Radio astrometry from the Moon

An array of three radio telescopes on the Moon, separated by 100-1000 km, could measure the positions of compact radio sources 50-100 times more accurately than can be done on Earth. These measurements would form an all-sky reference frame of extreme precision (5-10 micro-arcsec) and stability, with applications to the dynamics of the solar system, our galaxy, and nearby galaxies

The search for reference sources for delta VLBI navigation of the Galileo spacecraft

A comprehensive search was made in order to identify celestial radio sources that can be used as references for navigation of the Galileo spacecraft by means of VLBI observations. The astronomical literature was seached for potential navigation sources, and several VLBI experiments were performed to determine the suitability of those sources for navigation. The results of such work performed since mid-1983 is reported. A summary is presented of the source properties required, the procedures used to identify candidate sources, and the results of the observations of these sources. The lists of souces presented are not meant to be taken directly and used for VLBI navigation, but they do provide a means of identifying the radio sources that could be used at various positions along the Galileo trajectory. Since the reference sources nearest the critical points of Jupiter encounter and probe release are rather weak, it would be extremely beneficial to use a pair of 70-m antennas for the VLBI measurements

Very long baseline interferometry using a radio telescope in Earth orbit

Successful Very Long Baseline Interferometry (VLBI) observations at 2.3 GHz were made using an antenna aboard an Earth-orbiting spacecraft as one of the receiving telescopes. These observations employed the first deployed satellite (TDRSE-E for East) of the NASA Tracking and Data Relay Satellite System (TDRSS). Fringes were found for 3 radio sources on baselines between TDRSE and telescopes in Australia and Japan. The purpose of this experiment and the characteristics of the spacecraft that are related to the VLBI observations are described. The technical obstacles to maintaining phase coherence between the orbiting antenna and the ground stations, as well as the calibration schemes for the communication link between TDRSE and its ground station at White Sands, New Mexico are explored. System coherence results and scientific results for the radio source observations are presented. Using all available calibrations, a coherence of 84% over 700 seconds was achieved for baselines to the orbiting telescope

Tunneling Between Two-Dimensional Electron Gases in a Strong Magnetic Field

We have measured the tunneling between two two-dimensional electron gases at high magnetic fields $B$, when the carrier densities of the two electron layers are matched. For filling factors $\nu<1$, there is a gap in the current-voltage characteristics centered about $V=0$, followed by a tunneling peak at $\sim 6$~mV. Both features have been observed before and have been attributed to electron-electron interactions within a layer. We have measured high field tunneling peak positions and fitted gap parameters that are proportional to $B$, and independent of the carrier densities of the two layers. This suggests a different origin for the gap to that proposed by current theories, which predict a $\sqrt{B}$ dependence.Comment: 9 pages, cond-mat/yymmnn

Active Based-Metasurfaces for Mid-Infrared Optoelectronics Devices

We develop low-temperature (450°C) deposition conditions for vanadium di-oxide phase change material. It permits implementation of tunable mid-infrared meta-surfaces on quantum cascade lasers based heterostructures

Photothermoelastic response of zincblende crystals to radiation from a THz-frequency quantum cascade laser

We investigate the photothermoelastic response of ZnTe and GaP crystals irradiated by THz-frequency radiation from a quantum cascade laser. We present a full theoretical description of this interaction that agrees well with the measured response

Gas spectroscopy with integrated frequency monitoring, through self-mixing in a terahertz quantum-cascade laser

Terahertz-frequency quantum cascade lasers (THz QCLs) [1] have been used as compact, yet powerful sources of THz radiation in a range of gas spectroscopy techniques [2], including both in situ active sensing [3] and heterodyne radiometry [4]. A novel approach has recently been demonstrated, based on self-mixing interferometry (SMI) in a QCL [5]. This effect occurs when radiation is fed back into the QCL from an external reflector [6]. The resulting interference within the QCL perturbs the terminal voltage, and the absorption spectrum of a gas within the external cavity may be inferred from the amplitude of these perturbations. This eliminates the need for an external THz detector, doubles the interaction-length for absorption spectroscopy, and the scanning speed can potentially be raised to the time-scale of the QCL lasing dynamics (~10 GHz). A limitation reported in the previously published work is that the QCL emission frequency was inferred from prior FTIR measurements of the unperturbed laser. However, the actual system QCL frequency is perturbed by SMI feedback effects and is therefore dependent on the gas absorption crosssection, leading to apparent frequency shifts in the measured spectral lines. In this work, we demonstrate a technique to measure the frequency directly by extending the external cavity length modulation to 200-mm using a motorised linear translation stage [Fig. 1(a)]. The QCL in this system can be tuned by adjusting the drive current, over a 1.5 GHz bandwidth, around a centre frequency of 3.394 THz. Fig. 1(b) shows the transmitted radiation intensity through a 73-cm gas cell with TPX windows, filled with methanol vapour at a pressure of 2 Torr, as a function of drive current, measured using a pyroelectric detector. Two absorption lines are clearly resolved. By replacing the detector with a planar mirror, and recording the QCL voltage modulation as a function of stage position, a full interferogram can be acquired, and a Fourier transform can then be used to determine the laser frequency and the amplitude of the transmitted signal [Fig. 1(c)]. In this paper, we will demonstrate the reconstruction of the methanol absorption spectrum, with direct measurement of the laser frequency using this technique

