Carlo Williams

Picosecond superconducting single-photon optical detector

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

Intrinsic picosecond response times of Y–Ba–Cu–O superconducting photodetectors

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

Fabrication and properties of an ultrafast NbN hot-electron single-photon detector

A new type of ultra-high-speed single-photon counter for visible and near-infrared wavebands based on an ultrathin NbN hot-electron photodetector (HEP) has been developed. The detector consists of a very narrow superconducting stripe, biased close to its critical current. An incoming photon absorbed by the stripe produces a resistive hotspot and causes an increase in the films supercurrent density above the critical value, leading to temporary formation of a resistive barrier across the device and an easily measurable voltage pulse. Our NbN HEP is an ultrafast (estimated response time is 30 ps; registered time, due to apparatus limitations, is 150 ps), frequency unselective device with very large intrinsic gain and negligible dark counts. We have observed sequences of output pulses, interpreted as single-photon events for very weak laser beams with wavelengths ranging from 0.5 /spl mu/m to 2.1 /spl mu/m and the signal-to-noise ratio of about 30 dB.

Ultrafast photoresponse in microbridges and pulse propagation in transmission lines made from high-T/sub c/ superconducting Y-Ba-Cu-O thin films

We report our femtosecond time-resolved measurements of the photoresponse of microbridges in YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) thin films, performed using an electrooptic sampling technique. Our test structures consisted of 5-/spl mu/m-wide, 7-/spl mu/m-long microbridges, incorporated in 4-mm-long coplanar waveguides, fabricated in 100-nm-thick, high-quality epitaxial YBCO films grown on LaAlO/sub 3/ substrates by laser deposition. When varying the biasing conditions between the superconducting and switched states, we observed transients of single-picosecond duration that corresponded to the nonequilibrium kinetic-inductance and the electron-heating response mechanisms, respectively. In both cases, experimental waveforms could be accurately simulated using a nonequilibrium (two-temperature) electron-heating model. From the fits, the YBCO intrinsic temporal parameters associated with the nonequilibrium conditions were extracted. The electron thermalization time was found to be 0.56 ps in the state above the materials critical temperature (T/sub c/=89 K) and 0.9/spl plusmn/0.1 ps in the superconducting state at temperatures ranging from 20 to 80 K. The electron-phonon energy relaxation time was found to be 1.1 ps. The single-picosecond pulse distortion due to propagation on a YBCO coplanar waveguide was also studied. Our results show that a YBCO microbridge can intrinsically operate as a photodetector at rates exceeding 100 Gb/s, making it useful as an optical-to-electrical transducer for optoelectronic interfaces in YBCO digital electronics. Simultaneously, YBCO mixers, based on hot-electron effects, should exhibit an intrinsic bandwidth exceeding 100 GHz.

Ultrafast YBCO photodetector based on the kinetic-inductive process

We make use of the nonequilibrium kinetic-inductive effect in epitaxial YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) thin films to demonstrate an ultrafast, high-sensitivity, broadband photodetector. The photoresponse of a 5-/spl mu/m-wide, 10-/spl mu/m-long, 100-nm-thick YBCO microbridge embedded in a 20-/spl mu/m-wide coplanar strip transmission line was measured using a >1-THz, submillivolt-sensitivity electro-optic sampling system. We performed a comprehensive study of the optical power and wavelength dependency on the YBCO photodetector response. The quantum coherence Rothwarf-Taylor model together with the hot-electron relaxation effect were used to explain the measured data and to extract the number of generated quasiparticles for each absorbed photon, revealing the intrinsic quantum yield of our device to be /spl sim/450. The amplitude, response time, and the intrinsic gain of the YBCO photoresponses were observed to be spectrally flat over the tested range of 390 nm to 810 nm.

