We demonstrate for the first time the closure of an electronic phase lock loop for a continuous-wave quantum cascade laser (QCL) at 1.5 THz. The QCL is operated in a closed cycle cryo cooler. We achieved a frequency stability of better than 100 Hz, limited by the resolution bandwidth of the spectrum analyser. The PLL electronics make use of the intermediate frequency (IF) obtained from a hot electron bolometer (HEB) which is downconverted to a PLL IF of 125 MHz. The coarse selection of the longitudinal mode and the fine tuning is achieved via the bias voltage of the QCL. Within a QCL cavity mode, the free-running QCL shows frequency fluctuations of about 5 MHz, which the PLL circuit is able to control via the Stark-shift of the QCL gain material. Temperature dependent tuning is shown to be nonlinear, and of the order of -16 MHz/K. Additionally we have used the QCL as local oscillator (LO) to pump an HEB and perform, again for the first time at 1.5 THz, a heterodyne experiment, and obtain a receiver noise temperature of 1741 K.

In high temperature fusion plasmas the transport of energy and particles is commonly believed to be driven by turbulence. Turbulence quantities as correlation length and decorrelation time are important for the confinement properties of a plasma. Besides other diagnostics, correlation reflectometry has proven to be a suitable tool for the measurement of turbulence properties. At the medium sized Toroidal EXperiment for Technical Oriented Research (TEXTOR) the existing correlation reflectometry has been recently upgraded. A new reflectometer based on a microwave synthesizer has been developed and installed for the investigation of turbulence properties in a fusion plasma. Together with the existing reflectometer the measurement of radial correlation length and decorrelation time becomes available. Both reflectometers are computer controlled and allow to program individual frequency sequences and the duration of each frequency step. With the existing poloidal antenna array at θ=0° and on top of the vacuum vessel, the system allows the measurement of radial correlation and poloidal correlations at the same time. First experiments have been performed and the results on the radial correlation length of density fluctuations in a fusion plasma are presented.

A large-aperture design for terahertz traveling-wave photomixers, continuously pumped free space by two detuned diode lasers, is proposed and experimentally verified for devices based on low-temperature-grown GaAs ͑LT-GaAs͒. It combines the advantages of conventional interdigitated small-area structures and traveling-wave devices. An output power of 1 µW at the mixing frequency of 1 THz was measured in initial testing, which meets local oscillator power requirements for superconducting heterodyne mixer devices. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1884262͔ Continuous-wave photonic mixer terahertz ͑THz͒ radiation sources are basically fast photoconductive switches modulated with the beat frequency of two detuned nearinfrared ͑IR͒ diode lasers. 1 One of the fastest materials to follow the beat frequency in the THz range is lowtemperature-grown GaAs ͑LT-GaAs͒ with photocarrier trap times down to 100 fs. 2 Besides the corresponding unmatched tuning range, photomixers are also particularly attractive for their all-solid state, noncryogenic, low power consumption, and relative low-cost approach. Therefore, they are interesting as local oscillators ͑LOs͒ for heterodyne submillimeter or terahertz receivers 3,4 based on superconductor-insulatorsuperconductor junctions ͑SIS͒ or hot-electron bolometers ͑HEB͒. Providing the power necessary for these mixers above 1 THz is a challenge, but is within reach of the current development of LT-GaAs photomixers. The latest report for smallest-area SIS junctions is p pump Ϸ 0.1 W at 1 THz at the mixer 5 ͑with a theoretical frequency dependency of p pump ϳ f 2 ͒, and for HEBs p pump Ϸ 0.2 W at 1.8 THz at the mixer Si lens 6 ͑with an expected frequency dependency of p pump ϳ f͒.With small-area photonic mixers, consisting of interdigitated metal-semiconductor-metal electrode structures, hereafter MSM ͓Fig 1͑c͔͒, it proved to be difficult to routinely reach these power levels above 1 THz. If small-area mixers are used with broadband antennas of load resistance, R a , the uncompensated device capacitance, C, introduces a rolloff, 2,7 ϳ1/͓1+͑2 · R a C · ͒ 2 ͔, which is usually around 1 THz. However, if it is located at the footpoint of resonant antennas, capacitance up to a certain value may be tuned out by the antenna inductance, 8 and the terahertz power at the resonance frequency, res , follows just the unavoidable rolloff, ϳ1/͓1+͑2 · e · res ͒ 2 ͔, given by the effective response time, e , for the electronic current seen locally at the electrodes. This in turn is not identical to the photoelectron trap time, 1,9-11 but is elongated by the intrinsic transit time of a space-charge dominated current pulse initiated by the shortliving photoelectrons between the electrodes. 9,12 The restriction on the capacitance first imposes upper limits on the device area and number of fingers. Because of a destruction intensity of Ͻ1.5 mW/ m 2 for LT-GaAs this limits IR pump power to Ͻ100 mW, so that improved passive thermal sinking is essential for small-area mixers, 3,13 unless coolin...

Context. The THz atmospheric "windows", centered at roughly 1.3 and 1.5 THz, contain numerous spectral lines of astronomical importance, including three high-J CO lines, the [N II] line at 205 µm, and the ground transition of para-H 2 D + . The CO lines are tracers of hot (several 100 K), dense gas; [N II] is a cooling line of diffuse, ionized gas; the H 2 D + line is a non-depleting tracer of cold (∼20 K), dense gas. Aims. As the THz lines benefit the study of diverse phenomena (from high-mass star-forming regions to the WIM to cold prestellar cores), we have built the CO N + Deuterium Observations Receiver (CONDOR) to further explore the THz windows by ground-based observations. Methods. CONDOR was designed to be used at the Atacama Pathfinder EXperiment (APEX) and Stratospheric Observatory For Infrared Astronomy (SOFIA). CONDOR was installed at the APEX telescope and test observations were made to characterize the instrument.Results. The combination of CONDOR on APEX successfully detected THz radiation from astronomical sources. CONDOR operated with typical T rec = 1600 K and spectral Allan variance times of ∼30 s. CONDOR's "first light" observations of CO 13−12 emission from the hot core Orion FIR 4 (= OMC1 South) revealed a narrow line with T MB ≈ 210 K and ∆V ≈ 5.4 km s −1 . A search for [N II] emission from the ionization front of the Orion Bar resulted in a non-detection. Conclusions. The successful deployment of CONDOR at APEX demonstrates the potential for making observations at THz frequencies from ground-based facilities.

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