E5.2

Optical sensors

Roman Sobolewski and Donald P Butler

E5.2.1 Introduction

Ultrafast phenomena, optoelectronics, and superconductivity are acknowledged fields of technologicalimportance, and a very large amount of research has been performed in these areas in recent years. Inmodern, highly advanceddigital electronic circuits, the primary function of high-performance, high-speedsignal processing is given to superconducting single-flux-quantum (SFQ) logical devices, based onresistively shunted Josephson tunnel junctions (see chapter E4.5). The development of this branch ofdigital electronics requires ultrafast and ultra-sensitive detectors for fiber optic interconnects betweenthe outside world and the SFQ processor, which must be able to work at cryogenic temperatures andbe technologically compatible with SFQ integrated circuits. The superconducting optical-to-electricaltransducers can transform the input information coded in the form of a train of ultrafast opticalpulses to the electrical domain and subsequently, feed it into the ultrafast superconducting processor.Superconducting optical sensors find also applications in traditional areas of optoelectronics and infrared(IR) imaging. Mid-IR optical radiation spectrum is especially important, since 3–5mm and 10–13mmbands correspond to the transmittancewindows in theEarth atmosphere (minimalHO2 absorption), thus,they are crucial for, e.g., effective satellite telecommunication and sensing. Optical fibers for the few mmradiation spectrum with ultra low losses can also be fabricated and form basis for future advancedtelecommunication systems. Contrary to current semiconductor optoelectronic compounds that lackadequately low bandgap values, superconducting photodetectors have essentially no bandgap limitationsand are known to be very effective mixers in the terahertz frequency area, as well as sensitive X-rayradiation detectors (see chapter E5.1). In this Chapter, we review general concepts of photodetection inboth low- and high-temperature superconducting (LTS and HTS) materials, focusing our attention NbNhot-electron superconducting photodetectors (HESP’s) and superconducting hot-electronYBa2Cu3O72x

(YBCO) microbridges. For completeness, we also present IR YBCO superconducting bolometers anduncooled semiconducting phase YBCO pyroelectric thermal imaging sensors.

E5.2.2 Photodetection and photoresponse mechanisms in superconductors

Many well-known nonlinear phenomena in superconductors are observed only on a very weak scalein the optical part of the electromagnetic spectrum. Examples are the Josephson effect in the IR andoptical ranges, or the change in optical properties of a superconducting film induced by the pulsedlaser radiation. For this reason, the above phenomena are not considered practical for applicationsin ultrafast optical/IR detectors and broadband photomixers or, in general, in superconductingoptoelectronics.

Optical sensors 1

E5.2.2.1 Bolometric photoresponse

Two classes of practical first-order optical effects in superconductors have been identified and they arecommonly referred to as bolometric or thermal, and nonequilibrium hot-electron heating processes. Inthe case of the bolometric response, incident radiation is converted to heat in the absorbing medium andresults in a measurable change in the absorber’s electrical properties. The speed of the bolometricresponse is limited by the time required for transferring the excess heat from the absorber to a heat sink,while the transient temperature increase in the absorber determine the responsivity. The bolometricmechanism generally results in relatively slow but very sensitive detectors [1]. In superconductingbolometers, the experimentally observed response has been either transition-edge bolometric or kineticinductive. In the former, the device is current-biased at its superconducting resistive transition (in otherwords, at the critical temperature Tc) and acts as a very sensitive thermometer (bolometer) because of thefact that at Tc, the resistance versus temperature (R vs. T ) dependence is highly nonlinear (very steep).The generated voltage response VR for a current-biased resistive photodetector is given by:

VRðtÞ ¼ IðdR=dTÞDTðtÞ; ðE5:2:1Þ

where I is the detector bias current, DT is the change in absorber temperature and the derivative dR=dT isevaluated at the bias point of the R vs. T characteristic.

The photoresponse of a superconductor at temperature T, far below Tc, is due to a change in thekinetic inductance. The incident radiation is not able to increase the absorber’s temperature above Tc,but it does break enough Cooper pairs to reduce the superfluid density, leading to a change in the kineticinductance LKin. In the presence of a bias current, the change in LKin produces a fast voltage transient VL

given by:

VLðtÞ ¼ IðdLkin=dtÞ; ðE5:2:2Þ

Lkin can be evaluated knowing the time rate of change of the superconducting fraction of electrons fsc,which is given by f scðtÞ ¼ 1 2 ½TðtÞ=Tc�

2.

