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We demonstrate a photodetector sensitive to both optical and x-ray picosecond pulses based on our in-house grown cadmium magnesium telluride (Cd,Mg)Te single crystal. Specifically, we developed In-doped Cd0.96Mg0.04Te material and discuss its femtosecond optical photoresponse, as well as the detector performance, such as

We present our studies on both photoconductive (PC) and electro-optic (EO) responses of (Cd,Mg)Te single crystals. In an In-doped Cd0.92Mg0.08Te single crystal, subpicosecond electrical pulses were optically generated via a PC effect, coupled into a transmission line, and, subsequently, detected using an internal EO sampling scheme, all in the same (Cd,Mg)Te material. For photo-excitation and EO sampling, we used femtosecond optical pulses generated by the same Ti:sapphire laser with the wavelengths of 410 and 820 nm, respectively. The shortest transmission line distance between the optical excitation and EO sampling points was 75 μm. By measuring the transient waveforms at different distances from the excitation point, we calculated the transmission-line complex propagation factor, as well as the THz frequency attenuation factor and the propagation velocity, all of which allowed us to reconstruct the electromagnetic transient generated directly at the excitation point, showing that the original PC transi...

The thermoelectric response of amorphous, semiconducting Y–Ba–Cu–O thin films was investigated by illumination with 150 ps optical pulses at 1064 nm and with a continuous wave at 493 nm (argon laser). The measured unbiased voltage response was consistent with pyroelectricity. At high pulse energies, the voltage response saturated due to the saturation of the spontaneous polarization. The pulsed responsivity was limited by the short pulse duration compared to the thermal time constant of the structure. The electrical time constant for the change in the spontaneous polarization was estimated to be 50 ns.

An experimental study of the Hall effect in nonmetallic Y-Ba-Cu-O thin films is reported. Both epitaxial crystalline YBa2Cu3O6+x (x⩽0.5) and multiphase/amorphous Y-Ba-Cu-O thin films were studied. The structure of the samples was measured by x-ray diffraction and Raman microprobe. The amorphous Y-Ba-Cu-O samples were found to have a grain size of about 100 A. The conduction properties were studied and analyzed for the two types of samples over a wide temperature range including room temperature. The Hall effect measurements showed positive charge carriers with a concentration ranging from 1017 to 1020 cm−3 at room temperature. The mobility was found to decrease with higher Hall carrier concentration. The empirical relationship for the mobility dependence on impurity concentration agreed with the relationship between mobility and the experimental Hall carrier concentration, suggesting that the same localized states were responsible for both providing the carriers and reducing the mobility through scatterin...

We explore the application of the semiconducting phases of YBaCuO thin films as a bolometer for uncooled infrared detection. For this study, four different structures were built with different types of buffer layers: YBaCuO on a Si substrate with and without a MgO buffer layer, and on an oxidized Si substrate with and without a MgO buffer layer. These films were all amorphous without a detectable long range order. For comparison, crystalline tetragonal YBa 2 Cu 3 O 6.5 and YBa 2 Cu 3 O 6.3 thin films on a LaAlO 3 substrate were included into the study. All six films exhibited semiconducting resistance versus temperature characteristics. The bolometer figures of merit, responsivity, and detectivity were calculated from the measured temperature coefficient of resistance ~TCR! and the inherent noise characteristics of the temperature sensing element. The room temperature TCRs for all four amorphous films were greater than 2.5% K 21 . The highest TCR of 4.02% K 21 was observed on the amorphous YBaCuO thin film deposited on MgO/Si without a SiO2 layer. The TCR of the tetragonal films, on the other hand, remained 2% K 21 or less in the same temperature range. Noise measurements performed in the 1‐100 Hz frequency range revealed a quadratic dependence on the bias current as would be expected from ohmic electrical characteristics. The Johnson and 1/f regions were clearly identified in the noise spectrum. From TCR and noise measurements, we estimated the amorphous semiconducting YBaCuO bolometers would have a responsivity as high as 3.8310 5 V/W and a detectivity as high as 1.6310 9 cm Hz 1/2 /W for 1 mA bias current and frame frequency of 30 Hz if integrated with a typical air-gap thermal isolation structure. © 1996 American Institute of Physics.@S0021-8979~96!02924-6#

