Submillimeter Wave Radiation Research Articles

Joint Retrieval of Multiple Species of Ice Hydrometeor Parameters from Millimeter and Submillimeter Wave Brightness Temperature Based on Convolutional Neural Networks

Submillimeter wave radiometers are promising remote sensing tools for sounding ice cloud parameters. The Ice Cloud Imager (ICI) aboard the second generation of the EUMETSAT Polar System (EPS−SG) is the first operational submillimeter wave radiometer used for ice cloud remote sensing. Ice clouds simultaneously contain three species of ice hydrometeors—ice, snow, and graupel—the physical distributions and submillimeter wave radiation characteristics of which differ. Therefore, jointly retrieving the mass parameters of the three ice hydrometeors from submillimeter brightness temperatures is very challenging. In this paper, we propose a multiple species of ice hydrometeor parameters retrieval algorithm based on convolutional neural networks (CNNs) that can jointly retrieve the total content and vertical profiles of ice, snow, and graupel particles from submillimeter brightness temperatures. The training dataset is generated by a numerical weather prediction (NWP) model and a submillimeter wave radiative transfer (RT) model. In this study, an end to end ICI simulation experiment involving forward modeling of the brightness temperature and retrieval of ice cloud parameters was conducted to verify the effectiveness of the proposed CNN retrieval algorithm. Compared with the classical Unet, the average relative errors of the improved RCNN–ResUnet are reduced by 11%, 25%, and 18% in GWP, IWP, and SWP retrieval, respectively. Compared with Bayesian Monte Carlo integration algorithm, the average relative error of the total content retrieved by RCNN–ResUnet is reduced by 71%. Compared with BP neural network algorithm, the average relative error of the vertical profiles retrieved by RCNN–ResUnet is reduced by 69%. In addition, this algorithm was applied to actual Advanced Technology Microwave Sounder (ATMS) 183 GHz observed brightness temperatures to retrieve graupel particle parameters with a relative error in the total content of less than 25% and a relative error in the profile of less than 35%. The results show that the proposed CNN algorithm can be applied to future space borne submillimeter wave radiometers to jointly retrieve mass parameters of ice, snow, and graupel.

Coupling of terahertz light with nanometre-wavelength magnon modes via spin–orbit torque

Spin-based technologies can operate at terahertz frequencies but require manipulation techniques that work at ultrafast timescales to become practical. For instance, devices based on spin waves, also known as magnons, require efficient generation of high-energy exchange spin waves at nanometre wavelengths. To achieve this, a substantial coupling is needed between the magnon modes and an electro-magnetic stimulus such as a coherent terahertz field pulse. However, it has been difficult to excite non-uniform spin waves efficiently using terahertz light because of the large momentum mismatch between the submillimetre-wave radiation and the nanometre-sized spin waves. Here we improve the light–matter interaction by engineering thin films to exploit relativistic spin–orbit torques that are confined to the interfaces of heavy metal/ferromagnet heterostructures. We are able to excite spin-wave modes with frequencies of up to 0.6 THz and wavelengths as short as 6 nm using broadband terahertz radiation. Numerical simulations demonstrate that the coupling of terahertz light to exchange-dominated magnons originates solely from interfacial spin–orbit torques. Our results are of general applicability to other magnetic multilayered structures, and offer the prospect of nanoscale control of high-frequency signals.

Erratum to: Response of a SINIS Detector with Electron Cooling to Submillimeter-Wave Radiation

Page 542, the sentence under Fig. 3 should read as follows: According to Fig. 3, the maximum radiation response is 2 × 109 V/W, which is 2.5 times greater than that at approximately the same power absorbed by the detector, but given in [3] at a substrate temperature of 0.2 K.

Response of a SINIS Detector with Electron Cooling to Submillimeter-Wave Radiation

Response of a SINIS Detector with Electron Cooling to Submillimeter-Wave Radiation

Simulation of Microwave and Sub-millimeter Wave Radiation from 1 to 3000GHz of Planetary Atmosphere

为模拟行星大气在微波elax-elax亚毫米波波段的辐射传输特性，利用逐线积分方法计算行星大气中各气体成分在1~3000GHz的吸收系数.基于HITRAN数据库中各气体分子的跃迁频率及线强等参数，有效模拟了各气体分子在此频段内的吸收特征，并与常用的微波elax-elax亚毫米波大气辐射传输模式进行对比.分析地球大气的组成及特性，利用辐射传输方程模拟临边探测方式下不同频段的大气辐射亮温.研究结果可为后续地球乃至行星大气成分探测模拟、频带选择以及大气成分廓线反演提供模型及理论依据.