E5.2.2.2 Nonequilibrium, electron heating photoresponse

Nonequilibrium processes, in solids in general, and superconductors in particular, give much shorter (ascompared to bolometric) photodetector response times. In nonequilibrium superconducting detectors,the incoming radiation breaks Cooper pairs and directly heats up the electron gas. The nonequilibriumdetector responds on a very short-time scale, before mobile carriers have a chance to reach thermalequilibrium with the lattice. Provided the extrinsic noise floor is sufficiently low, the device is sensitive toQ1

single quanta of the incoming radiation [2].In superconductors, the nonequilibrium electron heating process can be illustrated in the following

way. Upon absorption of a light quantum by a Cooper pair, the highly excited electron, with the energyclose to the incident photon energy is created (due to a large physical size [coherence length] of a Cooperpair, only one electron absorbs a photon, while the second one becomes a low-energy quasiparticle[QP]). Next, this excited (very hot) electron very rapidly (on tens of femtoseconds time scale, accordingto all-optical pump/probe experiments [3, 4]) loses its energy via electron–electron (e–e) scattering andthe creation of secondary excited electrons. In ordinary, metallic superconductors like Pb or NbN, theabove process continues until approximately 0.1 eV (approximately the Debye energy), when the mostefficient mechanism for redistribution of energy within the electron subsystem becomes emission ofDebye phonons by electrons (e–ph process). The mean free path of those phonons is very small, andthey efficiently excite additional electrons and break Cooper pairs (ph–e process). As the average energyof the electrons in the avalanche decreases to approximately 1 meV (T approximately 10 K), their further

2 Optical sensors

multiplication due to the absorption of phonons is replaced by multiplication due to e–e collisions,either in the QP–QP, or QP-Cooper-pair form. At that time-moment of the relaxation, whichcorresponds to a thermalization time tct , 7 ps for NbN [5], the global electron temperature Te,somewhat above the sample phonon (lattice) temperature Tph is established. The above scenario was firstexperimentally studied by Chi et al. [6] in Pb tunnel junctions, and most recently used to explain the veryhigh quantum efficiency of NbN photodetectors by Il’in et al. [7]. In YBCO, tct was measured to bebelow 1 ps [8]; although, it is not clear if the above avalanche scenario directly applies to HTS. Thecommon wisdom dictates, however, that the QP multiplication processes in HTS should be similar, withthermalization occurring via both the e–e and e–ph scattering processes. On the other hand, theinterplay between these two energy relaxation channels, their relative strength, and the associated valuesof energy-dependent scattering times, may be substantially different. The theory by Sergeev and Reizer[9] predicts that the e–e interaction should be strongly modified in a d-wave superconductor, since thereis an increase of the e–e scattering time, according to 1=te–e / ðT=DÞð1=tnÞ where D is the materialenergy gap and tn is the e–e interaction time in the normal state. Thus, it suggests that the dominance ofthe e–e channel over the e–ph process in the QP avalanche in YBCO.

Once the thermal distribution of electrons with the effective Te, elevated as compared to phononsTph is established, further cooling of the electron subsystem towards the initial sample temperature Ts isdue to the QP relaxation and recombination processes. Relaxation occurs via the e–ph interaction, byemitting acoustic (long-wave) phonons, and can be described by either a two-temperature (2-T) model[10, 11] or, equivalently, by the set of Rothworf–Taylor (R–T) equations [12]. We note that the e–phcooling process is usually hampered by reabsorption of excited phonons by electrons (ph–e mechanism).Thus, the excited phonons must be actually transferred out of the superconductor and into the outsideword (in practice, from the superconducting film to a substrate) in order to maximize the nonequilibriumand minimize the thermal components of the response.

In the 2-T model, the time evolution of the electron and phonon subsystems is described by theirtime-dependent Te and Tph, respectively. The balance between Te and Tph is governed by the set of twocoupled differential equations:

CedTe

dt¼

aPinðtÞ

V2

Ce

te–phðTe 2 TphÞ;

CphdTph

dt¼

Ce

te–phðTe 2 TphÞ2

Cph

tesðTph 2 T sÞ; ðE5:2:3Þ

where Ce and Cph are the electron and phonon specific heats, a is the radiation absorption coefficient, V

is the volume of the sample, and Ts is the sample temperature. Pin(t ) is the incident optical power,modeled as a Gaussian-shaped pulse. The equations also contain the characteristic times te–ph for e–phrelaxation, and tes for phonon escape to the substrate. In deriving equaiton (E5.2.3), we used the energybalance equation te–ph ¼ tph–eðCe=CphÞ, where tph–e is ph–e scattering time. The time evolution of Te

and Tph resulting from numerically solving equation (E5.2.3) for a superconductor exposed to sub-psoptical excitation is shown in figure E5.2.1. We note that the large difference between Te and Tph isobserved only during the first few picoseconds (nonequilibrium electron heating) after the perturbation,and later there is only the bolometric (thermal) response associated with Te ¼ Tph . T s: As we will showlater, the 2-T model has been very successfully used to explain both qualitatively and quantitativelyexperimental, picosecond photoresponses of both LTS and HTS microbridges.

