Search

The recombination rate of quasiparticle excitations and metal film thickness are both important factors in determining the sensitivity of kinetic inductance detectors (KIDs). To maximize KID sensitivity, we aim to quantify the interdependence of these two detector attributes. We have measured the decay times of optical pulses produced by illuminating aluminum CPW resonators with an infrared LED. Measurements were made using both 1/4-wavelength and 1/2-wavelength resonators for film thicknesses between 20 and 50 nm for a range of temperatures and microwave readout powers. We observed several millisecond decay times for all thicknesses, with an elevated decay time ($$\sim$$ 5 ms) and critical temperature ($$\sim$$ 1.5 K) for the 20-nm-thick film.

TolTEC is a 3-band millimeter-wave imaging polarimeter scheduled for deployment to the Large Millimeter Telescope (LMT) in January 2020. TolTEC consists of three, kilopixel-scale, monolithic arrays of microwave kinetic inductance detectors (MKIDs), together comprising over 7,000 polarization sensitive detectors. Here we describe many of the unique aspects of the TolTEC all-silicon focal plane design. We then present both laboratory and fully integrated in-receiver measurements in the lab with which we characterize the optical, resonator, and noise properties of the arrays.

SuperSpec is a compact on-chip spectrometer operating at mm and sub-mm wavelengths which will enable the construction of sensitive multibeam spectrometers. SuperSpec employs a filter bank architecture, consisting of lithographically patterned niobium superconducting microstrip mm-wave resonators. The power admitted by each resonator is detected by a titanium nitride lumped-element kinetic inductance detector (KID) with resonant frequency from 100 to 200 MHz. We present a characterization of the detector noise performance down to 10 mK measured in a dark setting. We report a device NEP of $$2.7 \times 10^{-18}\, \hbox {W Hz}^{-1/2}$$ at 210 mK, which is below the expected photon noise level at high-altitude ground-based observatories. The NEP decreases to a constant value of approximately $$7.0 \times 10^{-19}\, \hbox {W Hz}^{-1/2}$$ below 130 mK. The white noise is well modeled by thermal generation–recombination noise (GR noise) down to 130 mK and a noise floor at low temperatures. Moreover, the addition of low-pass coaxial filters further reduces the noise floor to achieve an NEP of $$5.7 \times 10^{-19} \,\hbox {W Hz}^{-1/2}$$ below 100 mK. We discuss a photolithographic technique to adjust KID resonances that results in an $$f_{0}$$ designed versus measured scatter of $$1.7 \times 10^{-5}$$ , which will allow a significant reduction in resonators lost to clashes in full-scale designs. Finally, we present a demonstration of a new ROACH-2-based readout system operating below 500 MHz and show preliminary data indicating the suitability of this system for future highly multiplexed KID arrays.

SuperSpec is a new technology for mm and sub-mm spectroscopy. It is an on-chip spectrometer being developed for multi-object, moderate-resolution ( R∼300 ), large bandwidth survey spectroscopy of high-redshift galaxies for the 1 mm atmospheric window. This band accesses the CO ladder in the redshift range of z= 0–4 and the [CII] 158 μ m line from redshift z= 5–9. SuperSpec employs a novel architecture in which detectors are coupled to a series of resonant filters along a single microwave feedline instead of using dispersive optics. This construction allows for the creation of a full spectrometer occupying only ∼10cm2 of silicon, a reduction in size of several orders of magnitude when compared to standard grating spectrometers. This small profile enables the production of future multi-beam spectroscopic instruments envisioned for the millimeter band to measure the redshifts of dusty galaxies efficiently. The SuperSpec collaboration is currently pushing toward the deployment of a SuperSpec demonstration instrument in fall of 2018. The progress with the latest SuperSpec prototype devices is presented; reporting increased responsivity via a reduced inductor volume (2.6 μm3 ) and the incorporation of a new broadband antenna. A detector NEP of 3–4 ×10−18 W/Hz 0.5 is obtained, sufficient for background-limited observation on mountaintop sites. In addition, beam maps and efficiency measurements of a new wide-band dual bow-tie slot antenna are shown.

