Abstract: Direct measurements of the thermal boundary resistance were performed by means of the stationary method. In this approach the temperature of an electrically heated film is controlled by its dc resistance while an additional film on the same substrate is used as a thermometer monitoring substrate temperature. The temperature field in the substrate is then calculated to deduce the Kapitza temperature step at the interface between the heated strip and the substrate. The main statement of all afore-said papers is that experimental values of the thermal boundary resistance are too large to be explained by the acoustic mismatch model. In this paper we investigate transparency of YBaCuO film/substrate interfaces for thermal phonons by means of photoresponse measurements. We show that our data are in reasonable agreement with the acoustic mismatch theory.

Abstract: Fast-operating picosecond detector of electromagnetical radiation is developed on the basis of fine superconducting film of niobium nitride with high sensitivity within spectral range from millimetric waves up to visible light. Detector sensitive element represents structure covering narrow parallel strips with micron sizes included in the rupture of microstrip line. Detecting ability of the detector and time constant measured using amplitude-simulated radiation of reverse wave tubes and pulse radiation of picosecond gas and solid-body lasers, constitute D*≅1010 W-1·cm·Hz-1/2 and τ≤5 ps respectively, at 10 K temperature. The expected value of time constant of the detector at 10 K obtained via extrapolation of directly measured dependence that is, τ ∝ τ-1, constitutes 20 ps. Experimental data demonstrate that detection mechanism is linked with electron heating effect.

Abstract: The results of experimental investigation of fast-action 5HF-limiter are presented; the limiter is based on the utilization of electron hetaing phenomenon in thin superconducting films. The design of SHF-limiter, which is intended for operation at liquid helium temperatures and which has the form of a section of superconducting NbN microstrip line for 1-12 GHz rang, is described.

Abstract: Measurements of transmission and photoconductivity spectra in submillimeter wave length range as well as of capacity C and conductivity G in the region of acoustic frequencies of metal-dielectric-La2CuO4 system at low temperatures are performed using La2CuO4 monocrystals. Optical spectra posses a threshold character, a sharp decrease of transmission and photocoductivity signal occurs in the energy region hν>1.5 MeV. C(ω,T) and G(ω, T) dependences have a universal form typical of Debye type relaxation processes. Relaxation time dependence is of thermoactivated character τ(T)∼exp(ξ/T) with the gap value ξ≅2 meV. It is assumed that excitations with characteristic energy of ∼2 meV exist in La2CuO4. A possible nature of the detected low-energy excitations is discussed.

Abstract: We present a simple quantum detector tomography protocol, which allows, without ambiguities, to measure the two-spot detection efficiency and extract the hot-spot interaction length of superconducting nanowire single photon detectors (SNSPDs) with unity intrinsic detection efficiency. We identify a significant parasitic contribution to the measured two-spot efficiency, related to an effect of the bias circuit, and find a way to rule out this contribution during data post-processing and directly in the experiment. From the data analysis for waveguide-integrated SNSPD, we find signatures of the saturation of the two-spot efficiency and hot-spot interaction length of order of 100 nm.

Abstract: Superconducting single-photon detectors (SSPDs) are nanostructured devices made from ultrathin superconducting films. They are typically operated at liquid helium temperature and exhibit high detection efficiency, in combination with very low dark counts, fast response time, and extremely low timing jitter, within a broad wavelength range from ultraviolet to mid-infrared (up to 6 mu m). SSPDs are very attractive for applications such as fiber-based telecommunication, where single-photon sensitivity and high photon-counting rates are required. We review the current state-of-the-art in the SSPD research and development, and compare the SSPD performance to the best semiconducting avalanche photodiodes and other superconducting photon detectors. Furthermore, we demonstrate that SSPDs can also be successfully implemented in photon-energy-resolving experiments. Our approach is based on the fact that the size of the hotspot, a nonsuperconducting region generated upon photon absorption, is linearly dependent on the photon energy. We introduce a statistical method, where, by measuring the SSPD system detection efficiency at different bias currents, we are able to resolve the wavelength of the incident photons with a resolution of 50 nm.

