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We demonstrate generation of time-resolved THz transients in a FeCo/graphene nanobilayer, excited by femtosecond optical laser pulses. The signal sign dependency on the experimental geometry and the external magnetic field allows assigning the mechanism of the THz radiation to the inverse spin Hall effect with a spin-to-charge current conversation parameter of the order of 0.001 nm.

Cd 1-x Mn x Te crystals exhibit a strong optical rectification and electro-optic effects. We present our studies on Cd 0.55 Mn 0.45 Te single crystals, operating as efficient emitters and detectors of THz radiation transients, when excited by femtosecond optical pulses.

We report on design, fabrication, and characterization of silicon quantum photonic integrated circuits comprising superconducting nanostripe single-photon detectors integrated with dielectric optical waveguides. In order to enhance absorption of photons by the superconducting nanostripes, the detectors are located directly on the dielectric waveguide core.

We explored THz emission from laser illuminated Fe/heavy-metal bilayer spintronic emitters grown epitaxially on top of GaAs(001) substrates. The fabricated emitters show an overall enhancement of the transient THz radiation compared to structures fabricated on insulating substrates. In addition, the THz amplitude shows strongly vertically off-set hysteretic behavior in varying magnetic field. We ascribe these observations to the interplay of several mechanisms including inverse spin Hall effect, optical rectification, Lorentz force and chemical bonding at the epitaxial Fe/GaAs(001) interface.

We have generated intense electromagnetic transients by femtosecond laser pulse illumination of ferromagnet/metal (F/M) nanobilayers, in the presence of an external magnetic field. Fourier analysis revealed that the frequency content of these transients extended up to ~5 THz. We have also observed that upon the increase of the magnetic field, the entire THz transient shifts towards earlier times by up to 110 fs. We ascribe this magnetically tunable onset-time shift to extra acceleration of photoelectrons induced due to the Lorenz force.

We demonstrate that a nitrogen-implanted GaAs can be successfully implemented as a tunable, THz frequency range photomixer, optimized for the best performance for optical excitation in the 760–800 nm range. The latter was obtained by fabricating a metal-semiconductor-metal diode on top of a Bragg mirror structure and resulted in a clear enhancement of the THz radiation emission in photomixing experiments.

Irradiation of semiconductor surfaces with femtosecond optical laser pulses is one of the common techniques for broadband, free-space THz transient generation. We demonstrate that the amplitude of surface-emitted THz pulses scales linearly with an applied, external, in-plane magnetic field. We studied the effect in several highly resistive GaAs samples and ascribe it to the Lorentz force that additionally accelerates optically excited carriers.

We report on design, fabrication, and characterization of superconducting nanostripe single-photon detectors integrated with dielectric optical waveguides, whereby part of the upper cladding is removed to enhance absorption of photons by the superconducting nanostripes.

We present a new operation regime of NbN superconducting single-photon detectors (SSPDs) by integrating them with a low-noise cryogenic high-electron-mobility transistor and a high-load resistor. T ...

We generate terahertz (THz) transients by illuminating a few-nanometer-thick Ta/NiFe/Pt nanolayers with a train of linearly polarized 100-fs-wide laser pulses. The transients are ∼1-ps-wide free-space propagating bursts of electromagnetic radiations with amplitudes that are magnetically and optically tunable. Their spectral frequency content extends up to 5 THz, and the 3-dB cutoff is at 0.85 THz. The observed transient electromagnetic signals originate from the NiFe/Pt bilayer, and their amplitude dependence on the external magnetic field, applied in the sample plane, very closely follows the static magnetization versus magnetic field dependence of the NiFe film. For the same laser power, excitation with highly energetic, blue light generates THz transients with amplitudes approximately three times larger than the ones resulting from excitation by infrared light. In both cases, the transients exhibit the same spectral characteristics and are linearly polarized in the perpendicular direction to the sample magnetization. The polarization direction can be tuned by rotation of the magnetic field around the laser light propagation axis. The characteristics of our THz spintronic emitter signals confirm that THz transient generation is due to the inverse spin Hall effect in the Pt layer and demonstrate that ferromagnet/metal nanolayers excited by femtosecond laser pulses can serve as efficient sources of magnetically and optically tunable, polarized transient THz radiation.

