Search

Time-resolved transient pump-probe spectroscopy measurements on proximized ferromagnet/superconductor (F/S) hybrid structures are presented. We focused our attention on both low and high critical temperature superconductors such as Nb and YBCO, while for F the weak-ferromagnetic alloy Ni0.48Cu0.52 has been used. Dynamics of the electron-phonon relaxation process has been investigated as a function of both the temperature and the F-film thickness. In the case of NiCu/Nb bilayers the presence of a thin F overlayer reduces the slow bolometric component of the photoresponse, while in YBCO-based structures faster relaxation times were measured. F/S nanobilayers are very attractive toward the development of novel hybrid superconducting photodetectors.

We report our experimental and theoretical studies on generation and time-resolved detection of coherent acoustic phonons (CAPs) in bulk GaN single crystals, using the femtosecond optical pump-probe spectroscopy. A train of 100-fs-wide, ultraviolet laser pump pulses with the photon energy far above of the GaN bandgap induced electronic stress at the crystal surface and triggered propagation of CAP oscillations. Subsequently, time-delayed, low-intensity, 100-fs-wide, ultraviolet/infrared (one/two-colour setup) probe pulses monitored CAP propagation either near the surface or deeply inside the GaN crystal. The amplitude of CAP oscillations was on the order of 10-5-10-6 of the probe-beam transient differential reflectivity signal, and was only dependent on the pump-beam absorption coefficient. The CAP oscillation frequency was dispersionless (proportional to the probe-beam wave vector) with the slope corresponding to 8000 m/s-the speed of sound in GaN. In our two-colour arrangement, we could detect CAP oscillations as deeply as ~0.1 mm inside the crystal (the pump-probe delay ~13.2 ns), since the CAP signal attenuation was only limited by the absorption of the infrared probe light in GaN. The latter indicated that the intrinsic CAP lifetime in our crystals was actually ultra long and was estimated to be of the order of at least 100 ns.

The authors report on observation of pronounced coherent acoustic phonon (CAP) oscillations in diluted-magnetic Cd1−xMnxTe (x=0.09) single crystals by means of femtosecond, optical single-color, and two-color pump-probe spectroscopy. The observed properties of CAP oscillations are consistent with the propagating-strain-pulse model, with electronic stress as the main CAP generation mechanism. The experimental dispersion relation between the CAP oscillation frequency and the probe-beam wave vector allowed them to extract the Cd0.91Mn0.09Te refractive index dependence on wavelength, while the dependence of oscillation amplitudes on the photon energy near the Cd0.91Mn0.09Te band gap edge indicated that the material energy gap is 1.63eV. Two-color pump-probe studies showed that the speed of sound in our material is 3590m∕s and that the absorption depth of probe light, rather than the decay time of CAPs, is the dominant source of the experimentally observed oscillation dephasing time. The intrinsic lifetime of CAPs in CdMnTe crystals has been found to be at least in the nanosecond range.

We present a direct observation of time-resolved intervalley transitions of electrons between the conduction band Γ and L valleys in GaN single crystals using a two-color (ultraviolet and near-infrared), femtosecond, pump-probe spectroscopic technique. We have found that the threshold for the Γ↔L transition appears at the energy of 4.51±0.05eV and the electron scattering time from the Γ to L valley is comparable to the 170fs width of our pump pulses, while the return process of electrons from L to Γ is measurably slower. The L to Γ scattering time is 1.02ps, while the total depopulation time of the L valley is estimated to be ∼20ps. Our physical model, based on the three-state rate equations, fitted our experimental data very well and allowed us to calculate the optical phonon emission time of 290fs. The deformation potential between the two valleys was also obtained from the intervalley scattering modeling and the value was 0.92×109eV∕cm.

We have fabricated and tested a two-channel single-photon detector system based on two fiber-coupled superconducting single-photon detectors (SSPDs). Our best device reached the system quantum efficiency of 0.3% in the 1540-nm telecommunication wavelength with a fiber-to-detector coupling factor of about 30%. The photoresponse consisted of 2.5-ns-wide voltage pulses with a rise time of 250ps and timing jitter below 40ps. The overall system response time, measured as a second-order, photon cross-correlation function, was below 400ps. Our SSPDs operate at 4.2K inside a liquid-helium Dewar, but their optical fiber inputs and electrical outputs are at room temperature. Our two-channel detector system should find applications in practical quantum cryptography and in antibunching-type quantum correlation measurements.