As in the case of bolometric devices, HESP’s can work as either transition-edge or kinetic inductivesensors. Their voltage response is described by equation (E5.2.1) or (E5.2.2) with DT replaced by DTe

and T by Te, respectively. Thus, for the resistive hot-electron device, the transient voltage signalsimply follows the time evolution of Te presented in figure E5.2.1. On the other hand, an example of

Photodetection and photoresponse mechanisms in superconductors 3

the nonequilibrium Vkin response simulated for YBCO using equation (E5.2.3) and (E5.2.2), is shown infigure E5.2.2. The top panel displays the excitation laser pulse and the resulting Te and Tph transients(Ts is assumed 60 K). The next two graphs show the time evolution of fsc and Lkin, respectively. Finally,the last graph shows the voltage signal calculated as the derivative of Lkin multiplied by the bias current,in accordance with equation (E5.2.2). Note that the entire Vkin transient is only about 3-ps-wide, afterwhich the superconducting, zero-voltage state is promptly restored; thus, an oscilloscope with an ,1-pstime resolution is required to observe kinetic-inductive transients generated by femtosecond opticalpulses.

One can also note that for HESP’s, the semiconductor photodetector concept of quantum efficiencycannot be directly applied, since, as we have discussed above, a single absorbed photon creates, throughe–e and e–ph interactions, a very large number of secondary QP’s. In this context, the HESP effectivequantumyield canbeveryhigh (muchhigher than100%), significantly larger than that for semiconductingphotodiodes. In addition, the micron size of the sense element and very low electron specific heat resultsthat the device heat capacity is many orders of magnitude lower than that of conventional bolometer-typedetectors.

E5.2.3 Low-temperature superconducting optical sensors

Studies of optically induced nonequilibrium phenomena in LTS films have been a subject of intenseinvestigation for the last 20 years. Transient optical experiments were performed on LTS thin films usingintense nanosecond and picosecond pulses and were aimed towards the understanding of the dynamics ofthe photon-induced, superconducting-to-normal transition [13–16]. These studies have shown that theoptical-pulse-induced transition from the superconducting state to the normal state is a two-steptransition. First, a sudden jump leads to the nonstationary superconducting intermediate state, whichconsists of coexisting in the film normal and superconducting regions, followed by a gradual increase ofthe volume of the normal phase with the rate proportional to the rate of increase of the laser pulse energy.

Despite some early suggestions of possibility of implementation of optically driven superconductingthin films as photodetectors and picosecond electrical pulse generators [17], no systematic research in

Figure E5.2.1. Solution of the 2-T model (equation (E5.2.3)) with a Gaussian-shaped excitation pulse, illustrating

the nonequilibrium heating conditions when Te, exceeds Tph.

4 Optical sensors

this direction was performed until recently, when Kadin et al. [18] introduced the ultrathin NbN films asphotodetective elements. The authors proposed photofluxonic detection first, and the photon-inducedhotspot generation [19] later as possible mechanisms for fast and efficient IR detection. NbN is materialof choice, since it has the shortest among all metallic superconductors te–ph and can be grownas ultrathin high-quality films, assuring that tes , tph–e, which significantly reduces the bolometriceffect. Very recently, Il’in et al. [7] experimentally demonstrated that the photodetection mechanism inultrathin NbN films was actually due to the nonequilibrium hot-electron heating effect. They showedthat for the 0.79-mm radiation, the NbN HESP exhibited responsivity h < 220 AW21 ð4 £ 104 VW21Þ,significantly higher than semiconductor photodetectors and avalanche photodiodes. Simultaneously,the quantum yield reached 340, demonstrating a very large intrinsic gain of the device. The aboveparameters are quite remarkable and show that the NbN HESP’s are able to detect single quanta ofnot only visible, but also IR radiation and successfully compete with other designs (such as, e.g.,photomultipliers and superconducting-insulator-superconducting tunneling structures [20]) as single-photon detectors [21].

Figure E5.2.2. Waveforms resulting from solving equaion (E5.2.2) and (E5.2.3), illustrating the nonequilibrium

kinetic inductive photoresponse mechanism. Lkin ¼ ð1=10v2pÞð1=f scÞðl=wdÞ, where e0 is the vacuum permitivitty,

vp ¼ 1:67 £ 1015 s21 is the plasma frequency of YBCO, and l, w, and d are the bridge length, width, and film

thickness, respectively.

Low-temperature superconducting optical sensors 5

Il’in et al. [5] have also measured that the NbN HESP’s response time was below 30 ps near Tc, whilete–ph was found to be 10 ^ 2 ps, which indicated that the maximum intermediate frequency band of aNbN hot-electron phonon-cooled mixer could reach 16+4/23 GHz, if one eliminates bolometricphonon heating effects. Figure E5.2.3 shows a time-resolved response of the 3.5-nm-thick NbN HESP to,100-fs optical pulses, measured using the electro-optic (EO) sampling technique presented in detail inRef. [22]. We note that the signal (solid line) contains a large amount of noise, but it was acquired at verylow bias current, right at the superconducting resistive transition. The dashed-dotted line represents theresponse of the sample calculated for the single pulse input based on the 2-T model (equaitons (E5.2.1)and (E5.2.3)). Since the reflection-free time window for the experiments was only about 40 ps, thereflections from the sample ends contributed to the measured response and had to be included in thesimulations. The dashed line in figure E5.2.3 shows the result of the 2-T model calculations that tookinto account the end-line reflections. We note an excellent agreement between the experimental data andthe 2-T model.

