Fernando Rodriguez-Morales

A terahertz focal plane array using HEB superconducting mixers and MMIC IF amplifiers

We present a focal plane array (FPA) designed for operation at terahertz frequencies. The FPA is based on NbN phonon-cooled hot electron bolometer mixers directly coupled to wide-band microwave monolithic integrated circuit IF amplifiers. The array incorporates all the required dc-bias and IF circuitry in a compact split-block design. We present new experimental results describing the optical coupling efficiency to the array, as well as receiver noise temperature measurements. The measurements were performed at 1.6 THz, showing good agreement with theoretical predictions. This is the first low-noise heterodyne focal plane array to be reported for any frequency above 1 THz.

Development of integrated HEB/MMIC receivers for near-range terahertz imaging

We present measurement results for a new type of integrated terahertz receiver, as an extension to previous work by the authors. The receiver we developed integrates quasi-optically coupled phonon-cooled NbN hot electron bolometric (HEB) mixers in close proximity with InP monolithic microwave integrated circuit (MMIC) intermediate-frequency (IF) amplifiers. We have measured antenna radiation pattern, receiver noise temperature, and bandwidth, as well as short-term stability of the integrated receivers. The measurements were performed at 1.6 and 2.5 THz over a very broadband IF frequency range. We have been able to extend the effective bandwidth of these receivers up to 5 GHz, the widest reported for any integrated configuration operating above 1 THz. The suitability of the HEB/MMIC approach for imaging applications has been confirmed through the development of a prototype system for near-range scanning. The results presented here are very promising for the future development of heterodyne focal plane arrays for space-based receivers, medical applications, and surveillance

Direct and heterodyne detection of microwaves in a metallic single wall carbon nanotube

This letter reports measurements of microwave (up to 4.5GHz) detection in metallic single walled carbon nanotubes. The measured voltage responsivity was found to be 114V∕W at 77K. The authors also demonstrated heterodyne detection at 1GHz. The detection mechanism can be explained based on standard microwave detector theory and the nonlinearity of the dc IV curve. The authors discuss the possible causes of this nonlinearity. While the frequency response is limited by circuit parasitics in this measurement, the authors discuss evidence that indicates that the effect is much faster and that applications of carbon nanotubes as terahertz detectors are feasible.

Wideband IF-Integrated Terahertz HEB Mixers: Modeling and Characterization

This paper presents a detailed study of the IF properties of phonon-cooled NbN hot electron bolometer (HEB) terahertz mixers and receivers, performed over the widest intermediate frequency range reported so far. First, a broadband technique is used to obtain the IF small-signal output impedance of HEB mixers using network analyzer measurements. The impedance is measured over an IF range of 300 kHz to 8.5 GHz under the presence of dc bias and local oscillator (LO) illumination. The LO frequencies (f LO) used in this investigation range from 694 GHz to nearly 2 THz, covering frequencies below and above the superconducting bandgap frequency of the NbN film for the first time. The impedance (real and imaginary parts) can be fitted to the Standard model for HEB devices over almost the entire range of frequencies measured. The impedance data are then used in conjunction with a computer-aided design model for the monolithic micorwave integrated circuit IF amplifier to design and analyze the performance of integrated receivers (mixer/IF amplifier combinations). The measured and predicted variations of the receiver noise, mixer gain, and mixer output noise with IF are in good agreement. A record IF noise bandwidth of 8 GHz (obtained for f LO= 694 GHz) is demonstrated for a receiver designed using the technique described in this paper.

Imaging and spectroscopy at terahertz frequencies using hot electron bolometer technology

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.

Development of focal plane arrays with hot electron bolometer heterodyne detectors for 3 to 10 THz

Broadband tunable, hot electron bolometer (HEB) heterodyne detectors with receiver noise temperatures of the order of 10/spl times/hf/k in the frequency range 3 THz to 10 THz are required for future space systems. The HEB detectors have to be configured in focal plane arrays (FPA) with many elements (hundreds). We discuss the feasibility of such FPAs, especially the expected performance of HEB detectors at high terahertz frequencies, the role of quantum noise, and quasi-optical configurations. We show that a flys eye configuration with separate silicon lenses is preferable, and that MMIC IF amplifiers can be directly integrated with the HEB detectors. We also discuss suitable integrated antenna elements for the FPA and LO injection schemes.

TREND: a low-noise terahertz receiver user instrument for AST/RO at the South Pole

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.

Integrated THz receivers based on NbN HEB mixers and InP MMIC IF amplifiers

We are presenting design details for an inte- grated heterodyne terahertz receiver based on phonon-cooled NbN HEB mixers directly coupled to wide-band InP MMIC IF amplifiers as an extension to our previous work. Measured noise and stability performance are reported for 1.6 THz and 2.5 THz. Finally, we discuss how this configuration can be extended to be used in large focal plane arrays, using a multi- level array architecture. Index Terms— HEB heterodyne detectors, terahertz re- ceivers, MMIC low-noise amplifiers.

HEB FPA imaging technology for security and biomedical applications

We have demonstrated a low-noise heterodyne three-element focal plane array (FPA) at 1.6 THz consisting of NbN hot electron bolometric (HEB) detectors, intimately integrated with monolithic microwave integrated circuit (MMIC) IF amplifiers in a single block. 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. The use of FPAs is crucial for maximizing the detection speed in these applications.

NbN hot-electron bolometer receivers and focal plane arrays for the terahertz range

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.