Abstract: Hot-electron single-photon counters based on long superconducting nanowires are starting to become popular in optical and infrared technologies due to their ultimately high sensitivity and very high response speed. We investigate intrinsic fluctuations in long NbN nanowires in the temperature range of 4.2 K-20 K, i.e. above and below the superconducting transition. These fluctuations are responsible for fluctuation resistivity and also determine the noise in practical devices. Measurements of the fluctuation resistivity were performed at low current densities and also in external magnetic fields up to 5 T. Above the BCS critical temperature T co the resistivity is well described by the Aslamazov-Larkin (AL) theory for two-dimensional samples. Below T co the measured resistivity is in excellent agreement with the Langer-Ambegaokar-McCumber-Halperin (LAMH) theory developed for one-dimensional superconductors. Despite that our nanowires of 100 nm width are two-dimensional with respect to the coherence length, our analysis shows that at relatively low current densities the one-dimensional LAMH mechanism based on thermally induced phase slip centers dominates over the two-dimensional mechanism related to unbinding of vortex-antivortex pairs below the Berezinskii-Kosterlitz-Thouless transition.

Abstract: We present a new class of ultrafast single-photon detectors for counting both visible and infrared photons. The detection mechanism is based on photon-induced hotspot formation, which forces the supercurrent redistribution and leads to the appearance of a transient resistive barrier across an ultrathin, submicrometer-width, superconducting stripe. The devices were fabricated from 3.5-nm- and 10-nm-thick NbN films, patterned into <200-nm-wide stripes in the 4 /spl times/ 4-/spl mu/m/sup 2/ or 10 /spl times/ 10-/spl mu/m/sup 2/ meander-type geometry, and operated at 4.2 K, well below the NbN critical temperature (T/sub c/=10-11 K). Continuous-wave and pulsed-laser optical sources in the 400-nm-to 3500-nm-wavelength range were used to determine the detector performance in the photon-counting mode. Experimental quantum efficiency was found to exponentially depend on the photon wavelength, and for our best, 3.5-nm-thick, 100-/spl mu/m/sup 2/-area devices varied from >10% for 405-nm radiation to 3.5% for 1550-nm photons. The detector response time and jitter were /spl sim/100 ps and 35 ps, respectively, and were acquisition system limited. The dark counts were below 0.01 per second at optimal biasing. In terms of the counting rate, jitter, and dark counts, the NbN single-photon detectors significantly outperform their semiconductor counterparts. Already-identified applications for our devices range from noncontact testing of semiconductor CMOS VLSI circuits to free-space quantum cryptography and communications.

Abstract: Results of development of single-photon receiving systems of visible, infrared and terahertz range based on thin-film superconducting nanostructures are presented. The receiving systems are produced on the basis of superconducting nanostructures, which function by means of hot-electron phenomena.

Abstract: An experimental study is made of the effect of intense radiation in the millimeter and submillimeter ranges on thin and narrow Nb films in the resistive state. It is found that the excess resistance resulting from radiation and the dependence of its relaxation time on radiation intensity and transport current can be explained in terms of the effect of electron heating. Quantitative agreement is obtained between the experimental data and a homogeneous electron heating model.

Abstract: We present recent results of the development of phonon cooled hot-electron bolometric (HEB) mixers for airborne and balloon borne terahertz heterodyne receivers. Three iomportant issues have been addresses: the quality of NbN films the HEB mixers were made from, the spectral properties of the HEB mixers and the local oscillator power required for optical operation. Studies with an atomic force microscope indicate, that the performance of the HEB mixer might have been effected by the microstructure of the NbN film. Antenna gain and noise temperature were investigated at terahertz frequencies for a HEB embedded in either log-spiral or twin-slot feed antenna. Comparison suggests that at frequencies above 3 THz the spiral feed provides better overall performance. At 1.6 THz, a power of 2.5 µW was required from the local oscillator for optimal operation of the HEB mixer.

Abstract: The spectral dependence of the quantum efficiency of superconducting NbN single-photon detectors integrated with quarter-wave resonators based on Si3N4, SiO2, and SiO layers has been studied.

Abstract: The creation of low noise heterodyne receivers for frequencies above 1 THz is in the urgentneed for radio astronomy, laser spectroscopy, plasma diagnostic, etc. In this paper we discussthe nonlinear effect related to hot electrons in superconductors, and their potential use in lownoise submilimeter wave mixer. We also discuss results achieved so far as well as possible futuredevelopments.

