A superconducting infrared photodetector employing SQUID (Superconducting Quantum Interference Device) measurement of fluxon flow in thin superconducting granular films to provide sensitive, low-noise detection of infrared radiation. The superconducting infrared photodetector includes a plurality of superconducting detector elements connected in parallel or series, means for supplying a bias current to the detector elements, and a digital or analog SQUID readout circuit. Each detector element includes a thin granular film of superconducting material which forms a randomly connected array of weakly coupled superconductors. The weakly coupled superconductors promote the formation of oppositely-polarized fluxons, which are driven to opposite sides of the film when subjected to the bias current. Incident radiation causes an increase in this fluxon flow, generating a voltage change. The voltage change is measured by the SQUID readout circuit to provide a sensitive, low-noise measurement of the amount of radiation incident on the detector elements.

BACKGROUND OF THE INVENTION 
This invention relates generally to infrared photodetectors and, more 
particularly, to superconducting infrared photodetectors having thin 
granular film detector elements. 
Infrared photodetectors are widely used in surveillance, monitoring, and 
imaging systems and are of two general types. Bolometric or thermal 
photodetectors rely on detector elements that undergo a change in some 
temperature-dependent parameter, such as resistance, when uniformly heated 
by infrared radiation. Bolometric photodetectors are typically broadband, 
but tend to have either a slow response time or poor sensitivity. Quantum 
or nonequilibrium photodetectors do not rely on a uniform heating of the 
detector elements and, therefore, usually provide both a fast response 
time and good sensitivity. 
Quantum-type detector elements are frequently fabricated from either 
semiconducting or superconducting materials. Semiconducting materials 
generally provide good quantum detection of photons at energy levels 
corresponding to the energy gaps of these materials. The energy gaps of 
semiconducting materials are on the order of 1 eV, which is in the near 
infrared portion of the electromagnetic spectrum. Superconducting 
materials generally provide good quantum detection of photons at much 
lower energy levels because of the much smaller energy gaps of these 
materials. The energy gaps of low-temperature superconducting materials 
are on the order of 1 meV, which is in the millimeter wave portion of the 
spectrum. 
Thin superconducting granular films, however, have recently shown 
considerable promise in detecting radiation over a wide range of the 
electromagnetic spectrum, including the desirable infrared portion of the 
spectrum. These granular films contain small grains of superconducting 
material which form a randomly connected array of weakly coupled 
superconductors. The weakly coupled superconductors promote the formation 
of oppositely-polarized fluxons, which are driven toward opposite sides of 
the film when subjected to a bias current. Incident infrared radiation 
causes an increase in this fluxon flow, generating a measurable voltage 
change. Unfortunately, infrared photodetectors that utilize these granular 
films have typically suffered from poor sensitivity and low 
signal-to-noise ratios, and have been difficult to implement in focal 
plane arrays. Accordingly, there has been a need for a superconducting 
infrared photodetector having thin granular film detector elements that 
does not suffer from these limitations. The present invention is directed 
to this end. 
SUMMARY OF THE INVENTION 
The present invention resides in a superconducting infrared photodetector 
employing SQUID (Superconducting Quantum Interference Device) measurement 
of fluxon flow in thin superconducting granular films to provide 
sensitive, low-noise detection of infrared radiation. The superconducting 
infrared photodetector includes a plurality of superconducting detector 
elements connected in parallel or series, means for supplying a bias 
current to the detector elements, and a digital or analog SQUID readout 
circuit. Each detector element includes a thin granular film of 
superconducting material which forms a randomly connected array of weakly 
coupled superconductors. The weakly coupled superconductors promote the 
formation of oppositely-polarized fluxons, which are driven toward 
opposite sides of the film when subjected to the bias current. Incident 
infrared radiation causes an increase in this fluxon flow, generating a 
voltage change. The voltage change is measured by the SQUID readout 
circuit to provide a sensitive, low-noise measurement of the amount of 
infrared radiation incident on the detector elements. 
A preferred embodiment of the superconducting infrared photodetector of the 
present invention includes a plurality of superconducting detector 
elements connected in parallel, a constant current source connected in 
parallel with the detector elements, and a digital SQUID readout circuit 
that operates in the I-f (current-frequency) mode. Each detector element 
includes a thin granular film of superconducting material coupled to a 
planar antenna. The planar antenna focuses or concentrates the infrared 
radiation onto the granular film to maximize the coupling of the incident 
infrared radiation to the granular film. 
