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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 generated electro magnetic transients with a frequency content up to the terahertz regime from Co 2 Fe 0.4 Mn 0.6 Si Heusler alloy/heavy-metal bilayers by excitation with fs-laser pulses. We ascribe the generation process to the inverse spin Hall effect. We compared our results with ferromagnetic resonance measurements to investigate the efficiency of interfacial spin currents. We observed that the efficiency of the THz radiation can be described by the spin-orbit coupling, nonetheless the interface properties play an important role.

Paraffin-embedded murine pancreatic ductal adenocarcinoma and healthy pancreas tissues were investigated using pulsed terahertz time-domain spectroscopy in transmission geometry. A deconvolution procedure was employed to obtain the tissue impulse response from the raw Terahertz signals. We report a well-resolved differences between the tumor and healthy pancreas along with an enhanced absorbance in tumor tissue compared to its healthy counterpart.

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 generated pulses of electromagnetic radiation with a frequency content up to three terahertz (THz) by optical excitation of Co2Fe0.4Mn0.6Si Heusler alloy/heavy metal bilayers (CFMS/HM) using fs-laser pulses. We attribute the generation process to the conversion of a spin current, generated by the illumination with a fs-laser pulse, to a charge current via the inverse spin Hall effect. We compared the THz emission efficiency in CFMS/Pt and CFMS/Ta bilayers due to their high spin-orbit coupling of Pt and Ta. Surprisingly, our data reveal that CFMS/Pt shows substantially larger THz amplitudes compared to CFMS/Ta. Both bilayers exhibit a tunability of the THz amplitude by external magnetic field both at 300 K and 100 K. Ferromagnetic resonance measurements demonstrate that CFMS/Ta has a high effective spin mixing conductance, describing the efficiency of interfacial spin transport. We observe that the efficiency of the THz radiation cannot be solely described by the spin-orbit coupling strength and the spin diffusion length in the HM material plays an important role.

We report on the generation of electromagnetic transients of picosecond duration from La-Sr-Mn-O/Au and La-Sr-Mn-O/lr nanobilayers, illuminated by femtosecond laser pulses in the presence of an external magnetic field. Fourier analysis revealed that the frequency content of these transients extended up 4 THz. The amplitude of THz transients followed the functional dependence of La-Sr-Mn-O magnetization on the magnetic field and was also tunable by varying the intensity of the optical pulses. We ascribe the observed emission of THz transients to the inverse spin Hall effect, earlier demonstrated in metallic ferromagnet/metal spintronic nanobilayers.

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.

Thin films of YBa2Cu3O7–x (YBCO) oxygen-deficient superconductor were used to manufacture asymmetric microdiodes and investigate their asymmetric non-linear current–voltage (I–V) dependences at temperatures T > T c, where T c is the temperature of the onset of superconductivity in the material. The diodes are based on being asymmetrically narrowed down to a 10-μm-wide–neck mesa exhibit voltage asymmetry depending on the current bias conditions, the mesa's temperature, and a residual oxygen content in its neck region. The intrinsic electric field in the biased diode causes free-carrier drift velocity saturation, initiated in the mesa's neck and following its asymmetric enlarging towards the mesa's electrodes depending on the biasing pulsed current amplitude, polarity, and rise time/fall time of the pulse's leading/trailing edges. The variation of the I–V characteristics asymmetry has been associated with change in the mesa's electric resistivity, affected by the electric field's asymmetric distribution in the YBCO mesa. It has been also demonstrated that our asymmetric diodes with the reduced oxygen content are sensitive detectors of microwave radiation.

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.

Thermodynamic fluctuations of the superconducting order parameter in NbN/NiCu and NbTiN/NiCu superconductor/ferromagnet (S/F) thin bilayers patterned to microbridges are investigated. Plain NbN and NbTiN films served as reference materials for the analyses. The samples were grown using dc-magnetron sputtering on chemically cleaned sapphire single-crystal substrates. After rapid thermal annealing at high temperatures, the superconducting films were coated with NiCu overlays, using co-sputtering. The positive magnetoresistance of the superconducting single layers is very small in the normal state but with a sharp upturn close to the superconducting transition, a familiar signature of superconducting fluctuations. The fluctuation-enhanced conductivity (paraconductivity) of the NbN and NbTiN single layer films is slightly larger than the prediction of the parameter-free Aslamazov-Larkin theory for order-parameter fluctuations in two-dimensional superconductors. The addition of a ferromagnetic top layer, however, changes the magnetotransport properties significantly. The S/F bilayers show a negative magnetoresistance up to almost room temperature, while the signature of fluctuations is similar to that in the plain films, demonstrating the relevance of both ferromagnetic and superconducting effects in the S/F bilayers. The paraconductivity is reduced below theoretical predictions, in particular in the NbTiN/NiCu bilayers. Such suppression of the fluctuation amplitude in S/F bilayers could be favorable to reduce dark counts in superconducting photon detectors and lead the way to enhance their performance.

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.