Abstract: The 3.5 nm thick-film, meander-structured NbN superconducting single-photon detectors have been implemented in the CMOS circuit-testing system based on the detection of near-infrared photon emission from switching transistors and have significantly improved the performance of the system. Photon emissions from both p- and n-MOS transistors have been observed.

Abstract: We present superconducting single-photon detectors (SSPDs) based on NbN thin-film nanostructures and operated at liquid helium temperatures. The SSPDs are made of ultrathin NbN films (2.5-4 nm thick, Tc= 9-11K) as meander-shaped nanowires covering the area of 10× 10 µm2. Our detectors are operated at the temperature well below the critical temperature Tc and are DC biased by a current Ib close to the meander critical current Ic. The operation principle of the detector is based on the use of the resistive region in a narrow ultra-thin superconducting stripe upon the absorption of an incident photon. The developed devices demonstrate high sensitivity and response speed in a broadband range from UV to mid-IR (up to 6 µm), making them very attractive for advanced optical technologies, which require efficient detectors of single quanta and low-density optical radiation.

Abstract: The gain bandwidth of NbN hot-electron bolometer terahertz mixers on electrically thin Si3N4/SiO2 membranes was experimentally investigated and compared with that of HEB mixers on bulk substrates. A gain bandwidth of 3.5 GHz is achieved on bulk silicon, whereas the gain bandwidth is reduced down to 0.6–0.9 GHz for mixers on 1.5 μm Si3N4/SiO2 membranes. We show that application of a MgO buffer layer on the membrane extends the gain bandwidth to 3 GHz. The experimental data were analyzed using the film-substrate acoustic mismatch approach.

Abstract: We have measured the noise performance and gain bandwidth of 35 Å thin NbN hot-electron mixers integrated with spiral antennas on silicon substrate lenses at 620 GHz. The best double-sideband receiver noise temperature is less than 1300 K with a 3 dB bandwidth of ≈5 GHz. The gain bandwidth is 3.2 GHz. The mixer output noise dominated by thermal fluctuations is 50 K, and the intrinsic conversion gain is about −12 dB. Without mismatch losses and excluding the loss from the beamsplitter, we expect to achieve a receiver noise temperature of less than 700 K.

Abstract: A quasi-optical superconducting niobium nitride (NbN) hot electron bolometer (HEB) mixer has been fabricated and measured in the terahertz (THz) frequency range of 0.5~2.52 THz. A receiver noise temperature of 2000 K at 2.52 THz has been obtained for the mixer without corrections. Also, the effect of a Parylene C anti-reflection (AR) coating on the silicon (Si) lens has been studied.

Abstract: We consider recent results in development of hot-electron bolometer mixers. Special attention is paid to optimization of the contacts between the antenna and the active area of a superconducting film. An important result in the study of the parasitic effect of direct detection is obtained during the measurement of the noise temperatures by the hot/cold load method. The latest results of studies of the waveguide hot-electron bolometer mixers and their successful practical applications are considered. Progress in development of high-frequency (over 1.3 THz) heterodyne receivers for several important international projects is discussed and new submillimeter radio astronomy projects ESPRIT and SAFIR are described.

Abstract: This Letter presents, to the best of our knowledge, a novel optical configuration for direct time-resolved measurements of luminescence from singlet oxygen, both in solutions and from cultured cells on photodynamic therapy. The system is based on the superconducting single-photon detector, coupled to the confocal scanner that is modified for the near-infrared measurements. The recording of a phosphorescence signal from singlet oxygen at 1270 nm has been done using time-correlated single-photon counting. The performance of the system is verified by measuring phosphorescence from singlet oxygen generated by the photosensitizers commonly used in photodynamic therapy: methylene blue and chlorin e6. The described system can be easily upgraded to the configuration when both phosphorescence from singlet oxygen and fluorescence from the cells can be detected in the imaging mode. Thus, co-localization of the signal from singlet oxygen with the areas inside the cells can be done.