Besides very high sensitivity and picosecond response time, the other unique advantage of theultrathin NbN hot-electron device is that it operates at approximately 10 K and this is an appropriateoperating temperature for the NbN digital circuitry. Thus, the entire sensor/processor unit can be jointlycooled to 10 K, and this temperature is much more readily attained than the 4K required for Nb-basedsuperconducting devices. A commercial, single-stage cryogenic refrigerator can be used, making NbNHESP a portable and user friendly device. The performance of HESP does not degrade with frequencyand NbN devices are very fast and extremely sensitive in the very wide range of optical wavelengths.NbN HESP’s operating as ultrafast single-photon counters [21] are expected to find applications inareas ranging from testing VLSI integrated circuits [23] to satellite communication and quantumcryptography. In addition, they are proposed as input transducers for feeding optically coded informa-tion into ultrafast superconducting digital circuits [24].

E5.2.4 High-temperature superconducting optical sensors

The photoresponse experiments of HTS materials are usually performed using a pulsed laser source anda high-speed oscilloscope to study the transient signal generated by a light-illuminated, current-biasedsuperconducting microbridge. The relatively high-speed response of simple YBa2Cu3O72x (YBCO)

Figure E5.2.3. Time-resolved response of a NbN HESP to a 100-fs optical excitation pulse. Temperature was

10.5 K.

6 Optical sensors

transition-edge bolometers [1] was initially surprising andmany authors attributed nanosecond transientsto nonequilibrium response mechanisms [25, 26], while only a few claimed that all observations could beexplained by equilibrium heating of the whole microbridge [27, 28]. Fast photoresponse transients werealso observed in the superconducting state far below Tc, and were interpreted assuming nonequilibriumconditions [29, 30], although an equilibrium Lkin mechanism was also presented [31]. Finally, in someexperiments, a fast signal was observed to be superimposed on a large and broad thermal background [32,33]. Common disadvantages of the early studies were often poor quality HTS samples and the limitedbandwidth of the voltage-recording methods used in the experiments. Thus, the final results were alwaysobscured by large errors, and the difficulty to properly distinguish between the nonequilibrium andbolometric responses.

To avoid the aforementioned difficulties in the time domain measurements, the photoresponse in thefrequency domain was studied using modulated laser sources [34]. In order to extend the measurementsto as high a frequency as possible, two wavelength-matched laser diodes at 1.5-mm wavelength wereutilized to create a beating signal with the frequency up to 18 GHz. The nonequilibrium response wasshown to extend at least to 18GHz and, by comparing with the 2-T model, an intrinsic bandwidthexceeding 100 GHz was predicted [35].

E5.2.4.1 HTS hot-electron superconducting photodetectors

Very recently, new photoresponse experiments with femtosecond optical excitations and optoelectronicregistration of transient electrical waveforms have been developed and provided the most direct infor-mation on nonequilibrium processes in HTS [8, 22, 36–38]. The femtosecond photoconductive and EOmeasurement techniques exhibit up to 1 THz bandwidth (subpicosecond time resolution) and sub-millivolt amplitude sensitivity, thus, they enable direct determination of intrinsic thermalization andrelaxation times. In this context, they are also most relevant when evaluating the potential of YBCO forultrafast photodetector and photomixer optoelectronic applications. The results obtained by Lindgrenet al. [8, 22], using a subpicosecond EO sampling system, are shown in figures E5.2.4 and E5.2.5, anddemonstrate photogeneration of single-picosecond electrical transients in superconducting YBCOmicrobridges, excited with ,100-fs laser pulses (400-nm wavelength) and current-biased in the resistive(switched) or the superconducting (flux-flow) state, respectively. Since the time resolution of the EOsystem is below 200 fs, the waveforms, shown in figures E5.2.4 and E5.2.5, must be regarded as theintrinsic response of a YBCO superconductor.

