A. Jukna

Dark Counts in Nanostructured NbN Superconducting Single-Photon Detectors and Bridges

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.

Spectroscopy With Nanostructured Superconducting Single Photon Detectors

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.

Laser processed channels of easy vortex motion in YBa2Cu3O7−δ films

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

Electric transport properties of YBa2Cu3O7−δ thin-film bridges with laser-written channels of easy vortex motion

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

Time-resolved photoresponse in the resistive flux-flow state in Y–Ba–Cu–O superconducting microbridges

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.

Electric properties of contacts to HTS thin films at current densities J > J{sub c}

The role of superconducting film contacts when transporting critical current density was investigated using d.c. and electric short pulses. It was found that both d.c. and pulse current exceeding critical current Icdissipate at the film contacts, i.e. at the metal/superconductor planar interface containing an intermediate layer of degraded superconductor. Current I > Icdissipation at the contacts initiates electric-thermal destruction of the superconducting film cathode. The resulting damage to the film is explained in terms of the formation of a (p/i/n) junction at the normal-metal(n)/mixed-state super-conductor(p) interface containing (i) layer of low electric conductivity.

MgO by Injection CVD

Abstract Epitaxial YBa 2 Cu 3 O 7 layers with 45° in-plane orientation have been grown by Injection CVD on MgO substrates polished off-axis to within 1.4–1.9° of the [100] direction. This new single-source CVD process is based on computer-controlled injection of precise microdoses of a metal–organic precursor solution into a CVD reactor. A wide range of solution compositions was tested to investigate compositional effects on phase purity, surface morphology, texturing and superconducting properties of the prepared films. The highest quality films with pure 45° texture had a smooth surface, zero resistance T c ( R =0) of 88–89 K, and critical current density J c (77 K) above 10 6 A/cm 2 .

Noise evidence for intermittent channeled vortex motion in laser-processed YBaCuO thin films

Pronounced random telegraph signals have been observed in voltages measured across current-biased thin-film YBa2Cu3O7-δ superconducting bridges containing laser-processed channels for easy vortex motion. The appearance of two-level and three-level telegraph noise in bridges with single and double laser-written channels, respectively, is interpreted as experimental evidence for intermittent channeled vortex flow in current induced dissipative state in type-II superconductors.

Fast High-Voltage Light Triggered Superconducting Opening Switch

The dynamics of light induced switching from the super-conducting or mixed state to the normal state were investigated in thin Y-Ba-Cu-0 films biased by high-current nanosecond pulses. It was demonstrated that the subnanosecond duration voltage transients with amplitudes of up to several hundred volts which are generated by the light triggered super-conducting opening switch are the result of film heating by light pulses and the bias current.

Transient Dynamics of Pulsed-Current-Biased YBa2Cu3O7-δ Superconducting Photoswitches

Studies on time-resolved dynamics of optically-thick YBa2Cu3O7-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 Ic could driven by optical pulses of comparatively weak fluence in the range of few tens of mW.