During the last decade, research on nanowires as superconducting single-photon detectors (SNSPDs or SSPDs) has gained interest due to many experiments demonstrating their fascinating potentialities in several fields. The search for more appealing materials for SNSPDs is presently a hot topic. Several superconductors have been proposed as possible alternatives to NbN, which is presently the racehorse. Examples are WSi and MoSi, which have advantages such as amorphous morphology and narrow-gap and low quasi-particle diffusivity, which permits extending the application in the mid- and far-IR ranges. A strategy based on material hybridization of superconductors with ferromagnets can open a new direction in the development of SNSPD. We present results concerning the hybrid superconducting/ferromagnetic (S/F) single nanowires for SNSPDs. Experiments under laser illumination show a single-photon response of S/F-based nanowires. The ferromagnetic overlayer has the beneficial effect of an increase of the superconducting critical current density and a decrease of dark counts. We compare dark counts in hybrid S/F with pure S nanowires. The experimental results are explained in terms of the existing models based on the presence of magnetic vortices.

The asymmetric nano-channel diode (ANCD) is the 2-dimensional electron gas (2DEG) semiconductor nanodevice that, unlike a conventional diode, relies on the device nanostructure and field-controlled transport in a ballistic nanometerwidth channel instead of barriers to develop its asymmetric, diode-like current-voltage (I-V) characteristics. We focus on ANCD optoelectronic properties, and demonstrate that the devices can act as very sensitive, single-photon-level, visiblelight photodetectors. Our test structures consist of 2-μm-long and ~230-nm-wide channels and were fabricated using electron-beam lithography on a GaAs/AlGaAs heterostructure with a 2DEG layer, followed by reactive ion etching. The I-V curves were collected by measuring the transport current under the voltage-source biasing condition, both in the dark and under light illumination. The experiments were conducted inside a cryostat, in a temperature range from 300 K to 78 K. As an optical excitation, we used a 800-nm-wavelength, generated by a commercial Ti:sapphire laser operated either at a quasi-continuous–wave mode or as a source of 100-fs-wide pulses. The impact of the light illumination was very clear, and at low temperatures we observed a significant photocurrent Iph ~ 0.25 μA at temperature 78 K for the incident optical power as low as 1 nW, with a limited dark-current background. The magnitude of the device optical responsivity increased linearly with the decrease of the optical power, reaching for 1-nW optical excitation the value as high as ~400 A/W at room temperature and >800 A/W at 78K. The physics of the photoresponse gain mechanism in the ANCD arises from a vast disparity between the sub-picosecond transit time of photo-excited electrons travelling in the 2DEG nanochannel and the up to microsecond lifetime of photo-excited holes pushed towards the device substrate.

We present our studies of electric properties of micro-diodes based on asymmetrically narrowed, partially oxygen-depleted, semiconducting YBa 2 Cu 3 O 7−x thin-film mesas. A level of asymmetry of nonlinear current-voltage characteristics of our diodes increases with the decrease of the residual oxygen content in their neck region and with increasing the operating temperature. The largest asymmetry is observed for diodes with x ∼ 0.5 and at T = 300 K. An asymmetric distribution of the electric potential for different bias polarities initiate an asymmetric, non-uniform distribution of intrinsic electric field due to heating of carriers in the diode. The experimental results and possible diode's technology are discussed.