Electric properties of YBa2Cu3O7−δ (YBCO) superconducting thin-film bridges with artificial 5-μm-wide channels for easy motion of Abrikosov vortices have been investigated. Vortex channels have been laser written into the bridge constrictions by means of heat-induced, partial depletion of oxygen from the laser-illuminated areas. Insertion of the channel into a bridge structure increases its resistivity in the normal state, broadens the superconducting transition, and dramatically reduces the critical current density. Experimentally observed features have been ascribed to the vortex channeling mechanism, which reduces the pinning efficiency of the extended growth defects in YBCO.

We report on coherent acoustic phonon (CAP) oscillations studied in high-quality bulk GaN single crystals with a two-color femtosecond optical pump-probe technique. Using a far-above-the-band gap ultraviolet excitation (∼270nm wavelength) and a near-infrared probe beam (∼810nm wavelength), the long-lived, CAP transients were observed within a 10 ns time-delay window between the pump and probe pulses, with a dispersionless (proportional to the probe-beam wave vector) frequency of ∼45GHz. The measured CAP attenuation corresponded directly to the absorption of the probe light in bulk GaN, indicating that the actual (intrinsic) phonon-wave attenuation in our crystals was significantly smaller than the measured 65.8cm−1 value. The velocity of the phonon propagation was equal to the velocity of sound in GaN.

The transverse magnetoresistance (MR) of YBa 2 Cu 3 O 7 thin films is analyzed with respect to the geometric magnetoresistance (GEMR), which is introduced to the measurements in a transverse magnetic field by a finite length-to-width ratio of the sample structure. We show that this effect becomes important in high-temperature superconductors (HTSC) at temperatures far above T c in sample geometries typically used for single crystal measurements. The GEMR adds to the physical MR, which may lead to a misinterpretation of the high temperature part of the MR. Measurements of the MR and the Hall effect employing two different sample geometries, patterned in the same YBa 2 Cu 3 O 7 thin film, which allow for a separation of GEMR from physical MR are presented.

Vortex dynamics in laser-patterned channels for easy motion in YBa2Cu3O7−δ thin-film bridges has been investigated by electric transport and magneto-optical measurements. It has been found that the laser-writing technique, relying on selective deoxygenation of the illuminated areas of YBa2Cu3O7−δ films, enables manufacturing of channels with the decreased field of the first penetration and pinning strength. Current-induced vortices confined in such channels move coherently within a limited temperature and bias ranges. Coherence in vortex motion was confirmed by the direct observation of self-resonant, Josephson-like current steps on the bridge current-voltage characteristics.

We have fabricated and characterized ultrafast metal-semiconductor- metal photodetectors based on low-temperature-grown (LT) GaN. The photodetector devices exhibit up to 200 kV/cm electric breakdown fields and subpicosecond carrier lifetime. We recorded as short as 1.4-ps-wide electrical transients using 360-nm-wavelength and 100-fs-duration laser pulses, that is corresponding to the carrier lifetime of 720 fs in our LT GaN material. © 2005 American Institute of Physics.

Transport properties of YBa2Cu3O7−x (YBCO) step-edge weak links were measured in the absence and presence of the magnetic fieldB, both before and after the cw-laser annealing. Depending on the laser power density, either an increase (up to 75%) or decrease (up to 100%) of the junction critical currentIc was observed. TheIc(B) characteristics of the laser-treated junctions exhibited about 10% variations of theIc(B) periodicity, indicating permanent changes in the junction barrier and/or adjacent YBCO electrodes.

The photoresponse of current-biased YBa2Cu3O7−x (YBCO) microbridges, patterned in 100-nm-thick, high-quality epitaxial films on LaAlO3, has been characterized by means of an electro-optic sampling technique. The photoresponse from 100-fs laser pulses (390-nm wavelength) was measured to be in the form of Gaussian-shaped pulses with the width as short as 0.8 ps. The physical origin of the photoresponse signal can be attributed to a nonequilibrium electron heating mechanism. The measured signal represents, in our opinion, the intrinsic photoresponse of YBCO.

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.

We report our studies on the influence of hydrogen doping on the electrical properties of epitaxial YBa 2 Cu 3 O x (YBCO) thin films of various oxygen content. We have found that hydrogenation increased the YBCO's normal-state resistivity, as well as broadened its superconducting transition and decreased the T c . The above effects were most pronounced in YBCO with low oxygen contents. However, they could be reversed by a subsequent annealing of the samples in oxygen. We associate the observed behavior with the hydrogen-diffusion-induced modification of the free-hole concentration in YBCO CuO 2 planes.