This work reports on investigation of remnant oxygen content in optically-modified regions of 0.3-mm-thick YBa2Cu3O7–x films, patterned by a laser-writing technique in an inert ambient gas atmosphere at room temperature. A laser-treated region of weak superconductivity with dimensions depending on the size of a laser spot, laser power, and initial content of oxygen is characterized by a lower oxygen content, weaker critical magnetic field, and suppressed both the superconducting critical temperature and the critical current density, as compared to the laser untreated regions. Optically induced (cw-laser, 532-nm-wavelength) heating strongly affects a non-uniform distribution of remnant oxygen content in the film, depending both on the optical power and beam’s scanning velocity. A level of oxygen depletion and the size of the oxygen-deficient region have been directly estimated from scanning-electron-microscope spectra with the X-ray microanalysis technique. The results of our measurements were compared with results extracted from electric measurements, assuming a correlation between the remnant oxygen content and the electric transport properties of oxygen-deficient YBa2Cu3O7–x films. DOI: http://dx.doi.org/10.5755/j01.ms.20.2.6323

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.

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.

We report on ultrafast THz-pulse time-domain spectroscopy (TDS) and femtosecond optical-pump THz-probe (OPTP) studies of Hg-Ba-Ca-Cu-O (HBCCO) high-temperature, superconducting thin films. Our 500-nm-thick films were prepared by rf-magnetron sputtering of Re-Ba-Ca-Cu-O precursor films, followed by an ex-situ, high-temperature mercuration process. The resulting films were c axis oriented with a predominant Hg-1212 (plus some Hg-1223) phase. Their transition temperature T c had an onset at 122 K and zero resistance at 110 K. The THz TDS measurements demonstrated a sharp drop in the transmitted THz signal when the sample temperature was decreased below T c, which we directly related to a change in the imaginary component of the film complex conductivity. Simultaneously, the peak of the temperature-dependent real part of the conductivity was shifted toward lower frequencies at lower temperatures. The time-resolved OPTP spectroscopy experiments showed that the quasiparticle relaxation process exhibited an intrinsic single-picosecond dynamics with no phonon bottleneck, which is a unique feature among superconductors and makes the HBCCO material promising for ultrafast radiation detector applications.

We present results which describe procedures of the preparation and patterning of Hg,Re–Ba–Ca–Cu–O superconducting films suitable for coplanar structures usable as possible photodetectors and for microbridges. We compare structural and electrical properties of the final films from the point of their applicability for photodetectors. Our prepared Re-doped Hg-based films on LaAlO 3 substrate are continuous with a sufficient adhesion to the substrate and with a maximum zero resistance critical temperature T C0 ∼ 122 K. The X-ray diffraction of all the films confirmed the Hg-1212 phase beside the minor Hg-1223 phase and intergrowth phases in some cases. In spite of epitaxial character of the final Hg-based films, the microwave and magnetic measurements suggest a possible existence of nonstoichiometric material between the grains which we register as a presence of weak links near the transition into the superconducting state. We show some electrical properties of the prepared superconducting structures, too. The prepared coplanar structures show ultrafast photoresponse signal to incident laser pulse.

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 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.

Performance of superconducting single-photon detectors based on resistive hotspot formation in nanostripes upon optical photon absorption depends strongly on the critical current density J C of the fabricated nanostructure. Utilization of an ultrathin, weak-ferromagnet cap layer on the top of a superconducting film enhances of the structure’s J C due to an extra flux pinning. We have fabricated a number of both NbN/NiCu and NbTiN/NiCu superconductor/ferromagnet (S/F) ultrathin bilayers and microbridges. NbN and NbTiN underlayers with thicknesses varying from 4 to 7 nm were grown using dc-magnetron sputtering on chemically cleaned sapphire single-crystal substrates. After rapid thermal annealing at high temperatures, the S films were coated with Ni 0.54 Cu 0.46 overlayers with thicknesses of about 6 nm, using cosputtering. Compositions of the deposited films were confirmed by EDX spectroscopy analysis, while TEM studies demonstrated excellent epitaxial quality of our S layers with ~2-nm-thick F/S transition layer and atomically-sharp S/substrate interface. Magnetic properties of bilayers were studied using both the SQUID and Vibrating Sample Magnetometer techniques in low and high magnetic fields. Low-temperature tests confirmed that in all cases NiCu films were ferromagnetic with the Curie temperature of above 30 K. Below the bilayer critical temperature of approx. 12-13 K, the structures were fully proximitized with the strong superconducting signal. For superconducting transport properties characterization, we used bilayers patterned into 40-μm-long microbridges with the width varying from 0.4 μm to 2 μm. The same S/F nanostructures were also used to study their superconducting fluctuations. The temperature dependence of magnetoresistance demonstrated highly 2-dimensional character with an unusual negative region that extended almost to room temperature. In the S/F sample, the fluctuations were observed to be substantially below theoretical expectations.

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.