Safety Checkpoints

With the advent of new applications such as more sophisticated airport security scanners, people will increasingly be exposed to millimeter-and submillimeter-wave radiation. While the question of whether non-ionizing electromagnetic radiation has any nonthermal biological effects remains subject to debate, the thermal effects of electromagnetic radiation are clearly evident. Based on studies performed predominantly at lower frequencies, safety limits have been set by the International Commission for Non-Ionizing Radiation Protection [1]. Between 2 and 300 GHz, the safety limit in terms of power flux density is 1 mW/cm2. This safety limit is derived from the basic limits defined for a specific absorption rate (SAR) between 100 kHz and 10 GHz of 0.08 W/kg for a body overall and of 2 W/kg for a limb. These SAR values exhibit a safety factor of 50 with regard to previously proven thermal health effects.

The fundamental rotational interval of para-H2 (+) by MQDT-assisted Rydberg spectroscopy of H2.

Transitions from selected nd Rydberg states of H2 to n'p/f Rydberg series converging on the lowest two (N(+) = 0 and 2) rotational levels of the X(+) (2)Σg (+) (v(+) = 0) ground state of para-H2 (+) have been measured in the range 1-7.4 THz using a laser-based, pulsed, narrow-band source of submillimeter-wave radiation. The analysis of the spectra by multichannel quantum-defect theory (MQDT) has allowed a complete interpretation of the fine structures of the Rydberg series and their dependence on the principal quantum number. The extrapolation of the series to their limits with MQDT has enabled the determination of the first rotational interval of para-H2 (+), which is 174.236 71(7) cm(-1) (5 223 485.1(2.3) MHz).

Study on Optical Filter Heating in Background Limited Detector Experiments

Cryogenic test setups with controlled stray light environments capable of reaching ultra-low radiative background levels are required to test far infrared (FIR) and submillimeter (sub-mm) wave radiation detectors for future space based observatories. In recent experiments (Nature Commun 5:3130, 2014), in which 1.54 THz radiation was coupled onto an antenna-coupled kinetic inductance detector (KID), we found a higher than expected optical loading. We show that this can be explained by assuming heating of the metal mesh IR filters and re-radiation onto the KID. Note that the total power from the cryogenic black body source used in the experiments (at T = \(3\)–\(25\) K) is much larger than the power inside the \(1.5\)–\(1.6\) THz band we use to calibrate our detector. The out-of-band radiation can have up to 5 orders of magnitude more power than inside the \(1.5\)–\(1.6\) THz band of interest. A strategy to mitigate the filter heating problem is presented, and when it is implemented, the validated upper limit for stray light at the detector level is down to few aW.

High-resolution laser spectroscopy between 0.9 and 14.3 THz in a supersonic beam: Rydberg-Rydberg transitions of atomic Xe at intermediate n values

A laser-based, pulsed, narrow-band source of submillimeter-wave radiation has been developed that is continuously tunable from 0.1 THz to 14.3 THz. The source is based on difference-frequency mixing in the nonlinear crystal trans-4(')-(dimethylamino)-N-methyl-4-stilbazolium tosylate. By varying the pulse length, the bandwidth of the submillimeter-wave radiation can be adjusted between 85 MHz and 2.8 MHz. This new radiation source has been integrated in a vacuum-ultraviolet-submillimeter-ware double-resonance spectrometer, with which low-frequency transitions of atoms and molecules in supersonic beams can be detected mass-selectively by photoionization and time-of-flight mass spectrometry. The properties of the radiation source and spectrometer are demonstrated in a study of 33f ← nd Rydberg-Rydberg transitions in Xe with n in the range 16-31. The frequency calibration of the submillimeter-wave radiation was performed with an accuracy of 2.8 MHz. The narrowest lines observed experimentally have a full-width at half-maximum of ∼3 MHz, which is sufficient to fully resolve the hyperfine structure of the Rydberg-Rydberg transitions of (129)Xe and (131)Xe. A total of 72 transitions were measured in the range between 0.937 THz and 14.245 THz and their frequencies are compared with frequencies calculated by multichannel quantum defect theory.

