UC Santa Barbara

Terahertz (THz) heterodyne detectors, also known as mixers, are widely used for high-resolution (with frequency resolution $\nu / \Delta \nu > 10^{6}$) detection of THz radiation in space for astrophysics and planetary science applications, and important for applications in bio-medical imaging and future THz wireless communications. Above 1 THz, heterodyne detectors based on hot-electron effects in thin superconducting films (superconducting hot-electron bolometers, or HEBs) are the current state-of-the art mixers, with an excellent sensitivity (with single-sideband noise temperature $T_{SSB} \sim 1,000$ K at 2 THz), a wide intermediate-frequency (IF) bandwidth ($\sim$3 GHz), and a small required local-oscillator (LO) power ($\sim$1 $\mu$W). However, cryogenic operating temperature (4 K or below) required for the superconducting HEBs limits their use in certain applications (e.g. deep-space missions to planets and comets) that cannot afford the power and mass required for active cryogenic cooling. To date, the only option for such applications has been Schottky-diode mixers that work at ambient temperature, but at the cost of significantly lower sensitivity (T\textsubscript{SSB} $\sim 10,000 $ K above 1 THz) and a higher required LO power ($\sim$1 mW). The latter especially limits the use of Schottky mixers in heterodyne array applications, which is important for cost- and time-effective heterodyne observations in space.

In this work, we explore a new type of THz heterodyne detector based on intersubband transitions of high-mobility 2-dimensional electron gas (2DEG) confined in single GaAs/AlGaAs quantum wells. Named as Tunable Antenna-Coupled Intersubband Terahertz (TACIT) mixer, our device is predicted to be as sensitive as superconducting HEB mixers (with $T_{SSB} \sim 1,000$ K) at relatively high operating temperature (20--60 K), with a wide IF bandwidth ($\sim 10 $ GHz) and a small required LO power ($< 1 \mu$W). In addition, THz absorption frequency of TACIT mixers can be tuned with small ($<2$) DC voltage biases, offering wide in-situ tunability in the detection frequency (2--5 THz). The operating temperature of TACIT mixers can be accessible with passive cooling for deep-space missions or with compact, light-weight coolers for other applications, which, along with other useful mixer characteristics, makes TACIT mixers an attractive mixer technology for a low-noise, multi-pixel THz heterodyne receiver for applications in THz high-resolution spectroscopy in deep space and for other applications in which relaxed cryogenic and LO power requirements are advantageous.

Despite the impressive mixer characteristics of TACIT mixers, experimental realization and characterization of the TACIT mixers have been challenging due to difficulties in fabricating reliable dual-gate structures required for their operation. In this work, using an advanced flip-chip technique that we developed in-house, we successfully fabricate and demonstrate two versions of prototype TACIT mixers. In the first prototype TACIT mixer integrated with a single slot antenna, we demonstrate the tunability in the detection frequency (2.52--3.44 THz) with relatively small ($< 5$ V) DC bias voltages, as well as the heterodyne detection (mixing) capability at 60 K with a wide IF bandwidth ($\sim$ 6 GHz). The observed tunability in the direct detection responses is consistent with the model responses based on intersubband transitions. In the second prototype TACIT mixers integrated with a modified, broadband bow-tie antenna, we explore capacitive coupling for the read-out of the device IF response and investigate the capacitive IF read-out and other properties of the device using various characterization methods, including microwave measurements and noise power measurements. In addition, we demonstrate tunability in both the direct detection and heterodyne detection consistent with the results from the first prototype TACIT mixer and with our model responses. In both prototype devices, the mixer noise temperature, conversion loss, and required LO power were not measured due the unexpectedly large conversion losses. We attribute these large losses to poor radiation efficiency of the antenna structures caused by impedance mismatch and non-optimal radiation patterns, as well as to possible degradation in the IF response caused by diffusion and ballistic cooling of the high-mobility 2DEG at low temperatures. To address these issues, we briefly discuss our plans for designing more optimized antenna structures and employing a different read-out mechanism based on the thermoelectric effect of high-mobility 2DEGs for future iterations of TACIT mixers.