## 25. Integrated superconducting detectors for optical and IR astrophotonics

Benjamin A Mazin<fnref n="1" />, Alexander B Walter<fnref n="2" /> and Chang-Ling Zou<fnref n="3" />

### Status

Astrophotonics harnesses the power of PICs and other guided light technologies to create components or even entire instruments for astronomy. For all astrophotonic applications a detector is needed for the final conversion of photons into electrical signals. For example, detectors can be designed by placing the photoelectric materials on top of waveguides, such that the propagating photons can be efficiently absorbed through the evanescent field of the waveguide [656, 657]. Below we discuss two different superconducting detector technologies that are currently being pursued for integration with astrophotonics: SNSPDs [658–660] and MKIDs [661–663]. There is also ongoing work using optical transition edge sensors [664, 665] and quantum capacitance detectors (QCDs) [666] are emerging as a promising detector technology for the near future. Superconducting detectors are naturally compatible with SM waveguide spectrometers due to the sub-micron dimensions of an SNSPD hairpin or an MKID inductor (see figures 46(b) and (d)). While superconducting detectors can add complexity, the sensitivity of these technologies can enable new capabilities, reduce the size, weight, and power (SWaP) and cost of instruments, and improve performance.

Superconducting detectors like SNSPDs or MKIDs detect individual photons that are absorbed in a superconducting film. They are known for high speed and low noise. The critical advantage of superconducting detectors over conventional technologies is the lack of read noise of a true photon counting detector without after-pulsing and their negligible dark counts ($<10^{-5}$ cps for SNSPDs and $<10^{-3}$ cps for MKIDs). This makes them ideal detectors for low-flux applications involving a SM spectrometer, such as characterization of exoplanet atmospheres or observation of faint extragalactic sources.

SNSPDs were first fabricated on a GaAs waveguide over a decade ago [656]. Soon after [657], directly coupled SNSPDs to a Si waveguide on-chip photonic network and demonstrated 91% detection efficiency. Since then, SNSPDs have been integrated on many waveguide platforms including Si, SiN, AlN, GaAs, LiNbO3, and diamond, as well as coupled to AWGs, on-chip echelle gratings, Rowland circle spectrometer, quantum dots, on-chip photonic crystal cavities, MEMS switches, and other photonic components. See [667] for a review. The relatively simple structure of SNSPDs and their exquisite timing performance with UV to Mid-IR demonstrations of <5 ps jitter and up to $10^7$ Hz maximum count rate make them favored detectors for many science and commercial applications of integrated quantum photonics systems [668]. While MKIDs lack this history in photonics, their similarly high performance and their natural multiplexability into large arrays make them interesting for astronomical applications.

The complex, high speed, power-hungry, and often custom readout electronics required for superconducting detectors, as well as their cryogenic cooling requirements, can make them undesirable if their high performance is not required. SNSPDs are notoriously difficult to multiplex into the large arrays required for astronomical imaging or spectroscopy [268, 669, 670]. MKIDs on the other hand, are naturally frequency multiplexable and the 20 440 pixel MEC instrument at the Subaru Telescope combined with the SCExAO instrument is the largest superconducting camera in the world [663]. Both technologies require expensive cryogenic cooling with NbN, NbTiN, WSi, or MoSi SNSPDs operating at ~<4 K while TiN or PtSi MKIDs operate even colder at ~0.10 K. Recently, efficient SNSPDs working at 7 K have been reported [671], opening up several applications with a low-cost cryostat for ground- and space-based telescopes.