We report our studies on electrical and structural properties of an interface between the oxygen-poor (semiconducting) and oxygen-rich (superconducting, Tc = 89.5 K) YBa2Cu3Oy. Our thin-film test structures were fully monolithic and prepared by a laser-writing patterning technique. Scanning electron and optical microscopy measurements revealed that the interface was very sharp (

Pronounced changes in low-frequency noise power spectra have been observed, close to the transition temperature, in current biased high-Tc superconducting thin films. Generally, the spectra scale as 1 f α , where 0.5 1 f power spectra.

We report a detailed study of the influence of thermodynamic fluctuations of the superconducting order parameter on various normal-state transport properties in 2223-(Bi,Pb)SrCaCuO thin films at temperatures near the superconducting transition. Measurements of the electrical resistivity, the magnetoresistance and the Hall effect were analyzed with regard to fluctuation contributions, using theories for 2-dimensional, layered superconductors. We obtained a consistent set of parameters, which fit all magneto-transport measurements above 118 K, but observed a remarkable enhancement of both excess Hall effect and negative magnetoconductivity closer to Tc, whereas the zero-field fluctuation conductivity follows the theoretical predictions down to 110 K. No significant contributions from the indirect (Maki-Thompson) fluctuation process were found.

We report our studies on the electric-field effect in partially oxygen-depleted YBa 2 Cu 3 O y (YBCO) thin-film test structures fabricated by a laser-writing patterning technique. Our preliminary results indicate substantial, field-induced changes of the sample critical current.

A novel superconducting single-photon detector (SSPD), intrinsically capable of high quantum efficiency (up to 20%) over a wide spectral range (ultraviolet to infrared), with low dark counts (

Measurements of the Hall effect and the resistivity in twinned ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ thin films in magnetic fields B oriented parallel to the crystallographic c axis and to the twin boundaries reveal a double sign reversal of the Hall coefficient for $Bl~1$ T. In high transport current densities, or with B tilted off the twin boundaries by $5\ifmmode^\circ\else\textdegree\fi{},$ the second sign reversal vanishes. The power-law scaling of the Hall conductivity to the longitudinal conductivity in the mixed state is strongly modified in the regime of the second sign reversal. Our observations are interpreted as strong, disorder-type dependent vortex pinning and confirm that the Hall conductivity in high-temperature superconductors is not independent of pinning.

Influence of the initial lead content in Bi(Pb)-Sr-Ca-Cu-O thin films on the annealing conditions, which make the films fully superconducting above 100 K is reported. We have found that the optimum annealing temperature was very close to the temperature of the film partial melting and decreased with the increase of the lead content in the film. The increase of the lead content also dramatically accelerated formation of the 110-K phase, resulting in short (1 hour) annealing times.

Fabrication and characterization of Bi 2 Sr 2 Ca 1 Cu 2 O x superconducting thin films, dc-magnetron sputtered from a stoichiometric oxide target on MgO and CaNdAlO 4 single crystals is reported. Despite a very short annealing time (5 minutes at 900 °C in air), the films are highly textured with a dominant 80-K phase. They exhibit a 5-K-wide superconducting transition, and sustain supercurrents densities above 10 5 A/cm 2 at temperatures below 55 K. We also demonstrate that the CaNdAlO 4 substrates are suitable for deposition of epitaxial Bi-based superconducting films.

We demonstrate a simple quasi-optical technique for spatiotemporal shaping of half-cycle terahertz-radiation pulses. We show, both experimentally and theoretically, that properly polarized half-cycle pulses can be modulated temporally by diffraction through a conductive aperture of finite thickness. We use the finite-difference time-domain method to solve Maxwell’s equations for such a geometry and show that it can explain all the experimentally observed features. In the case of thick aperture, a planar waveguide model can also be used to describe the propagation of the pulse through the aperture, with excellent agreement with the experimental results.

