Shannon M. Duff

On-chip optical interconnects made with gallium nitride nanowires.

In this Letter we report on the fabrication, device characteristics, and optical coupling of a two-nanowire device comprising GaN nanowires with light-emitting and photoconductive capabilities. Axial p-n junction GaN nanowires were grown by molecular beam epitaxy, transferred to a non-native substrate, and selectively contacted to form discrete optical source or detector nanowire components. The optical coupling demonstrated for this device may provide new opportunities for integration of optical interconnects between on-chip electrical subsystems.

Proximity-coupled Ti/TiN multilayers for use in kinetic inductance detectors

We apply the superconducting proximity effect in TiN/Ti multi-layer films to tune the critical temperature, T_C, to within 10 mK with high uniformity (less than 15 mK spread) across a 75 mm wafer. Reproducible T_C’s are obtained from 0.8 to 2.5 K. These films had high resistivities, > 100 µΩ cm, and internal quality factors for resonators in the GHz range, on the order of 100 k and higher. Trilayers of both TiN/Ti/TiN and thicker superlattice films were prepared, demonstrating a well controlled process for films over a wide thickness range. Detectors were fabricated and shown to have single photon resolution at 1550 nm. The high uniformity and controllability coupled with the high quality factor, kinetic inductance, and inertness of TiN make these films ideal for use in frequency multiplexed kinetic inductance detectors and potentially other applications such as nanowire detectors, transition edge sensors, and associated quantum information applications.

Microwave SQUID multiplexer demonstration for cosmic microwave background imagers

Key performance characteristics are demonstrated for the microwave SQUID multiplexer (µmux) coupled to transition edge sensor (TES) bolometers that have been optimized for cosmic microwave background (CMB) observations. In a 64-channel demonstration, we show that the µmux produces a white, input referred current noise level of [Formula: see text] at -77 dB microwave probe tone power, which is well below expected fundamental detector and photon noise sources for a ground-based CMB-optimized bolometer. Operated with negligible photon loading, we measure [Formula: see text] in the TES-coupled channels biased at 65% of the sensor normal resistance. This noise level is consistent with that predicted from bolometer thermal fluctuation (i.e. phonon) noise. Furthermore, the power spectral density is white over a range of frequencies down to ~ 100 mHz, which enables CMB mapping on large angular scales that constrain the physics of inflation. Additionally, we report cross-talk measurements that indicate a level below 0.3%, which is less than the level of cross-talk from multiplexed readout systems in deployed CMB imagers. These measurements demonstrate the µmux as a viable readout technique for future CMB imaging instruments.

Hotspot Relaxation Dynamics in a Current Carrying Superconductor

We experimentally studied the dynamics of optically excited hotspots in current-carrying WSi superconducting nanowires as a function of bias current, bath temperature, and excitation wavelength. We observed that the hotspot relaxation time depends on bias current, temperature, and wavelength. We explained this effect with a model based on quasiparticle recombination, which provides insight into the quasiparticle dynamics of superconductors.

The design and characterization of wideband spline-profiled feedhorns for Advanced ACTPol

Advanced ACTPol (AdvACT) is an upgraded camera for the Atacama Cosmology Telescope (ACT) that will measure the cosmic microwave background in temperature and polarization over a wide range of angular scales and five frequency bands from 28-230 GHz. AdvACT will employ four arrays of feedhorn-coupled, polarization- sensitive multichroic detectors. To accommodate the higher pixel packing densities necessary to achieve Ad- vACT’s sensitivity goals, we have developed and optimized wideband spline-profiled feedhorns for the AdvACT multichroic arrays that maximize coupling efficiency while carefully controlling polarization systematics. We present the design, fabrication, and testing of wideband spline-profiled feedhorns for the multichroic arrays of AdvACT.

