High-temperature superconducting staring arrays are potentially important for both space and terrestrial applications which require the combination of high sensitivity over a broad wavelength range and relatively high temperature operation. In many such array applications sensitivity is more important than speed of response. Thus, it is desirable to design low- thermal-mass pixels that are thermally isolated from the substrate. To this end, Johnson, et al. at Honeywell have fabricated meander lines of YBa₂Cu₃O_7-x (YBCO) sandwiched between layers of silicon nitride on silicon substrates. The silicon was etched out from under each YBCO meander line to form low-thermal-mass, thermally isolated microbolometers. These 125 micrometers X 125 micrometers devices are estimated to have a noise equivalent power of 1.1 X 10^-12 W/Hz^1/2 near 5 Hz with a 5 (mu) A bias (neglecting contact noise). A drawback of this original Honeywell design is that the YBCO is grown on an amorphous silicon nitride underlayer, which precludes the possibility of epitaxial YBCO growth. The YBCO therefore has a broad resistive transition, which limits the bolometer response, and the grain boundaries lead to excess noise. We discuss the potential performance improvement that could be achieved by using epitaxial YBCO films grown on epitaxial yttria-stabilized zirconia buffer layers on silicon. This analysis shows a significant signal to noise improvement at all frequencies in devices incorporating epitaxial YBCO films. Progress toward fabricating such devices is discussed.

Silicon wafers have shown promise for the fabrication of photothermal IR detectors (i.e., bolometers) from epitaxial HTS thin films of YBa₂Cu₃O_(7-(delta )) (YBCO). Conventional IC-grade wafers, ultrathin wafers, and micromachined-silicon membrane windows in conventional wafers, are all suitable, but the latter provides considerable advantage for bolometer performance. The high thermal conductivity and strength of silicon make it ideal for submicron-thick window designs. Epitaxy in the HTS film is advantageous, since it reduces granular disorder, the primary cause of dark noise (resistance-fluctuations) in the detector. Mid-to-far IR transparency of Si at 90 K is unique among those substrates that support high-quality epitaxial YBCO films. This Si transparency to IR can be used for various improvements in the optical design of these devices. We review the thermal and optical advantages of silicon substrates, device fabrication issues, and bolometer modeling. Thermal modeling of membrane bolometers indicates that the steady- state temperature-rise profile is nonuniform, but that this does not degrade the response linearity of the bolometer. Certain size limits and trade-offs in the design, will be important in the final device performance. We also discuss applications to FTIR instruments, and extensions of this technology to arrays including a novel on-chip transform spectrometer design.

The OH Interferometer Observations (OHIO) concept is a Fabry-Perot/grating based spectrometer for measuring the OH radical in the earth's atmosphere from a satellite platform. OHIO uses technology which is all presently available to provide a reasonably simple, small, and robust instrument which will specialize in stratospheric OH measurements, and will operate without stored cryogens, except that an order of magnitude improvement is still necessary in detector sensitivity. High-T_c bolometers are a promising technology that may provide this improvement. We present a baseline design for the instrument and the underlying atmospheric spectroscopy supporting the choice of this type of instrument. We also show that a far infrared satellite instrument is chiefly suitable for making OH measurements; measurements of other species which are of interest in ozone layer photochemistry are either significantly more difficult to make, or can be measured well by existing techniques. The more difficult measurement of the next HO_x species of importance, the HO₂ radical, is briefly discussed.

We report bolometric photoresponse of YBa₂Cu₃O_7-x thin film and YBa₂Cu₃O_{7- x}/La₂CuO₄ multilayers at temperature of liquid nitrogen irradiated by a semiconductor laser diode at 0.83 microns. We develop a heat transfer model which takes into account the lateral diffusion of heat in the substrates. Using this model we explain the time dependence of the photoresponse. From the analysis of the photoresponse voltage versus time we calculate the optical absorption of the YBa₂Cu₃O_{7- x}/La₂CuO₄ multilayers and of the YbaCuO monolayer.