Detector-free gas spectroscopy, with integrated frequency monitoring, through self-mixing in a terahertz quantum-cascade laser

Terahertz-frequency quantum cascade lasers (THz QCLs) have been used as compact, yet powerful THz radiation sources in a range of gas spectroscopy techniques, including both in situ active sensing and heterodyne radiometry. However, all such approaches require external THz instrumentation (detectors or mixers) in addition to the QCL, thus raising the system complexity and cost. A partial solution has recently been demonstrated, based on self-mixing interferometry (SMI) in a QCL, which occurs when radiation is fed back into the QCL from an external reflector. The resulting interference within the QCL perturbs the terminal voltage, and the absorption spectrum of a gas within the external cavity may be inferred from the amplitude of these perturbations. This both eliminates the need for an external THz detector or mixer, doubles the interaction-length for absorption spectroscopy, and the scanning speed can potentially be raised to the time-scale of the QCL lasing dynamics (~10 GHz). A limitation reported in the previous work is that the QCL emission frequency must be inferred from prior spectral measurements of the unperturbed laser, which introduces two principal problems: (1) additional THz instrumentation is still required, and (2) the system QCL frequency is itself perturbed by feedback effects, leading to apparent frequency shifts in the measured spectral lines. In this work, we demonstrate a technique to measure the QCL frequency directly by extending the external cavity length modulation to 400-mm using a motorised linear translation stage. By recording the QCL voltage modulation as a function of stage position, a full interferogram can be acquired, and a Fourier transform can then be used to determine the laser frequency and the amplitude of the transmitted signal. The QCL was shown to be tunable by adjusting the drive current over a 1.5-GHz bandwidth, around a centre frequency of 3.4052 THz. To demonstrate gas spectroscopy, a 1-m gas cell with TPX windows was filled with methanol vapour, and the transmitted QCL power was measured as a function of drive current through SMI analysis. Two absorption lines are clearly resolved. The technique was found to be accurate to partial methanol pressures of < 10 mTorr. In conclusion, we have demonstrated an accurate and low-cost THz gas spectroscopy technique based on self-mixing in a THz QCL, without the need for any external THz mixer or detector, or a priori calibration of the QCL emission frequency

A test of water vapor radiometer-based troposphere calibration using VLBI observations on a 21-kilometer baseline

Simultaneous very long baseline interferometry (VLBI) and water vapor radiometer (WVR) measurements on a 21 km baseline showed that calibration by WVRs removed a significant fraction of the effect of tropospheric delay fluctuations for these experiments. From comparison of the residual delay variations within scans and between scans, the total tropospheric contribution t the delay residuals for each of the three 5 to 20 hour sessions was estimated as 1, 17, and 10%, with the first value being uncertain. The observed improvement in rms residual delay from WVR calibration during these three sessions was 4, 16, and 2%, respectively. The improvement is consistent with the estimated 2 to 3 mm path delay precision of current WVRs. The VLBI measurements, of natural radio sources, were conducted in April and May 1993 at Goldstone, California. Dual-frequency (2.3 and 8.4 GHz) observations were employed to remove the effects of charged particles from the data. Measurements with co-pointed WVRs, located within 50 m of the axis of each antenna, were performed to test the ability of the WVRs to calibrate line-of-sight path delays. Factors that made WVR performance assessment difficult included (1) the fact that the level of tropospheric fluctuations was smaller than is typical for Goldstone during these experiments and (2) VLBI delay variations on longer time scales (i.e., over multiple scans) contained uncalibrated instrumental effects (probably a result of slow temperature variations in the VLBI hardware) that were larger than the tropospheric effects