High-resolution submillimeter/terahertz spectroscopy is important for studying atmospheric and interstellar molecular gaseous species. It typically uses heterodyne receivers where an unknown (weak) signal is mixed with a strong signal from the local oscillator (LO) operating at a slightly different frequency. The non-linear mixer devices for this frequency range are unique and are not off-the-shelf commercial products.
Three types of THz mixers are commonly used: Schottky diode, superconducting hot-electron bolometer (HEB), and superconductor-insulation-superconductor (SIS) junction. The latter two are the most sensitive and require very small LO power to be driven to the desired operating point. These mixers require deep cryogenic cooling to at least 4 K. Schottky mixers are less sensitive and require stronger LO sources. However, they can be used at any ambient temperature.
A HEB mixer based on the two-dimensional electron gas (2DEG) formed at the interface of two slightly dissimilar semiconductors was developed. This mixer can operate at temperatures between 100 and 300 K, and thus can be used with just passive radiative cooling available even on small spacecraft. It requires small LO power (1–10 microwatt) and, therefore, can be driven by the existing LOs, even above 1 THz.
The mixer device is a micron-sized patch of the 2DEG formed in the AlInN/GaN heterostructure grown on sapphire substrate. The device operates as a bolometer with a temperaturedependent resistance (mobility of the 2DEG). Free electrons in the device absorb THz radiation received by a micro-antenna coupled to the mixer device. This changes the temperature of electrons and the bolometer resistance. The maximum speed of the mixer device of this type is set by the combination of the electron-phonon relaxation in the material and the diffusion of hot electrons through the device ends, and corresponds to several GHz. This is what is usually required for the intermediate frequency (IF) bandwidth of a typical THz mixer. One can say that this 2DEG HEB mixer combines the best qualities of the superconducting HEB mixer (low LO power, low noise) and of the Schottkydiode mixer (ambient temperature operation).
The main innovation here is the use of GaN-based heterostructures. Com - pared to the much better known GaAsbased heterostructures, the new material system provides nearly ideal conditions for strong Drude absorption of radiation by electrons. This allows for the very short momentum relaxation time (time between collisions) of electrons. Since this time is shorter than a period of the THz field oscillation, the electrons absorb THz radiation well. In the GaAs structures, the momentum relaxation time is usually much longer, so the electrons move in the field without collisions for a long time. This reduces their ability to absorb radiation and makes the mixer device much less sensitive.
This work was done by Boris S. Karasik, John J. Gill, Imran Mehdi, and Timothy J. Crawford of Caltech, and Andrei V. Sergeev and Vladimir V. Mitin of SUNY Buffalo for NASA’s Jet Propulsion Laboratory. NPO-47796
This Brief includes a Technical Support Package (TSP).
Terahertz Radiation Heterodyne Detector Using Two- Dimensional Electron Gas in a GaN Heterostructure
(reference NPO-47796) is currently available for download from the TSP library.
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Overview
The document is a Technical Support Package from NASA's Jet Propulsion Laboratory (JPL) detailing the development of a Terahertz Radiation Heterodyne Detector that employs a Two-Dimensional Electron Gas (2DEG) in a GaN heterostructure. This technology is particularly relevant for applications in atmospheric science, where it can measure trace constituent abundances and physical properties under various climate conditions, including high dust loading. The detector leverages the strong submillimeter transitions of polar molecules, allowing for the detection of numerous trace species with sensitivities ranging from parts per trillion to parts per billion.
The document discusses the challenges associated with traditional materials used in 2DEG structures, such as AlGaAs/GaAs, which exhibit poor coupling of the electron gas to radiation and significant contact resistance. In contrast, newly synthesized InAlN/AlN/GaN heterostructures present a more favorable combination of parameters for implementing 2DEG mixers. These materials demonstrate electron mobilities around 10^3 cm²/(V s) and high electron densities (~10^13 cm⁻²), resulting in moderate sheet resistance (200-300 Ohm). This allows for effective impedance matching with planar microantennas and 50-Ω intermediate frequency (IF) amplifiers.
The document also highlights the expected performance of the mixer, with a noise temperature (T_N) of less than 1000 K, significantly lower than that of Schottky mixers, which can reach several thousand K at terahertz frequencies. The short device length facilitates cooling of hot electrons through diffusion, with an IF bandwidth of approximately 7 GHz, which can be increased further by reducing the device length to submicron scales.
The research was conducted by a team from JPL and SUNY at Buffalo, under NASA sponsorship, and references several key studies that support the findings. The document emphasizes the potential of this technology for advancing low-noise THz receivers and enhancing the capabilities of atmospheric measurements, thereby contributing to a better understanding of environmental conditions and trace gas concentrations.
Overall, this Technical Support Package provides a comprehensive overview of the advancements in terahertz detection technology, showcasing its significance for both scientific research and practical applications in aerospace and environmental monitoring.