Toward high-sensitivity and high-resolution submillimeter-wave video imaging

Against a background of newly emerged security threats, the well-established idea of utilizing submillimeter-wave radiation for personal security screening applications has recently evolved into a promising technology. Possible application scenarios demand sensitive, fast, flexible and high-quality imaging techniques. At present, best results are obtained by passive imaging using cryogenic microbolometers as radiation detectors. Building upon the concept of a passive submillimeter-wave stand-off video camera introduced previously, we present the evolution of this concept into a practical application-ready imaging device. This has been achieved using a variety of measures such as optimizing the detector parameters, improving the scanning mechanism, increasing the sampling speed, and enhancing the image generation software. The camera concept is based on a Cassegrain-type mirror optics, an optomechanical scanner, an array of 20 superconducting transition-edge sensors operated at a temperature of 450 to 650 mK, and a closed-cycle cryogen-free cooling system. The main figures of the system include: a frequency band of 350±40 GHz, an object distance of 7 to 10 m, a circular field of view of 1.05 m diameter, and a spatial resolution in the image center of 2 cm at 8.5 m distance, a noise equivalent temperature difference of 0.1 to 0.4 K, and a maximum frame rate of 10 Hz.

Optical characterization of the quantum capacitance detector at 200 μm

We present the optical characterization at 200 μm wavelength of an antenna coupled quantum capacitance detector (QCD), a cryogenic detector based on a single Cooper pair box (SCB). The response of the device to a cryogenic blackbody source and the device noise have been measured giving an optical noise-equivalent power (NEP) at a readout frequency of 10 kHz of 2 × 10−17 W/Hz1/2 at an optical loading of 17 fW. For optical loadings of a few pW, the optical NEP was on the order of 10−16 W/Hz1/2, demonstrating that QCDs could already work as detectors for far-infrared and submillimeter wave radiation in a ground based telescope.

Multichannel Radiometric System for Registering Submillimeter-Wave Radiation at Beam-Plasma Interaction

We describe a 4-channel radiometric system developed for registering pulses of submillimeter-wave radiation in the frequency band 210-530 GHz. The system employs a quasi-optical scheme with polarization demultiplexed channels, wherein subsequent frequency filtering and spectral signal detection are provided by quasi-optical bandpass filters and high-speed Schottky detectors respectively. The system has frequency resolution of the channels ~30-50 GHz and response time better than 2 ns and destined for registering emission of electromagnetic radiation in the vicinity of a double plasma frequency, which is produced at turbulent heating of dense plasma in the multimirror magnetic trap GOL-3 by a powerful relativistic electron beam of 10 microsecond duration.

Jumping the Gunn

A new solid-state source of submillimetre-wave radiation has been created at the University of Leeds in the UK. By extracting the third-harmonic from an InP Gunn device, Dr Heribert Eisele at the Institute of Microwaves and Photonics is the first to demonstrate that a single room-temperature semiconductor device, which can simply be connected to a power supply such as a battery, can generate significant power at 480 GHz. This result could lead to inexpensive, reliable and compact oscillators which would greatly benefit a range of applications, including wireless networks with greatly increased bandwidth.

Compact two-stream generator of millimeter- and submillimeter-wave radiation

A novel approach to producing short-wavelength radiation is proposed, where two offset electron beams of slightly different energies are merged within a dipole magnet and interact. With proper modulation of the two beams, the two-stream instability can be exploited to efficiently generate narrow-band millimeter-wave radiation. This type of source dispenses with the problematic structure required in other generation mechanisms and eliminates complex machining, delicate alignment, expensive parts, and catastrophic failures. Preliminary simulations have shown promise for a 100-GHz design, which is also scalable to 1THz. This concept requires only low-voltage, low-current electron beams and could lead to a device capable of generating up to 100W at 1THz, while remaining simple and compact.

Coherent Multiple Charge Transfer in a Superconducting NbN Tunnel Junction

We measured shot noise and submillimeter-wave response in a superconducting NbN tunnel junction that had a subharmonic gap structure on the current-voltage (I-V) curve. We found that the observed effective charge, defined from the noise-current ratio, tends to a steplike function of voltage. In the presence of submillimeter-wave radiation of frequency v, novel step structures spaced by hv/2e below and above the half-gap voltage clearly appeared on the I-V curve, overlapping the ordinary photon-assisted tunneling steps spaced by hv/e. Observation of these features provides clear evidence that coherent multiple Andreev reflection processes occur in the NbN tunnel junction with low barrier transparency.