The high sensitivity requirements set by future cosmic microwave background instruments are pushing the current technologies to produce highly performant focal plane arrays with thousands of detectors. The coupling of the detectors to the telescope optics is a challenging task. Current implemented solutions include phased-array antenna-coupled detectors, platelet horn arrays, and lenslet-coupled planar antennas. There are also recent developments of flat graded-index lenses based on etched silicon. However, there are strong requirements in terms of electromagnetic performance, such as coupling efficiency and bandwidth, as well as requirements in terms of easy manufacturing and scalability, and it is very challenging to meet all these requirements with one of the above solutions. Here, we present a novel approach for producing flat metal-mesh lenslet arrays based on devices previously realized using the mesh-filter technology. We have now adapted the polypropylene-based mesh lens design to silicon substrates, thus providing a good mechanical match to the silicon-based detector arrays. The measured performance of prototype pixels operating at millimeter wavelengths is presented.

We demonstrate microwave kinetic inductance detectors (MKIDs) whose sensitivity is limited by photon noise at signal frequencies as low as 50 mHz. The subhertz part of the detection spectrum is important for contemporary millimeter-wave science, yet photon noise below 1 Hz in MKIDs has not previously been unambiguously demonstrated. These feedhorn-coupled detectors are sensitive over a 40% fractional bandwidth centered on λ = 2 mm, are dual-polarization-sensitive, and consist of lumped-element superconducting resonators fabricated from a hybrid of stoichiometric TiN and Al films together with an amorphous-Si passivation layer. When observing a 7 K, 3.8 pW thermal load and employing no signal modulation scheme, the measured noise spectrum is white down to 50 mHz and has an amplitude consistent with photon noise. These results have implications for the development of large-format, polarization-sensitive, millimeter-wave imagers.

Future generations of far-infrared (FIR) telescopes will need detectors with noise-equivalent powers on the order of 5 x 10^(-20) W/Hz^(1/2) in order to be photon background limited by astrophysical sources. One such mission concept in development is the Galaxy Evolution Probe (GEP), which will characterize galaxy formation and evolution from z=0 to beyond z=4. Kinetic inductance detectors (KIDs) have been baselined for the GEP for spectroscopy and imaging science between 10 μm and 400 μm due to their intrinsic frequency multiplexability and simple readout schemes. We focus on quasiparticle recombination times as a strategy for increasing detector responsivities to move towards the NEP requirements of the GEP. We present a new model for quantifying time constants from the responses of detectors to pulses of light, and test this model on a 40 nm thick ¼ λ Al coplanar waveguide KID. We intend to use this measurement scheme to quantify the dependence of the quasiparticle recombination time on Al thickness.