### Current and future challenges

The first kilo-pixel SNSPD array, a 32 × 32 array of WSi nanowires, was demonstrated by [268] using a row-column multiplexing technique. Conventionally, SNSPDs are DC biased near their critical current such that when a photon is absorbed a local hotspot forms. The highly resistive hotspot temporarily redirects the bias current into the readout circuit thus generating an AC pulse signal. In the row-column multiplexing scheme this signal is split with a resistive-inductive network such that half is propagated along the row and column feedlines respectively. Since photon events are recorded by searching for coincidence events in the rows and columns this readout scheme is susceptible to confusion during multi-photon events. Furthermore, the splitting of the signal as well as parasitics in the resistive network reduce the signal-to-noise ratio presenting a major challenge for scaling to megapixel arrays. More recently [670], have demonstrated a kilo-pixel SNSPD array using an orthogonal time-amplitude multiplexing method. Other multiplexing schemes exist including thermal row-column [673], frequency multiplexing [674], and SFQ readouts [675]

<footnote n="1">¹ Department of Physics, University of California, Santa Barbara, CA, United States of America</footnote>

<footnote n="2">² Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, United States of America</footnote>

<footnote n="3">³ CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei, Anhui, People's Republic of China</footnote>
<figure><img src="image_59.png" /></figure>

but these are less mature and have increasingly complex fabrication steps that generally include multiple lithographic layers with complicated electrical wiring. Coupling light directly on-chip with integrated photonics adds to this complexity due to the experimentally challenging growth or transfer of high-quality functional material, including semiconductors, two-dimensional material layers, and also superconducting nanofilms, to dielectric photonic materials. These functional materials also introduce complexities in the nanofabrication processes.

MKIDs consist of lithographed microwave resonators whose resonant frequency temporarily changes when a photon interacts with the superconducting metal of the detector. The magnitude of this frequency change, usually measured as the phase shift of a microwave probe tone, encodes both the energy (R = 35 at 400 nm) [676] and arrival time (1 µs) of the photon. Since an MKID resonator has high quality factor Q and unity transmission off-resonance, many resonators can be read out over a single microwave feedline allowing for easy fabrication into large arrays. MKIDs require a very clean and uniform substrate to deposit their superconducting film on in order to attain the very high internal Q's (>2e5) and low two level system (TLS noise, see [677]) required for optimal performance. One challenge with realizing waveguide-integrated MKIDs is to use dielectric materials which simultaneously have low optical loss and low TLS density, and to mitigate the effects of TLS noise from amorphous dielectrics such as SiO₂. Maintaining high cleanliness and inter-material compatibility at interfaces represent the primary challenges when simultaneously fabricating photonic components.

A cryogenic operating temperature creates additional complications. First, the optical properties of PICs can change slightly from room temperature, adding another variable to fabrication. This is not well explored in literature. Second, efficient self-aligned coupling from a SM optical fiber to a PIC at sub-Kelvin temperatures has remained a challenge for the community, as the maintenance of sub-micron alignment of the optical fiber is necessary to maintain efficient coupling. A space-based astrophotonics instrument must have an alignment that survives the vibrations of the launch, and then the autonomous cooling en-route to the final orbit. Thirdly, increased cost, complexity, and SWaP is needed to cool these superconducting detectors. This may be prohibitive for optical astronomy but useful for mid-IR astronomy where cryogenic cooling is always required for reduced thermal backgrounds.
<figure><img src="image_60.png" /><figcaption>**Figure 47.** Light from a waveguide is dispersed with a photonic Echelle grating onto a superconducting nanowire implementing an on-chip spectrometer. Reproduced from [678]. CC BY 4.0.</figcaption></figure>

### Advances in science and technology to meet challenges

A novel application of integrated photonics could implement on-chip waveguides into the readout circuit of an imaging array of SNSPDs. In [672] they demonstrate a UV sensitive MoSi SNSPD pixel electrically coupled to a photonic MRR as depicted in figure 46(c). The voltage pulse from a photon event in the detector is used to forward bias the resonator influencing its refractive index and imprinting the signal on an optical carrier propagated along a Si waveguide. The signal is then coupled to a SMF through a vertical grating coupler and readout at room temperature. This approach allows for wavelength division multiplexing of SNSPDs which avoids the system tradeoffs of other multiplexing techniques by preserving high sensitivity, high efficiency, high maximum count rates/dynamic range, and a robustness to crosstalk or pickup noise. Future developments will need to demonstrate many pixels multiplexed to a single optical fiber and mitigate the thermal power dissipation of the MRRs. Ultimately, an imaging array of SNSPDs suitable for astronomy may utilize several different multiplexing techniques which could be tailor fit to the application. Megapixel arrays of SNSPDs could emerge by the end of the decade, but a more dedicated research effort would produce them considerably sooner. Apart from light intensity measurements, the single-photon detections by SNSPDs might also benefit the 'intensity correlation measurement' for astronomy applications [679]. For example, the second-order correlation and even high-order correlations of photons have been demonstrated within a single photonic chip recently [680, 681].