We report on femtosecond optical pump-probe studies of proximized ferromagnet/superconductor (F/S) hybrids, consisting of Ni0.5Cu0.5 layers deposited on top of Nb films. The weak ferromagnetic nature of the completely proximised Ni0.5Cu0.5 film makes possible to observe the dynamics of the nonequilibrium superconductivity through the near-surface optical reflectivity change measurements. The time-resolved photoresponse transient of the NiCu(21 nm)/Nb bilayer in the superconducting state shows strongly suppressed slow bolometric component. The fast relaxation time is also discussed accordingly to current theories on S/F heterostructures. The proposed S/F nanobilayers represent a new, artificially designed superconductor with the features (sub-picosecond photoresponse with suppressed bolometric component) very desirable for superconducting photodetectors and photon counters. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

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.

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 report the transient optical pump-probe reflectivity measurements on single and double layer cuprate single crystals and thin films of ten different stoichiometries. We find that with sufficiently low fluence the relaxation time (τR) of all samples exhibits a power law divergence with temperature (T ): τR ∝ T �3±0.5 . Further, the divergence has an onset temperature above the superconducting transition temperature for all superconducting samples. Possible causes of this divergence are discussed.

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 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.

We present measurements of ferromagnet/superconductor (NiCu/NbN) and plain superconducting (NbN) nanostripes with the linewidth ranging from 150 to 300 nm. The NiCu (3 nm)/NbN (8 nm) bilayers, as compared to NbN (8 nm), showed a up to six times increase in their critical current density, reaching at 4.2 K the values of 5.5 MA/cm2 for a 150 nm wide nanostripe meander and 12.1 MA/cm2 for a 300 nm one. We also observed six-time sensitivity enhancement when the 150 nm wide NiCu/NbN nanostripe was used as an optical detector. The strong critical current enhancement is explained by the vortex pinning strength and density increase in NiCu/NbN bilayers and confirmed by approximately tenfold increase in the vortex polarizability factor.

We present the latest generation of our superconducting single-photon detector (SPD), which can work from ultraviolet to mid-infrared optical radiation wavelengths. The detector combines a high speed of operation and low jitter with high quantum efficiency (QE) and very low dark count level. The technology enhancement allows us to produce ultrathin (3.5 nm thick) structures that demonstrate QE hundreds of times better, at 1.55 μm, than previous 10 nm thick SPDs. The best, 10 × 10 μm2, SPDs demonstrate QE up to 5% at 1.55 μm and up to 11% at 0.86 μm. The intrinsic detector QE, normalized to the film absorption coefficient, reaches 100% at bias currents above 0.9 Ic for photons with wavelengths shorter than 1.3 μm.

Using a femtosecond pump-probe optical technique, we measured the time-resolv ed change in reflectivity of three single-layer cuprate samples, including Bi2Sr 2 − x La x CuO 6 + z, La2CuO 4 + y and La 2 − x Sr x CuO4 grown on SrTiO3. We analyze the data in terms of a nonthermal electron relaxation model, and obtain quantitative agreement between the model and the data. The initial electron-electron thermalization process is much longer in the superconducting samples as opposed to the overdoped nonsuperconducting sample, suggesting a bottleneck in electron thermalization process due to the presence of the superconducting gap.

We report on fabrication and characterization of MgB 2 thin films and tunnel junction structures. The MgB 2 films were prepared on Al 2 O 3 , Si, glass, and plastic foil substrates by either vacuum codeposition of boron and magnesium, or high-temperature magnesium annealing of boron films. The crystalline structure of our films depended directly on the method of preparation. The films prepared by codeposition and postannealed in Ar atmosphere were amorphous with nanocrystal inclusions and were characterized by very smooth surfaces. On the other hand, the boron-precursor films annealed in magnesium vapor were rough, polycrystalline with approximately 1-μm-diameter single-crystal blocks. Because of their surface quality, the amorphous films were used for preparation of point contact junctions and for optical characterization. The point-contact spectra of tested junctions exhibited a two-gap structure. The MgB 2 polycrystalline films was used for bulk transport studies. The best films were characterized by the critical temperature T c of up to 39 K and the current density j c at 4.2 K of about 10 7 A/cm 2 .

A new microwave technique was successfully applied for direct studies of energy relaxation times in two-dimensional AlGaAs/GaAs structures under quasi-equilibrium conditions in the nanosecond and picosecond time scale. We report our results of energy relaxation time measurements in the temperature range 1.6-50 K, in quantum Hall effect regime in magnetic fields up to 4 T.

This work presents results of terahertz time-domain spectroscopy (THz-TDS) investigations of two types of polymer-based nanocomposites, consisting of bulk samples of graphene nanoflakes embedded in a polymer matrix and thin films where single-walled carbon nanotubes (SWCNTs) with a chiral index (7,5) were wrapped in single strains of a polymer. Our THz-TDS setup is a room-temperature system with a frequency range of 0.1 to 3.5 THz, based on photoconductive switches as both the emitter and detector, excited by 100-fs-wide optical pulses. We have studied THz spectra of both types of samples and compared them to the ones obtained for the pure polymer reference specimens and fitted the experimental complex conductivity data to the Drude–Smith model. The Drude–Smith model fits our data well, demonstrating that graphene nanoflakes and SWCNTs, exhibit highly localized intra grain/tube electron backscattering with a femtosecond relaxation time.