Abstract: We report the results of our study on the noise performance of a fast THz detector based on the repurpose of hot electron bolometer mixer (HEB). Instead of operating with an elevated bath temperature, microwave power is injected into the HEB device, which enhances the sensitivity of the detector and at the same time provide a mechanism for reading out impedance changes of the device induced by the modulated incident THz radiation [1]. We have demonstrated an improvement of the detector’s optical noise equivalent power (NEP). Furthermore, by introducing a homodyne readout scheme based on a room temperature microwave mixer, the dynamic range of the detector is increased. The HEB devices used in this work were made of 4 nm thick NbN film. The detector chips were installed into a waveguide mixer block fitted with a corrugated horn, mounted on the cold plate of a liquid helium cryostat. The HEBs were operated at a bath temperature of 4.2 K. The signal beam was terminated on black bodies at ambient and liquid nitrogen temperatures. A chopper wheel placed in front of the cryostat window operating at a frequency of 1.48 kHz modulated the input load temperature of the detector. A cold mesh filter, centered at 830 GHz, was used to define the input signal power bandwidth. Microwave was injected through a broadband directional coupler inside the cryostat. Our experiments were mostly conducted at a pump frequency of 1.5 GHz. The reflected microwave power from the HEB device was fed into a cryogenic low noise amplifier (LNA). The output of the LNA was connected to the RF input port of a room temperature microwave mixer, which beat the reflected signal from the HEB using a copy of the original 1.5 GHz injection signal in a homodyne demodulation scheme. The amplitude of the detected power was measured by a lock-in amplifier, which was synchronized to the chopper frequency. Preliminary results yield an optical NEP of ~1 pW/ Hz 1/2 which corresponds to an improvement of a factor of 3 compared to [1], driven mainly by a lowering of the system noise floor. The dynamic range was also increased by similar amount. References 1. A. Shurakov et al. “A Microwave Pumped Hot Electron Bolometric Direct Detector,” submitted on Oct 18, 2013 to Appl. Phys. Let.

Abstract: Using time-resolved terahertz (THz) spectroscopy we performed direct studies of the photoinduced suppression and recovery of the SC gap in a conventional SC NbN. Both processes are found to be strongly temperature and excitation density dependent. The analysis of the data with the established phenomenological Rothwarf-Taylor model enabled us to determine the important microscopic constants: the Cooper pair-breaking rate via phonon absorption and the bare quasiparticle recombination rate. From the latter we were able to extract the dimensionless electron-phonon coupling constant, λ=1.1±0.1, in excellent agreement with theoretical estimates. The technique also allowed us to determine the absorbed energy required to suppress SC, which in NbN equals the thermodynamic condensation energy (in cuprates the two differ by an order of magnitude). Finally, we present the first studies of dynamics following resonant excitation with intense narrow band THz pulses tuned to above and below the superconducting gap. These suggest an additional process, particularly pronounced near Tc, that could be attributed to amplification of SC via effective quasiparticle cooling.

Abstract: We describe a novel method of operation of the HEB direct detector for use with a Fourier Transform Spectrometer. Instead of elevating the bath temperature, we have measured the RF response of waveguide HEB mixers by applying microwave radiation to select appropriate bias conditions. In our experiment, a microwave signal is injected into the HEB mixer via its IF port. By choosing an appropriate injection level, the device can be operated close to the desired operating point. Furthermore, we have shown that both thermal biasing and microwave injection can reproduce the same spectral response of the HEB mixer. However, with the use of microwave injection, there is no need to wait for the mixer to reach thermal equilibrium, so characterisation can be done in less time. Also, the liquid helium consumption for our wet cryostat is also reduced. We have demonstrated that the signal- to-noise ratio of the FTS measurements can be improved with microwave injection.

Abstract: Millimetron (official RosKosmos name ”Spectrum-M”) is a part of ambitious program called Spectrum intended to cover the whole electromagnetic spectrum with world class facilities. It is an approved mission included in Russian space program with the launch date in 2017..2019 time frame. The Millimetron satellite has a deployable 12 m diameter antenna with inner solid 4..6 m dish and a rim of petals. The mirror design is largely based on Radioastron mission concept that will be launched in 2009. If the antenna is passively cooled by radiation to open space, it would operate at approx. 50 K surface temperature, due to presence of a deployable three layer radiation screen. As a goal, there is a consideration of active cooling of antenna to 4 K, but this will depend on resources available to the project. Lagrangian libration point L2 considered for Millimetron orbit. There are four groups of scientific instruments envisioned: SVLBI instruments Space-Earth VLBI. It will allow to achieve unprecedented spatial resolution. Millimetron mission will attempt to achieve a mm/submm wave SVLBI. For that purpose, a SVLBI instrument covering selected ALMA bands and a standard VLBI band is envisioned, accompanied by a maser reference oscillator, a data digitizing and memory system, and a high speed data transmission link to ground. The ALMA bands can be extended to cover water lines if detector technology allows. Type of detector – heterodyne. Photometer/polarimeter. Recent progress in direct detector cameras with low spectral resolution, allows to propose a large format (5-10 kPixel) photometer camera on board of Millimetron mission. This camera can cover 0.1 – 2 THz region (with adequate amount of pixels per each subband). Wide band moderate resolution imaging spectrometer. Wide band moderate R = 1000 imaging spectrometer type instrument similar to SPICA SAFARI is planned, taking advantage of large cooled dish. It will cover the adequate spectral range allowable by antenna and will also work below 1 THz, as no ground instrument can have a cold main dish. High resolution spectrometer. For high resolution spectroscopy a heterodyne instrument is proposed, conceptually similar to HIFI on Herschel. This instrument will cover interesting frequency spots in 0.5..4 THz frequency range (using central part of antenna for higher frequency). It is sure that advances in LO and mixer technology will allow this frequency coverage.