Publisher's version (útgefin grein), The terahertz time-domain spectroscopy (THz-TDS) technique has been used to obtain transmission THz-radiation spectra of polymer nanocomposites containing a controlled amount of exfoliated graphene. Graphene nanocomposites (1 wt%) that were used in this work were based on poly(ethylene terephthalate-ethylene dilinoleate) (PET-DLA) matrix and were prepared via a kilo-scale (suitable for research and development, and prototyping) in-situ polymerization. This was followed by compression molding into 0.3-mm-thick and 0.9-mm-thick foils. Transmission electron microscopy (TEM) and Raman studies were used to confirm that the graphene nanoflakes dispersed in a polymer matrix consisted of a few-layer graphene. The THz-radiation transients were generated and detected using a low-temperature-grown GaAs photoconductive emitter and detector, both excited by 100-fs-wide, 800-nm-wavelength optical pulses, generated at a 76-MHz repetition rate by a Ti:Sapphire laser. Time-domain signals transmitted through the nitrogen, neat polymer reference, and 1-wt% graphene-polymer nanocomposite samples were recorded and subsequently converted into the spectral domain by means of a fast Fourier transformation. The spectral range of our spectrometer was up to 4 THz, and measurements were taken at room temperature in a dry nitrogen environment. We collected a family of spectra and, based on Fresnel equations, performed a numerical analysis, that allowed us to extract the THz-frequency-range refractive index and absorption coefficient and their dependences on the sample composition and graphene content. Using the Clausius-Mossotti relation, we also managed to estimate the graphene effective dielectric constant to be equal to ~7 ± 2. Finally, we extracted from our experimental data complex conductivity spectra of graphene nanocomposites and successfully fitted them to the Drude-Smith model, demonstrating that our graphene nanoflakes were isolated in their polymer matrix and exhibited highly localized electron backscattering with a femtosecond relaxation time. Our results shed new light on how the incorporation of exfoliated graphene nanoflakes modifies polymer electrical properties in the THz-frequency range. Importantly, they demonstrate that the complex conductivity analysis is a very efficient, macroscopic and non-destructive (contrary to TEM) tool for the characterization of the dispersion of a graphene nanofiller within a copolyester matrix., This research was funded in part by the PumpPrimerII Program at the University of Rochester. A.N.V. acknowledges support from the Air Force Office of Scientific Research (FA9550-16-1-0020). M.E.F. acknowledges support from the Polish National Centre for Research and Development (PBS1/A5/2/2012).

We developed a transfer and integration technique for freestanding Ge 1 Sb 2 Te 4 nano-membranes. A so-called laser micro annealing process for the precisely-local formation of low resistance Ge 1 Sb 2 Te 4 /Ti/Au ohmic contacts was optimized. Ultrafast switching effects were studied on nano-membrane devices after their implementation into a group III-nitride based optoelectronic circuit. Highly resistive switching was observed without any noticeable structural deterioration. Charge transport measurements indicate that current densities exhibit values in the "READ resp. SET/RESET" regime from 10−6 down to 10−11A/μm3 after ∼100 switching cycles. Our study on freestanding Ge 1 Sb 2 Te 4 nano-membranes contributes to a better understanding of charge transport related physical phenomena in novel chalcogenide material systems with high potential for future low energy consumption data storage devices.

We present transient THz spectroscopy studies of graphene nano-flakes imbedded in a polymer medium, forming a nanocomposite with 1% graphene content. We measured both the pure polymer and nanocomposite transmission spectra extending up to 4 THz and by comparing them obtained the absorption, the complex index of reflection, and conductivity of the embedded graphene. The conductivity results follow the Drude-Smith model with the imaginary part being negative, a property that indicates the total backscattering of carriers within the graphene grains.

We present exploratory studies of digital circuit design using the recently proposed ballistic deflection transistor (BDT) devices. We demonstrate a variety of possible logic functions through simple reconfiguration of two drain-connected BDTs. We further propose the creation of a three-BDT logic cell to yield differential versions of each logic function, improving overall flexibility of BDT circuit design. Each of the proposed gate configurations has been verified through extensive numerical calculations using an in-house Monte Carlo simulator. Simulation results show that the proposed gate arrangements are capable of achieving 400-GHz operating frequencies at room temperature. A compact fit-based analytical model to aid circuit design using BDTs is also introduced.

The successful application of finite element analysis to ultrafast optoelectronic devices is demonstrated. Finite element models have been developed for both an alloyed- and surface-contact metal–semiconductor–metal photodetectors. The simulation results agree with previously reported experimental data. The alloyed device, despite having a somewhat larger capacitance, has a non-illuminated region of lower resistance with a more-uniform and deeper-penetrating electric field and carrier transport current. The latter explains, in terms of the equivalent lumped parameters, the experimentally observed faster response of the alloyed device. The model is further used to predict improved responsivity, based on electrode spacing and antireflective coating. We project that increasing the depth of the alloyed contact beyond approximately half of the optical penetration depth will not yield significantly improved responsivity.