We have deposited superconducting YBCO (YBa₂Cu₃O₇) thin films on polycrystalline diamond using a composite Si/YSZ (yttria stabilized zirconia) diffusion barrier. 10 - 20 micron thick, free standing diamond films were coated with a micron thick layer of amorphous Si by plasma CVD. This was vacuum annealed at 700 degree(s)C to crystallize Si prior to the in-situ deposition of YSZ and YBCO by laser ablation. A primary layer of Si was found essential to avoid the peeling of the multilayer structure during the cooling of the substrates. A T_c-zero equals 50 K was observed for the best films. A bolometer fabricated from this film exhibited a 18 V/W responsivity and NEP equals 6.3 X 10^-9 W/(root)Hz at 60 K. Both the buffer layers and YBCO were polycrystalline. We believe single crystal Si backing CVD grown diamond can be used instead of a polycrystalline Si layer to further improve the quality of the YBCO deposited on polycrystalline diamond.

The transient thermal photoresponse of YBCO thin epitaxial films is calculated for conditions comparable to those frequently used in actual photoresponse measurements. At low light fluences and low bias currents, the calculations are in accord with a linear bolometric response. At higher fluences and bias currents, the thermal response displays behaviors often attributed to a non-bolometric mechanism. This is particularly true for the response decay time, which can decrease as the temperature falls through T_c. Some comparisons with experiment are presented, and it is concluded that a careful analysis is required to distinguish non-bolometric from bolometric response, especially for conditions of high light fluence.

Electrical photoresponse measurements on a Corbino type structure with thin epitaxial YBaCuO layer have been previously reported. They showed a very fast inductive reaction (rise time <EQ 12 ps, width 29 ps), followed by the bolometric heating effect. New detector structures based on coplanar lines have been experimented with similar experimental apparatus. The performance of the different geometries of detector will be compared in terms of magnitude and sharpness of the inductive non-equilibrium voltage peek, and of bolometric relaxation constants. The influence of bias currents, incident power, and operating temperature on the photoresponse mechanisms will be discussed. The feasibility of applying this type of fast superconducting detectors to far infra-red radiation measurements has been demonstrated by using a Free Electron Laser pulses at a wavelength of 20 micrometers . The detector may be used as two simultaneous different sensors for describing very fast optical pulse: the fast part of the response voltage represents the envelope of the incident pulse, while the bolometric part is significant for the incident energy.

The transmission probability of the interface between YBaCuO film and different substrates for thermal phonons incident both from the film and from the substrate are obtained from photoresponse measurements performed at the transition temperature of YBaCuO. The experimental values are compared with the predictions of the acoustic mismatch model and the diffusive mismatch model taking account of optical phonon modes as well as the difference between the speed of sound and the group velocity of acoustic phonons. Although absolute values of transmission probabilities derived from these two models are close to each other, the experimental data rather indicate the validity of the acoustic mismatch model.

Nonequilibrium picosecond and bolometric responses of YBCO films 500 angstroms thick patterned into 20 X 20 micrometers ² size structure to 17 ps laser pulses and modulated radiation of GaAs and CO₂ lasers have been studied. The modulation frequencies up to 10 GHz for GaAs laser and up to 1 GHz for CO₂ were attained. The use of small radiation power (1 - 10 mW/cm² for cw radiation and 10 - 100 nJ/cm² for pulse radiation) in combination with high sensitive read-out system made possible to avoid any non-linear transient processes caused by an overheating of sample above a critical temperature or S-N switching enhanced by an intense radiation. Responses due to the change of kinetic inductance were believed to be negligible. The only signals observed were caused by a small change of the film resistance either in the resistive state created by a bias current or in the normal state. The data obtained by means of pulse and modulation techniques are in agreement. The responsivity about 1 V/W was measured at 1 GHz modulation frequency both for 0.85 micrometers and 10.6 micrometers wavelengths. The sensitivity of high-T_c fast wideband infrared detector is discussed.

Fast resistive response of structured TiBa₂Ca₂Cu₃O₉ thin film to visible and far-infrared pulsed radiation has been observed for the first time. The voltage response of a current biased film was induced by a Nd:YAG laser in the visible and by an optically pumped Raman laser in the far-infrared range. Fast components of the response with a duration limited by the temporal resolution of the readout systems were observed in both spectral ranges. Each signal pulse consisted of a fast component that was followed by a slower decaying signal. We attributed the slow decay, with a time constant of few ns, to bolometric effects. The fast component of the response was most likely due to the depression of superconductivity by light induced nonequilibrium quasiparticles. The high temporal resolution, a large range of responsivity of few mV/W of the structured film represent a very powerful detector performance.