Patterns on YBa2Cu3O6.87 films, laser written in O2 or N2 atmospheres, and with various writing power densities and scanning speeds, have been studied by Raman scattering. For laser writing in O2 at atmospheric pressure, with low power density and fast scanning speed, it is found systematically that the oxygen content of YBa2Cu3O x can be reduced. Only when the writing power density is high and the scanning speed is low (e.g., 2.3 mW/μm2 and 1 μm/s), do the laser‐irradiated lines in an O2 atmosphere approach full oxygenation (x→7). For laser writing in N2 at atmospheric pressure, the oxygen stoichiometry always decreases, rendering the lines nonsuperconducting. The structural phase transition from orthorhombic to tetragonal can be identified with laser writing in a N2 atmosphere with a writing power density of 1.9 mW/μm2 and a scanning speed of 20 μm/s. The oxygen O(4) line (∼500 cm−1) and the Ba line (115 cm−1) disappear, and the intensity of the Cu(2) line (144 cm−1) rises sharply when the phase transition occurs. This study shows how Raman scattering can be used as a routine technique, which is fast, sensitive, and nondestructive, and is able to measure oxygen content on a fine scale (a few μm) during high‐T c superconductor device processing.

We present our studies on a subpicosecond Faraday effect in diluted magnetic semiconducting Cd1−xMnxTe single crystals and its application in a magneto-optical (MO) sampling system for time-resolved detection of ultrafast current pulses. The measurements were performed at 10 K. We used the Cd0.38Mn0.62Te crystal as the active MO medium and a low-temperature-grown free-standing GaAs photoconductive switch integrated into a coplanar transmission line as the picosecond electrical pulse generator. We observed subpicosecond MO transients that correspond to the intrinsic MO low-temperature response in the Cd1−xMnxTe system with the high Mn concentration. The current sensitivity of our MO system was found to be ∼0.1mA. We have demonstrated that the MO sampling technique using the Cd0.38Mn0.62Te transducer is as fast as the standard LiTaO3, electro-optical sampling technique and allows for a complementary (magnetic-field component) characterization of electrical picosecond transients in ultrafast switching device...

We have measured the quantum efficiency (QE), GHz counting rate, jitter, and noise-equivalent power (NEP) of nanostructured NbN superconducting single-photon detectors (SSPDs) in the visible to infrared radiation range. Our 3.5-nm-thick and 100- to 200-nm-wide meander-type devices (total area 10×10μm2), operating at 4.2K, exhibit an experimental QE of up to 20% in the visible range and ∼10% at 1.3 to 1.55μm wavelength and are potentially sensitive up to midinfrared (∼10μm) radiation. The SSPD counting rate was measured to be above 2GHz with jitter

We have fabricated and tested metal–semiconductor–metal (MSM) photodetectors based on nitrogen-ion-implanted GaAs. Nitrogen ions with energy of 700 and 880 keV, respectively, were implanted into epitaxial GaAs films at an ion concentration of 3×1012 cm−2. Ti/Au MSM photodetectors with 1-μm-wide fingers were fabricated on top of the implanted GaAs. In comparison to low-temperature-grown GaAs photodetectors, produced in parallel in identical MSM geometry, the 880 keV N+-implanted photodetectors exhibited almost two orders of magnitude lower dark current (10 nA at 1 V bias) and the responsivity more than doubled (>20 mA/W at 20 V bias). Illumination with 100-fs-wide, 810 nm wavelength laser pulses, generated ∼2.5-ps-wide photoresponse signals with amplitudes as high as 2 V. The 2.5 ps relaxation time was the same for both the ion-implanted and low-temperature-grown devices and was limited by the MSM capacitance time constant.