This paper studies integrated silicon photodiodes (PDs) implemented in standard CMOS technologies. A new PIN PD structure utilizing deep n-well is presented, and compared with conventional vertical and lateral PIN PDs at 850-nm wavelength and different bias conditions. Prototype PDs were fabricated in a 0.18-mum standard CMOS technology, and their DC, impulse and frequency responses were characterized. A 70 times 70 mum2 PD with the new structure achieved a 3-dB bandwidth of 2.2 GHz in small signal at 5-V bias, whereas conventional lateral and vertical PIN PDs could only operate up to 0.94 GHz and 1.15 GHz, respectively. At 5-V bias, the impulse response of the new PD exhibited a full-width at half-maximum pulsewidth of 127 ps, versus 175 and 150 ps for the conventional lateral and vertical ones, respectively. At 15.5-V bias, the bandwidth of this new PD reached 3.13 GHz, with an impulse response pulsewidth of 102 ps. The responsivity of all prototype PDs was measured at approximately 0.14 A/W up to 10-V bias, which corresponded to a quantum efficiency of 20%. The responsivity of the new PD could be further increased to 0.4 A/W or 58% quantum efficiency, when operating in the avalanche region at 16.2-V bias.

Dark counts in superconducting single-photon detectors (SSPDs) manifest themselves as spontaneous, transient voltage pulses, typically indistinguishable from photon counts. We present here a new readout technique based on integrating the SSPD with a low-noise, cryogenic high-electron-mobility transistor (HEMT) with high-input impedance. This arrangement allowed us to achieve amplitude resolution of the recorded output transients. In two-dimensional superconducting nanostripes, the physics of photon counting is based on the hotspot formation mechanism, while the dark counts correspond to voltage transients triggered by the vortex-antivortex motion and/or phase-slip centers. Thus, their respective transients can be distinguished by comparing the output pulse amplitude distributions. Our scheme also allowed us to perform photon-energy-resolution studies by comparing the SSPD output pulse amplitude distributions (the mean pulse amplitude and the distribution width) collected for incident single photons with different energies.

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 mum). 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.

We report on time-resolved, transient photoimpedance measurements performed on current-biased microbridges excited by 100-fs wide, 800-nm wavelength, and 76-MHz repetition-rate optical pulses. Our bridges had dimensions of 20 mum by 40 mum and were patterned by conventional photolithography and ion milling from 150-nm-thick, epitaxial MgB2 films grown by a hybrid physical-chemical vapor-deposition technique. For high-speed detection experiments, the microbridges were embedded into coplanar-strip transmission lines and connected to a 20-GHz-bandwidth sampling oscilloscope via a semirigid coaxial cable. The test structures were placed inside an optical cryostat and studies were performed in the temperature range between 15 K and 40 K. At temperatures, e.g., 20 K, far below the bridge critical temperature and under low optical excitation, large and fast (130 ps in duration) photoresponse transients were observed, which we associate with the kinetic-inductive (Cooper-pair breaking) mechanism. At temperatures approaching the MgB2 critical temperature and/or under intense light illumination, an additional slow component of the photoresponse with a nanosecond decay time was measured. We interpret this second photoimpedance component as a resistive response, resulting from the hot-electron heating effect. The observed picosecond kinetic photoresponse makes our MgB2 bridges attractive candidates for fast and efficient optical detectors and photon counters.

We present our studies on dark counts, observed as transient voltage pulses, in current-biased NbN superconducting single-photon detectors (SSPDs), as well as in ultrathin (~4 nm), submicrometer-width (100 to 500 nm) NbN nanobridges. The duration of these spontaneous voltage pulses varied from 250 ps to 5 ns, depending on the device geometry, with the longest pulses observed in the large kinetic-inductance SSPD structures. Dark counts were measured while the devices were completely isolated (shielded by a metallic enclosure) from the outside world, in a temperature range between 1.5 and 6 K. Evidence shows that in our two-dimensional structures the dark counts are due to the depairing of vortex-antivortex pairs caused by the applied bias current. Our results shed some light on the vortex dynamics in 2D superconductors and, from the applied point of view, on intrinsic performance of nanostructured SSPDs.

We present an overview of the state-of-the-art of NbN superconducting single-photon detectors (SSPDs). Our devices exhibit quantum efficiency (QE) of up to 30% in near-infrared wavelength and 0.4% at 5 mum, with a dark-count rate that can be as low as 10-4 s-1. The SSPD structures integrated with lambda/4 microcavities achieve a QE of 60% at telecommunication, 1550-nm wavelength. We have also developed a new generation of SSPDs that possess the QE of large-active-area devices, but, simultaneously, are characterized by low kinetic inductance that allows achieving short response times and the GHz-counting rate with picosecond timing jitter. The improvements presented in the SSPD development, such as fiber-coupled SSPDs, make our detectors most attractive for high-speed quantum communications and quantum computing.

