Millimeter-Wave Lumped Element Superconducting Bandpass Filters for Multi-Color Imaging

The opacity due to water vapor in the Earth's atmosphere obscures portions of the sub-THz spectrum (mm/sub-mm wavelengths) to ground based astronomical observation. For maximum sensitivity, instruments operating at these wavelengths must be designed to have spectral responses that match the available windows in the atmospheric transmission that occur in between the strong water absorption lines. Traditionally, the spectral response of mm/sub-mm instruments has been set using optical, metal-mesh bandpass filters [1]. An alternative method for defining the passbands, available when using superconducting detectors coupled with planar antennas, is to use on-chip, superconducting filters [2]. This paper presents the design and testing of superconducting, lumped element, on-chip bandpass filters (BPFs), placed inline with the microstrip connecting the antenna and the detector, covering the frequency range from 209–416 GHz. Four filters were designed with pass bands 209–274 GHz, 265–315 GHz, 335–361 GHz and 397–416 GHz corresponding to the atmospheric transmission windows. Fourier transform spectroscopy was used to verify that the spectral response of the BPFs is well predicted by the computer simulations. Two-color operation of the pixels was demonstrated by connecting two detectors to a single broadband antenna through two BPFs. Scalability of the design to multiple (four) colors is discussed

SAMBA: Superconducting antenna-coupled, multi-frequency, bolometric array

We present a design for a multipixel, multiband (100 GHz, 200 GHz and 400 GHz) submillimeter instrument: SAMBA (Superconducting Antenna-coupled, Multi-frequency, Bolometric Array). SAMBA uses slot antenna coupled bolometers and microstrip filters. The concept allows for a much more compact, multiband imager compared to a comparable feedhorn-coupled bolometric system. SAMBA incorporates an array of slot antennas, superconducting transmission lines, a wide band multiplexer and superconducting transition edge bolometers. The transition-edge film measures the millimeter-wave power deposited in the resistor that terminates the transmission line

Experimental evidence for a surface distribution of two-level systems in superconducting lithographed microwave resonators

We present measurements of the temperature-dependent frequency shift of five niobium superconducting coplanar waveguide microresonators with center strip widths ranging from 3 $\mu$m to 50 $\mu$m, taken at temperatures in the range 100-800 mK, far below the 9.2 K transition temperature of niobium. These data agree well with the two-level system (TLS) theory. Fits to this theory provide information on the number of TLS that interact with each resonator geometry. The geometrical scaling indicates a surface distribution of TLS, and the data are consistent with a TLS surface layer thickness of order a few nm, as might be expected for a native oxide layer.Comment: 3 figures, submitted to AP

Integrated Focal Plane Arrays for Millimeter-wave Astronomy

We are developing focal plane arrays of bolometric detectors for sub-millimeter and millimeter-wave astrophysics. We propose a flexible array architecture using arrays of slot antennae coupled via low-loss superconducting Nb transmission line to microstrip filters and antenna-coupled bolometers. By combining imaging and filtering functions with transmission line, we are able to realize unique structures such as a multi-band polarimeter and a planar, dispersive spectrometer. Micro-strip bolometers have significantly smaller active volume than standard detectors with extended absorbers, and can realize higher sensitivity and speed of response. The integrated array has natural immunity to stray radiation or spectral leaks, and minimizes the suspended mass operating at 0.1 - 0.3 K. We also discuss future space-borne spectroscopy and polarimetry applications

Measurement of loss in superconducting microstrip at millimeter-wave frequencies

We have developed a new technique for accurate measurement of the loss of superconducting microstrips at mm-wave frequencies. In this technique, we optically couple power to slot antenna, which is connected to one port of a hybrid coupler. One of the output ports of the hybrid delivers power to a series of mm-wave microstrip resonators which are capacitively coupled to a feedline followed by an MKID (microwave kinetic inductance detector) that measures the transmitted power. Two other MKIDs are connected to the remaining ports of the hybrid to measure the total incident optical power and the power reflected from the mm-wave resonators, allowing |S_(21)|^2 and |S_(11)|^2 to be accurately determined and resonance frequency fr and quality factor Q to be retrieved. We have fabricated such a Nb/SiO_2/Nb microstrip loss test device which contains several mm- wave resonators with f_r~100 GHz and measured it at 30 mK. All the resonators have shown internal quality factor Qi~500–2000, suggesting a loss tangent of ~5×10^(−4)−2×10^(−3) for the SiO_2 in use. For comparison, we have also fabricated a 5 GHz microstrip resonator on the same chip and measured it with a network analyzer. The loss tangent at 5 GHz derived from fitting the f_0 and Q data to the two-level system (TLS) model is 6×10^(−4), about the same as from the mm-wave measurement. This suggests that the loss at both microwave and mm-wave frequencies is probably dominated by the TLS in SiO_2. Our results are of direct interest to mm/submm direct detection applications which use microstrip transmission lines (such as antenna-coupled MKIDs and transition-edge sensors), and other applications (such as on-chip filters). Our measurement technique is applicable up to approximately 1 THz and can be used to investigate a range of dielectrics