We present an electro-optic (EO) sampling system based on an organic ionic salt crystal, 4-N,N-dimethylamino-4′-N′-methyl-4-stilbazolium tosylate (DAST) transducer. Compared to LiTaO3, the DAST lower dielectric permittivity and much higher electro-optic coefficient dramatically improveme electric-field coupling into the EO crystal, which results in a much better signal-to-noise ratio of the sampling system. Submillivolt signals can be easily measured with the DAST sensor. Time resolution of the DAST-based EO system is the same as that of the LiTaO3-based sampler.

We have fabricated and tested freestanding low-temperature-grown GaAs photoconducting devices exhibiting subpicosecond photoresponse. The epitaxially grown thin-film microswitches were transferred on top of sapphire and Si/SiO2 substrates and integrated with Ti/Au coplanar strip transmission lines. Our devices exhibited above 200 V breakdown voltages and dark currents at 100 V below 3×10−7 A. The photoresponse of our microswitches was measured using electro-optic sampling technique with 100-fs-wide laser pulses at wavelengths of 810 nm and 405 nm as the excitation sources. For 810-nm excitation, we measured 0.55-ps-wide electrical transients with voltage amplitudes of up to 1.3 V. The ultraviolet excitation resulted in 1.35-ps-wide transients due to the hot carriers generation and intraband relaxation.

The picosecond electrical pulse‐propagation characteristics of microchannel‐plate detectors, used in diagnosing laser‐driven inertial‐confinement‐fusion experiments, were measured with an electro‐optic sampling system. We determined propagation velocity and signal distortion of the microchannel‐plate microstrip, as well as its characteristic impedance and the substrate relative dielectric permittivity. These parameters are critical for proper calibration of the detectors and serve as a guide for their improved designs.

We experimentally demonstrate a supercurrent-assisted, hotspot-formation mechanism for ultrafast detection and counting of visible and infrared photons. A photon-induced hotspot leads to a temporary formation of a resistive barrier across the superconducting sensor strip and results in an easily measurable voltage pulse. Subsequent hotspot healing in ∼30 ps time frame, restores the superconductivity (zero-voltage state), and the detector is ready to register another photon. Our device consists of an ultrathin, very narrow NbN strip, maintained at 4.2 K and current-biased close to the critical current. It exhibits an experimentally measured quantum efficiency of ∼20% for 0.81 μm wavelength photons and negligible dark counts.

We report time-resolved characterization of superconducting NbN hot-electron photodetectors using an electro-optic sampling method. Our samples were patterned into micron-size microbridges from 3.5-nm-thick NbN films deposited on sapphire substrates. The devices were illuminated with 100 fs optical pulses, and the photoresponse was measured in the ambient temperature range between 2.15 and 10.6 K (superconducting temperature transition TC). The experimental data agreed very well with the nonequilibrium hot-electron, two-temperature model. The quasiparticle thermalization time was ambient temperature independent and was measured to be 6.5 ps. The inelastic electron–phonon scattering time τe–ph tended to decrease with the temperature increase, although its change remained within the experimental error, while the phonon escape time τes decreased almost by a factor of two when the sample was put in direct contact with superfluid helium. Specifically, τe–ph and τes, fitted by the two-temperature model, were equal to 11.6 and 21 ps at 2.15 K, and 10(±2) and 38 ps at 10.5 K, respectively. The obtained value of τe–ph shows that the maximum intermediate frequency bandwidth of NbN hot-electron phonon-cooled mixers operating at TC can reach 16(+4/−3) GHz if one eliminates the bolometric phonon-heating effect.

We report our time-resolved measurements of switching dynamics of grain-boundary, 5-μm-wide Y–Ba–Cu–O (YBCO) Josephson junctions, excited by 2-ps-wide electrical pulses optically generated by a voltage-biased, 5-μm-wide, 10-μm-long YBCO microbridge. The time-resolved transients were recorded at 20 K using a subpicosecond cryogenic electro-optic sampling system. The intrinsic junction response was separated from the circuit-related feedthrough, and we observed both experimentally and in numerical simulations a 0.8-ps-wide, single-flux-quantum (SFQ) pulse, generated by the switching of the junction with the IcRn product equal to ∼2 mV. At the same time, the measured turn-on delay time was almost three times shorter than that obtained from simulations, questioning applicability of the resistively shunted tunnel junction model to nonhysteretic, high-temperature superconducting weak links. Our test structure's power consumption associated with the single SFQ pulse generation was ∼0.1 μW, leading to a (switchin...

