Towards Photon Counting Kinetic Inductance Detectors for Far-IR Spectroscopy

Jake Connors (NASA-GSFC, jake.Connors@nasa.gov)Emily Barrentine, Jochem Baselmans, Ari Brown, Michael Cyberey, Aveek Gangopadhyay, Larry Hess, Arthur Lichtenberger,

Harvey Moseley, Thomas Stevenson, Edward Wollack, Kongpop U-yen, Omid Noroozian

The prototype photon counting detectorwill consist of an on-chip antenna tocouple light onto a NbTiN microstripline. A double half-wave bandpass filterin concert with quasi-optical filteringthen defines a narrow band to simulateone channel of a medium resolutionspectrometer. This signal is fed to asmall volume Al inductor at the centerof a lumped element resonator. Thecapacitive elements in the resonator areNbTiN parallel plate capacitors, whichare patterned with a photonic choke toreflect pair breaking photons back tothe inductor area. Expected loadinglevels for photon counting at 850GHzare in the 0.1-5 attowatt range given aquasi-particle lifetime of ~1ms. Toincrease responsivity, the aluminumvolume is kept in the range of 0.1-1 μm3

using demonstrated high-Qi 20nm thickfilms.

We have developed a dual-band (2-4GHz and 4-8GHz)microwave I/Q Up/Down converter system to interfaceKIDs and μMux chips with I/Q-based baseband DSPsystems such as ROACH2 or RFSoC. System consists ofan input amplifier chain, input demodulator, outputmodulator, output filtering and levelling, input andoutput power monitoring and LO distribution sub-systems. Both input and output chains have 60dB ofvariable gain/attenuation to accommodate varyingresonator read powers. Input and output chains areisolated to better than 100dB through the use ofisolators and a high isolation Wilkinson splitter. Allcomponents are COTS except a single custom equalizerper band to flatten the input gain. The entire readoutdissipates < 50W and fits in a 2U rack-mounted box.Variable attenuator settings are controlled through asingle EMI-filtered USB input. Eight temperaturesensors and the four power detectors are read usingan Arduino which is also command-able via USB.

Why Photon Counting Detectors?

Photon-Counting Detector Concept

Dark Test Volume and Integrated Blackbody CalibratorDC and Microwave Powder Filters

NbTiN & Al Film Characterization

Photon-counting detectors are an enabling technology for future space-based far-infraredspectroscopic instruments such as those proposed as part of the Origins Space Telescope (OST) andwould greatly increase the sensitivity and mapping speed of potential instruments. Microwavekinetic inductance detectors (KID) are a promising technology for these instruments, where largearrays of detectors with noise equivalent powers (NEP) less than 3x10-20 W/Hz-1/2 will be required toachieve photon-noise background limited performance. In contrast to superconducting transitionedge sensors (TES), KIDs are naturally frequency multiplexed allowing for the simple readout oflarge arrays, but a factor of nearly ten improvement in NEP is needed to meet the needs of futurespace-based spectrometers. Our project seeks to develop KID technology and achieve NEPs suitablefor future space missions through the use of a novel lumped element KID design with opticalcoupling implemented at 850GHz. Utilizing ultra-low volume Al inductors to increase responsivityand photonically-choked NbTiN parallel plate capacitors on single crystal silicon to minimizeinterface defect-driven two-level system (TLS) noise, our design seeks to approach the photoncounting limit, where detector baseline calibration is obviated and full duty-cycle observation canbe achieved. We are currently studying both NbTiN and Al deposition parameters to achieve filmswith low TLS densities and high internal quality factors, while simultaneously developing low-defectcompatible fabrication processes and an ultra-low background measurement facility with anintegrated cryogenic blackbody calibrator to characterize our high sensitivity devices.

Far-IR Spectroscopy Science

OST Mission ConceptOST Detector Needs

HFSS design of a double slotantenna in 300nm thick NbTiN ona 450nm thick SOI device layer.The antenna is fed by twomicrostrip radial probe cross-overs, which are combined in-phase to form a single 12Ωmicrostrip input. The designachieves a 22% bandwidth (η >90%) at 850GHz with feature sizes≥ 1.4μm.

Assuring light-tightness of a dark test-volumerequires all electrical penetrations into thatvolume be filtered to reduce the flux of pair-breaking photons to well below the rateassociated with the quasi-particle lifetime.Thermometry, heaters, and other readout wiringcan be effectively filtered using a heavilyconductor loaded epoxy in a compact packagewithout affecting 120dB of attenuation at > 90GHz.

Microwave IF Readout System

The blackbody calibrator is designedin the many-mode limit to workfrom 400GHz to 5THz with areflection < -30dB. The blackbodyuses a cast non-magneticconductively loaded epoxy as anabsorptive layer over a 15deg half-angle folded copper cone. Thermalmass is kept to a minimum in orderto achieve a time constant of < 15minutes up to 10K.

Detectors for space-based Far-IR spectroscopy will bedesigned with very low saturation powers and thus requirevery dark environments, 106) but the effects of a hybrid NbTiN/Alfabrication process and aging on them have not beenfully studied. Additionally both NbTiN-compatible Aletch processes and NbTiN liftoff processes are indevelopment.