Microwave Kinetic Inductance Detectors (MKIDs) have held promise as the focal plane sensing elements in large-format imaging arrays for over a decade and have now found application in several ground-based instruments. In this presentation, we discuss the first implementation of MKIDs for a suborbital instrument in the Balloon-borne Large Aperture Submillimeter Telescope – The Next Generation (BLAST-TNG), a suborbital imaging array designed to study the role magnetic fields play in star formation and bridge the angular scales between Planck’s low resolution all-sky maps and ALMA’s ultra-high resolution narrow fields. BLAST-TNG is scheduled to launch from Antarctica in December 2018. This experiment will utilize 8 times as many polarization sensitive detectors and will have 16 times greater mapping speed compared to its predecessor BLASTpol. This will also be a demonstration for future MKID instruments for ground based telescopes, e.g. TolTEC arrays on the LMT, as well as proposed space based missions. We have built three, large-format MKID arrays for BLAST-TNG. Each monolithic 100mm diameter array is sensitive to a different waveband centered at 250 micron, 350 micron, or 500 micron; together comprising 3318 individual polarization-sensitive detectors. The detector arrays are read out with high levels of multiplexing, with each microwave feedline addressing between 466 and 938 unique resonators depending on the array. Designing for space-like low photon loads, polarization-sensitivity, different frequency bands, and 275 mK operation all pose unique challenges. We address these challenges by employing feedhorn-coupled, dual-polarization sensitive pixels fabricated from TiN/Ti multilayers that combine high responsivity, high uniformity, low loss, and a tunable superconducting Tc. Here, we present the detailed design and fabrication of these arrays, which includes an optimized quarter wavelength silicon backshort for each band realized by micromachining a silicon on insulator (SOI) wafer, aluminum patching of the TiN/Ti absorbing inductor to increase response and tune the effective optical coupling impedance, and a semi-automated layout scheme to make a stepper-compliant lithography process that tiles identical stepper images across the array and then trims them individually to minimize their frequency scatter and crosstalk. This results in high quality, easily reconfigurable, and uniform arrays of MKIDs. We show measurements that demonstrate high pixel yield, > 98% polarization isolation, and a noise equivalent power (NEP) limited by photon noise at the expected in-flight photon load.

SuperSpec is a new technology for millimeter and submillimeter spectroscopy. It is an on-chip spectrometer being developed for multi-object, moderate resolution (R = ~300), large bandwidth survey spectroscopy of high-redshift galaxies for the 1 mm atmospheric window. SuperSpec targets the CO ladder in the redshift range of z = 0 to 4, the [CII] 158 um line from z = 5 to 9, and the [NII] 205 um line from z = 4-7. All together these lines offer complete redshift coverage from z = 0 to 9. SuperSpec employs a novel architecture in which detectors are coupled to a series of resonant filters along a single microwave feedline instead of using dispersive optics. This construction allows for the creation of a full spectrometer occupying only 20 cm squared of silicon, a reduction in size of several orders of magnitude when compared to standard grating spectrometers. This small profile enables the production of future multi-object spectroscopic instruments required as the millimeter-wave spectroscopy field matures. SuperSpec uses a lens-coupled antenna to deliver astrophysical radiation to a microstrip transmission line. The radiation then propagates down this transmission line where upon proximity coupled half wavelength microstrip resonators pick off specific frequencies of radiation. Careful tuning of the proximity of the resonators to the feedline dials in the desired resolving power of the SuperSpec filterbank by tuning the coupling quality factor. The half wavelength resonators are then in turn coupled to the inductive meander of kinetic inductance detectors (KIDs), which serve as the power detectors for the SuperSpec filterbank. Each SuperSpec filter bank contains hundreds of titanium nitride TiN KIDs and the natural multiplexibility of these detectors allow for readout of the large numbers of required detectors. The unique coupling scheme employed by SuperSpec allows for the creation of incredibly low volume (2.6 cubic microns), high responsivity, TiN KIDs. Since responsivity is proportional to the inverse of quasiparticle-occupied volume, this allows SuperSpec to reach the low NEPs required by moderate resolution spectroscopy to be photon limited from the best ground-based observing sites. We will present the latest results from SuperSpec devices. In particular, detector NEPs, measured filter bank efficiency (including transmission line losses), and spectral profiles for a full ~ 300-channel filterbank. Finally, we will report on our system end to end efficiency and total system NEP.