Abstract: We perform measurements of the switching current distributions of three w approximately 120 nm wide, 4 nm thick NbN superconducting strips which are used for single-photon detectors. These strips are much wider than the diameter of the vortex cores, so they are classified as quasi-two-dimensional (quasi-2D). We discover evidence of macroscopic quantum tunneling by observing the saturation of the standard deviation of the switching distributions at temperatures around 2 K. We analyze our results using the Kurkijarvi-Garg model and find that the escape temperature also saturates at low temperatures, confirming that at sufficiently low temperatures, macroscopic quantum tunneling is possible in quasi-2D strips and can contribute to dark counts observed in single photon detectors. At the highest temperatures the system enters a multiple phase-slip regime. In this range single phase-slips are unable to produce dark counts and the fluctuations in the switching current are reduced.

Abstract: We report on the characterization of a superconducting nanowire detector for ions at low kinetic energies. We measure the absolute single-particle detection efficiency eta and trace its increase with energy up to eta = 100%. We discuss the influence of noble gas adsorbates on the cryogenic surface and analyze their relevance for the detection of slow massive particles. We apply a recent model for the hot-spot formation to the incidence of atomic ions at energies between 0.2 and 1 keV. We suggest how the differences observed for photons and atoms or molecules can be related to the surface condition of the detector and we propose that the restoration of proper surface conditions may open a new avenue for SSPD-based optical spectroscopy on molecules and nanoparticles.

Abstract: We report the observation of an ultrafast photoresponse of a high-T(c), film to far-infrared radiation pulses. The response of a sample, consisting of a current-carrying structured YBa(2)Cu(3)O(7-delta) film cooled to liquid-nitrogen temperature, was studied by use of ultrashort laser pulses from an optically pumped far-infrared laser in the frequency range from 0.7 to 7 THz. We found that the response time was limited by the time resolution, 120 ps, of our electronic registration equipment.

Abstract: We have carried out a high-resolution spectroscopic study of the absorption of submillimeter radiation by free excitons in germanium compressed along the [ 1 11 ] axis in a magnetic field parallel to the compression axis. In particular, we studied the splitting of the 1s- 2p transition in fields up to 6 kOe at T = 1.6 K, and observed a complex pattern in the Zeeman splitting which we believe is related to the effect of thermal motion of the excitons in a magnetic field on their internal structure (the magneto-Stark effect). The calculated submillimeter spectrum of excitons agrees with the experimental data. We predict that in a magnetic field the energy of the 2p, term is a minimum at a finite value of the exciton momentum perpendicular to the field-that is, the energy minimum forms a ring in momentum space. It follows that the density of states for this term must be a nonmonotonic function of the energy. A theory is developed of analogous phenomena in positronium.

Abstract: The effect of heating of electrons relative to phonons is observed and investigated in a superconducting film that is made resistive by current and by an external magnetic field. The effect is manifested by an increase of the film resistance under the influence of the electromagnetic radiation, and is not selective in the frequency band 10^10-10^15 Hz. The independence of the effect of frequency under conditions of strong scattering by static defects is attributed to the decisive role of electron-electron collisions in the distribution function. The experimentally obtained characteristic time of resistance variation near the superconducting transition corresponds to the relaxation time of the order parameter, while at lower temperatures and fields it corresponds to the time of the inelastic electron-phonon interaction.

Abstract: Measurements were made of the lifetimes rf of free carriers and the relaxation time 7, of the submillimeter impurity photoconductivity when carriers are captured by attracting shallow donors and acceptom in Ge. It is nod that in samples with capture-center concentration N,Z 10"cm-' the relaxation time 7, greatly exceeds rf in the temperature range 4.2-12 K. The measured values of 7,- are compared with the calculation of cascade recombination by the classical model. To evaluate the data on T,, the distinguishing features of this model are considered for the nonstationary case. The substantial difference betweea the values of rf and T, is attributed to re-emission of the carriers from the excited states of the shallow impurities.

Abstract: We investigated the spectrum of the submillimeter photoconductivity of p-Ge at helium temperatures and the effects of a magnetic field up to 40 kOe on the spectrum. A large number of lines of transitions between the excited states of the acceptors was observed, some of the lines were identified, and the energies of a number of spectral levels B, Al, Ga, In, and TI in Ge were identified. The results are compared with calculations and with experimental data obtained from the spectra of the photoexcitation of the ground state of the impurities. Using one transition as an example, we discuss the splitting of the excited states of acceptors in the magnetic field and under uniaxial compression.

