A. M. Kadin

Nonequilibrium photon‐induced hotspot: A new mechanism for photodetection in ultrathin metallic films

A new class of photodetectors is proposed, based on localized heating of an ultrathin metallic film due to absorption of individual photons. For a 1 eV photon, the transient temperature rise can be of order 10 K or greater in a nonequilibrium ‘‘hotspot’’ on the nm spatial scale and ps timescale. If current is flowing in a metallic film with a temperature‐dependent resistance, such a hotspot can give rise to a voltage pulse. This can provide the basis for an ultrafast photodetector with spectral sensitivity, in contrast to a conventional bolometer. Prospects for practical realization of infrared photodetectors based on this mechanism are discussed.

Interaction of picosecond optical pulses with high Tc superconducting films

Infrared pulses (1.06 μm wavelength, ≊150 ps long) from a Nd:YAG laser illuminated a superconducting YBa2Cu3O7 granular film, 0.7 μm thick. This caused the film to switch from the superconducting to the normal (resistive) state in less than 2 ns, followed by a much slower recovery ∼1 μs. The initial fast switching may be explained by nonequilibrium hot‐electron energy transfer from the top absorbing layer (≊100 nm thick) through the film, followed by an essentially bolometric response and cooldown via equilibrium thermal conduction out of the film.

Photofluxonic detection: A new mechanism for infrared detection in superconducting thin films

A new model is proposed in which a photon of energy E=hf is absorbed by a superconducting film to create a pair of equal and opposite fluxons (or vortices), each with quantized flux Φ0=h/2e. An applied current sweeps these fluxons to opposite edges of the film, causing a voltage pulse with time‐integrated magnitude Φ0, and leading to a time‐averaged voltage responsivity Rv = Φ0/E = 1/(2ef). This is directly analogous to photoconductive detection in a semiconductor via creation of electron‐hole pairs. Data on an ultrathin granular NbN film are presented which indicate a responsivity of 6000 V/W in red light, in agreement with the model. This is promising for the development of a sensitive, high‐speed infrared detector using thin films of either low or high Tc superconductors.

High temperature superconducting films by RF magnetron sputtering

The authors have produced sputtered films of Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O by RF magnetron sputtering from an oxide target consisting of loose reacted powder. The use of a large 8-in stoichiometric target in the magnetron mode permits films located above the central region to be free of negative-ion resputtering effects, and hence yields reproducible, uniform stoichiometric compositions for a wide range of substrate temperatures. Superconducting YBCO films have been obtained either by sputtering at low temperatures followed by an 850 degrees C oxygen anneal, or alternatively by depositing onto substrates heated to approximately=600-650 degrees C and cooling in oxygen. Films prepared by the former method on cubic zirconia substrates consist of randomly oriented crystallites with zero resistance above 83 K. Those deposited on zirconia at medium temperatures without the high-temperature anneal contain smooth partially oriented crystallites, with a slightly depressed T/sub c/ approximately=75 K. Finally, superconducting films have been deposited on MgO using a BiSrCaCu/sub 2/O/sub x/ powder target. >

Superior microwave properties by post‐annealing YBa2Cu3O7 thin films at low oxygen partial pressure

YBa2Cu3O7 (YBCO) thin films with improved properties have been recently reported by post‐annealing in a low partial pressure of oxygen, similar to that used by in situ methods, compared to the usual post‐annealing in 1 atm of oxygen. Here it is shown that the improvements extend to the microwave surface resistance. The surface resistance was measured at close to 10 GHz; the scaled value at 10 GHz and at 77 K is 240 μΩ. This value is as low as has been reported for YBCO thin films measured around 10 GHz made by any method, and is two orders of magnitude lower than the surface resistance of copper at the same temperature and frequency.

Hall effect and giant magnetoresistance in lanthanum manganite thin films

Magnetoresistance and Hall coefficient have been investigated as a function of temperature in patterned thin films of La–Ca–Mn–O. Our data on magnetoresistance show a maximum magnetoresistance ratio Δρ/ρ(0) of ≊−60% at 1.5 T and 240 K. The Hall coefficient indicates that the carriers are predominantly holes, with apparent density that varies as a function of temperature. Although changes in mobility were also observed, our results indicate that changes in carrier density become more important than changes in mobility at temperatures lower than the Curie temperature. We interpret our data in terms of a simple picture that includes two parallel contributions to the carrier density and the conductivity.

Bolometric and nonbolometric infrared photoresponses in ultrathin superconducting NbN films

The photoresponse of 10 nm thick superconducting NbN meander lines is measured using an amplitude modulated infrared semiconductor laser operating at a wavelength of 1300 nm. The response time of the film is found to be less than 1 ns with a measured responsivity of up to 1500 V/W of absorbed power at 100 kHz. Thermal properties of the film are extracted from current–voltage characteristics using a self‐heating hot spot model. At temperatures well below the superconducting transition, the magnitude of the photoresponse is found to be an order of magnitude too large to be purely bolometric, even when electron heating and effects due to intergranular weak links are taken into account. The photoresponse is seen to be bolometric near Tc. Other contributions to photoresponse are discussed, including kinetic inductance and the photofluxonic effect.

Microlithography of high-temperature superconducting films: laser ablation vs. wet etching

Narrow lines and microbridge structures have been etched in sputtered superconducting films of Y-Ba-Cu-O by variations of two methods. The first uses standard photolithography followed by wet etching in weak acid. The second uses a maskless process involving focused pulsed YAG (yttrium-aluminium-garnet) laser together with a computer-controlled x-y stage to produce local ablation of the superconducting film. Issues relating to limits of resolution, annealing of films, and degradation of superconducting properties are critically discussed for the two approaches. >

Film thickness dependence of microwave surface resistance for YBa2Cu3O7 thin films

Microwave surface resistance close to 10 GHz has been measured as a function of temperature for epitaxial thin films of YBa2Cu3O7 (YBCO) on LaAlO3 in the film thickness range of 0.2–0.8 μm. The films were made by a reduced‐temperature post‐anneal technique. The surface resistance (Rs) scaled to 10 GHz decreases with increasing film thickness as is expected due to the finite film thickness with respect to the magnetic penetration depth. Below about 70 K there is an increase in Rs for the thickest films, attributed to a change in microstructure from c axis normal to the substrate plane, to c axis in the plane of the substrate; the resulting minimum in Rs occurs at a film thickness of 0.6 μm. The critical current density (Jc) at 77 K is highest for the thinner films, so that films with the highest Jc do not have the lowest measured Rs. These results suggest that the optimum YBCO film thickness for microwave devices patterned from these films may be about 0.6 μm, depending on operating temperature.

Optically triggered switching of optically thick YBCO films

Thin films of high-T/sub c/ superconducting material were used to construct two different types of opening switches: the photoresistive switch and the inductively coupled switch. The former show two distinct switching components in 8000-AA films consisting of a slow bolometric response and a faster nonbolometric response. The fast nonbolometric component had risetimes of approximately 4 ns and falltimes of approximately 6 ns. The slower component was identified to be thermal in origin and was in good agreement with the one-dimensional heat flow model used to simulate the thermal photoresponse. The inductively coupled switch is a contactless switch where the superconducting film screens the flux coupling between two coils of a transformer. The optically induced switching was achieved with risetimes of approximately 50 ns. with a multiturn output coil. Short (150-ps) laser pulses containing energies of up to 3 mJ are used in both cases to perform the switching.<<ETX>>