Investigation of the 110‐K Bi2Sr2Ca2Cu3Ox phase formation in superconducting thin films of Bi‐based cuprates is reported. The films were dc magnetron sputtered from single Bi(Pb)‐Sr‐Ca‐Cu‐O targets of various stoichiometries, and subsequently annealed in air at high temperatures. The influence of the initial Pb content, annealing conditions, as well as the substrate material on the growth of the 110‐K phase was investigated. We found that the films, fully superconducting above 100 K could be reproducibly fabricated on various dielectric substrates from Pb‐rich targets by optimizing annealing conditions for each initial Pb/Bi ratio. Heavy Pb doping considerably accelerated formation of the 110‐K phase, reducing the film annealing time to less than 1 h. Films containing, according to the x‐ray measurement, more than 90% of the 110‐K phase were obtained on MgO substrates, after sputtering from the Bi2Pb2.5Sr2Ca2.15Cu3.3Ox target and annealing in air for 1 h at 870 °C. The films were c‐axis oriented, with 4.5...

We report our femtosecond time-resolved measurements on the photoresponse of an epitaxial YBa2Cu3O7−x (YBCO) thin-film photodetector, patterned into a microbridge geometry. By varying the current–voltage biasing conditions between the superconducting and resistive (hot spot) states, we observed transients that correspond to the nonequilibrium kinetic-inductance and the nonequilibrium electron-heating response mechanisms, respectively. The two-temperature model and the Rothwarf–Taylor theory have been used to simulate the measured wave forms and to extract the temporal parameters. The electron thermalization time and the electron–phonon energy relaxation time were determined by the electron temperature rise and decay times, which were found to be 0.56 and 1.1 ps, respectively, in the resistive state. We have also measured the ratio between the phonon and electron specific heats to be 38, which corresponds to a phonon–electron scattering time of 42 ps. No phonon-trapping effect (typical for low-temperature ...

The quantum efficiency and current and voltage responsivities of fast hot-electron photodetectors, fabricated from superconducting NbN thin films and biased in the resistive state, have been shown to reach values of 340, 220 A/W, and 4×104 V/W, respectively, for infrared radiation with a wavelength of 0.79 μm. The characteristics of the photodetectors are presented within the general model, based on relaxation processes in the nonequilibrium electron heating of a superconducting thin film. The observed, very high efficiency and sensitivity of the superconductor absorbing the photon are explained by the high multiplication rate of quasiparticles during the avalanche breaking of Cooper pairs.

A low-temperature electro-optic sampling system was implemented to study the crosstalk of picosecond pulses between on-chip microstrip interconnects. With a submillivolt sensitivity and a subpicosecond temporal response, this system has allowed noninvasive, nodal testing of superconducting Nb integrated circuits. We have characterized pulse propagation and the crosstalk arising from two microstrip waveguides crossing perpendicular to each other, with one signal line crossing above the other. These results provided direct feedback for improving computer simulations of superconducting electronic circuits, but they are representative of any high-speed interconnect in today’s very large scale integrated circuit technology.

We report on the laser‐induced permanent changes of the critical current (Ic) and normal resistance (Rn) of YBa2Cu3O7−x (YBCO) step‐edge Josephson junctions. The 2‐ to 20‐μm‐wide junctions were prepared from a 200‐nm‐thick YBCO film deposited by a pulsed KrF excimer laser onto 300‐nm‐high steps etched in the LaAlO3 substrate. The laser modification experiments were performed by illuminating the junctions at 50 K with a focused Ar‐ion laser beam of various intensities. Depending on the illumination power density, either increase or decrease of the junction Ic has been observed. In particular, after illumination at the 0.6×105 W/cm2 power level, a 75% enhancement of Ic and increase of the IcRn product up to 25% were obtained without a measurable change in the junction critical temperature. The laser‐induced modifications were very reproducible and remained unchanged even after a subsequent room‐temperature/helium thermal cycling of the sample. Photoassisted, thermally activated oxygen redistribution in the ...

