Millimeter-wave to near-terahertz sensors based on reversible insulator-to-metal phase transition in Vanadium dioxide

Millimeter-wave (mm-wave) and terahertz (THz) detection has opened a significant research direction in imaging, sensing, spectroscopy, and ultrafast wireless communication.The sensitivity of the phase transition to electromagnetic waves in Vanadium dioxide (VO2) offers solutions to highly-sensitive radio frequency (RF), mm-wave, and THz detectors. We present the concept of uncooled mm-wave detection based on the sensitivity of field-induced insulator-to-metal transition (IMT) threshold voltage to electromagnetic waves in VO2 thin film devices. After exploring potential principles for implementation of a VO2-based detector, we present our experimental results on two main approaches: first, threshold voltage shifting in VO2 two-terminal switches along with its readout methods, and second, utilizing non-linearity in VO2 electrical characteristics close to the transition point by electrically biased devices.Based on an accumulative effect, the static threshold voltage is down-shifted in presence of different types of excitation, such as elevated temperatures or electromagnetic waves. We first demonstrate the detection concept through actuation of biased VO2 two-terminal switches encapsulated in a pair of coupled antennas on a Si/SiO2 substrate. We also study the behavior of VO2 switches interrupting coplanar waveguide (CPW)s for broadband measurement purposes. Then we propose a mm-wave-sensitive voltage-controlled spike generator based on the two-terminal VO2 devices in an astable circuit. By the shift in the threshold voltage, one reads out the impact on the frequency of oscillations in the astable output. Based on experimental statistical data, we report our approach to harness the stochastic behavior and noise limits of VO2-based spiking sensors, which are expected to form a new class of energy efficient transducers.The fast dynamics of the non-linear impedance characteristics of VO2 provides capability of large-bandwidth (BW), broadband detection of electromagnetic waves. We scale down the threshold voltage and the hysteresis width by implementation of miniaturised vertical switches. The film operates in sub-threshold regime, exploiting local nonlinearity and partial switching. The implementation is by growing VO2 on top of a metallic surface and overlapping a top electrode to realize the switching through the thickness of the VO2 film. The two-terminal vertical devices can detect the envelop of an intensity modulated sub-THz signal.They are encapsulated seamlessly inside CPWs (for broadband measurements) as well as bowtie antennas (for free-space experiments). The optimum responsivity can be achieved at a certain bias point on the devices close to the IMT, where the nonlinearity reaches its highest level.The antennas are optimized to operate at sub-THz carrier frequencies according to the availability of sources, however the design can adapt for higher frequencies, as VO2 shows reception for carrier frequencies up to the visible range. The responsivity of the proposed devices to mm-wave radiation along with its large BW characteristics makes it a viable detector for a wide range of high-data-rate applications.