Abstract: Results are presented of an investigation of free excitons in Ge in the submillimeter wavelength range for low as well as for high excitation levels when interaction between the excitons becomes important. The free-exciton energy spectrum is discussed. It is shown that the drop radii and their concentrations can be determined by measuring the temperature dependence of the free-exciton concentration. A section of the phase diagram is obtained in the 0.5-2.8 K temperature range for the free excitons+condensate system.

Abstract: NbN phonon-cooled hot electron bolometer mixers (HEBs) have been realized with negligible contact resistance between the bolometer itself and the contact structure. Using a combination of in situ cleaning of the NbN film and the use of an additional superconducting interlayer of a 10 nm NbTiN layer between the Au of the contact structure and the NbN film superior noise temperatures have been obtained as low as 950 K at 2.5 THz and 750 K at 1.9 THz. Here we address in detail the DC characterization of these devices, the interface transparencies between the bolometers and the contacts and the consequences of these factors on the mixer performance.

Abstract: Planar spiral antenna coupled and directly lens coupled NbN HEB mixer structures are studied. An additional MgO buffer layer between the superconducting film and Si substrate is introduced. The buffer layer enables us to increase the gain bandwidth of a HEB mixer due to better acoustic transparency. The gain bandwidth is widened as NbN film thickness decreases and amounts to 5.2 GHz. The noise temperature of antenna coupled mixer is 1300 and 3100 K at 2.5 and 3.8 THz respectively. The structure and composition of NbN films is investigated by X-ray diffraction spectroscopy methods. Noise performance degradation at LO frequencies more than 3 THz is due to the use of a planar antenna and signal loss in contacts between the antenna and the sensitive NbN bridge. The mixer is reconfigured for operation at higher frequencies in a manner that receiver’s noise temperature is only 2300 K (3 times of quantum limit) at LO frequency of 30 THz.

Abstract: We report on a superconducting hot electron bolometer coupled to radiation via a broadband antenna. The bolometer, a structured NbN film, was patterned on a thin dielectric membrane between terminals of a gold slotline antenna. We investigated the response to submillimeter radiation (wave-lengths ∼ 0.1 mm to 0.7 mm) in the fundamental Gaussian mode. We found that the directivity of the antenna was constant within a factor of 2.5 through the whole experimental range. The noise equivalent power of the bolometer at 119 µm was ∼ 3 · 10−13 W/Hz1/2; a time constant of ∼ 160 ps was estimated.

Abstract: Picosecond nonequilibrium and slow bolometric responses to infrared radiation from a patterned high-T c superconducting (HTS) film in resistive and normal states deposited onto LaAlO3, NdGaO3, and MgO substrates were investigated using both pulse and modulation techniques. The response time of 35 ps to a laser pulse of 17 ps FWHM has been observed. The intrinsic response time of the fast process is expected to be about a few picoseconds. The modulation technique, being free from the disadvantages of pulse methods (poor sensitivity, limited dynamic range), makes the detailed study of a number of relaxation processes possible. Besides the nonequilibrium response, two kinds of bolometric processes, namely phonon transport through the film-substrate interface and phonon thermal diffusion in a substrate, manifest themselves in certain frequency dependences.

Abstract: The transient voltage response in both epitaxial and granular YBaCuO thin films to 80 ps pulses of YAG∶Nd laser radiation of wavelength 0.63 and 1.54 μm was studied. In the normal and resistive states both types of films demonstrate two components: a nonequilibrium picosecond component and a bolometric nanosecond one. The normalized amplitudes are almost the same for all films. In the superconducting state we observed a kinetic inductive response and two-component shape after integration. The normalized amplitude of the response in granular films is up to five orders of magnitude larger than in epitaxial films. We interpret the nonequilibrium response in terms of a suppression of the order parameter by the excess of quasiparticles followed by the change of resistance in the normal and resistive states or kinetic inductance in the superconducting state. The sharp rise of inductive response in granular films is explained both by a diminishing of the cross section for current percolation through the disordered network of Josephson weak links and by a decrease of condensate density in neighboring regions.