The digital SQUID readout circuit includes an I-f SQUID quantizer and a 
binary counter. The I-f SQUID quantizer generates high-frequency pulses 
which vary in frequency with the voltage change induced across the 
detector elements. The binary counter then counts these pulses over some 
sampling interval. In this manner, the digital SQUID readout circuit 
functions as an analog-to-digital converter by converting the analog 
voltage change to digital form. A superconducting inductor is connected in 
parallel with the detector elements to inductively couple a current change 
to the I-f SQUID quantizer that is proportional to the voltage change 
induced across the detector elements. 
The digital SQUID readout circuit provides high sensitivity, fast response 
and a digital readout. The digital SQUID readout circuit also improves the 
signal-to-noise ratio of the infrared photodetector by providing an 
impedance matching function. The signal-to-noise ratio of the infrared 
photodetector can also be improved by increasing the number of detector 
elements. 
Another preferred embodiment of the superconducting infrared photodetector 
of the present invention includes a plurality of detector elements 
connected in series, rather than in parallel, and the digital SQUID 
readout circuit. A constant voltage source is connected in series with the 
detector elements to provide the bias current. The series connection of 
detector elements provides maximum voltage responsivity, while the 
parallel connection of detector elements provides maximum current 
responsivity. 
Still another preferred embodiment of the superconducting infrared 
photodetector of the present invention includes a digital SQUID readout 
circuit that operates in the tracking mode and either the series or 
parallel arrangement of detector elements. This digital SQUID readout 
circuit includes a SQUID amplifier, a tracking SQUID quantizer and a 
bidirectional binary counter. The tracking SQUID quantizer generates a 
pulse whenever the current inductively coupled to the quantizer increases 
or decreases by a flux quantum. The binary counter then counts these 
pulses over some sampling interval. The tracking SQUID quantizer is more 
linear and has a higher dynamic range than the I-f SQUID quantizer, but 
the I-f SQUID quantizer is more sensitive. 
Yet another preferred embodiment of the superconducting infrared 
photodetector of the present invention includes an analog SQUID readout 
circuit and either the series or parallel arrangement of detector 
elements. The analog SQUID readout circuit includes a SQUID amplifier, a 
transformer for inductively coupling the amplified current change to a 
room-temperature amplifier, and a voltmeter for measuring the voltage 
output of the room-temperature amplifier. 
The detector elements are preferably arranged as sensing elements in a 
focal plane array. The detector elements are fabricated on multiple 
substrate layers, preferably of silicon or germanium, to allow for back 
illumination of the detector elements. In a focal plane array, the 
detector elements can be easily integrated on a single chip for reduced 
cost and increased reliability. 
It will be appreciated from the foregoing that the present invention 
represents a significant advance in the field of superconducting infrared 
photodetectors. Other features and advantages of the present invention 
will become apparent from the following more detailed description, taken 
in conjunction with the accompanying drawings, which illustrate, by way of 
example, the principles of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in the drawings for purposes of illustration, the present 
invention is embodied in a superconducting infrared photodetector 
employing SQUID (Superconducting Quantum Interference Device) measurement 
of fluxon flow in thin superconducting granular films to provide 
sensitive, low-noise detection of infrared radiation. Superconducting 
granular films have recently shown considerable promise in detecting 
radiation in the infrared region of the electromagnetic spectrum. 
Unfortunately, infrared photodetectors that utilize these granular films 
have typically suffered from poor sensitivity and low signal-to-noise 
ratios, and have been difficult to implement in focal plane arrays. 
In accordance with the present invention, a superconducting infrared 
photodetector includes a plurality of superconducting detector elements 
connected in parallel or series, means for supplying a bias current to the 
detector elements, and a digital or analog SQUID readout circuit. Each 
detector element includes a thin granular film of superconducting material 
which forms a randomly connected array of weakly coupled superconductors. 
The weakly coupled superconductors promote the formation of 
oppositely-polarized fluxons, which are driven toward opposite sides of 
the film when subjected to the bias current. Incident infrared radiation 
causes an increase in this fluxon flow, generating a voltage change. The 
voltage change is measured by the SQUID readout circuit to provide a 
sensitive, low-noise measurement of the amount of infrared radiation 
incident on the detector elements. 
As illustrated in FIG. 1, a preferred embodiment of the superconducting 
infrared photodetector of the present invention includes a plurality of 
superconducting detector elements 10 connected in parallel, a constant 
current source 12 connected in parallel with the detector elements 10, and 
a digital SQUID readout circuit 14 that operates in the I-f 
(current-frequency) mode. Each detector element 10 includes a thin 
granular film 16 of superconducting material coupled to a planar antenna 
18. The planar antenna 18 focuses or concentrates infrared radiation onto 
the granular film 16 to maximize the coupling of the incident infrared 
radiation to the granular film 16. 
