A. D. Semenov

Broadband ultrafast superconducting NbN detector for electromagnetic radiation

An ultrafast detector that is sensitive to radiation in a broad spectral range from submillimeter waves to visible light is reported. It consists of a structured NbN thin film cooled to a temperature below Tc (∼11 K). Using 20 ps pulses of a GaAs laser, we observed signal pulses with both rise and decay time of about 50 ps. From the analysis of a mixing experiment with submillimeter radiation we estimate an intrinsic response time of the detector of ∼12 ps. The sensitivity was found to be similar for the near‐infrared and submillimeter radiation. Broadband sensitivity and short response time are attributed to a quasiparticle heating effect.

Subnanosecond photoresponse of a YBaCuO thin film to infrared and visible radiation by quasiparticle induced suppression of superconductivity

We observed subnanosecond photoresponse of a structured superconducting YBa2Cu3O7−δ thin film to infrared and visible radiation. We measured the voltage response of a current biased film (thickness 700 A) in a resistive state to radiation pulses. From our results we conclude a response time of about 90 ps and a responsivity of about 4×1010 Ω/J. We attribute the response to Cooper pair breaking and suppression of the superconducting energy gap induced by nonequilibrium quasiparticles.

Sensitive picosecond NbN detector for radiation from millimetre wavelengths to visible light

The authors report on the application of a broad-band NbN film detector which has high sensitivity and picosecond response time for detection of radiation from millimetre wavelengths to visible light. From a study of amplitude modulated radiation of backward-wave tubes and picosecond pulses from gas and solid state lasers at wavelengths between 2 mm and 0.53 mu m, they found a detectivity of 1010 W-1 cm Hz-1/2 and a response time of less than 50 ps at T=10 K. The characteristics were provided by using a 150 AA thick NbN film patterned into a structure of micron strips. According to the proposed detection mechanism, namely electron heating, they expect an intrinsic response time of approximately 20 ps at the same temperature.

Electromagnetic radiation mixer based on electron heating in resistive state of superconductive Nb and YBaCuO films

A theory of an electron-heating mixer which makes it possible to calculate all the characteristics of the device is developed. It is shown that positive conversion gain is possible for such a mixer in the millimeter to near-infrared wavelength range. The dynamic range and the optimum heterodyne power can be selected from a very wide interval by varying the mixing element volume. Measurements made for Nb within the frequency range of 120-750 GHz confirm the theory. The conversion loss obtained at T=1.6 K and normalized to the element reaches 0.3 dB in the intermediate frequency band of 40 MHz; the possible noise temperature is 50 K. The estimation of noise temperature and output band for YBaCuO at T=77 yields 200 K and more than 10 GHz, respectively.

Mechanism of picosecond response of granular YBaCuO films to electromagnetic radiation

Abstract The ultrafast mechanisms of radiation detection in granular YBaCuO films are studied in the wide wavelength range from millimeter to near infrared. With the rise of radiation frequency the Josephson detection at the grain boundary weak links is replaced by electron heating into the grains. This change occurs in the submillimeter wavelength range. Electron-phonon relaxation time τ eph is determined by direct measurements and analyses quasistationary electron heating. Temperature dependence of τ eph at T ≤ 40 K was found to be τ eph ∼ T −1 . The results show that detectors with the response time of few picoseconds at nitrogen temperature are attainable.

Coupling of terahertz radiation to a high‐Tc superconducting hot electron bolometer mixer

We report on efficient coupling of THz radiation to a high‐Tc superconducting hot electron bolometer that is suitable for heterodyne detection. Our quasi‐optical system consisted of a planar self‐complementary spiral antenna on a dielectric substrate clamped to an extended hyperhemispherical lens. The antenna was integrated into a co‐planar line for broadband intermediate frequency matching. Measurements in the homodyne regime at a frequency of 2.5 THz showed a radiation pattern with a beam width of 1° and a coupling efficiency of ≊0.1. We measured, at an intermediate frequency of 1.5 GHz, an output noise temperature of ≊160 K and estimated for the device, operated in the heterodyne regime, a system noise temperature of ≊3×105 K. We also discuss possibilities of significant improvement of the sensitivity.

Fast far-infrared to visible light response of a YBa2Cu3O7−δ film measured with subnanosecond radiation pulses

Abstract We have measured the resistance response of a current biased YBa2Cu3O7-δ thin film (thickness ∼ 700 A) at temperatures below Tc to pulsed laser radiation in a large spectral range from far-infrared waves to visible light. For generation of short pulses we used an optically pumped D2O Raman far-infrared laser and infrared high pressure CO2 laser, both delivering pulses with durations less than 500 ps, and a Nd: YAG picosecond pulse laser. We found that the responde time was less than 0.5 ns at far-infrared frequencies and increased strongly (to ∼ 4 ns) at a critical frequency of about 7 THz. The slower response is consistent with heat escape from the film while the fast response may be connected with a nonequilibrium electronic effect.

Processes of electron-phonon interaction in thin YBaCuO films

Abstract The ultrafast voltage response of YBaCuO films to laser radiation is studied and compared with previously investigated quasiparicles response to radiation of submillimeter wavelength range. Voltage shift under the visible light radiation has two components. Picosecond response realized as suppression superconductivity by nonequilibrium excess quasiparticles, response time is determined by quasiparticles recombination rate. Nanosecond response is probably due to bolometric effect.

Nonthermal mixing mechanism in a diffusion-cooled hot-electron detector

We present an analysis of a diffusion-cooled hot-electron detector fabricated from clean superconducting material with low transition temperature. The distinctive feature of a clean material, i.e., material with large electron mean free path, is a relatively weak inelastic electron scattering that is not sufficient for the establishment of an elevated thermodynamic electron temperature when the detector is subjected to irradiation. We propose an athermal model of a diffusion-cooled detector that relies on suppression of the superconducting energy gap by the actual dynamic distribution of excess quasiparticles. The resistive state of the device is caused by the electric field penetrating into the superconducting bridge from metal contacts. The dependence of the penetration length on the energy gap delivers the detection mechanism. The sources of the electric noise are equilibrium fluctuations of the number of thermal quasiparticles and frequency dependent shot noise. Using material parameters typical for A...

Spiral antenna NbN hot-electron bolometer mixer at submm frequencies

We have studied the phonon-cooled hot-electron bolometer (HEB) as a quasioptical mixer based on a spiral antenna designed for the 0.3-1 THz frequency band and fabricated on sapphire and high resistivity silicon substrates. HEB devices were produced from superconducting 3.5-5 nm thick NbN films with a critical temperature 10-12 K and a critical current density of approximately 10/sup 7/ A/cm/sup 2/ at 4.2 K. For these devices we reached a DSB receiver noise temperature below 1500 K, a total conversion loss of L/sub t/=16 dB in the 500-700 GHz frequency range, an IF bandwidth of 3-4 GHz and an optimal LO absorbed power of /spl sime/4 /spl mu/W. We experimentally analyzed various contributions to the conversion loss and obtained an RF coupling factor of about 5 dB, internal mixer loss of 10 dB and IF mismatch of 1 dB.