Erich N. Grossman

Superconducting multiplexer for arrays of transition edge sensors

We report the design and testing of an analog superconducting time-division multiplexer to instrument large format arrays of low-temperature bolometers and microcalorimeters. The circuit is designed to multiplex an array of superconducting quantum interference devices, thereby simplifying wiring and room temperature electronics. We have fabricated a prototype 8×1 multiplexer chip and show a switching rate of 1 MHz. We calculate that a 32×32 array or larger is feasible.

Lithographic Spiral Antennas at Short Wavelengths

We have extended the high efficiency of lithographic antennas to mid‐infrared wavelengths. Pattern measurements made at 9.5 μm wavelength on a 65°, self‐complementary, spiral antenna exhibit a ratio of response to orthogonal linear polarizations of 1.35 dB, a beamwidth of 85° (3 dB full width), a directivity of 8.2 dB, and surprisingly, a close resemblance to the theoretical pattern for a 65° spiral in free space. Direct detection measurements made with an ambient temperature blackbody source yield an antenna efficiency of 52±7%, when corrected for incomplete filling of the antenna beam by the source, at a mean effective wavelength of 19 μm.

Passive terahertz camera for standoff security screening

We describe the construction and performance of a passive, real-time terahertz camera based on a modular, 64-element linear array of cryogenic hotspot microbolometers. A reflective conical scanner sweeps out a 2 m x 4 m (vertical x horizontal) field of view (FOV) at a standoff range of 8 m. The focal plane array is cooled to 4 K in a closed cycle refrigerator, and the signals are detected on free-standing bridges of superconducting Nb or NbN at the feeds of broadband planar spiral antennas. The NETD of the focal-plane array, referred to the target plane and to a frame rate of 5 s(-1), is 1.25 K near the center of the array and 2 K overall.

Spectral transmittance of lossy printed resonant-grid terahertz bandpass filters

In this paper, we present terahertz bandpass filters composed of resonant arrays of crossed slots in lossy metal films deposited on dielectric membranes. The filters exhibit insertion loss as low as 1.9 dB at room temperature and 1.2 dB at 77 K at a center frequency of 2.2 THz. It is found that the dielectric substrate introduces a downward shift in frequency not predicted by standard mean dielectric-constant approximations. This shift is proportional to the permittivity and thickness of the substrate, and is accurately modeled for polyester, fused quartz and silicon substrates using a finite-difference time-domain (FDTD) model. It is also found that the insertion loss and Q-factors of the filters vary with the product of the thickness and conductivity of the metal film for lead and gold films, even in cases when the thickness is several skin depths at the center frequency. The FDTD theory presented here accounts for some of the conductor losses.

THz Metrology and Instrumentation

This paper gives an overview of measurement techniques used in the THz region of the electromagnetic spectrum, from about 100 GHz to several THz. Currently available components necessary for THz metrology, such as sources, detectors and passives, are briefly described. A discussion of power measurements, vector network analysis and antenna measurements as well as the limitations of these measurements at THz frequencies is given. The paper concludes with a summary of available components and instrumentation for THz metrology at the time of writing.

Detection and Segmentation of Concealed Objects in Terahertz Images

Terahertz imaging makes it possible to acquire images of objects concealed underneath clothing by measuring the radiometric temperatures of different objects on a human subject. The goal of this work is to automatically detect and segment concealed objects in broadband 0.1-1 THz images. Due to the inherent physical properties of passive terahertz imaging and associated hardware, images have poor contrast and low signal to noise ratio. Standard segmentation algorithms are unable to segment or detect concealed objects. Our approach relies on two stages. First, we remove the noise from the image using the anisotropic diffusion algorithm. We then detect the boundaries of the concealed objects. We use a mixture of Gaussian densities to model the distribution of the temperature inside the image. We then evolve curves along the isocontours of the image to identify the concealed objects. We have compared our approach with two state-of-the-art segmentation methods. Both methods fail to identify the concealed objects, while our method accurately detected the objects. In addition, our approach was more accurate than a state-of-the-art supervised image segmentation algorithm that required that the concealed objects be already identified. Our approach is completely unsupervised and could work in real-time on dedicated hardware.

A W -Band Polarization Converter and Isolator

A 95-GHz printed low-loss linear-to-circular polarizer is designed as a component of an active direct-detection millimeter-wave imaging system. The periodic printed grid structure presents different reactances to the TE and TM polarizations, resulting in equal amplitude and phase quadrature upon transmission through four parallel grids. The polarizer is measured in both a Gaussian beam system and a plane wave system, and demonstrates an axial ratio of 0.23 dB, polarization isolation of 38 dB, and transmission loss of 0.3 dB for normal incidence. The quarter-wave plate is characterized up to plusmn35deg off the optical axis, and exhibits an axial ratio better than 1 dB up to plusmn17deg off the optical axis.

Micro-Fabricated 130–180 GHz Frequency Scanning Waveguide Arrays

This paper describes frequency scanning slot arrays operating from 130 to 180 GHz. The arrays are micro-fabricated using the PolyStrata sequential copper deposition process. Measured reflection coefficient and radiation patterns agree with HFSS full-wave simulations. The voltage standing wave ratio is less than 1.75:1 over the entire frequency range, and the measured scanning is 1.04°/ GHz from 130 to 150 GHz and 32.5° over the full frequency range. The measured gain is 15.5 dBi for a 10-element array at 150 GHz and 18.9 dBi for a 20-element array at 150 GHz with about 3 dB of variation over the scan range.

An Ultra-Low Noise Superconducting Antenna-Coupled Microbolometer With a Room-Temperature Read-Out

In this letter, we report the electrical and optical characteristics of a superconducting vacuum-bridge microbolometer with an electrical noise equivalent power of 26fW radicHz and an effective time constant of 380 ns, when operated at a bath temperature of 4K. We employ a novel room temperature external negative feedback readout architecture, that allows for noise matching to the device without bulky stepup transformers or cooled electronics. Both the detector and the readout lend themselves to be scaled to imaging arrays. The directly measured noise equivalent temperature difference over a 100-1000-GHz bandwidth is 125 mK in a 30-ms integration time

Antenna‐coupled high‐Tc air‐bridge microbolometer on silicon

An antenna‐coupled high‐Tc superconducting microbolometer on a silicon substrate, operating at infrared wavelengths, is described. This detector incorporates a silicon‐micromachined yttria‐stabilized zirconia air bridge at the feed of a planar lithographic antenna to simultaneously minimize the thermal conductance and the heat capacity of the bolometer. At an operating temperature of 87.4 K, the optical responsivity measured using a 300‐K blackbody source over a 0.2–2.9 THz bandwidth is 2900 V/W, the optical noise‐equivalent power (NEP) is 9×10−12 W/Hz1/2, and the time constant is <10 μs. This NEP is nearly a factor of 2 lower than the previous record for a liquid‐nitrogen‐cooled thermal detector, and the time constant is several orders of magnitude shorter.