The digital SQUID readout circuit 14 includes an I-f SQUID quantizer 20 and 
a binary counter 22. The I-f SQUID quantizer 20 generates high-frequency 
pulses which vary in frequency with the voltage change induced across the 
detector elements 10. The binary counter 22 then counts these pulses over 
some sampling interval. In this manner, the digital SQUID readout circuit 
14 functions as an analog-to-digital converter by converting the analog 
voltage change to digital form. A superconducting inductor 24 is connected 
in parallel with the detector elements 10 to inductively couple a current 
change to the I-f SQUID quantizer 20 that is proportional to the voltage 
change induced across the detector elements 10. A load resistor 26 is 
connected in series with the superconducting inductor 24. 
As shown in detail in FIG. 4, the superconducting granular film 16 consists 
of small grains 28 of superconducting material. The granular nature of the 
film 16 creates voids in the superconducting material which weaken the 
ability of the film to conduct supercurrent. When the current limit of an 
electrical contact between adjacent grains is exceeded and the contact 
becomes resistive, fluxons or vortices 30 are formed. The fluxons 30 are 
usually formed as one or more pairs of oppositely-circulating or 
oppositely-polarized fluxons, but one or more fluxons of the same polarity 
may also be formed (only one polarization of fluxons is shown). Each fluxon 
30 is bound by a loop of superconducting electrons and supports one or more 
quanta of magnetic flux (.PHI..sub.0 =h/2e, where e is the electron charge 
and h is Planck's constant). 
Incident infrared radiation causes additional fluxons 30 to be formed and 
also weakens the intergrain coupling that pins the fluxons in place. This 
weakening of the intergrain coupling allows the oppositely-polarized 
fluxons 30 to be more easily swept by the bias current to the opposite 
sides of the film 16. The fluxons 30 flow at right angles to the bias 
current due to the Lorentz force. The increase in fluxon flow generates a 
voltage change, which causes a corresponding current change through the 
inductor 24. This current change is then measured by the digital SQUID 
readout circuit 14. 
As shown in FIG. 1, the I-f SQUID quantizer 20 includes two Josephson 
junctions 32 connected in a superconducting loop 34. A bias current, on 
line 36, biases the two Josephson junctions 32 into their voltage states. 
Applying a voltage to a Josephson junction adds energy to the junction 
electrons that causes the electrons to flow across the junction in 
discrete pulses, creating an ac current that radiates the energy away. 
This phenomenon is known as the ac Josephson effect and is useful because 
the frequency of these pulses varies with the voltage (f=2eV/h) applied to 
the junction. 
In the I-f SQUID quantizer 20 of the present invention, the current change 
inductively coupled by the inductor 24 is received by the superconducting 
loop 34. This current change combines with the bias current to cause a 
change in the voltage across the Josephson junctions 32. This voltage 
change causes a corresponding change in the frequency of the pulses, which 
is measured by the counter 22. 
The digital SQUID readout circuit 14 provides high sensitivity, fast 
response and a digital readout. The digital SQUID readout circuit 14 also 
improves the signal-to-noise ratio of the infrared photodetector by 
providing an impedance matching function. The signal-to-noise ratio of the 
infrared photodetector can also be improved by increasing the number of 
detector elements 10. 
Another preferred embodiment of the superconducting infrared photodetector 
of the present invention includes a plurality of detector elements 10 
connected in series, as shown in FIG. 2, and the digital SQUID readout 
circuit 14 shown in FIG. 1. A constant voltage source 38 is connected in 
series with the detector elements 10 to provide the bias current. The 
series connection of detector elements 10 provides maximum voltage 
responsivity, while the parallel connection of detector elements 10 
provides maximum current responsivity. 
Still another preferred embodiment of the superconducting infrared 
photodetector of the present invention includes a digital SQUID readout 
circuit 40 that operates in the tracking mode, shown in FIG. 2, and either 
the series or parallel arrangement of detector elements 10 shown in FIGS. 1 
and 2. The digital SQUID readout circuit 40 includes a SQUID amplifier 42, 
a tracking SQUID quantizer 20' and a bidirectional binary counter 22'. The 
tracking SQUID quantizer 20' generates a pulse whenever the current 
inductively coupled to the quantizer increases or decreases by a flux 
quantum. The binary counter 22' then counts these pulses. The SQUID 
amplifier 42 provides amplification of the current change prior to 
quantization. The tracking SQUID quantizer 20' is more linear and has a 
higher dynamic range than the I-f SQUID quantizer 20, but the I-f SQUID 
quantizer 20 is more sensitive. 