The transient voltage response in both epitaxial and granular YBaCuO thin films to 20 ps pulses of YAG:Nd laser radiation with 0.63 micrometers and 1.54 micrometers was studied. In normal and resistive states both types of films demonstrate two components: nonequilibrium picosecond component and following bolometric nanosecond. The normalized amplitudes are almost the same for all films. In superconducting state we observed a kinetic inductive response and two-component shape after integration. The normalized amplitude of the response in granular films is up to several orders of magnitude larger than in epitaxial films. We interpret the nonequilibrium response in terms of a suppression of order parameter by the excess of quasiparticles followed by the change of resistance in normal and resistive states or kinetic inductance in superconducting state. The sharp rise of inductive response in granular films is explained both by a diminishing of the crossection for current percolation through the disordered network os Josephson weak links and by a decrease of condensate density in neighboring regions.

A general trend is observed in the photoresponse of current biased epitaxial YBa₂Cu₃O_7-(delta ) thin film bridge structures exposed to picosecond laser pulses. Both fast and slow components are seen in the photoresponse near the transition region. The slow component, which has a decay over several nanoseconds, is a resistive bolometric response due to heating of the film by the laser pulse in the resistive transition region. At lower temperatures, only the fast component is observed with an amplitude which is linear with bias current. The fast component has been observed in films ranging in thickness from 30 nm to 260 nm. Using 100 ps, 532 nm laser pulses, the origin of the fast component can be explained by a kinetic inductive bolometric response where the superfluid fraction is rapidly decreased by the laser pulse heating the bridge. Recent results using 5 ps, 820 nm laser pulses on samples maintained at liquid nitrogen temperature (77.4 K) in a high speed measurement setup have revealed fast components in the photoresponse as short as 16 ps full width at half maximum. To our knowledge, this is the fastest photoresponse signal observed to date from YBa₂Cu₃O_7-(delta ) thin films. A large portion of this fast response can be attributed to a kinetic inductive bolometric response. The possibility of a nonbolometric component over this short time scale is discussed.

A process is described for fabricating antenna-coupled resistive-edge microbolometers based on the high-T_c superconductor YBa₂Cu₃O₇ (YBCO) on silicon. The YBCO and a buffer layer of yttria-stabilized zirconia (YSZ) were grown epitaxially on silicon to minimize excess electrical noise. A silicon-micromachined YBCO/YSZ air-bridge was incorporated to minimize the thermal conductance and the heat capacity. The thermal conductance of the air-bridge was measured to be 3 X 10^-6 W/K at a temperature of 100 K. At an operating temperature of 89 K, the detector is estimated to have a response time of 2 microsecond(s) , a responsivity of the 1000 V/W range, and a noise-equivalent power in the 10^-12 W/Hz^1/2 range at 1000 Hz.

We report systematic studies of the femtosecond transient reflectivity in Y-Ba-Cu-O (YBCO) thin films at temperatures ranging from 12 to 300 K. By using a 2-eV-pump/white-light-probe technique, we have very accurately measured the ultrafast optical response of YBCO as a function of the optical frequency, film oxygen content, and excitation intensity. Our results shed new light on the thermomodulation model, routinely used to interpret time-domain reflectivity data in high-temperature superconductors. We also review the current state of the art in the ultrafast optoelectronic response of YBCO.

We investigate the superfluid and quasiparticle response for high temperature superconductor films of YBa₂Cu₃O₇ and Ba_0.6K_0.4BiO₃ under optical radiation. This is done through direct measurement of the complex conductivity of the illuminated superconductors using a coherent terahertz-bandwidth spectroscopy technique. The nature of the detected signals is discussed in the context of bolometric and nonbolometric response.

We have investigated the optical response of single grain-boundary weak-links in superconducting YBCO thin films. At the superconducting transition of a film containing a single grain-boundary weak-link we observed an enhanced optical response from the weak- link. Below the transition temperature, we measure the bias current and the temperature dependence of the optical response of the grain-boundary in the temperature range of 15 K < T < 80 K. Using a Resistively-Shunted-Junction model for the grain-boundary weak- link, we find that the optical response in this temperature range can be described by radiation induced thermal modulation of the critical currents.