We report temporal shaping of few-cycle terahertz pulses, using a slit in a conductive screen as a high-pass filter. The filter’s cutoff frequency was tuned by changing the width of the slit; the slope of the cutoff transition was altered by changing the thickness of the screen. We measured the transmission function of the filters, using large-aperture photoconducting antennas to create and detect the incident and transmitted electric field. Our experimental results were in excellent agreement with the performed finite-difference time-domain simulations of the propagation of the pulse through the slit. When the screen thickness was greater than the slit width, the filter was well modeled by a short, planar waveguide. Using a simple transfer function, we accurately describe the sharp cutoff and dispersion of such a filter. © 1997 Optical Society of America Recently terahertz (THz) pulses have proved useful for a wide variety of applications, including far-infrared 1 / time-domain spectroscopy, 2 study and control of Rydberg atoms, 3 T-ray imaging of optically opaque materials, 4 and impulse-ranging studies. 5 Applications exploit the features of THz pulses that distinguish them from optical pulses, namely their broad bandwidth and long center wavelength. The broad bandwidth allows for the exciting possibility for sculpting the pulses to meet a desired goal by modifying their spectra. Although shaped THz pulses have been made by modifying the optical pump intensity profile 6 or by manufacturing more complex emitters, 7 there are

Transient response of a 2500 A thick superconducting Nb film to the impact of a single 129Xe50+ ion of total energy 3.2 GeV, transferring about 2.4 keV/A of electronic excitation energy, was observed by means of a superconductor-insulator-superconductor tunnel junction attached to the film some distance away from the impact point. The response had rise time of less than 150 ps (resolution limit of the electronic detection system) and decay time varying from 2 to 1.3 ns as function of temperature. Main experimental features, if not finer details, are in good agreement with Rothwarf and Taylor theory of nonequilibrium processes in superconductors.

We report our studies on the fabrication and superconducting properties of thin films in the YBaCuO and BiSrCaCuO systems. The films were manufactured on ZrO2:Y, ZrO2:Gd, MgO, and LaAlO3 substrates, by chemical deposition of properly mixed nitrate precursors, as well as by d.c. magnetron sputtering from a single oxide target. In all cases, as-deposited films required a high temperature heat treatment to become superconducting. After annealing, our films were granular with the thickness ranging from 0.5 to 5 μm, and exhibited sharp superconducting transitions with the zero resistivity state reached at 89 K and 77 K for YBaCuO and BiSrCaCuO films respectively. Patterning techniques, most suitable for our deposition methods, were also developed and tested.

We report experimental measurements of the voltage response of current-biased superconducting thin films excited by 30-ps-FWHM (full width at half-maximum) pulsed laser radiation. Experiments performed with Pb and Sn films irradiated by a full train of laser pulses provide direct evidence for the existence of a nonstationary intermediate state. The state was observed during the superconducting-to-normal transition, as well as during the recovery of the film from the nonequilibrium normal state into the superconducting state. Single-pulse laser experiments enabled us to study the superconducting-to-normal transition in detail. Depending on the optical power, both triangular and quadruangular voltage pulses were observed. The triangular pulses had amplitudes dependent on the laser power and reflected the transition to the intermediate state, where superconducting and normal domains coexisted. The quadrangular pulses were associated with the full transition into the normal state. Very short triangular voltage pulses with full widths of the order of hundreds of picoseconds and rise times as short as about 100 ps were observed for thin Pb films illuminated by a low, near-threshold, laser power. The pulse rise time was almost constant and it only depended weakly on the film thickness, while the pulse width and the fall time were verymore » sensitive to the optical input power as well as other experimental conditions (e.g., film thickness, Pb-substrate interface). The pulse decay was determined to be linear. The linear decay reflects a geometric relaxation of the intermediate state. An experiment performed with a stretched laser pulse (about 470 ps FWHM) demonstrated an initial thresholding effect and indicated that the subsequent voltage is proportional to the time integral of the laser pulse (optical energy).« less

The authors report investigations of picosecond transient propagation on normal and superconducting transmission lines and results of a variety of lines that include YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) coplanar lines, a superconducting coaxial cable, and a dielectric-matched gold-line structure. A previously developed algorithm for analyzing transient propagation was used to identify the dominant mechanisms for signal distortion in most of these cases, and the essential properties of all tested to date are summarized for a direct comparison. >

We have developed a fully cryogenic electro-optic sampler by integrating and immersing in superfluid helium both the photoconductive switch and the birefringent lithium tantalate sensor. The ultimate temporal response of the system as determined by measuring the transient onset of photoconductivity in a GaAs switch was less than 400 fs. The system was arranged into a coplanar transmission line geometry and used to probe the switching of a 30 × 30 μm2Josephson tunnel junction. Both time-dependent waveforms of the transmitted signal and current-voltage characteristics of the junction switched by a picosecond input pulse were measured. The results were compared to transient simulations based on the resistively and capacitively shunted junction model, and showed that: the initial junction response was limited only by the time integral of the input pulse, the switched voltage was proportional to the total charge delivered by the pulse, and there was no turn-on delay time. This represents the first, in situ measurement of the response of an unbiased Josephson tunnel junction to a stepped, picosecond pulse.