A highly desired photonic technology for astronomy is the monolithic integration of spectrographs with detectors that offer a compact solution and high efficiency by avoiding losses when imaging the guided light to on-chip detectors. They would consume significantly less SWaP, enjoy higher stability, be cheaper to produce, and be easier to use. For the faint sources targeted by astronomers, photon counting, low dark rates, low read noise, broadband sensitive detectors like SNSPDs or MKIDs are ideal. A moderate resolution (2.2 nm) but very small bandwidth (24 nm at 1550 nm wavelengths) has already been demonstrated with SNSPDs coupled to narrowband nanophotonic circuits [267]. Kong et al [682] demonstrated a single-pixel SNSPD with broadband responsivity ranging from 660–1900 nm, which was able to achieve a spectral resolution of <10 nm at telecom wavelengths by varying the quantum efficiency over time. Cheng et al [678] demonstrated the first broadband on-chip spectrometer with 200 effective spectral channels between 600 and 2000 nm wavelengths. Light manipulated by a waveguide is dispersed by a lithographically defined Si3N4 Echelle grating onto a NbN SNSPD which is read out with time-of-flight multiplexing (figure 47). The design allows for easy scalability to higher spectral resolution. For this proof of concept the system detection efficiency was ~<0.1% at 1550 nm although future implementations could reach 50% with some significant optimizations of the 8–9 dB nanowire absorption loss and 7–8 dB diffraction efficiency loss in the grating. Dedicated efforts are also needed in scaling the number of on-chip detectors integrated with the PICs (e.g. several thousands for spectrographs).
Work is on-going at UCSB to build waveguide integrated MKIDs with the goal of integrating these detectors with an AWG spectrograph, as shown in figure 46(d). This PIC spectrometer uses the intrinsic spectral resolution ($R \sim 35$ at 400 nm now, up to $R \sim 100$ in the future) of the MKIDs to determine what order a photon came from [683], allowing an octave or greater bandwidth spectrometer with significantly higher spectral resolution than the MKIDs alone can deliver ($R = 1-100k$). This on-chip spectrometer could be extremely compact, allowing tens to hundreds of them to reside in a single dewar.

## Concluding remarks

Combining photonics with on-chip superconducting detectors allows instrument designers to bring extremely powerful light processing tools together with the most powerful photon counting detectors available. We are at the very start of this field, but the promise is considerable. Potential instruments include high-resolution fiber-fed integral field spectrographs for exoplanets direct imaging using high dispersion coronagraphy, inexpensive time-resolved high-resolution spectrometers, mid-IR space-based exoplanet transit spectrometers, WFSs [53], on-chip interferometric instruments [546], and significantly less expensive and more stable EPRV instruments.

## Acknowledgments

Thanks to Matthew Shaw, Miguel Daal, and Majid Mohammed for comments and advice. A portion of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (NASA) (80NM0018D0004).

## Data availability statement

The data that support the findings of this study are available upon reasonable request from the authors.