Photoresponse signals with widths as short as 1.5 ps are observed from epitaxial YBa2Cu3O7−δ thin films using electro‐optic sampling techniques. Voltage transients less than 2 ps wide are seen in 100‐ and 200‐nm films exposed to 150‐fs laser pulses and cooled to 79 K. At low bias currents, the amplitude of the fast response varies linearly with the bias current, suggesting a kinetic inductive mechanism. A negative transient about 15‐ps long is also seen that may provide evidence for nonequilibrium recombination of excited quasiparticles into Cooper pairs. At high bias currents or large laser fluences, a fast tail with a decay time of about 10 ps appears in the response followed by a slow, resistive bolometric component due to sample heating. Nonequilibrium aspects of the photoresponse and the origin of the fast tail are discussed.

Using a femtosecond pulsed laser, ultrafast electrical pulses were optoelectronically generated on silicon and indium phosphide by edge illumination of a coplanar transmission line. Backing up theory with experiment, we demonstrate that this pulse‐generation method is material independent, thus providing a powerful tool for broadband characterization of devices made on a wide range of semiconductor substrates. We also demonstrate that edge illumination enables the generation of 550 fs electrical pulses on indium phosphide and 800 fs pulses on silicon—the fastest pulses to date on bulk silicon.

We report on the direct observation of a single‐flux‐quantum (SFQ) pulse. The response of a metal–semiconductor–metal photodiode to a femtosecond laser pulse was used to switch Josephson junctions and to generate an SFQ voltage pulse on a superconducting microstrip line. The detailed shape of the pulse was measured optoelectronically, using a cryogenic electro‐optic sampling system. The measured SFQ pulse had a width of 3.2 ps, an amplitude of 0.67 mV, and a total pulse content of 2.1±0.2 mV×ps, corresponding to the quantum of magnetic flux h/2e. With larger excitation, multiple SFQ pulses were observed. Numerical simulations are shown to be qualitatively similar to our experimental results.

We report systematic studies of the femtosecond transient reflectivity at 2 eV in partially oxygen‐depleted Y‐Ba‐Cu‐O thin films exhibiting the resistive superconducting transition at 60 K. Our measurements, performed at room temperature, reveal that in 60 K films, the Fermi level in the Cu‐O2 plane lies approximately 2 eV above the filled copper d9/d10 band. By mapping the transient optical response across a 1 cm2 film, we show that the position of the Fermi level is very sensitive to the apparent local variations of the film oxygen content, which gives rise to dramatically different transient optical responses. We also observe a strong transient reflectivity dependence on intensity, which can be attributed to the shift of the Fermi level caused by nonequilibrium hole heating.

A picosecond time separation between the two sides of a single electrical transient propagated on a bent coplanar waveguide has been observed using a subpicosecond electro‐optic sampling system. The separation corresponds to the geometrical path difference introduced by a 90° coplanar waveguide bend. Contrary to the straight waveguide where the transient propagates in the odd mode only, the bend forced the signal to emerge as a superposition of both the odd and even waveguide modes. Introduction of proper local grounding in the bend area, by wirebonding the ground lines on both sides of the transmission line, suppressed the unwanted even mode while preserving the standard odd‐mode propagation.

Single‐crystal perovskite calcium‐neodymium aluminate CaNdAlO4 has been grown and tested as a substrate material for high Tc superconducting thin films. The material is chemically stable, can be fabricated into the form of large, twin‐free wafers, permits deposition of epitaxial Y‐Ba‐Cu‐O and Bi‐Sr‐Ca‐Cu‐O films, and exhibits low dielectric permittivity (e≂20) and very low losses (tan δ≂10−3 at 100 K) at sub‐THz frequencies. Its high potential in microwave applications of thin‐film high Tc superconductors is clearly demonstrated.