We report our time-resolved transient photoresponse studies of current-biased Hg-Ba-Ca-Cu-O superconducting microbridges excited by 100-fs-wide and 800-nm-wavelength laser pulses. Our devices consisted of 20-mum wide and 30-mum long bridges, incorporated into a 0.1-mm-wide signal line of the coplanar strip transmission line. The structures were patterned in 400-nm-thick precursor films grown on substrates by room-temperature magnetron sputtering and, subsequently, exposed to an ex-situ high-temperature mercuration process. The resulting structures consisted of a c-axis-oriented, predominantly Hg-1212 compound with the onset superconducting critical temperature Tc,on around 110 K. At low temperatures (far below Tc ), under low bias current and low optical excitation, the microbridge photoresponse signal was a positive, ~90-ps-wide (measurement- limited) pulse followed by a negative component and was related to the kinetic-inductive response. At high temperatures, or when the bias current was near the bridge critical current, we observed an additional slow resistive response due to the bolometric effect. The observed ultrafast photoresponse dynamics in our Hg-Ba-Ca-Cu-O bridges make this material very promising for optical photodetector and mixer applications.

We present a novel, two-channel, single photon receiver based on two fibre-coupled, NbN, superconducting, single photon detectors (SSPDs). The SSPDs are nanostructured superconducting meanders and are known for ultrafast and efficient detection of visible-to-infrared photons. Coupling between the NbN detector and optical fibre was achieved using a micromechanical photoresist ring placed directly over the SSPD, holding the fibre in place. With this arrangement, we obtained coupling efficiencies up to ∼30%. Our experimental results showed that the best receiver had a near-infrared system quantum efficiency of 0.33% at 4.2 K. The quantum efficiency increased exponentially with the photon energy increase, reaching a few percent level for visible-light photons. The photoresponse pulses of our devices were limited by the meander high kinetic inductance and had the rise and fall times of approximately 250 ps and 5 ns, respectively. The receiver's timing jitter was in the 37 to 58 ps range, approximately 2 to 3 t...

The possible use of high-temperature superconductors (HTS) for realizing superconducting nanowire single-photon detectors is a challenging, but also promising, aim because of their ultrafast electron relaxation times and high operating temperatures. The state-of-the-art HTS nanowires with a 50-nm thickness and widths down to 130 nm have been fabricated and tested under a 1550-nm wavelength laser irradiation. Experimental results presenting both the amplitude and rise times of the photoresponse signals as a function of the normalized detector bias current, measured in a wide temperature range, are discussed. The presence of two distinct regimes in the photoresponse temperature dependence is clearly evidenced, indicating that there are two different response mechanisms responsible for the HTS photoresponse mechanisms.

Selected possible applications of (Cd, Mn)Te, a classical wide-bandgap semimagnetic semiconductor, will be described. Both the semimagnetic character and the other physical properties of this material give rise to its wide applications. As a semimagnetic semiconductor (Cd, Mn)Te demonstrates “giant Faraday rotation” (of the direction of polarization in a strong static magnetic field) for photons with energy a little below that of the free exciton energy. This effect, which is strongest at low temperatures, is also significant at room temperature. (Cd, Mn)Te-based “Faraday rotators” can be used in optical telecommunications as “optical isolators” between the source of radiation (a strong laser) and the rest of the optical transmission line to avoid having parasitic light reflected back to the laser and disturbing its operation. In (Cd, Mn)Te crystals the Faraday effect is extremely fast. The results of the experiments, showing that the response time of the (Cd, Mn)Te crystals to a pulsed magnetic field is in the range 300–500 fs, will be presented, as well as the possible applications of this super-fast Faraday effect. As a wide-bandgap semiconductor with high stopping power for X- and γ-rays, (Cd, Mn)Te can be applied to the construction of radiation detectors. The required parameters: high resistivity (∼10 10 Ω cm) and a high mobility–lifetime product ( μτ = 3 × 10 −3 cm 2 /V) have already been achieved and the results will be described.

