Imaging and spectroscopy at terahertz frequencies have great potential for healthcare, plasma diagnostics, and homeland security applications. Terahertz frequencies correspond to energy level transitions of important molecules in biology and astrophysics. Terahertz radiation (T-rays) can penetrate clothing and, to some extent, can also penetrate biological materials. Because of their shorter wavelengths, they offer higher spatial resolution than do microwaves or millimeter waves.
We are developing hot electron bolometer (HEB) mixer receivers for heterodyne detection at terahertz frequencies. HEB detectors provide unprecedented sensitivity and spectral resolution at terahertz frequencies. We describe the development of a two-pixel focal plane array (FPA) based on HEB technology. Furthermore, we have demonstrated a fully automated, two-dimensional scanning, passive imaging system based on our HEB technology operating at 0.85 THz. Our high spectral resolution terahertz imager has a total system noise equivalent temperature difference (NEΔT) value of better than 0.5 K and a spatial resolution of a few millimeters. HEB technology is becoming the basis for advanced terahertz imaging and spectroscopic technologies for the study of biological and chemical agents over the entire terahertz spectrum.
Imaging and spectroscopy at terahertz frequencies (defined roughly as 300 GHz - 3 THz) have great potential for both healthcare and homeland security applications. Terahertz frequencies correspond to energy level transitions of important molecules in biology and astrophysics. Terahertz radiation (T-rays) can penetrate clothing and, to some extent, can also penetrate biological materials, and because of their shorter wavelengths they offer higher spatial resolution than microwaves or millimeter waves. We describe the development of a novel two-dimensional scanning, passive, terahertz imaging system based on a hot electron bolometer (HEB) detector element. HEB mixers are near quantum noise limited heterodyne detectors operating over the entire terahertz spectrum. HEB devices absorb terahertz radiation up to the visible range due to the very short momentum scattering times. The terahertz imaging system consists of a front-end heterodyne detector integrated with a state-of-the-art monolithic microwave integrated-circuit low-noise amplifier (MMIC LNA) on the same mixer block. The terahertz local oscillator (LO) signal is provided by a commercial harmonic multiplier source.
We have achieved the first demonstration of a low-noise heterodyne array operating at a frequency above 1 THz (1.6 THz). The prototype array has three elements, consisting of NbN hot electron bolometer (HEB) detectors on silicon substrates. We use a quasi-optical design to couple the signal and local oscillator (LO) power to the detector. We also demonstrate, for the first time, how the HEB detectors can be intimately integrated in the same block with monolithic microwave integrated circuit (MMIC) IF amplifiers. Such focal plane arrays can be increased in size to a few hundred elements using the next generation fabrication architecture for compact and easy assembly. Future HEB-based focal plane arrays will make low-noise heterodyne imaging systems with high angular resolution possible from 500 GHz to several terahertz. Large low-noise HEB arrays are well suited for real-time video imaging at any frequency over the entire terahertz spectrum. This is made possible by virtue of the extremely low local oscillator power requirements of the HEB detectors (a few hundred nanowatts to a microwatt per pixel). The operating temperature is 4 to 6 K, which can be provided by a compact and mobile cryocooler system, developed as a spin-off from the space program. The terahertz HEB imager consists of a computer-controlled optical system mounted on an elevation and azimuth scanning translator which provides a two-dimensional image of the target. We present preliminary measured data at the symposium for a terahertz security system of this type.
The next generation of hot electron bolometric (HEB) mixer receivers for terahertz frequencies is under development. In order to improve sensitivity and integration time, terahertz focal plane arrays with HEB elements are required. We have designed, fabricated, and tested a three-element focal plane array with HEB devices. We implemented a quasi-optical power coupling scheme using three elliptical silicon lenses. Recently developed wideband (0.5 GHz to 12 GHz) MMIC low noise amplifiers were directly integrated with HEB devices in a single block. The array was tested using an FIR laser as the LO source and a side band generator as the signal source. This is the first heterodyne array for a frequency above 1 THz, and the suitability of HEB elements in a terahertz FPA has thus been demonstrated. This development is also geared toward investigating new architectures for much larger arrays utilizing HEB elements. Additional issues to be resolved include an improved antenna design for efficient LO injection, compact and low power IF amplifiers, and cryogenic optimization.
Based on the excellent performance of NbN HEB mixer receivers at THz frequencies which we have established in the laboratory, we are building a Terahertz REceiver with NbN HEB Device (TREND) to be installed on the 1.7 meter diameter AST/RO submillimeter wave telescope at the Amundsen/Scott South Pole Station. TREND is scheduled for deployment during the austral summer season of 2002/2003. The frequency range of 1.25 THz to 1.5 THz was chosen in order to match the good windows for atmospheric transmission and interstellar spectral lines of special interest. The South Pole Station is the best available site for THz observations due to the very cold and dry atmosphere over this site. In this paper, we report on the design of this receiver. In particular, we report on HEB mixer device performance, the quasi-optical coupling design using an elliptical silicon lens and a twin-slot antenna, the laser local oscillator (LO), as well as the mixer block design and the plans for coupling the TREND receiver to the sky beam and to the laser LO at the AST/RO telescope site.
HEB technology continues to extend the sate-of-the-art for THz low-noise receivers. This talk discusses recent measured noise temperatures for NbN HEB receivers form which we infer intrinsic noise temperatures which approach the quantum noise limit within a factor of 3-5. We discuss the feasibility of achieving noise temperatures even close to the quantum limit noting that this limit has been reached both at lower frequencies and at higher frequencies. Another approach for considerably enhancing the speed with which THz receivers collect data is to employ a focal plane array system. We will discuss our design approach and general constraints for such a system.
We have developed prototype HEB receivers using thin film superconducting NbN devices deposited on silicon substrates. The devices are quasi-optically coupled through a silicon lens and a self-complementary log-specific toothed antenna. We measured DSB receiver noise temperatures of 500 K (13 X hf/2k) at 1.56 THz and 1,100 K (20 X hf/2k) at 2.24 THz. Noise temperatures are expected to fall further as devices and quasi-optical coupling methods are being optimized. The measured 3 dB IF conversion gain bandwidth for one device was 3 GHz, and it is estimated that the bandwidth over which the receiver noise temperature is within 3 dB of its minimum value is 6.5 GHz which is sufficient for a number of practical applications. We will discuss our latest results and give a detailed description of our prototype setup and experiments. We will also discuss our plans for developing focal plane arrays with tens of Hot Electron Bolometric mixer elements on a single silicon substrate which will make real time imaging systems in the THz region feasible.