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Progress in high-performance computing demands significant advances in memory technology. Among novel memory technologies that promise efficient device operation on a sub-ns timescale, resistance switching between charge ordered phases of the 1T-TaS$_2$ has shown to be potentially useful for the development of high-speed, energy efficient non-volatile memory device. While ultrafast switching was previously reported with optical pulses, determination of the intrinsic speed limits of actual devices that are triggered by electrical pulses is technically challenging and hitherto still largely unexplored. A new optoelectronic laboratory-on-a-chip, designed for measurements of ultrafast memory switching, enables an accurate measurement of the electrical switching parameters with 100 fs temporal resolution. A photoconductive response is used for ultrashort electrical pulse generation, while its propagation along a coplanar transmission line is detected using electro-optical sampling using a purpose-grown highly-resistive electro-optic (Cd,Mn)Te crystal substrate. By combining the transmission line and the 1T-TaS$_2$ device in a single optoelectronic circuit a non-volatile resistance switching with a single 1.9 ps electrical pulse is demonstrated, with an extremely small switching energy density per unit area E$_A$ = 9.4 fJ/$\mu$m$^2$. The experiments demonstrate ultrafast, energy-efficient circuits utilizing switching between non-volatile charge-ordered states offers a new technological platform for cryogenic memory devices., Comment: 13 pages, 4 figures

We report on SPICE circuit modeling of traveling-wave superconducting-nanowire single-photon detectors integrated with Si3N4/SiO2 optical waveguides.

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 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 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.

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.

The phenomenon of dark counts in nanostripes of different superconductor systems such as high-temperature superconducting YBa 2 Cu 3 O 7-x and superconductor/ferromagnet hybrids consisting of either NbN/NiCu or YBa 2 Cu 3 O 7-x /L 0.7 Sr 0.3 MnO 3 bilayers have been investigated. For NbN/NiCu the rate of dark-count transients have been reduced with respect to pure NbN nanostripes and the events were dominated by a single vortex entry from the edge of the stripe. In the case of nanostripes based on YBa 2 Cu 3 O 7-x , we have found that thermal activation of vortices was also, apparently, responsible for triggering dark-count signals.

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

We report our results of investigation of electric and magnetic properties of partially oxygen-depleted channels for easy vortex motion in YBa2Cu3O7−x (YBCO) superconducting, 50-μm-wide, and 100-μm-long microbridges at temperatures below the onset of the superconducting state critical temperature T c on . The channels were produced by means of a laser-writing technique. The writing was performed using a 0.1–0.3 W power, continuous-wave laser radiation focused down to a ~ 5 μm spot on the surface of a superconducting film in a nitrogen gas atmosphere, and resulted in perpendicular stripes (channels) with partial (x ~ 0.2) reduction of the oxygen content in the YBCO stripe. The oxygen-depleted channels exhibit a depressed T c and lower both the critical current density and the first critical magnetic field, as compared with the laser-untreated areas. The bias current applied to the bridge self-produced a magnetic flux that penetrated the channels in a form of Abrikosov magnetic vortices that, subsequently, moved coherently (a quasi-Josephson effect) along the channels in the narrow temperature range of 0.943 T c on –0.98 T c on and manifested themselves as steps on the current–voltage characteristics of our microbridges. Our results demonstrate that laser-induced formation of artificial channels of the flux flow can be used for a precise control of vortex nucleation and their coherent motion in pre-assigned regions of thin-film YBCO devices.