Abstract: We have measured the quantum efficiencysQEd, GHz counting rate, jitter, and noise-equivalentpowersNEPdof nanostructured NbN superconducting single-photon detectorssSSPDsdin thevisible to infrared radiation range. Our 3.5-nm-thick and 100- to 200-nm-wide meander-typedevices(total area 10310mm2), operating at 4.2 K, exhibit an experimental QE of up to 20% inthe visible range and,10% at 1.3 to 1.55mm wavelength and are potentially sensitive up tomidinfrareds,10mmdradiation. The SSPD counting rate was measured to be above 2 GHz withjitter,18 ps, independent of the wavelength. The devices’ NEP varies from,10−17W/Hz1/2for1.55mm photons to,10−20W/Hz1/2for visible radiation. Lowering the SSPD operatingtemperature to 2.3 K significantly enhanced its performance, by increasing the QE to,20% andlowering the NEP level to,3310−22W/Hz1/2, both measured at 1.26mm wavelength.

Abstract: For the first time, measurements have been made on the electron energy relaxation time due to the electron--phonon interaction in films of the YBaCuO superconductor. The results indicate a significant intensification of the electron--phonon interaction in this compound as compared with normal superconducting metals.

Abstract: The energy relaxation time of 2D electrons, Te, has been measured under quasiequilibrium conditions in AlGaAs—GaAs heterojunctions over the temperature range T= 1.5—20 K. At T> 4 K, Te depends only weakly on the temperature, while at T< 4 K 7;'(T) there is a dependence fr; lNT. A linear dependence 7: 1 (T) in the Bloch—-Grfineisen temperature region (T< 5 K) is unambiguous evidence that a piezoacoustic mechanism of an electron—phonon interaction is predominant in the inelastic scattering of electrons. The values of T6 in this temperature range agree very accurately with theoretical results reported by Karpus [Sov. Phys. Semicond. 22 (1988)]. At higher temperatures, where scat—tering by deformation acoustic phonons becomes substantial, there is a significant discrepancy between the experimental and theoretical re-sults.

Abstract: In this paper, we describe a superconductive hot-electron-bolometer mixer receiver designed to operate in the partially transmissive 350-μm atmospheric window. The receiver employs an NbN thin-film microbridge as the mixer element, in which the main cooling mechanism of the hot electrons is through electron-phonon interaction. At a local-oscillator frequency of 808 GHz, the measured double-sideband receiver noise temperature is TRX=970 K, across a 1-GHz intermediate-frequency bandwidth centered at 1.8 GHz. We have measured the linearity of the receiver and the amount of local-oscillator power incident on the mixer for optimal operation, which is PLO&ap;1 μW. This receiver was used in making observations as a facility instrument at the Heinrich Hertz Telescope, Mt. Graham, AZ, during the 1998-1999 winter observing season.

Abstract: A superconducting hot-electron bolometer mixer-receiver operating from 1 to 1.26 THz has been developed. This heterodyne receiver employs two solid-state local oscillators each consisting of a Gunn oscillator followed by two stages of varactor frequency multiplication. The measured receiver noise temperature is 1350 K at 1.035 THz and 2700 K at 1.26 THz. This receiver demonstrates that tunable solid-state local oscillators, supplying only a few micro-watts of output power, can be used in terahertz receiver applications.

Abstract: NbN Hot Electron Bolometric (HEB) mixers have produced promising results in terms of DSB receiver noise temperature (2800 K at 1.56 THz). The LO source for these mixers is a gas laser pumped by a CO/sub 2/ laser and the device is quasi-optically coupled through an extended hemispherical lens and a self-complementary log-periodic toothed antenna. NbN HEBs do not require submicron dimensions, can be operated comfortably at 4.2 K or higher, and require LO power of about 100-500 nW. IF noise bandwidths of 5 GHz or greater have been demonstrated. The DC bias point is also not affected by thermal radiation at 300 K. Receiver noise temperatures below 1 THz are typically 450-600 K and are expected to gradually approach these levels above 1 THz as well. NbN HEB mixers thus are rapidly approaching the type of performance required of a rugged practical receiver for astronomy and remote sensing in the THz region.