The algorithm suitable for the computer-aided design of transmission lines is used to model the propagation of picosecond and subpicosecond electrical signals on superconducting planar transmission lines. Included in the computation of a complex propagation factor are geometry-dependent modal dispersion and the frequency-dependent attenuation and phase velocity which arise as a result of the presence of a superconductor in the structure. The results of calculations are presented along with a comparison to experimental data. The effects of modal dispersion and the complex surface conductivity of the superconductor are demonstrated, with the conclusion that it is necessary to incorporate both phenomena for accurate modeling of transient propagation in strip transmission lines. >

The response of a thin sputtered film of YBaCuO to both microwave and DC magnetic fields has been investigated. A number of unexpected features have been observed and explanations of these in terms of two conceptually different models are presented. The first assumes that there are just a few active intergrain Josephson junctions in the film, whereas the second assumes a robust 'fluxon lattice' is present, which moves transversely to a transport current and whose motion is correlated with an applied microwave field.

Statistical properties of resonance fluorescence of a three-level atom, interacting with a monochromatic, strong laser field are examined. Analytical formulas, describing first- and second-order correlation functions are presented. The effect of antibunching is predicted and the nontrivial difference between the statistical properties of differently polarized fluorescence is discussed.

Studies are reported of the interaction of Y-Ba-Cu-O and Bi-Ca-Sr-Cu-O thin films with microwaves. The films were prepared on cubic zirconia and magnesium oxide substrates using a modified spray deposition technique. The tested films were about 1-3 mu m thick and exhibited good superconducting properties. Several test structures have been designed by spraying a mixture of nitrate precursors on heated substrates through very thin stencil marks. The supercurrent dependence on millimeter wavelength microwave illumination was measured, and microwave detection and mixing experiments were performed. The films of both materials exhibited properties characteristic of granular superconductors, with Josephson-type coupling at the intergrain connections. The results also indicate that these materials can be successfully used as very sensitive millimeter radiation detectors operational at liquid-nitrogen temperatures. >

We report a sub-picosecond photoresponse and THz transient generation of GaAs single-crystal mesoscopic platelets excited by femtosecond optical pulses. Our structures were fabricated by a top-down technique, by patterning an epitaxial, 500-nm-thick GaAs film grown on top of an AlAs sacrificial layer and then transferring the resulting etched away 10 × 20-μm2 platelets onto an MgO substrate using a micropipette. The freestanding GaAs devices, incorporated into an Au coplanar strip line, exhibited extremely low dark currents and ~0.4 % detection efficiency at 10 V bias. The all-optical, pump–probe carrier dynamics analysis showed that, for 800-nm-wavelength excitation, the intrinsic relaxation of photocarriers featured a 310-fs-wide transient with a 290 fs fall time. We have also carried out a femtosecond, time-resolved electro-optic characterization of our devices and recorded along the transmission line the electrical transients as short as ~600 fs, when the platelet was excited by a train of 100-fs-wide, 800-nm-wavelength optical laser pulses. The platelets have been also demonstrated to be very efficient generators of free-space propagating THz transients with the spectral bandwidth exceeding 2 THz. The presented performance of the epitaxial, freestanding GaAs meso-structured photodevices makes them uniquely suitable for THz-frequency optoelectronic applications, ranging from ultrafast photodetectors to THz-bandwidth optical-to-electrical transducers and photomixers.

Microwave properties of an Y‐Ba‐Cu‐O microstrip detector have been studied at 24 GHz frequency band at the liquid‐nitrogen temperature. The amplitude of the video detection signal was found to be proportional to the incident microwave power. Direct mixing action was also demonstrated, setting the upper limit for the film response time as short as 40 ps. Occurrence of microwave mixing in the absence of dc current bias was observed, suggesting the presence of the inverse Josephson effect. Physical properties of the detector response were very similar to the enhanced mode of detection, characteristic for granular superconducting films.