Generation of Submillimeter-Wave Radiation with GaAs Tunnett Diodes and InP Gunn Devices in a Second or Higher Harmonic Mode

Efficient second-harmonic power extraction was demonstrated recently with GaAs TUNNETT diodes up to 235 GHz and with InP Gunn devices up to 325 GHz. This paper discusses the latest theoretical and experimental results from second-harmonic power extraction at submillimeter-wave frequencies and explores the potential of using power extraction at higher harmonic frequencies to generate continuous-wave radiation with significant output power levels at frequencies above 325 GHz. Initial experimental results are more than 50 /spl mu/W at 356 GHz from a GaAs TUNNETT diode in a third-harmonic mode and at least 0.2-5 /spl mu/W in the frequency range 400-560 GHz from InP Gunn devices in a third or higher harmonic mode. The spectral output of these submillimeter-wave sources was analyzed with a simple Fourier-transform terahertz spectrometer and, up to 415 GHz, with a spectrum analyzer and appropriate harmonic mixers.

Application of the Gyrotron FU II Submillimeter Wave Radiation Source to Plasma Scattering Measurements

Gyrotron FU II has been successfully applied as a submillimeter wave radiation source to plasma scattering measurements on the Compact Helical System (CHS) in National Institute for Fusion Science (NIFS) in Japan. The gyrotron operates in a long pulse mode (the pulse width is about 600 ms) at a frequency of about 350 GHz (the corresponding wavelength is 0.85 mm). The output power is about 110 W. The output power is transmitted along a circular waveguide system and converted to a Gaussian-like beam by a quasi-optical antenna. After that, the beam is directed onto the CHS plasma and the scattered signal is detected by a homodyne detection system. The frequency and the wave number of the scattered signal are analyzed. The results suggest that a broad band low frequency (several tens to several hundreds kHz) density fluctuation is excited in the CHS plasma only during neutral beam injection (NBI) or ion cyclotron resonance (ICR) heating.

Design Considerations for Powerful Continuous-Wave Second-Cyclotron-Harmonic Coaxial-Cavity Gyrotrons

The mode-selection ability of coaxial gyrotron cavities with a longitudinally corrugated inner conductor is systematically examined, as regards powerful, continuous-wave (CW) operation at the second harmonic of the electron-cyclotron frequency. Suitable cavity geometries for the excitation of a high-order mode of eigenvalue around 100 at the second harmonic have been identified, employing coaxial inserts both of high and of low conductivity. Focusing on submillimeter-wave radiation, we investigate frequency and CW limitations on efficiency, output power, and cavity machining and we present pertinent design considerations. The feasibility of powerful and efficient, CW, submillimeter-wave operation of coaxial cavities at the second harmonic is illustrated by two realistic designs of a second-harmonic gyrotron delivering a power over 100 kW CW at 340 GHz with electronic efficiency around 29%. The performance of the designs is confirmed by independent numerical simulations, which in addition show weak sensitivity to a spread in the electron velocity ratio.

Development of High-Frequency, Highly Stable Gyrotrons as Millimeter to Submillimeter Wave Radiation Sources

High-frequency, frequency-tunable, medium-power gyrotrons (Gyrotron FU series) are being developed in Fukui University as millimeter to submillimeter wave sources. The gyrotron series has achieved frequency tuneability in a wide range from 38 to 889 GHz and medium output power from 0.1 kW to several kilowatts in pulse mode (the pulse width is 1 ms and repetition rate 1 Hz) and several tens of watts in CW mode. Stabilizations of both amplitude and frequency have been achieved, for the application to many fields. High purity mode operations by installation of carefully designed cavity were also achieved for highly efficient conversion to Gaussian beam. In this paper, the advances in recent few years are summarized.

Frequency multiplication of microwave radiation by propagating space-charge domains in a semiconductor superlattice

We report on frequency multiplication of microwave radiation by propagating space-charge domains in a semiconductor superlattice; the domains were due to a negative differential mobility of miniband electrons. We irradiated an arrangement of two GaAs/AlAs superlattices, mounted in a rectangular waveguide system, with microwave radiation (frequency near 70 GHz) and observed the generation of harmonics; the conversion of radiation power to the third harmonic showed a remarkable efficiency (5%). A theoretical study, based on a drift-diffusion model for the miniband electrons, suggests that the microwave radiation has driven the formation and annihilation of space-charge domains and that the resulting anharmonic current was the source of the frequency-multiplied radiation. Our results indicate that frequency multiplication by space-charge domains in a semiconductor superlattice can be exploited for efficient generation of submillimeter-wave radiation.