The need for larger arrays of millimeter and submillimeter wavelength detectors for Cosmic Microwave Background (CMB) experiments is driving a demand for focal planes which can field large numbers of detectors with both high sensitivity and wide bandwidth. Current CMB experiments have $\sim 10^{4}$ detectors, with next generation focal planes requiring $\sim 10^{5}$ or more. One challenge of expanding the array size is coupling the detectors to instrument optics with a method that is broadband, low loss, and scalable. Current state of the art methods of coupling incident radiation include phased array antenna-coupled detectors, corrugated feedhorn arrays, and hemispherical lenslet array-coupled planar antennas. Phased array antennas are fabricated using planar lithography techniques and therefore easily scalable, but are typically narrow band ($\sim 30\%$). Silicon platelet feedhorns are scalable and low loss, but typically achieve only an octave of bandwidth. Lenslets have been produced using silicon hemispheres stacked on silicon plates to approximate an elliptical lens. Low loss and broadband behavior is accomplished by individually molding anti-reflection layers made of materials with appropriate refractive indices and individually glued to arrays; however this approach does not easily scale to larger arrays. We are developing planar lenslet arrays using metamaterials fabricated with standard microlithograpy techniques on silicon wafers. Instead of using difficult to manufacture curved optical surfaces, the lenslets consist of stacks of silicon wafers which are each patterned with an array of sub-wavelength features to produce optical features which form a well defined beam at measurement wavelengths. These arrays are being developed using two approaches: GRadient INdex (GRIN) lenslets which are fabricated by etching holes on a sub-wavelength grid to produce a spatially varying effective index of refraction, and metal-mesh lenslets which are produced by depositing spatially varying metallic features which act as a series of Transmission Line (TL) lumped element features to control phase delay across the wafer. GRIN lenslets are fabricated by etching sub-wavelength holes on a periodic, sub-wavelength grid using standard microlithography techniques. The wafers can be stacked, allowing the spatial index to be altered along all dimensions, which allows for arbitrary anti-reflective coatings to be integrated in the lenslet design. Simulations in finite element modeling (FEM) software have been used to both evaluate the effective index of an individual element and simulate full lenslet structures. Dielectrically embedded mesh-lenses are based on existing mesh-filter technology. Differently from the mesh-filters, the grids are inhomogeneous and their geometry is designed in such a way to impart variable phase shifts across the surface. The local phase shifts reproduce those that would be introduced by a classical dielectric lens. In this work we are developing mesh-lenslets on silicon substrates. The metal grids are supported by silicon nitride (SiN) membranes and kept at specific distances in an air-gap configuration. Finite element analysis is used to quantify and optimize the performance of these devices. We report on progress in both lenslet design approaches. In each case we have developed a set of design equations which guide the design of the full lenslet structure. These structures are simulated using finite element modeling simulations. We report on measurements and efficacy of the design and simulation process and agreement with laboratory measurements of prototype lenslet arrays.

Microwave Kinetic Inductance Detectors (MKIDs) provide a technological path towards the high-yield, large-format detector arrays needed for the next generation of experiments. The intrinsically integrated readout components of MKIDs generally give rise to high multiplexing factors, simplified assembly, and streamlined experimental integration. We describe the first MKID arrays fabricated and tested on monolithic 150 mm diameter silicon substrates – a crucial scaling in fabrication capacity that is necessary for future large-scale experiments aiming to incorporate hundreds of thousands of detectors in the coming years. The arrays described here are being developed for the TolTEC millimeter-wave imaging polarimeter being constructed for the 50-meter Large Millimeter Telescope (LMT), with observations planned to begin in early 2019. TolTEC uses dichroic filters to define three physically independent focal planes for operation in observational bands centered at 1.1, 1.4, and 2.0 mm. Each focal plane observes in just one wavelength band, allowing the use of simple to produce, direct-absorption pixel designs with each pixel comprising two detectors that are sensitive to orthogonal states of linear polarization. TolTEC comprises approximately 7,000 polarization sensitive MKIDs designed to operate at a base temperature of 100 mK. The primary working material used for these devices are TiN/Ti/TiN multilayer films, which have several advantageous qualities including: low two-level system noise at the TiN-silicon interface; linear responsivity; uniformity in deposition; and tunable transition temperature, sheet resistance and sheet inductance. We describe the detailed pixel and array layout designs, including focal plane integration and optical coupling via spline-profiled, silicon-platelet, feedhorn-coupled waveguide. We present measurements of full arrays and/or prototype small arrays of devices operating in each of the three observation bands and compare the observed noise and optical performance to that predicted from models and simulations. We also describe the fabrication methods used to produce these large-format arrays with high yield and uniformity.