Infrared pulses (1.06 μm wavelength, ≊150 ps long) from a Nd:YAG laser illuminated a superconducting YBa2Cu3O7 granular film, 0.7 μm thick. This caused the film to switch from the superconducting to the normal (resistive) state in less than 2 ns, followed by a much slower recovery ∼1 μs. The initial fast switching may be explained by nonequilibrium hot‐electron energy transfer from the top absorbing layer (≊100 nm thick) through the film, followed by an essentially bolometric response and cooldown via equilibrium thermal conduction out of the film.

A new patterning method suitable for spray-deposited Y-Ba-Cu-O films has been demonstrated. The method is a single-step process, based on the application of metal stencil masks to define a desired pattern. Lines less than 50 mu m wide, with superconducting properties identical to these of unpatterned films, have been successfully deposited.

We report our studies on fabrication and superconducting properties of YBa2 Cu3 O7-y thin films prepared by d.c. magnetron sputtering from a single "1-2-3" target and subsequent heat treatment. The films exhibited sharp resistive transitions and zero-resistance temperatures above 85 K. XPS results indicate that the stoichiometry of our films was close to the target composition.

Using a cryogenic electro‐optic sampling technique, we have studied the transient propagation characteristics of superconducting and normal indium lines in the picosecond regime. Transient dispersion effects, including increased rise time and increased pulse width, the introduction of ringing on the waveform, and a novel ‘‘pulse sharpening’’ were observed. A model that takes into consideration the effects of modal dispersion and superconducting complex conductivity was established, and an algorithm was developed that accurately describes all of the experimental findings.

We report experimental and numerical studies of the dynamics of the Josephson tunnel junction driven by picosecond current pulses. Both single pulse switching, and the response to the train of pulses are discussed. The observed behavior is explained in terms of the “ccritical pulse charge” concept.

A fully integrated superconducting electro-optic sampler, designed for subpicosecond measurements of the transient response of devices operating in a cryogenic environment, is reviewed in detail. Several different configurations of the sampling head, including a coplanar transmission line geometry with a photoconductive switch fabricated on semi-insulating GaAs and sensor transmission lines fabricated on lithium tantalate, are discussed. The optical part of the system consists of two 80-fs laser pulse trains at a 100 MHz repetition rate, generated by a colliding-pulse mode-locked laser. One pulse train was used to generate a picosecond electrical input pulse, while the second probed the induced change in birefringence of the electro-optic crystal (LiTaO3). By changing the relative delay between the optical excitation and sampling pulses, the temporal evolution of the electrical transient was recorded. The limit of the temporal response of our sampler, as determined by measuring the transient onset of photoconductivity in a GaAs switch, was less than 400 fs. We discuss two applications of this sampler for the study of the picosecond pulse propagation on superconducting transmission lines and for probing the switching of a Josephson tunnel junction.

Since the first successful deposition of the YBa2Cu3O7-y (hereafter referred as to YBCO) superconducting thin-film1 with properties approaching those of the bulk material, a wide spectrum of methods and techniques (both high-vacuum and chemical) have been successfully implemented for fabrication of thin film high-TC superconductors.2 Currently, high-vacuum deposition at elevated temperatures (at least 650 °C) appears to have a decisive advantage in growing epitaxial or textured films with high transport-current densities. On the other hand chemical methods give a precise control over the stoichiometry of the deposited material and produce the best-quality granular films.

We report experimental and numerical studies of chaotic behavior in Josephson tunnel junctions driven by a train of picosecond electrical pulses. Dramatic changes were observed in both the experimental and simulated current-voltage characteristics, demonstrating that periodic wide-band excitations can temporarily alter the junction dynamics, leading to intermittent chaotic behavior. We relate the observed chaotic behavior to the junction switching processes in the regime of very short current pulses.

Superconducting single-photon ultrathin NbN film detectors are studied. The development of manufacturing technology of detectors and the reduction of their operating temperature down to 2 K resulted in a considerable increase in their quantum efficiency, which reached in the visible region (at 0.56 μm) 30%—40%, i.e., achieved the limit determined by the absorption coefficient of the film. The quantum efficiency exponentially decreases with increasing wavelength, being equal to ~20% at 1.55 μm and ~0.02% at 5 μm. For the dark count rate of ~10-4s-1, the experimental equivalent noise power was 1.5×10-20 W Hz-1/2; it can be decreased in the future down to the record low value of 5×10-21 W Hz-1/2. The time resolution of the detector is 30 ps.