We report on our progress in development of superconducting single-photon detectors (SSPDs), specifically designed for secure high-speed quantum communications. The SSPDs consist of NbN-based meander nanostructures and operate at liquid helium temperatures. In general, our devices are capable of GHz-rate photon counting in a spectral range from visible light to mid-infrared. The device jitter is 18 ps and dark counts can reach negligibly small levels. The quantum efficiency (QE) of our best SSPDs for visible-light photons approaches a saturation level of ~30-40%, which is limited by the NbN film absorption. For the infrared range (1.55µm), QE is ~6% at 4.2 K, but it can be significantly improved by reduction of the operation temperature to the 2-K level, when QE reaches ~20% for 1.55-µm photons. In order to further enhance the SSPD efficiency at the wavelength of 1.55 µm, we have integrated our detectors with optical cavities, aiming to increase the effective interaction of the photon with the superconducting meander and, therefore, increase the QE. A successful effort was made to fabricate an advanced SSPD structure with an optical microcavity optimized for absorption of 1.55 µm photons. The design consisted of a quarter-wave dielectric layer, combined with a metallic mirror. Early tests performed on relatively low-QE devices integrated with microcavities, showed that the QE value at the resonator maximum (1.55-µm wavelength) was of the factor 3-to-4 higher than that for a nonresonant SSPD. Independently, we have successfully coupled our SSPDs to single-mode optical fibers. The completed receivers, inserted into a liquid-helium transport dewar, reached ~1% system QE for 1.55 µm photons. The SSPD receivers that are fiber-coupled and, simultaneously, integrated with resonators are expected to be the ultimate photon counters for optical quantum communications.

We present our magnetooptical Kerr effect (MOKE) studies on picosecond spin dynamics in closely spaced rectangular Co dots with sizes ranging from 2/spl times/6 /spl mu/m/sup 2/ down to 100 /spl times/ 300 nm/sup 2/ under in-plane, picosecond, magnetic pulse excitation. A low-temperature-grown GaAs photoconductive switch, excited by femtosecond laser pulses, was used to produce /spl sim/25-ps-long magnetic transients along the surface of the coplanar-waveguide centerline, which contained arrays of patterned Co dots. The resulting spin dynamics in the dots was detected by a time-synchronized train of femtosecond optical probe pulses employing the time-resolved MOKE. Our experimental technique allowed us to measure the initial picosecond dynamics of spins in Co nanodots, followed by damped oscillations. We ascribe the observed results to the small-angle coherent spin precession and show that it depends on the size of magnetic dots.

We present our studies on the quantum efficiency (QE) and the noise equivalent power (NEP) of the latest-generation, nanostructured, superconducting, single-photon detectors (SSPDs) in the wavelength range from 0.5 to 5.6 /spl mu/m, operated at temperatures in the 2.0- to 4.2-K range. Our detectors are designed as 4-nm-thick and 100-nm-wide NbN meander-shaped stripes, patterned by electron-beam lithography and cover a 10/spl times/10-/spl mu/m/sup 2/ active area. The best-achieved QE at 2.0 K for 1.55-/spl mu/m photons is 17%, and QE for 1.3-/spl mu/m infrared photons reaches its saturation value of /spl sim/30%. The SSPD NEP at 2.0 K is as low as 5/spl times/10/sup -21/ W/Hz/sup -1/2/. Our nanostructured SSPDs, operated at 2.0 K, significantly outperform their semiconducting counterparts, and, together with their GHz counting rate and picosecond timing jitter, they are devices-of-choice for practical quantum key distribution systems and free-space (even interplanetary) quantum optical communications.

We have developed a fabrication process for GHz-counting-rate, single-photon, high-detection-efficiency, NbN, nanowire detectors. We have demonstrated two processes for the device patterning, one based on the standard polymethylmethacrylate (PMMA) organic positive-tone electron-beam resist, and the other based on the newer hydrogen silsesquioxane (HSQ) negative-tone spin-on-glass resist. The HSQ-based process is simple and robust, providing high resolution and the prospect of high fill-factors. Initial testing results show superconductivity in the films, and suggest that the devices exhibit photosensitivity.