The SQUID amplifier 42 includes two Josephson junctions 44 connected in a 
superconducting loop 46. The current change inductively coupled by the 
inductor 24 is received by the superconducting loop 46 and amplified. The 
amplified current change is then inductively coupled to the tracking SQUID 
quantizer 20' by a superconducting inductor 48 connected in parallel with 
the superconducting loop 46. The SQUID amplifier 42 may include one or 
more additional amplifiers in series and/or parallel to provide the 
necessary gain and current levels required for operation of the quantizer 
in the tracking mode. 
The tracking SQUID quantizer 20' includes two Josephson junctions 32' 
connected in a superconducting loop 34'. The superconducting loop 34' 
receives the amplified current change inductively coupled by the inductor 
48 A bias current, on line 36', biases the two Josephson junctions 32' 
such that the junctions remain in their zero voltage states. The induced 
current in the loop 34' combines positively with the bias current in one 
Josephson junction 32' and negatively in the other Josephson junction 32'. 
Each time the induced current increases or decreases by a flux quantum, the 
current through one of the Josephson junctions 32' is raised momentarily 
above the critical current of the junction, causing the junction 32' to 
generate a pulse. Positive incremental changes in the induced current 
result in the generation of pulses across one Josephson junction 32' and 
negative incremental changes result in the generation of pulses across the 
other Josephson junction 32'. The bidirectional binary counter 22' counts 
these up-count and down-count pulses, increasing the binary count when 
up-count pulses are received and decreasing the binary count when 
down-count pulses are received. 
As shown in FIG. 2, a static magnetic field may be applied by 
superconducting inductors 50 to the series or parallel arrangement of 
detector elements 10 to improve detector responsivity. FIG. 5 is a graph 
of photoconductivity as a function of magnetic field strength for a 
superconducting granular film of Y--Ba--Cu--O. Photoconductivity is a 
measure of responsivity and is expressed as a voltage, measured in 
arbitrary units. Photoconductivity is shown to increase as a function of 
the strength of the applied magnetic field, measured in Teslas (1 
Tesla=10,000 Gauss). 
Yet another preferred embodiment of the superconducting infrared 
photodetector of the present invention includes an analog SQUID readout 
circuit 52, shown in FIG. 3, and either the series or parallel arrangement 
of detector elements 10 shown in FIGS. 1 and 2. The analog SQUID readout 
circuit 52 includes a SQUID amplifier 42, a transformer 54 for inductively 
coupling the amplified current change to a room-temperature amplifier 56, 
and a voltmeter 58 for measuring the voltage output of the amplifier 56. 
As illustrated in FIG. 6, the detector elements 10 of the present invention 
are preferably arranged as sensing elements in a focal plane array. The 
detector elements 10 are fabricated on multiple substrate layers 60, 
preferably of silicon or germanium, to allow for back illumination of the 
detector elements 10. Each detector element 10 shown in FIG. 6 is a series 
or parallel arrangement of detector elements 10, as illustrated in FIGS. 1 
and 2. Sensor electronics 62 further process the signals received from the 
digital or analog SQUID readout circuits 14, 40, 52. In a focal plane 
array, the detector elements 10 can be easily integrated on a single chip 
for reduced cost and increased reliability. 
The superconducting granular films 16 used in the detector elements 10 are 
fabricated from either low or high temperature superconducting materials. 
Typical low temperature materials include BaPb.sub.1-x Bi.sub.x O.sub.3 
and NbN and a typical high temperature material is YBa.sub.2 Cu.sub.3 O. 
The planar antenna 18 preferably includes two triangular-shaped, gold 
antenna elements coupled to either side of the granular film 16. However, 
other shapes and materials may be used. Although radiation in the infrared 
region of the electromagnetic spectrum has been described, the 
photodetector is also suitable for detecting radiation over other regions 
of the spectrum, such as millimeter wave. 
From the foregoing, it will be appreciated that the present invention 
represents a significant advance in the field of superconducting infrared 
photodetectors. Although several embodiments of the invention have been 
shown and described, it will be apparent that other adaptations and 
modifications can be made without departing from the spirit and scope of 
the invention. Accordingly, the invention is not to be limited, except as 
by the following claims.