In the switched state (figure E5.2.4), the microbridge was in the normal state because of theformation of a hot spot at the bridge area [39], and its photoimpedance increased due to the raised Te, sothe bias current produced a voltage spike (see also figure E5.2.1). The main figure displays the 1.57-ps-wide waveform (dots) with a low-noise baseline. The bath temperature was 80 K, but very similarwaveforms with pulse widths ranging from 1.1 to 1.6 ps were observed at all tested temperatures in the20–80 K range. This suggests that the response was not directly related to the hot-spot temperature. Thephotoresponse was modeled (solid line) by numerically solving equation (E5.2.3) and substituting Te intoequation (E5.2.1). We note that the agreement between the experiment and the 2-T model is excellent.The transient rise time corresponds to tet ¼ 0:56 ps; while the fall time is governed by te–ph ¼ 1:1 ps. Theresponse shown in figure E5.2.4 was superimposed on a bolometric signal, which on the picosecond scaleof figure E5.2.4, led to a ,250-mV constant-level signal after the pulse. The 250-mV-high bolometricsignal (0.3-K increase of Tph) was also directly observed (see inset in figure E5.2.4) on the fastoscilloscope and exhibited a few-ns-long fall time, consistent with tes. We note that the nonequilibriumhot-electron photoresponse is more than an order of magnitude larger and three orders of magnitudefaster than the phonon (bolometric) signal.

High-temperature superconducting optical sensors 7

Figure E5.2.5. Measured voltage transient (dots) and the fitted nonequilibrium kinetic-inductive response based on

the 2-T model (solid line),\ when the bridge was biased in the superconducting state. Temperature was 60 K.

Figure E5.2.4. Measured voltage transient (dots) and the fitted nonequilibrium Te (solid line), when the bridge was

biased in the resistive hot-spot state. The inset shows the bolometric part of the photoresponse, registered with the

help of a 14-GHz-bandwidth oscilloscope.

8 Optical sensors

In the superconducting state (figure E5.2.5), light-induced pair-breaking led to a rapid change ofsuperfluid density, which in the presence of a bias current gave rise to a bipolar waveform (dots),characteristic for the nonequilibrium Lkin response (see also figure E5.2.2), with the positive 0.9-ps-widepart representing the process of Cooper-pair breaking and the negative part corresponding to the pairrecombination. The solid line in figure E5.2.5 represents the theoretical fit based on the 2-T model(equations (E5.2.2) and (E5.2.3)). We note that the model describes the main features of the transientvery well, but the fit to the negative part of the transient is quite poor. The main transient (withoutoscillations) lasts only about 2 ps with the recombination part essentially as fast as the pair breaking;thus, there is clearly no phonon-trapping effect present in the photoresponse of YBCO [40]. Thisobservation is contrary to the standard response of a LTS thin film, but is in direct agreement with thetheoretical prediction [9] stating that, since the tph–e in YBCO is very long and much longer than tph–ph,phonons do not participate in the secondary Cooper-pair breaking and the QP lifetime is the real QPrecombination time tR. The absence of the phonon-trapping effect can also be attributed to the d-wave-pairing model and the existence of allowed states at the gap nodes, where QP’s can recombine.

From the application point of view, HTS HESP’s are ideally suited for digital and communicationapplications because of their high absorption coefficient at essentially any wavelength and the ultrafastresponse. As we have already shown, simple YBCO microbridges exhibit single-picosecond responsetimes, making them not only the cheapest, but also one of the fastest optoelectronic switches. Since theintrinsic bit rate exceeds 300 Gbit s21, the YBCO photodetectors are ideal as ultrafast optical-to-electrical transducers. They can transform to the electrical domain the input information coded in theform of a train of ultrafast optical pulses and, subsequently, feed it into the ultrafast superconductingprocessor, based, e.g., on a SFQ logic. Simultaneously, HTS operating temperature range enables thesuperconducting optoelectronics to be fully integrated with the conventional (cooled) semiconductorelectronics. Finally, hot-electron HTS mixers can reach an intermediate frequency bandwidth greaterthan 100 GHz.

E5.2.4.2 HTS bolometric photodetectors

The discovery of HTS has also prompted interest in the development of superconducting bolometricsensors that can operate at liquid nitrogen temperatures and above [1]. As we mentioned in sectionE.5.2.2.1, the bolometric process is a general property of conducting solids, but superconductingsensors tend to be some of the most sensitive bolometric devices, since they use the sharp resistivetransition dependence near Tc. According to equation (E5.2.1), the generated voltage is proportional toI, DT, and dR/dT. The dR/dT is determined by the width of the superconducting transition DTc—thesharper the transition, the more sensitive is the bolometer element. At the same time, however, thedevice response saturates when DT approaches 0.5DTc, thus, a very sharp transition severely limits tothe sensor dynamical range. In addition, one would like to maximize the magnitude of I, but I shouldbe small enough to avoid excessive self-heating of the element and the heat-related broadening ofthe transition. The above constrains illustrate the difficult trade-offs that must be made in designingthe transition-edge superconducting bolometer. The way to circumvent the dc biasing trade-offs isto apply a voltage bias to the bolometer element, by virtue of the very small shunt resistance (about 1/10of the bolometer resistance) [41, 42]. In this scenario, called a negative electrothermal feedback, thetemperature increase associated with the radiation absorption causes the increase of the bolometerresistance, but this leads to the decrease of the current and results in the temperature decrease,stabilizing the device operation.

