The opening of Concordia Station at Dome C is now offering a unique opportunity to the international submm astronomical community to perform ground-breaking astronomical science. To take advantage of these unique conditions, we are proposing the realization of an international Antarctic Submillimeter Observatory (ASO) based on the ALMA antenna prototype, to carry out both continuum and spectral line observations, across the MIR and submm wavebands. The telescope would be able to operate in the MIR using the innermost surface (2-4 m in diameter) of the primary reflector, which would be accurate enough to operate at wavelengths as short as ~5 micron.

The dark side of galaxy formation.

I discuss the discovery of a population of extremely luminous, but very dusty and very distant, galaxies in the submillimetre (submm) waveband. Almost all the light emitted by the stars in these galaxies is absorbed by interstellar dust (which is produced by the same stars) and re-radiated in the far-infrared. This leaves little to be detected at optical wavelengths and results in most of these galaxies being effectively invisible in even the deepest optical images obtainable with the Hubble space telescope. Yet this population contributes most of the light emitted by galaxies at wavelengths of lambda > or approximately equal 100 microm over the lifetime of the Universe. Together with other observations, this suggests that perhaps up to half of all the stars seen in galaxies today were formed in very dusty regions in the early Universe. Hence, studying the galaxies detected in the submm wavebands is critical for developing and testing models of galaxy formation and evolution. Individually, these luminous submm galaxies are forming stars a thousand times faster than our Galaxy is at the present-day, sufficiently fast to form all the stars in the most luminous galaxy in the local Universe within a short period, up to ca. 0.1-1 Gyr. Detailed study of a handful of examples of this population confirm these estimates and unequivocally identify the bulk of this submm-selected population with dusty, star-burst galaxies in the very distant Universe. The extreme faintness of this population in the optical and near-infrared wavebands, resulting from their obscuration by dust, means that our understanding of the detailed nature of these galaxies is only slowly growing. I give a brief summary of the properties of these highly obscured systems and describe the wide range of facilities currently being developed that will greatly aid in their study.

Flux-flow resonant current steps in intrinsic Josephson junctions on Bi2Sr2CaCu2O8 single crystal

A mesa with a-b plane dimensions of 12 µm × 12 µm was patterned on Bi2Sr2CaCu2O8 single crystal, after which two separate electrical contacts were carefully fabricated on the top of the mesa to ensure a standard four-probe arrangement. With contact resistance excluded, the measured current-voltage (I-V) curves featured high voltage resolution. Voltage jumps up to 20 mV and remarkable hysteresis at 4.2 K indicated typical superconductor-insulator-superconductor properties. A few sharp current steps started to manifest themselves clearly when a magnetic field of about 10 kOe was applied parallel to the a-b plane. Carefully studied were the dependences of the current steps on the magnetic fields and the operating temperatures. We attributed the field-induced steps to the flux-flow resonances, and the multisteps implied different modes existing in the stacked intrinsic junctions. The voltage region at which the steps occurred was of the order of millivolts, indicating possible high-frequency applications at mm and sub-mm wavebands.

Low-noise millimeter- and submillimeter-wave receivers

A review of the current state in the development of receivers with an extremely low noise level in the short-wave part of the millimeter (mm) and submillimeter (submm)-wave bands is presented. A superheterodyne with a mixer at the input remains the main type of such receivers. The mixers using superconductor-insulator-superconductor (SIS) tunnel contacts and having noise temperatures very close to the quantum limit dominate at frequencies of up to ~1 THz. At the higher frequencies, the best results were obtained with hot-electron bolometers as mixers where the strong dependence of the semiconductor resistance on the temperature Tc is employed. Examples of the SIS receivers and cooled Schottky-barrier diode (SBD) receivers developed at the Institute of Applied Physics of the Russian Academy of Sciences are slightly inferior to SIS receivers in noise temperature but are useful for many applications. The prospects of low-noise reception in the mm and submm-wave bands, in particular the prospects of using integrated receivers and multipath systems, are discussed.

Numerical analysis of the coherent radiation emission by two stacked Josephson flux-flow oscillators

The numerical investigation of the radiation emission by a system of two magnetically coupled, long Josephson junctions is reported. Time-dependent synchronized voltage response in the flux-flow regime is analyzed for the case of in-phase and out-of-phase oscillations in the junctions. Simulations show that Josephson junctions operating in the in-phase flux-flow mode may generate rf radiation power by a factor of more than 4 larger than that of a single Josephson junction. The radiation in the out-of-phase flux-flow mode is characterized by nearly completely suppressed amplitudes of odd harmonics and considerably damped even harmonics as compared to that of a single barrier junction. The dependence of the radiation power on the parameter spread between the junctions is investigated. The advantages of using stacked Josephson junctions as oscillators for the sub-mm wave band are discussed.

Hail formation onset in a cloud, determined by a “window” in the ice absorption index in submillimeter wavelength

Three hail-formation phases in a cloud may be detected, by means of MM and SubMM wave propagation analysis in Convective clouds. These phases are size increase of super-large(85 to 1500Μm in radius, and larger) droplets, their transition into supercooled state, and immediate droplets transition into ice or small hail, or their coating by ice Crust. Ice transparency “window” in absorption index, existing in SubMM wave band, is shown to serve as indicator of ice creation in a cloud. Radar reflection coefficient is shown to be increased by two orders of magnitude in the ice transparency “window” under super-large droplets transition into ice.