We review advances in low temperature detector (LTD) arrays for Cosmic Microwave Background (CMB) polarization experiments, with a particular emphasis on imaging arrays. We briefly motivate the science case, which has spurred a large number of independent experimental efforts. We describe the challenges associated with CMB polarization measurements and how these challenges impact LTD design. Key aspects of an ideal CMB polarization imaging array are developed and compared to the current state-of-the-art. These aspects include dual-polarization-sensitivity, background-limited detection over a 10:1 bandwidth ratio, and frequency independent angular responses. Although existing technology lacks all of this capability, today's CMB imaging arrays achieve many of these ideals and are highly advanced superconducting integrated circuits. Deployed arrays map the sky with pixels that contain elements for beam formation, polarization diplexing, passband definition in multiple frequency channels, and bolometric sensing. Several detector architectures are presented. We comment on the implementation of both transition-edge-sensor bolometers and microwave kinetic inductance detectors for CMB applications. Lastly, we discuss fabrication capability in the context of next-generation instruments that call for $\sim 10^6$ sensors., Comment: Proceedings for the 17th international workshop on Low Temperature Detectors (LTD17)

SuperSpec is a new spectrometer-on-a-chip technology for submm/mm-wave spectroscopy. SuperSpec stands out from other direct-detection submm spectrometer technologies in that the detectors are coupled to a series of resonant filters along a single microwave feedline instead of using dispersive optics. SuperSpec makes use of kinetic inductance detectors (KIDs) to detect radiation in this filter bank. The small profile of this design makes SuperSpec a natural choice to produce a multi-object spectrometer for tomographic mapping or galaxy redshift surveys. We have recently fabricated a device that is a 50 channel subset of a full 280 channel filter bank, which would cover the 190 - 310 GHz range at R = 275. Analysis of the data from this device informs us of the potential design modifications to enable a high-yield background-limited SuperSpec spectrometer. The results indicate that this subset filter bank can scale up to a full filter bank with only a few collisions in readout space and less than 20% variation in responsivity for the detectors. Additionally, the characterization of this and other prototype devices suggests that the noise performance is limited by generation-recombination noise. Finally, we find that the detectors are sufficiently sensitive for ground-based spectroscopy at R = 100, appropriate for tomographic mapping experiments. Further modifications are required to reach the background limit for R = 400, ideal for spectroscopy of individual galaxies.

To characterize further the cosmic star formation history at high redshifts, a large-area survey by a cryogenic 4-6 meter class telescope with a focal plane populated by tens of thousands of far-infrared (FIR, 30-300 μm) detectors with broadband detector noise equivalent powers (NEPs) on the order of 3×10^(-9) W/√ Hz is needed. Ideal detectors for such a surveyor do not yet exist. As a demonstration of one technique for approaching the ultra-low NEPs required by this surveyor, we present the design of an array of 96 350 µm KIDs that utilize phonon recycling to boost responsivity. Our KID array is fabricated with TiN deposited on a silicon-on-insulator (SOI) wafer, which is a 2 μm thick layer of silicon bonded to a thicker slab of silicon by a thin oxide layer. The backside thick slab is etched away underneath the absorbers so that the inductors are suspended on just the 2 μm membrane. The intent is that quasiparticle recombination phonons are trapped in the thin membrane, thereby increasing their likelihood of being re-absorbed by the KID to break additional Cooper pairs and boost responsivity. We also present a Monte-Carlo simulation that predicts the amount of signal boost expected from phonon recycling given different detector geometries and illumination strategies. For our current array geometry, the simulation predicts a measurable 50% boost in responsivity.