The best reported values for detectivity D* for YBCO bolometers are on the order of 1010 [1], whichis well above108 reported for the best uncooled devices. However, since D* generally increases with thetemperature decrease, LTS bolometers operating at helium temperatures or below are consistently better

High-temperature superconducting optical sensors 9

than YBCO detectors at 90 K. The time response of YBCO bolometers is governed by the Tph evolutionand, as we discussed in connection with figure E5.2.4, is limited by tes. We note that while a few-ns-longresponse is quite remarkable for bolometers, it is three orders of magnitude slower that the response of aYBCO HESP.

Despite their lower sensitivity and significantly reduced speed of response, as compared tosuperconducting devices, most recently developed uncooled IR YBCO bolometric detectors have severalimportant practical advantages and quickly become devices-of-choice for low-cost, high performanceIR multipixel detectors in both civilian and military applications [43]. Their main advantages, makingthem so appealing for mass-market production, are much lower operating cost and weight due to lack ofcooling systems and compatibility with Si-based processing and fabrication. The latter point is of specialimportance, since for e.g., monolithic focal plane arrays and IR cameras, sensors must be directlyintegrated into Si read-out circuitry.

The semiconducting phases of YBCO are attractive for uncooled IR detection as bolometers, dueto their relatively high temperature coefficient of resistance (TCR) ranging from 23 to 24% K21

near room temperature [44], and as pyroelectric detectors, due to high pyroelectric coefficient up to20mC cm22 K21 [45]. Recently, self-supporting multi-pixel YBCO detectors have been developed [46].The YBCO membrane was supported solely by the metal electrode arms. This allowed for a significantreduction in the overall thickness and thermal mass of the structure. These structures have been operatedas detectors without current bias, i.e., via the pyroelectric effect. Due to their low thermal mass, the devicethermal time constant was less than 0.2 ms, while the electrical time constant was less than 40ms [47]. Themaximum D* of the gang of four pyroelectric pixels was 5£108 cm Hz1/2 W21 and the noise equivalenttemperature difference (NETD) was calculated to be 14 mK.

E5.2.5 Conclusions

We have reviewed general concepts of photodetection in superconducting materials, presented both thebolometric (thermal) and nonbolometric (nonequilibrium) mechanisms of response, and introduced thetwo-temperature model used to describe the nonequilibrium electron heating effect. We have alsopresented examples of superconducting optical sensors, focusing our attention on the most importantfrom the practical point of view, phonon-cooled NbN HESP’s and superconducting hot-electron YBCOmicrobridges. NbN HESPs are ideally suitable for both fast digital and communication applications, andspace and atmospheric sensing, because of their high absorption coefficient in the entire visible-to-millimeter wavelength radiation range, picosecond response, and extremely high responsivity. SimpleYBCO microbridges exhibit single-picosecond response times and intrinsic bit rate in excess of300 Gbit s21, making HTS films one of the fastest (as well as the cheapest) optoelectronic switches.Simultaneously, the photoresponse is spectrally flat and detection from ultraviolet (0.4mm) to far IR(.10mm) wavelengths has been experimentally demonstrated. Latter represents, in general, the uniqueadvantage of implementation of superconductors in optoelectronics. Finally, we briefly reviewed theperformance of superconducting YBCO bolometers and mentioned the implementation of semi-conducting YBCO phases for effective bolometric and pyroelectric photodetection at room temperature.Uncooled IR YBCO detectors, are currently, the most attractive HTS-based devices for mass-marketcommercial applications.

Acknowledgments

This research was supported by the U.S. Office of Naval Research grant N00014-02-1-0026 and theNational Science Foundation grant DMR-0073366 (Rochester), and the National Science Foundationgrant ECS-9800062 and Army Research Office grant ARO-38673PH (SMU).

10 Optical sensors

References

[1] Richards P L 1994 Bolometers for infrared and millimeter waves J. Appl. Phys. 76 1[2] Semenov A, Gol’tsman G N and Sobolewski R 2002 Hot-electron effect in superconductors and its applications for radiation

sensors—topical review article Supercond. Sci. Technol. 15 R1[3] Gong T, Zheng L X, Xiong W, Kula W, Kostoulas Y, Sobolewski R and Fauchet P M 1993 Ultrafast optical and

optoelectronic response of Y–Ba–Cu–O Phys. Rev. B 47 14495[4] Sobolewski R, Shi L, Gong T, Xiong W, Weng X, Kostoulas Y and Fauchet P M 1994 Femtosecond optical response of

Y–Ba–Cu–O films and their applications in optoelectronics Proc. SPIE 2159 110 and references thereinQ2