We present fabrication and optical time-resolved photoresponse characterization of Hg-based high-temperature superconducting (HTS) thin films. The films were prepared from RF-magnetron-sputtered precursor films by an ex-situ mercuration process. Femtosecond, time-resolved, optical pump-probe spectroscopy measurements gave insight into the details of the Cooper-pair breaking and formation processes in our Hg-based films at temperatures far below the material's critical temperature. We observed that the Cooper-pair dynamics is not limited by the phonon bottleneck, which makes this HTS compound the material-of-choice for ultrafast optoelectronics applications.

We report on the GHz counting rate and jitter of our nanostructured superconducting single-photon detectors (SSPDs). The devices were patterned in 4-nm-thick and about 100-nm-wide NbN meander stripes and covered a 10-/spl mu/m/spl times/10-/spl mu/m area. We were able to count single photons at both the visible and infrared telecommunication wavelengths at rates of over 2 GHz with a timing jitter of below 18 ps. We also present the model for the origin of the SSPD switching dynamics and jitter, based on the time-delay effect in the phase-slip-center formation mechanism during the detector photoresponse process. With further improvements in our readout electronics, we expect that our SSPDs will reach counting rates of up to 10 GHz. An integrated quantum communications receiver based on two fiber-coupled SSPDs and operating at 1550-nm wavelength is also presented.

We present our progress on the research and development of NbN superconducting single-photon detectors (SSPD's) for ultrafast counting of near-infrared photons for secure quantum communications. Our SSPD's operate in the quantum detection mode based on the photon-induced hotspot formation and subsequent development of a transient resistive barrier across an ultrathin and submicron-width superconducting stripe. The devices are fabricated from 4-nm-thick NbN films and kept in the 4.2- to 2-K temperature range. The detector experimental quantum efficiency in the photon-counting mode reaches above 40% for the visible light and up to 30% in the 1.3- to 1.55-µm wavelength range with dark counts below 0.01 per second. The experimental real-time counting rate is above 2 GHz and is limited by our readout electronics. The SSPD's timing jitter is below 18 ps, and the best-measured value of the noise-equivalent power (NEP) is 5 × 10–21 W/Hz1/2 at 1.3 µm. In terms of quantum efficiency, timing jitter, and maximum counting rate, our NbN SSPD's significantly outperform semiconductor avalanche photodiodes and photomultipliers in the 1.3- to 1.55-µm range. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

M. Marsoa,∗, M. Mikulics, R. Adam, S. Wu, X. Zheng, I. Camara, F. Siebe, A. Forster, R. Gusten, P. Kordos and R. Sobolewski Institute of Thin Films and Interfaces, Research Center Julich 52425 Julich, Germany Max-Planck-Institut fur Radioastronomie, 53121 Bonn, Germany Department of Electrical and Computer Engineering and Laboratory for Laser Energetics University of Rochester, Rochester, NY 14627-0231, USA

We present ultrafast optical characterization of Cd/sub 1-x/Mn/sub x/Te single crystals with high (x>0.5-Mn) concentration, studied by magnetooptical sampling and time-resolved magnetization modulation spectroscopy. We have demonstrated that the dynamics of both Mn spins and carrier spins in Cd/sub 1-x/Mn/sub x/Te is extremely fast (in the subpicosecond range), making the nanostructures based on this material very promising for applications in spintronics and magnetooptics.

The paper reports progress on the design and development of niobium-nitride, superconducting single-photon detectors (SSPDs) for ultrafast counting of near-infrared photons for secure quantum communications. The SSPDs operate in the quantum detection mode, based on photon-induced hotspot formation and subsequent appearance of a transient resistive barrier across an ultrathin and submicron-width superconducting stripe. The devices are fabricated from 3.5 nm thick NbN films and kept at cryogenic (liquid helium) temperatures inside a cryostat. The detector experimental quantum efficiency in the photon-counting mode reaches above 20% in the visible radiation range and up to 10% at the 1.3–1.55 μn infrared range. The dark counts are below 0.01 per second. The measured real-time counting rate is above 2 GHz and is limited by readout electronics (the intrinsic response time is below 30 ps). The SSPD jitter is below 18 ps, and the best-measured value of the noise-equivalent power (NEP) is 2 × 10−18 W/Hz1...