Toward Discrete Axial p–n Junction Nanowire Light-Emitting Diodes Grown by Plasma-Assisted Molecular Beam Epitaxy

In this paper we investigate axial p–n junction GaN nanowires grown by plasma-assisted molecular beam epitaxy (MBE), with particular attention to the effect of Mg doping on the device characteristics of individual nanowire light-emitting diodes (LEDs). We observe that a significant fraction of single-nanowire LEDs produce measurable band-gap electroluminescence when a thin AlGaN electron blocking layer (EBL) is incorporated into the device structure near the junction. Similar devices with no EBL typically yield below-detection-limit electroluminescence, despite diode-like I–V characteristics and optically measured internal quantum efficiencies (IQEs) of ∼1%. I–V measurements of the p-regions in p–n junction nanowires, as well as nanowires doped with Mg only, indicate low p-type conductivity and asymmetric Schottky-like p-contacts. These observations suggest that imbalanced carrier injection from the junction and p-contact can produce significant nonradiative losses.

Amplifier-free slab-coupled optical waveguide optoelectronic oscillator systems

We demonstrate a free-running 3-GHz slab-coupled optical waveguide (SCOW) optoelectronic oscillator (OEO) with low phase-noise (<-120 dBc/Hz at 1-kHz offset) and ultra-low sidemode spurs. These sidemodes are indistinguishable from noise on a spectrum analyzer measurement (>88 dB down from carrier). The SCOW-OEO uses high-power low-noise SCOW components in a single-loop cavity employing 1.5-km delay. The noise properties of our SCOW external-cavity laser (SCOWECL) and SCOW photodiode (SCOWPD) are characterized and shown to be suitable for generation of high spectral purity microwave tones. Through comparisons made with SCOW-OEO topologies employing amplification, we observe the sidemode levels to be degraded by any amplifiers (optical or RF) introduced within the OEO cavity.

Readout of two-kilopixel transition-edge sensor arrays for Advanced ACTPol

Advanced ACTPol is an instrument upgrade for the six-meter Atacama Cosmology Telescope (ACT) designed to measure the cosmic microwave background (CMB) temperature and polarization with arcminute-scale angular resolution. To achieve its science goals, Advanced ACTPol utilizes a larger readout multiplexing factor than any previous CMB experiment to measure detector arrays with approximately two thousand transition-edge sensor (TES) bolometers in each 150 mm detector wafer. We present the implementation and testing of the Advanced ACTPol time-division multiplexing readout architecture with a 64-row multiplexing factor. This includes testing of individual multichroic detector pixels and superconducting quantum interference device (SQUID) multiplexing chips as well as testing and optimizing of the integrated readout electronics. In particular, we describe the new automated multiplexing SQUID tuning procedure developed to select and optimize the thousands of SQUID parameters required to readout each Advanced ACTPol array. The multichroic detector pixels in each array use separate channels for each polarization and each of the two frequencies, such that four TESes must be read out per pixel. Challenges addressed include doubling the number of detectors per multiplexed readout channel compared to ACTPol and optimizing the Nyquist inductance to minimize detector and SQUID noise aliasing.

Characterization of the Mid-Frequency Arrays for Advanced ACTPol

The Advanced ACTPol upgrade on the Atacama Cosmology Telescope aims to improve the measurement of the cosmic microwave background anisotropies and polarization, using four new dichroic detector arrays fabricated on 150-mm silicon wafers. These bolometric cameras use AlMn transition-edge sensors, coupled to feedhorns with orthomode transducers for polarization sensitivity. The first deployed camera is sensitive to both 150 and 230 GHz. Here, we present the laboratory characterization of the thermal parameters and optical efficiencies for the two newest fielded arrays, each sensitive to both 90 and 150 GHz. We provide assessments of the parameter uniformity across each array with evaluation of systematic uncertainties. Lastly, we show the arrays’ initial performance in the field.

Low-noise RF-amplifier-free slab-coupled optical waveguide coupled optoelectronic oscillators: physics and operation

We demonstrate a 10-GHz RF-amplifier-free slab-coupled optical waveguide coupled optoelectronic oscillator (SCOW-COEO) system operating with low phase-noise (<-115 dBc/Hz at 1 kHz offset) and large sidemode suppression (>70 dB measurement-limited). The optical pulses generated by the SCOW-COEO exhibit 26.8-ps pulse width (post compression) with a corresponding spectral bandwidth of 0.25 nm (1.8X transform-limited). We also investigate the mechanisms that limit the performance of the COEO. Our measurements indicate that degradation in the quality factor (Q) of the optical cavity significantly impacts COEO phase-noise through increases in the optical amplifier relative intensity noise (RIN).