[5] Il’in K S, Lindgren M, Currie M, Semenov A D, Gol’tsman G N, Sobolewski R, Cherednichenko S I and Gershenzon E M2000 Picosecond hot-electron energy relaxation in NbN superconducting photodetectors Appl. Phys. Lett. 76 2752

[6] Chi C C, Loy M M T and Cronemeyer D C 1981 Transient responses of superconducting lead films measured withpicosecond laser pulses Phys. Rev. B 23 124

[7] Il’in K S, Milostnaya I I, Verevkin A A, Gol’tsman G N, Gershenzon E M and Sobolewski R 1998 Ultimate quantumefficiency of a superconducting hot-electron photodetector Appl. Phys. Lett. 73 3938

[8] Lindgren M, Currie M, Williams C, Hsiang T Y, Fauchet P M, Sobolewski R, Moffat S H, Hughes R A, Preston J S andHegmann F A 1999 Intrinsic photoresponse of Y–Ba–Cu–O superconducting photodetectors Appl. Phys. Lett. 74 853

[9] Sergeev A V and Reizer M Y u 1996 Photoresponse mechanisms of thin superconducting films and superconductingdetectors Int. J. Mod. Phys. 10 635

[10] Anisimov S I, Kapeliovich B L and Perelman T L 1974 Emission of electrons from the surface of metals induced by ultrashortlaser pulses Zh. Eksp. Teor. Fiz. 66 776 (Engl. Transl. 1974 Sov. Phys-JETP 39, 375)Q3

[11] Gusev V E and Wright O B 1998 Ultrafast nonequilibrium dynamics of electrons in metals, Phys. Rev. B 57 2878[12] Rothwarf A and Taylor B N 1967 Measurement of recombination lifetimes in superconductors Phys. Rev. Lett. 19 27[13] Testardi L R 1971 Destruction of superconductivity by laser light Phys. Rev. B 4 2189[14] Elesin V F and Kopaev Yu V 1981 Superconductors with excess quasiparticles Usp. Fiz. Nauk 133 259 Sov. Phys. Usp. 24 116

1981 and references thereinQ2

[15] Sobolewski R, Butler D P, Hsiang T Y, Stancampiano C V and Mourou G A 1986 Dynamics of the intermediate state innonequilibrium superconductors Phys. Rev. B 33 4604 and references thereinQ2

[16] Hu X-H, Juhasz T and Bron W E 1991 Transient electric-field generated by nonequilibrium states in superconducting Pbfilms Appl. Phys. Lett. 59 3333

[17] Sobolewski R, Stancampiano C V, Mourou G A and Butler D P 1984 Picosecond pulse generation capability of optically-driven superconducting thin films Phys. Status Solidi A 83 K203

[18] Kadin A M, Leung M, Smith A D and Murduck J M 1990 Photofluxonic detection: a new mechanism for infrared detectionin superconducting thin films Appl. Phys. Lett. 57 2847

[19] Kadin A M and Johnson M W 1996 Nonequilibrium Photon-induced hotspot: a new mechanism for photodetection inultrathin metallic films Appl. Phys. Lett. 69 3938

[20] Verhoeve P, Rando N, Peacock A, van Dordrecht A, Poelaert A and Goldie D J 1997 Superconducting tunnel junctions asphoton counting detectors in the infrared to the ultraviolet IEEE Trans. Appl. Supercond. 7 3359

[21] Gol’tsman G N, Okunev O, Chulkova G, Lipatov A, Semenov A, Smirnov K, Voronov B, Dzardanov A, Williams C andSobolewski R 2001 Picosecond superconducting single-photon optical detector Appl. Phys. Lett. 79 705

[22] Lindgren M, Currie M, Williams C, Hsiang T Y, Fauchet P M, Sobolewski R, Moffat S H, Hughes R A, Preston J S andHegmann FA 1996 Ultrafast photoresponse in microbridges and pulse propagation in transmission lines made from high-Tc

superconducting Y–Ba–Cu–O thin films IEEE J. Select. Top. Quant. Electr. 2 668 Special Issue on Ultrafast Electronics,Optoelectronics, and Photonics

[23] Somani S, Kasapi S, Wilsher K, Lo W, Sobolewski R and Gol’tsman G N 2001 New photon detector for device analysis:superconducting single-photon detector based on a hot electron effect J. Vac. Sci. Technol. B 19 2766

[24] Sobolewski R 2001 Ultrafast optoelectronic interface for digital superconducting electronics Supercond. Sci. Technol. 14 994[25] Frenkel A, Saifi M A, Venkatesan T, Chinlon L, Wu X D and Inam A 1989 Observations of fast nonbolometric optical

response of nongranular high Tc YBa2Cu3O72x superconducting thin films Appl. Phys. Lett. 54 1594[26] Kwok H S, Zheng J P, Ying Q Y and Rao R 1989 Nonthermal optical response of Y–Ba–Cu–O thin films Appl. Phys. Lett.