Superconducting kinetic inductance photon detectors

We are investigating a novel superconducting detector and readout method that could lead to photon counting, energy resolving focal plane arrays. This concept is intrinsically different from STJ and TES detectors, and in principle could deliver large pixel counts, high sensitivity, and Fano-limited spectral resolution in the optical/UV/X-ray bands. The readout uses the monotonic relation between the kinetic surface inductance L_s of a superconductor and the density of quasiparticles n, which holds even at temperatures far below T_c. This allows a sensitive readout of the number of excess quasiparticles in the detector by monitoring the transmission phase of a resonant circuit. The most intriguing aspect of this concept is that passive frequency multiplexing could be used to read out ~10^4 detectors with a single HEMT amplifier. Single x-ray events have been observed in prototype detectors

Design of broadband filters and antennas for SAMBA

We present a design for multipixel, multiband submillimeter instrument: SAMBA (Superconducting Antenna-coupled, Multi-frequency, Bolometric Array). SAMBA uses antenna coupled bolometers and microstrip filters. The concept allows for a much more compact, multiband imager compared to a comparable feedhorn-coupled bolometric system. SAMBA incorporates an array of slot antennas, superconducting transmission lines, a wide band multiplexer and superconducting transition edge bolometers. The transition-edge film measures the millimeter-wave power deposited in the resistor that terminates the transmission line

Superconducting kinetic inductance photon detectors

We are investigating a novel superconducting detector and readout method that could lead to photon counting, energy resolving focal plane arrays. This concept is intrinsically different from STJ and TES detectors, and in principle could deliver large pixel counts, high sensitivity, and Fano-limited spectral resolution in the optical/UV/X-ray bands. The readout uses the monotonic relation between the kinetic surface inductance L_s of a superconductor and the density of quasiparticles n, which holds even at temperatures far below T_c. This allows a sensitive readout of the number of excess quasiparticles in the detector by monitoring the transmission phase of a resonant circuit. The most intriguing aspect of this concept is that passive frequency multiplexing could be used to read out ~10^4 detectors with a single HEMT amplifier. Single x-ray events have been observed in prototype detectors

Transition-edge superconducting antenna-coupled bolometer

We report test results for a single pixel antenna-coupled bolometric detector. Our device consists of a dual slot microstrip antenna coupled to an Al/Ti/Au voltage-biased transition edge superconducting bolometer (TES). The coupling architecture involves propagating the signal along superconducting microstrip lines and terminating the lines at a normal metal resistor colocated with a TES on a thermally isolated island. The device, which is inherently polarization sensitive, is optimized for 140 GHz band measurements. In the thermal bandwidth of the TES, we measure a noise equivalent power of 2.0 × 10^(-17) W/√Hz in dark tests that agrees with calculated NEP including only contributions from thermal, Johnson and amplifier noise. We do not measure any excess noise at frequencies between 1 and 200 Hz. We measure a thermal conductance G ~5.5 × 10^(-11) W/K. We measure a thermal time constant as low as 437μs at 3μV bias when stimulating the TES directly using an LED

A semi-empirical model for two-level system noise in superconducting microresonators

We present measurements of the low--temperature excess frequency noise of four niobium superconducting coplanar waveguide microresonators, with center strip widths $s_r$ ranging from 3 $\mu$m to 20 $\mu$m. For a fixed internal power, we find that the frequency noise decreases rapidly with increasing center strip width, scaling as $1/s_r^{1.6}$. We show that this geometrical scaling is readily explained by a simple semi-empirical model which assumes a surface distribution of independent two-level system fluctuators. These results allow the resonator geometry to be optimized for minimum noise.Comment: 3 fig