We demonstrate a 64-pixel free-space-coupled array of superconducting nanowire single photon detectors optimized for high detection efficiency in the near-infrared range. An integrated, readily scalable, multiplexed readout scheme is employed to reduce the number of readout lines to 16. The cryogenic, optical, and electronic packaging to read out the array, as well as characterization measurements are discussed.
There is a growing interest in developing systems employing large arrays of SNSPDs. To make such instruments practical, it is desirable to perform signal processing before transporting the detector outputs to room temperature. We present a cryogenic eight-channel pixel combiner circuit designed to amplify, digitize, edge detect, and combine the output signals of an array of eight SNSPDs. The circuit has been fabricated and measurement results agree well with expectation. The paper will conclude with a summary of ongoing work and future directions.
We report on the development of waveguide-based mixers for operation beyond 2 THz. The mixer element is a
superconducting hot-electron bolometer (HEB) fabricated on a silicon-on-insulator (SOI) substrate. Because it is beyond
the capability of conventional machining techniques to produce the fine structures required for the waveguide embedding
circuit for use at such high frequencies, we employ two lithography-based approaches to produce the waveguide circuit:
a metallic micro-plating process akin to 3-D printing and deep reactive ion etching (DRIE) silicon micromachining.
Various mixer configurations have been successfully produced using these approaches. A single-ended mixer produced
by the metal plating technique has been demonstrated with a receiver noise temperature of 970 K (DSB) at a localoscillator
frequency of 2.74 THz. A similar mixer, produced using a silicon-based micro-machining technique, has a
noise temperature of 2000 K (DSB) at 2.56 THz. In another example, we have successfully produced a waveguide RF
hybrid for operation at 2.74 THz. This is a key component in a balanced mixer, a configuration that efficiently utilizes
local oscillator power, which is scarce at these frequencies. In addition to allowing us to extend the frequency of
operation of waveguide-based receivers beyond 2 THz, these technologies we employ here are amenable to the
production of large array receivers, where numerous copies of the same circuit, precisely the same and aligned to each
other, are required.
Reception of faint optical communications signals from deep space presumes multi-meter diameter optical receivers
coupled to high detection efficiency photon counting detectors. Superconducting nanowire detectors presently offer the
highest performance for photon starved optical communications at near-infrared optical communications wavelengths.
Square-millimeter sized arrays are required due to atmospheric turbulence and the classical solid angle-area invariant of
an optical system, but most development of superconducting nanowire detectors has been for small pixels and arrays of
less than 0.001 square-millimeter area. One deep space receiver approach is to partition detector area across multiple
receive apertures (multiple telescopes) to use these small detectors, but this carries performance and cost penalties
compared to use of a single large aperture. At JPL we are pursuing the development of large superconducting nanowire
arrays for free space coupling to multi-meter telescopes and have developed a facility for testing large area free-space
coupled nanowire arrays and fabricated arrays up to 64 pixels in the NbTiN and W<sub>(1-x)</sub>Si<sub>x</sub> material systems.
We summarize the development and the delivery of two SIS mixers for the 1.1-1.25 THz band of the heterodyne
spectrometer of Herschel Observatory (HSO). The quasi-optical SIS mixer has two Nb/AlN/NbTiN junctions with
the area of 0.25 um<sup>2</sup>. The Josephson critical current density in the junction is 30-50 kA/cm<sup>2</sup>. The tuning circuit
integrated with SIS junction has the base electrode of Nb and a gold wire layer.
With the new SIS mixers the test receiver maximum Y factor is 1.41. The minimum receiver uncorrected DSB
noise temperature is 450 K. The SIS receiver noise corrected for the loss in the optics is 350-450 K across the
1100-1250 GHz band. The receiver has a uniform sensitivity in the full IF range of 4-8 GHz. The sub-micron
sized SIS junction shape is optimized to ease the suppression of the Josephson current, and the receiver operation
is stable. The measured mixer beam pattern is symmetrical and, in a good agreement with the requirements, has
the f/d =4.25 at the central frequency of the operation band. The minimum DSB SIS receiver noise is close to
6 hv/k, the lowest value achieved thus far in the THz frequencies range.