We present the ultrafast (THz-bandwidth) photoresponse from GaAs single-crystal mesoscopic structures, such as freestanding whiskers and platelets fabricated by the top-down technique, transferred onto a substrate of choice, and incorporated into a coplanar strip line. We recorded electrical transients as short as similar to 600 fs from an individual whisker photodetector. Analysis of the frequency spectrum of the photoresponse electrical signal showed that, intrinsically, our device was characterized by an similar to 150-fs carrier lifetime and an overall 320-fs response. The corresponding 3-dB frequency bandwidth was 1.3 THz-the highest bandwidth ever reported for a GaAs-based photodetector. Simultaneously, as high-quality, epitaxially grown crystals, our GaAs structures exhibited mobility values as high as similar to 7300 cm(2)/V.s, extremely low dark currents, and similar to 7% intrinsic detection efficiency, which, together with their experimentally measured photoresponse repetition time of similar to 1 ps, makes them uniquely suitable for terahertz-frequency optoelectronic applications, ranging from ultrafast photon detectors and counters to THz-bandwidth optical-to-electrical transducers and photomixers. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4737442]

[EN] We present a Monte Carlo time-domain study of nanostructured ballistic three-branch junctions (TBJs) excited by both step-function and Gaussian picosecond transients. Our TBJs were based on InGaAs 2-dimensional electron gas heterostructures and their geometry followed exactly the earlier experimental studies. Time-resolved, picosecond transients of both the central branch potential and the between-the-arms current demonstrate that the bandwidth of the intrinsic TBJ response reaches the THz frequency range, being mainly limited by the large-signal, intervalley scattering, when the carrier transport regime changes from ballistic to diffusive., ROOTHz (FP7-243845)

We report our experimental studies on time-resolved pump–probe spectroscopy in high-quality Al0.86Ga0.14N single crystals, grown using a solution technique in a high-nitrogen-gas-pressure system. Our optical measurements were performed using a non-traditional, two-beam [one ultraviolet (UV) and one infrared (IR)], femtosecond pump–probe approach, in which the photon energies of both beams were below the bandgap of the sample and each electron–hole pair was generated by a multi-photon process of absorption of a pump photon together with another photon produced by second harmonic generation from two probe photons. Temporal scanning of the probe while monitoring the normalized transient differential transmissivity (ΔT/T) signal, produced a 310-fs-wide, Gaussian-shaped correlation signal caused by the multi-photon absorption process, followed by a >100-ps-long relaxation of photo-excited carriers. By studying the ΔT/T correlation signal amplitude dependence on the pump-power intensity and wavelength, the mult...

Ultrafast time-domain electro-optical characterization of the ballistic three-branch junctions (TBJs) has been performed to investigate their electrical response at terahertz (THz) frequencies. A TBJ rectifier consisting of two TBJs in parallel was placed in a coplanar waveguide (CPW) in which subpicosecond electrical pulses were generated by a photoconductive switch. A LiTaO3-based electro-optic transducer measured the waveform of the incident and transmitted voltage signals with a subpicosecond temporal resolution. This experimental configuration allowed the rectifier’s electrical response to be studied with a bandwidth of up to 1 THz. Detailed analysis of the TBJ rectifier and the photoconductive switch was performed in both the time domain and frequency domain. This analysis revealed poor coupling of the excitation signal into the rectifier because of a severe impedance mismatch between the rectifier and CPW as well as a parasitic capacitive coupling between the input and output ports of the rectifier...

Nonlinear electrical characteristics of nanostructured T-branch junctions (TBJs) made of two-dimensional electron gas in an InGaAs∕InAlAs heterostructure were studied by a systematic variation of both the device size and the operating temperature. We have found that two distinct mechanisms are responsible for the electronic transport in TBJs and their resulting nonlinear characteristics, namely, the nonlinear ballistic effect at low applied voltages and the intervalley transfer at high voltages. Detailed experimental analysis for each mechanism and their contributions with respect to the TBJ’s nanochannel length and operating temperature are discussed.

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.

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.

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.

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.