54 2473[27] Hegmann F A and Preston J S 1993 Origin of the fast photoresponse of epitaxial YBa2Cu3O72x thin films Phys. Rev. B 48

16023[28] Carr G L, Quijada M, Tanner D B, Hirschmug C J, Williams G P, Etemad S, Dutta B, DeRosa F, Inam A, Venkatesan T and

Xi X 1990 Fast bolometric response by high Tc detectors measured with subnanosecond synchrotron radiation Appl. Phys.

Lett. 57 2725[29] Bluzer N 1991 Temporal relaxation of nonequilibrium in Y–Ba–Cu–O measured from transient photoimpedance response

Phys. Rev. B 44 10222[30] Ghis A, Villegier J C, Pfister S, Nail M and Gibert P 1993 Electrical picosecond measurements of the photoresponse in

YBa2Cu3O72x Appl. Phys. Lett. 63 551

References 11

[31] Hegmann F A, Hughes R A and Preston J S 1994 Picosecond photoresponse of epitaxial YBa2Cu3O72x thin films Appl. Phys.

Lett. 64 3172[32] Donaldson W R, Kadin A M, Ballentine P H and Sobolewski R 1989 Interaction of picosecond optical pulses with high Tc

superconducting films Appl. Phys. Lett. 54 2470[33] Johnson M 1991 Nonbolometric photoresponse of YBa2Cu3O72x films Appl. Phys. Lett. 59 1371[34] Lindgren M, Trifonov V, Zorin M, Danerud M, Winkler D, Karasik B S, Gol’tsman G N and Gershenzon E M 1994

Transient resistive photoresponse of YBa2Cu3O72d films using low power 0.8 and 10.6mm laser radiation Appl. Phys. Lett.

64 3036[35] Lindgren M, Zorin M A, Trifonov V, Danerud M, Winkler D, Karasik B S, Gol’tsman G N and Gershenzon E M 1994

Optical mixing in a patterned YBa2Cu3O72d thin film Appl. Phys. Lett. 65 3398[36] Hegmann F A, Jacobs-Perkins D, Wang C-C, Moffat S H, Hughes R A, Preston J S, Currie M, Fauchet P M, Hsiang T Y

and Sobolewski R 1995 Electro-optic sampling of 1.5-ps photoresponse signal from YBa2Cu3O72x thin films Appl. Phys.

Lett. 67 285[37] Jaekel C, Roskos H G and Kurz H 1996 Emission of picosecond electromagnetic pulses from optically excited

superconducting bridges Phys. Rev. B 54 R6889[38] Hangyo M, Tomozawa S, Murakami Y, Tonouchi M, Tani M, Wang Z, Sakai K and Nakashima S 1996 Terahertz radiation

from superconducting YBa2Cu3O72d thin films excited by femtosecond optical pulses Appl. Phys. Lett. 69 2122[39] Poulin G D, Lachapelle J, Moffat S H, Hegmann F A and Preston J S 1995 Current-voltage characteristics of dc voltage

biased high temperature superconducting microbridges Appl. Phys. Lett. 66 2576[40] Sobolewski R 1998 Ultrafast dynamics of nonequilibrium quasiparticles in high-temperature superconductors Proc. SPIE

3481 480 and references thereinQ2

[41] Irwin K D 1995 An application of electrothermal feedback for high resolution cryogenic particle detection Appl. Phys. Lett. 661998

[42] Lee A T, Richards P L, Nam S W, Cabrera B and Irwin K D 1996 A superconducting bolometer with strong electrothermalfeedback Appl. Phys. Lett. 69 1801

[43] Frost, Sullivan 1995 US commercial and military infrared systems market.[44] Shan P C, Celik-Butler Z, Butler D P, Jahanzeb A, Travers C M, Kula W and Sobolewski R 1996 Investigation of

semiconducting YBaCuO thin films: a new room temperature bolometer J. Appl. Phys. 80 7118[45] Jahanzeb A, Travers C M, Butler D P, Celik-Butler Z and Gray J E 1997 Strong pyroelectric response in semiconducting

Y–Ba–Cu–O and its application to uncooled infrared detection Appl. Phys. Lett. 70 3495[46] Jahanzeb A, Travers C M, Celik-Butler Z, Butler D P and Tan S G 1997 A semiconductor YBCO microbolometer for room

temperature IR imaging IEEE Trans. Elect. Dev. ED-44 1795[47] Butler D P, Celik-Butler Z, Adam R and Sobolewski R 1999 Pyroelectric effect in Y–Ba–Cu–O thin films under laser

illumination J. Appl. Phys. 85 1075

12 Optical sensors

Author QueriesJOB NUMBER: E5.2

JOURNAL: HSM

Q1 Revised sentence Ok?. Please check.

Q2 Please update reference.

Q3 Author: kindly provide the article title for this reference.