Jam Farhoomand

Generation of tunable laser sidebands in the far‐infrared region

Continuously tunable laser sidebands have been generated by mixing radiation from an optically pumped far infrared (FIR) molecular laser, operated at 693, 762, 1627, and 1839 GHz, with that from millimeter‐wave klystrons in a Schottky‐barrier diode. An enhancement in conversion efficiency over similar systems reported previously is obtained by using a Michelson interferometer to separate the sidebands from the carrier and by placing the Schottky diode in an open structure corner cube mount. With 4 mW of laser power at 693 and 762 GHz the sideband power was measured to be 3.0 μW. This is at least an order of magnitude better than the previously reported results. At higher frequencies, 22 mW of 1627‐GHz laser power produced about 2.5 μW of sideband output, while 3mW of 1839‐GHz laser power generated about 100 nW of sideband radiation. The lower efficiency at the higher frequencies is due primarily to the mismatch between the laser radiation and the fixed‐length diode antenna. To demonstrate the tunability of the generated far‐infrared radiation, the laser sidebands were swept through absorption lines of HDO and H_2CO near 600 and 800 GHz. The absorption signals were easily seen, using either video or lock‐in detection techniques.

Performance of metal meshes as a function of incidence angle

An inductive mesh was measured for transmission as a function of frequency, incidence angle, and polarization. The experimental data agree well with Chen’s waveguide theory for meshes as long as an adequate number of modes are included in the calculations. To simplify calculations of mesh transmission a lumped circuit model has been developed to fit Chen’s theory. This model predicts transmission for the two major polarizations and a variety of angles to within 1%.

Stable 1.25 watts CW far infrared laser radiation at the 119 μm methanol line

We report the generation of 1.25 watts of CW laser power at the 119 μm (2522.8 GHz) methanol line. The maximum frequency fluctuation of the free running laser is less than ±100 KHz per hour.

DIRECT MEASUREMENT OF THE FUNDAMENTAL ROTATIONAL TRANSITIONS OF THE OH RADICAL BY LASER SIDEBAND SPECTROSCOPY

We report for the first time the direct (zero-field) spectra of the fundamental rotational transitions of the OH radical in its Ω = 3/2 and 1/2 states at 2509.9 and 1834.7 GHz using a recently developed far-infrared laser sideband spectrometer. These measurements have verified and refined the predictions of previous laser magnetic resonance (LMR) work, thereby confirming the far-infrared detection of interstellar OH. The increased accuracy of these direct measurements will be useful to future astronomical and atmospheric studies of these important transitions.

Latest progress in developing large format Ge arrays for far-IR astronomy

Germanium photoconductors offer excellent sensitivity in the 50-140μm spectral range. Coupled with their modest cooling requirements and their compatibility with the silicon cryo-CMOS readout technology, these detectors are the most attractive candidates for far IR astronomy in this wavelength range. Over the years we have been pursuing the advancement of this technology and our initial effort has produced a 2x16 Ge:Sb array with an NEP in the low 10-18 W/√Hz range, rivaling the best far IR arrays currently available. Further work has resulted in design and fabrication of a low noise, 2-side buttable 32x32 (64x64 mosaic) CTIA readout, the first 1k-pixel Ge:Sb fully assembled focal-plane array, a new hybrid design better suited for far IR photoconductors, and the preliminary design of a 2-side buttable 64x64 (128x128 mosaic) CTIA readout. Our developmental work continues and we believe that sensitivity levels below 10-18 W/√Hz are within reach. This paper presents an overview of our progress so far and outlines our roadmap for further work.

Characterization of high purity GaAs far-infrared photoconductors

In this paper we report the results of an extensive study on the far-infrared photoconductivity of high purityn-type GaAs. The crystal, which was grown at Max-Plank-Institute for Solid State Physics using liquid-phase epitaxy, exhibited the fine structures of the excited state transitions of the residual shallow level impurities. The major peak in the spectral response belongs to the 1s-2p transition, with its responsivity about thirty five times higher than the continuum. At 3.4K detector temperature, 625 mV bias, and 100 Hz chopping frequency the detector responsivity at 35.4 cm−1 (279 µm) was measured to be 0.017 A/W. Under these same conditions, the NEP was 5.9×10−14 W/√Hz. The (DC) dark current at 25 mV bias was 5.6×10−14 A.

Testing of the SB349: a 32x32 CTIA readout multiplexer for far-IR focal-plane arrays

The SB349 is a 32x32 readout multiplexer specifically designed for far IR photodetectors and is capable of operating at cryogenic temperatures at least as low as 1.8K. This readout is a capacitive-transimpedance amplifier multiplexed to eight outputs and is buttable on two sides to form a 64x64 mosaic array. It features eight selectable gain settings, auto zero for better input uniformity, sample-and-hold circuitry, and provisions to block the readout glow. A special, 2-micron cryo-CMOS process was adopted to prevent freeze out and ensure low noise and proper operation at deep cryogenic temperatures. An overview of the design and the results of the tests performed on this device are reported in this paper.

Design of a 1k pixel Ge:Sb focal-plane array for far-IR astronomy

Development of large format, far infrared focal-plane arrays has been identified as a pressing need for future astronomical instruments. In particular, array sizes as large as 128x128 with sensitivities equal to or better than 10-18 W/√Hz are the goals to be achieved within the next fifteen years. As part of our continuing effort to further this technology, we are developing a 32x32 Ge:Sb photoconductor FPA with a CTIA cryogenic readout multiplexer. A new, layered-hybrid architecture is employed to block the readout glow, improve heat dissipation and temperature uniformity across the array, and alleviate the potential problems associated with the large CTE mismatch between the Ge detector and the Si readout. This is the first 1k-pixel photoconductor FPA of its kind and is meant to be a pathfinder for future large format FPAs. Based on the test results of a prototype 2x16 Ge:Sb array of similar design, we expect the sensitivity of this FPA to be as low as 10-18 W/√Hz. This paper presents the design, characteristics, and the expected performance of this array.

Microwave-assisted far-infrared photoconductivity in high-purity GaAs

We have observed, for the first time, microwave-assisted photoconductivity in high purity GaAs. The enhancement of response appears to be dictated by two distinct mechanisms. First, a broadband enhancement which is believed to be due to detrapping of the free carriers and, therefore, increased photoconductive gain. Secondly, microwave-ionization of the excited states. We expect that both of these mechanisms contribute very little, if any, to the detector noise and, therefore, improve the detectors NEP. In this paper, we report the results of our preliminary tests showing broadband enhancement in response and an indication of enhancement of the excited-state response. Further investigation is currently underway.

A Prototype 1×32 Ge:Ga/Ge:Sb detector array with SBRC-190 CTIA readout multiplexer

We have constructed a far infrared detector array consisting of a 1×16 Ge:Ga and a 1×16 Ge:Sb placed side-by-side to form a linear, 1×32 array. An SBRC-190 readout multiplexer, which is a 1×32, multi-gain, capacitive transimpedance amplifier (CTIA) manufactured by Raytheon Infrared Operation, is wire bonded to the array. The array has been tested in the temperature range of 3.2K to 2.6K under various infrared radiation levels. In this presentation we will discuss the design and will report on the results of the preliminary tests conducted on this array at 3.0K. This developmental effort is intended to test the viability of the SBRC-190 unit-cell design for far infrared detector arrays and help foster improvement and further development of readout electronics based on the CTIA design. In addition, we hope to extend the detector design and develop a two-dimensional, monolithic array.