## ORCID iDs

Nemanja Jovanovic https://orcid.org/0000-0001-5213-6207

Pradip Gatkine https://orcid.org/0000-0002-1955-2230

Narsireddy Anugu https://orcid.org/0000-0002-2208-6541

Ritoban Basu Thakur https://orcid.org/0000-0002-3351-3078

Chad F. Bender https://orcid.org/0000-0003-4384-7220

Jean-Philippe Berger https://orcid.org/0000-0001-5025-0428

Azzurra Bigioli https://orcid.org/0000-0003-1008-3682

Joss Bland-Hawthorn https://orcid.org/0000-0001-7516-4016

Guillaume Bourdarot https://orcid.org/0000-0002-6777-6386

Charles M Bradford https://orcid.org/0000-0001-5261-7094

Julia Bryant https://orcid.org/0000-0003-1627-9301

Kevin Bundy https://orcid.org/0000-0001-9742-3138

Ross Cheriton https://orcid.org/0000-0003-0522-0187

Nick Cvetojevic https://orcid.org/0000-0002-7465-4176

Momen Diab https://orcid.org/0000-0001-6556-9378

Scott A Diddams https://orcid.org/0000-0002-2144-0764

Aline N Dinkelaker https://orcid.org/0000-0002-4111-4763

Jeroen Duis https://orcid.org/0009-0003-3230-3209

Stephen Eikenberry https://orcid.org/0000-0002-1332-5061

Simon Ellis https://orcid.org/0000-0002-0742-379X

Akira Endo https://orcid.org/0000-0003-0379-2341

Donald F Figer https://orcid.org/0000-0002-4206-733X

Michael P. Fitzgerald https://orcid.org/0000-0002-0176-8973

Itandehui Gris-Sanchez https://orcid.org/0000-0001-5756-5107

Simon Gross https://orcid.org/0000-0001-5130-183X

Ludovic Grossard https://orcid.org/0000-0002-4022-5736

Olivier Guyon https://orcid.org/0000-0002-1097-9908

Sebastiaan Y Haffert https://orcid.org/0000-0001-5130-9153

Samuel Halverson https://orcid.org/0000-0003-1312-9391

Jinping He https://orcid.org/0000-0002-1899-3384

Tobias Herr https://orcid.org/0000-0002-5966-6697
Philipp Hottinger https://orcid.org/0000-0002-9489-1160

Michael Ireland https://orcid.org/0000-0002-6194-043X

Laurent Jocou https://orcid.org/0000-0001-9376-1818

Stefan Kraus https://orcid.org/0000-0001-6017-8773

Lucas Labadie https://orcid.org/0000-0001-5342-5713

Sylvestre Lacour https://orcid.org/0000-0002-6948-0263

Romain Laugier https://orcid.org/0000-0002-2215-9413

Jonathan Lin https://orcid.org/0000-0001-8542-3317

Stephanie Leifer https://orcid.org/0000-0002-8980-7825

Sergio Leon-Saval https://orcid.org/0000-0002-5606-3874

Guillermo Martin https://orcid.org/0000-0002-6531-5615

Marc-Antoine Martinod https://orcid.org/0000-0002-0989-9302

Benjamin A Mazin https://orcid.org/0000-0003-0526-1114

John D Monnier https://orcid.org/0000-0002-3380-3307

Denis Mourard https://orcid.org/0000-0001-7425-5055

Abani Shankar Nayak https://orcid.org/0000-0002-4666-9282

Ewelina Obrzud https://orcid.org/0000-0001-9856-0484

Karine Perraut https://orcid.org/0000-0003-3099-757X

François Reynaud https://orcid.org/0000-0002-6402-9748

Steph Sallum https://orcid.org/0000-0001-6871-6775

David Schiminovich https://orcid.org/0000-0003-2666-4430

Christian Schwab https://orcid.org/0000-0002-4046-987X

Sherif Soliman https://orcid.org/0009-0008-8938-2880

Andreas Stoll https://orcid.org/0000-0001-7105-2930

Peter Tuthill https://orcid.org/0000-0001-7026-6291

Gautam Vasisht https://orcid.org/0000-0002-1871-6264

Sylvain Veilleux https://orcid.org/0000-0002-3158-6820

Alexander B Walter https://orcid.org/0000-0002-4427-4551

Edward J Wollack https://orcid.org/0000-0002-7567-4451

Yinxi Xin https://orcid.org/0000-0002-6171-9081

Zongyin Yang https://orcid.org/0000-0003-2869-406X

Stephanos Yerolatsitis https://orcid.org/0000-0002-6268-3674

Yang Zhang https://orcid.org/0000-0003-1754-8796

Chang-Ling Zou https://orcid.org/0000-0003-2484-7292

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