The Heterodyne Instrument for Far Infrared (HIFI) on ESA's Herschel Space Observatory utilizes a variety of novel RF components in its five SIS receiver channels covering 480- 1250 GHz and two HEB receiver channels covering 1410-1910 GHz. The local oscillator unit will be passively cooled while the focal plane unit is cooled by superfluid helium and cold helium vapors. HIFI employs W-band GaAs amplifiers, InP HEMT low noise IF amplifiers, fixed tuned broadband planar diode multipliers, high power W-band Isolators, and novel material systems in the SIS mixers. The National Aeronautics and Space Administration through the Jet Propulsion Laboratory is managing the development of the highest frequency (1119-1250 GHz) SIS mixers, the local oscillators for the three highest frequency receivers as well as W-band power amplifiers, high power W-band isolators, varactor diode devices for all high frequency multipliers and InP HEMT components for all the receiver channels intermediate frequency amplifiers. The NASA developed components represent a significant advancement in the available performance. This paper presents an update of the performance and the current state of development.
We present a low noise SIS mixer developed for the 1.2 THz band of the heterodyne spectrometer of the Herschel Space Observatory. With the launch of the Herschel SO in 2007, this device will be among the first SIS mixers flown in space. This SIS mixer has a quasi-optical design, with a double slot planar antenna and an extended spherical lens made of pure Si. The SIS junctions are Nb/AlN/NbTiN with a critical current density of about 30 KA/cm2 and with the junction area of a quarter of a micron square. Our mixer circuit uses two SIS junctions biased in parallel. To improve the simultaneous suppression of the Josephson current in each of them, we use diamond-shaped junctions. A low loss Nb/Au micro-strip transmission line is used for the first time in the mixer circuit well above the gap frequency of Nb. The minimum uncorrected Double Sideband receiver noise is 550 K (Y=1.34). The minimum receiver noise corrected for the local oscillator beam splitter and for the cryostat window is 340 K, about 6 hv/k, the lowest value achieved thus far in the THz frequencies range.
The Heterodyne Instrument for Far Infrared (HIFI) on ESA's Herschel Space Observatory is comprised of five SIS receiver channels covering 480-1250 GHz and two HEB receiver channels covering 1410-1910 GHz. Two fixed tuned local oscillator sub-bands are derived from a common synthesizer to provide the front-end frequency coverage for each channel. The local oscillator unti will be passively cooled while the focal plane unit is cooled by superfluid helium and cold helium vapors. HIFI employs W-band GaAs amplifiers, InP HEMT low noise IF amplifiers, fixed tuned broadband planar diode multipliers, and novel material systems in the SIS mixtures. The National Aeronautics and Space Administration's Jet Propulsion Laboratory is managing the development of the highest frequency (1119-1250 GHz) SIS mixers, the highest frequency (1650-1910 GHz) HEB mixers, local oscillators for the three highest frequency receivers as well as W-band power amplifiers, varactor diode devices for all high frequency multipliers and InP HEMT components for all the receiver channels intermediate frequency amplifiers. The NASA developed components represent a significant advancement in the available performance. The current state of the art for each of these devices is presented along with a programmatic view of the development effort.
SIS heterodyne mixer technology based on niobium tunnel junctions has now been pushed to frequencies over 1 THz, clearly demonstrating that the SIS junctions are capable of mixing at frequencies up to twice the energy gap frequency (4(Delta) /h). However, the performance degrades rapidly above the gap frequency of niobium (2(Delta) /h approximately equals 700 GHz) due to substantial ohmic losses in the on-chip tuning circuit. To solve this problem, the tuning circuit should be fabricated using a superconducting film with a larger energy gap, such as NbN; unfortunately, NbN films often have a substantial excess surface resistance in the submillimeter band. In contrast, the SIS mixer measurements we present in this paper indicate that the losses for NbTiN thin films can be quite low.