Abstract: We report our studies on the response of ultrathin superconducting NbN hot-electron photodetectors. We have measured the photoresponse of few-nm-thick, micron-size structures, which consisted of single and multiple microbridges, to radiation from the continuous-wave semiconductor laser and the femtosecond Ti:sapphire laser with the wavelength of 790 nm and 400 nm, respectively. The maximum responsivity was observed near the film's superconducting transition with the device optimally current-biased in the resistive state. The responsivity of the detector, normalized to its illuminated area and the coupling factor, was 220 A/W(3/spl times/10/sup 4/ V/W), which corresponded to a quantum efficiency of 340. The responsivity was wavelength independent from the far infrared to the ultraviolet range, and was at least two orders of magnitude higher than comparable semiconductor optical detectors. The time constant of the photoresponse signal was 45 ps, when was measured at 2.15 K in the resistive (switched) state using a cryogenic electro-optical sampling technique with subpicosecond resolution. The obtained results agree very well with our calculations performed using a two-temperature model of the electron heating in thin superconducting films.

Abstract: THE 25th International Symposium on Space Terahertz Technology (ISSTT) was held in Moscow, Russia, between April 27–30, 2014. The conference was organized by Moscow State Pedagogical University and the Higher School of Economics (National Research University) and Chaired by Professor Gregory Gol'tsman of Moscow State Pedagogical University. The conference was attended by roughly 150 participants from 15 countries. The technology covered by ISSTT includes detectors, devices, circuits and systems in various areas of THz science and technology. Each year this symposium brings together the global THz space science technology community, and as such, emphasizes the broad international collaboration that is required to execute these large complicated instrument programs that dominate this field. However, talks covering technologies for balloon, aircraft, and ground-based telescopes were also presented.
In this special section of IEEE Transactions on Terahertz Science and Technology, we include eight expanded papers from the 25th ISSTT symposium. The papers range from development of SIS mixers to optical adjustment systems for radio telescopes. The 26th ISSTT will be held in Boston, MA, USA, during March 16–18, 2015. Researchers and scientist involved in THz research are invited to attend this symposium (more details are at http://www.cfa.harvard.edu/events/2015/isstt2015/).
You can access the full list of papers presented at the ISSTT symposia from the National Radio Astronomy Observatory website: http://www.nrao.edu/meetings/isstt/index.shtml
Yours sincerely

Abstract: We report the results of our study of the stability of an 800 GHz hot electron bolometer (HEB) mixer cooled with a pulse-tube cryocooler. Pulse-tube cryocoolers introduce temperature fluctuations as well as mechanical vibrations at a frequency of ~1 Hz, both of which can cause receiver gain fluctuations at that frequency. In our system, the motor of the cryocooler was separated from the cryostat to minimize mechanical vibrations, leaving thermal effects as the dominant source of the receiver gain fluctuations. We measured root mean square temperature variations of the 4 K stage of ~7 mK. The HEB mixer was pumped by a solid state local oscillator at 810 GHz. The root mean square current fluctuations at the low noise operating point (1.50 mV, 56.5 μA) were ~0.12 μA, and were predominantly due to thermal fluctuations. To stabilize the bias current, microwave radiation was injected to the HEB mixer. The injected power level was set by a proportional-integral-derivative controller, which completely compensates for the bias current oscillations induced by the pulse-tube cryocooler. Significant improvement in the Allan variance of the receiver output power was obtained, and an Allan time of 5 s was measured.

Abstract: We present thorough measurements of the intrinsic detection efficiency in the wavelength range from 350 to 2500 nm for meander-type TaN and NbN superconducting nanowire single-photon detectors with different widths of the nanowire. The width varied from 70 nm to 130 nm. The open-beam configuration allowed us to accurately normalize measured spectra and to extract the intrinsic detection efficiency. For detectors from both materials the intrinsic detection efficiency at short wavelengths amounts at 100% and gradually decreases at wavelengths larger than the specific cut-off wavelengths, which decreases with the width of the nanowire. Furthermore, we show that applying weak magnetic fields perpendicular to the meander plane decreases the smallest detectable photon flux.