The preparation and characterization of a new generation of hybrid optical sensors fabricated from high-temperature superconductor thin films coated with organic dye overlayers is described herein. These dye-coated superconductor structures respond selectively to those wavelengths of light which are absorbed strongly by the molecular dye. Methods for preparing such optical sensors are details. Scanning electron microscopy, resistivity vs. temperature and optical measurements are exploited to characterize the hybrid devices.

We report here a model of a high temperature superconducting detector response to microwaves. The goal of the model is to approximate, based on microscopic theoretical results, the measured response of a superconducting detector to microwave frequency over a wide temperature range (from zero up to the critical temperature of the superconductor, T_c). In this work, the nonbolometric response is emphasized because its detector performance is better than the bolometric response. The nonbolometric response model is based on microwave enhancement of a thermal fluctuation voltage occurring in the networks of inherent Josephson junctions. Modeling the film as a distribution of granular connections with varying critical currents and temperatures yields results similar to those observed in microwave response measurements in granular Bi-Sr-Ca-Cu-O thin films. We also report here initial measurements and modeling of the microwave (9 GHz) response of a bi-epitaxial thin film showing the simultaneous presence of bolometric and nonbolometric response peaks in the same sample. The response in this Y-Ba-Cu-O thin films gives further evidence of microwave detection in the networks of inherent Josephson junctions presented at the grain boundaries of two epitaxial layers.

YBCO microthermometers to be used in composite bolometers are working either on the resistive transition or on the critical current transition, as shown earlier. YBCO microbridges obtained by different techniques and processes, of thicknesses varying from 30 nm to 200 nm, and of widths between 10 micrometers and 30 micrometers have been thus qualified. Results are in good accordance with those predicted by the simplified thermal model. A noise measurement has been made on the voltage fluctuations using a conventional current bias. The previously published results have been greatly improved in terms of sensitivity and detectivity, using either a constant current or a constant voltage DC bias.

Laser pulses were applied to granular, and c-axis oriented, YBa₂Cu₃O_x films current-biased in a resistive state, and the decay of the transient voltage was monitored as a function of time. At low enough temperatures and fluences (approximately equals 1 nJ per cm² per pulse), the decay rate follows a T³-dependence characteristic of electron energy loss to acoustic phonons. Above about 7 K, the response time of 300 angstroms films stays constant at 2.4 nsec, in agreement with the bolometric response observed by others. In the range of dominant electron-phonon interaction, the response time contains direct information about the coupling constant (lambda) , via a formula derived by P.B. Allen. However, as in ultrasonic attenuation, the limitation of the electron mean free path must be taken into account. A support for this procedure is the approximate proportionality of the relaxation time upon the room temperature resistivity, i.e. the electron mean free path. We thus obtain a value of (lambda) appropriate to the acoustic mode interaction.

Dry and wet photolithography has been used to define metal elements with YBCO thin film microbolometers. I-NaI concentrated solution was found suitable to etch gold layouts on the top of YBCO layer without noticeable reaction. The wet and dry processes were found to give comparable devices performances, while the wet process is easier to reproduce.

New ultrafast and sensitive superconducting high T_c detectors are proposed. Based on the analysis of the mechanism of the optical response it is suggested that the speed limit of the photoresponse is governed by the phonon escape time. Furthermore, the intrinsic speed limit is determined by the relaxation time of quasiparticles generated by photons through the electron- electron and electron-phonon scattering which is on the order of a picosecond. The device structure is presented for the optimization of the phonon escape time. It is also proposed to increase the detector responsivity by improving the efficiency of the flux activation (unpinning of vortices by the high energy quasiparticles and phonons) and increasing the critical current density which can be accomplished simultaneously by the increasing pinning site (or defect) density. A superconducting ultrafast flux activation detector is proposed with a picosecond time response and significantly improved responsivity.

The construction and parameters of array bolometer modules of nitrogen level cooling for the reception of high-frequency radiation mm-, submm- and infrared band of wavelength is presented. The offered modules can be used for measuring power of high-frequency radiation at the radioastronomy, as the sensors of the space localization of the radiation source at the ground and cosmic systems of navigation, high-frequency and infrared vision.