Ultrafast voltage transients from optically thick YBa2Cu3O7−x (YBCO) microbridges, dc-biased and triggered with femtosecond optical pulses at temperatures below the YBCO critical temperature were used to excite radiation of a transmitting gigahertz-frequency antenna. The shape of the power spectrum of the antenna radiation depended on the time evolution and amplitude of the YBCO photoresponse transient, which was controlled by the incident laser fluence and the microbridge temperature and bias current. The main contribution to the above 10 GHz radiation was attributed to the photoresponse rising edge, generated due to the initial supercurrent redistribution and the kinetic-inductive effect. Low-frequency antenna radiation was excited by the photoresponse component associated with heating and cooling effects generated in the superconducting microbridge. Our antenna radiation approach can be used for time-resolved investigation of electric current distributions in photoactivated YBCO thin films.

We present a new, simple to manufacture superconducting single-photon detector operational in the range from ultraviolet to mid-infrared radiation wavelengths. The detector combines GHz counting rate, high quantum efficiency and very low level of dark (false) counts. At 1.3-1.5 µm wavelength range our detector exhibits a quantum efficiency of 5-10%. The detector photoresponse voltage pulse duration was measured to be about 150 ps with jitter of 35 ps and both of them were limited mostly by our measurement equipment. In terms of quantum efficiency, dark counts level, speed of operation the detector surpasses all semiconductor counterparts and was successfully applied for CMOS integrated circuits diagnostics.

Studies on time-resolved dynamics of optically-thick YBa 2 Cu 3 O 7-δ microbridges biased with supercritical current pulses I of nanosecond duration and synchronously excited by femtosecond optical pulses have been carried out. The switching process is explained by the transient vortex dynamics in the superconducting stripe. It was found that the amplitude of the photoresponse increased linearly with the increase of laser fluence up to < 30 μ J / cm 2 and with the increase of the bias current up to ~ 1.9Ic. Calculations of quantum responsivity showed that our YBa 2 Cu 3 O 7-δ bridges biased with I > Ic could driven by optical pulses of comparatively weak fluence in the range of few tens of mW.

We report our studies on ultrafast voltage transients of optically thick superconducting YBa2Cu3O7−x microbridges biased with nanosecond supercritical current pulses (I > Ic) and, simultaneously, illuminated with 100 fs optical pulses (810 nm wavelength) from a Ti:sapphire laser. The pulsed current bias created a resistive flux-flow state in the superconductor, while the laser pulse transferred it into a state of more intensive flow of vortices within a time of less than 100 ps. The light-enhanced flux state remained constant until the end of the biasing pulse. The amplitude of the photoresponse signal increased rapidly with the increase of laser fluence in the range from 8 × 106 to 2 × 108 photons per pulse, as well as with the current-bias increase up to 2Ic, exhibiting the maximum, when the light illuminated the entire area of the microbridge. Maximal repetition rate of optically excited YBa2Cu3O7−x photoresponse signals was found to be in the GHz range, appropriate for applications of YBa2Cu3O7−x thin-film microbridges as high-power ultrafast switches.

Fabrication of NbN superconducting single-photon detectors, based on the hotspot effect is presented. The hotspot formation arises in an ultrathin and submicrometer-width superconductor stripe and, together with the supercurrent redistribution, leads to the resistive detector response upon absorption of a photon. The detector has a meander structure to maximally increase its active area and reach the highest detection efficiency. Main processing steps, leading to efficient devices, sensitive in 0.4-5 /spl mu/m wavelength range, are presented. The impact of various processing steps on the performance and operational parameters of our detectors is discussed.

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 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.

We report our time-resolved measurements of NbN-based superconducting single-photon detectors. The structures are meander-type, 10-nm thick, and 200-nm wide stripes and were operated at 4.2 K. We have shown that the NbN devices can count single-photon pulses with below 100-ps time resolution. The response signal pulse width was about 150 ps, and the system jitter was measured to be 35 ps.

We present the fabrication and optical time-resolved photoresponse characterization of MgB/sub 2/ superconducting thin films. The films were prepared on crystalline and flexible substrates by vacuum co-deposition of B and Mg precursors and high-temperature annealing in an Ar atmosphere. The postannealed films exhibited very smooth surfaces and amorphous morphology with nanocrystal inclusions. Optical pump-probe measurements gave the first insight into the carrier dynamics in MgB/sub 2/ by time-resolved experimental studies of the Cooper-pair breaking and thermalization mechanisms.