Nonequilibrium kinetic inductive response of Y-Ba-Cu-O photodetectors

We present a comprehensive study of the light-induced transient nonequilibrium kinetic inductive response of high-quality, epitaxial Y-Ba-Cu-O (YBCO) thin films. The test structures consisted of 20-30 µm wide coplanar strip (CPS) transmission lines with 7-10 µm separation, patterned in 100 nm thick YBCO films grown on MgO and LaAlO3 substrates. Each CPS structure contained a 5 µm wide, 7-10 µm long microbridge. The photoresponse of the microbridge, biased far below the film critical current and temperature, and excited by 100 fs laser pulses, was measured using a cryogenic subpicosecond electro-optic sampling system. The physical origin of the photoresponse is attributed to the nonequilibrium quasiparticle generation and recombination effect and fitted with the Rothwarf-Taylor model. Our measurements show 1.9 ps wide bipolar waveforms for the LaAlO3-based samples and 2.2 ps wide waveforms for the MgO-based samples. We regard the measured microbridge response times and the corresponding material time constants to be the intrinsic dynamics of YBCO. Our results show that the quasiparticle recombination time is very weakly temperature dependent and there is no phonon-trapping effect in YBCO. The picosecond response of YBCO makes it a suitable material for THz digital and communication applications.

Direct observation of subpicosecond single-flux-quantum generation in pulse-driven Y–Ba–Cu–O Josephson junctions

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

YBa/sub 2/Cu/sub 3/O/sub 7-x/ thin-film picosecond photoresponse in the resistive state

Using a subpicosecond electro-optic sampling technique, we have characterized the photoresponse of current-biased YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) photodetectors, designed as 5-/spl mu/m-wide and 7-/spl mu/m-long microbridges patterned in 100-nm-thick, high-quality epitaxial films grown on LaAlO/sub 3/ substrates by pulsed laser deposition. The bridges were centered in a coplanar waveguide structure, allowing the photogenerated pulses to be measured 20 /spl mu/m from the detector. The experiments were conducted in the temperature range between 20 and 80 K; however, the bridges were biased in the switched (resistive) state, which corresponded to a hot-spot formation at the center of the microbridge. The photoresponse from 100-fs laser pulses (395-nm wavelength) was measured to be in the form of a single spike with the width as short as 1.3 ps. The physical origin of this ultrafast response is attributed to nonequilibrium electron heating, We extracted the intrinsic temporal parameters of the YBCO response-the electron thermalization time equal to 0.56 ps and electron-phonon energy relaxation time equal to 1.1 ps, Our measurements demonstrate that a simple YBCO microbridge can operate as a >100-GHz bandwidth photodetector, e.g., as an optical-to-electrical transducer for optoelectronic interface in YBCO digital electronics.

Experiments and simulations of picosecond pulse switching and turn-on delay time in Y-Ba-Cu-O Josephson junctions

We report our studies on single-picosecond electrical pulse switching of YBa2Cu3O7-x (YBCO) grain-boundary bicrystal Josephson junctions. The test structures consisted of coplanar strip (CPS) transmission lines that were patterned in 50 nm thick YBCO films prepared on (100) MgO bicrystal substrates. Each CPS contained a 5 ?m wide, 10 ?m long microbridge and a 5 ?m wide grain-boundary Josephson junction. A train of 100 fs wide optical pulses from a Ti:sapphire laser was used to excite the microbridge and generate ~2 ps wide electrical pulses, which were then applied to switch the junction. The junction response was studied at 20 K using a cryogenic electro-optic sampling system. We measured a 0.65 ps wide, single-flux-quantum pulse generated by the junction as well as a 0.7 ps junction turn-on delay time. Junction switching and turn-on delay time have been compared with numerical simulations.

Picosecond photoresponse of YBa2Cu3O7−x thin films

The photoresponse of current-biased YBa2Cu3O7−x (YBCO) microbridges, patterned in 100-nm-thick, high-quality epitaxial films on LaAlO3, has been characterized by means of an electro-optic sampling technique. The photoresponse from 100-fs laser pulses (390-nm wavelength) was measured to be in the form of Gaussian-shaped pulses with the width as short as 0.8 ps. The physical origin of the photoresponse signal can be attributed to a nonequilibrium electron heating mechanism. The measured signal represents, in our opinion, the intrinsic photoresponse of YBCO.