Terahertz (THz) electromagnetic spectrum draws wide attention for nondestructive and/or biocompatible sensing. In order to be widely applicable to the THz sensing, it is of prime importance to develop THz sensors that can be operated at room temperature and have high sensitivity and fast operation speed. However, conventional room-temperature THz thermal sensors fall short of expectations in these characteristics required in various applications of THz sensing, including THz cameras. Utilizing a thermomechanical transduction scheme, we have developed an uncooled, sensitive, and fast THz bolometer by using a doubly clamped GaAs microelectromechanical system (MEMS) beam resonator as a sensitive thermistor. Owing to its ultrahigh temperature sensitivity (the noise equivalent temperature difference of ∼1 μK/√Hz), the present bolometer achieves not only high sensitivity but also an operation bandwidth of several kHz, which is more than 100 times faster than other uncooled THz thermal sensors. The obtained electrical noise equivalent power is as low as ∼90 pW/√Hz, which is close to the limit set by the thermal fluctuation noise. The MEMS bolometers are fabricated by the standard semiconductor fabrication processes and are well suited for making detector arrays for realizing THz cameras.

We propose a room temperature, all electrical driving and detecting, very sensitive thermometer structure using a microelectromechanical (MEMS) resonator for bolometer applications. We have fabricated a GaAs doubly clamped MEMS beam resonator whose oscillation can be excited and detected by the piezoelectric effect. When a heating power is applied to a NiCr film deposited on the MEMS beam surface, internal thermal stress is generated in the beam, leading to a reduction in the resonance frequency. The present device detects the shift in the resonance frequency caused by heating and works as a very sensitive thermometer. When the resonator was driven by a voltage slightly below the threshold for the nonlinear, hysteretic oscillation, the thermometer showed a voltage responsivity of about 3300 V/W, while keeping a low noise spectral density of about 60 nV/Hz^1/2, demonstrating a noise equivalent power of <20 pW/Hz^1/2 even at room temperature. The observed effect can be used for realizing high-sensitivity terahertz bolometers for room-temperature operation.

We have investigated the thermal response of the doubly clamped microelectromechanical (MEMS) beam resonators as a function of an input heating power. When a small amount of heat (<1 mW) is applied to the MEMS beam, reduction in resonance frequency is observed. As the heating power is further increased, however, the resonance frequency levels off and starts to increase, which originates from the buckling of the beam. We find that the slope of the frequency increase at large input powers is 2–3 times steeper than that of the frequency decrease at small input powers and that the buckled device can take larger heating powers without degrading the linearity. These observations suggest that both the thermal sensitivity and the dynamic range can be improved by introducing a proper amount of buckling. Using this insight, we have fabricated a buckled MEMS beam resonator by using a strained In_xGa_1-xAs (x = 0.001) beam structure on a GaAs substrate. We have found that the frequency responsivity of the In_xGa_1-xAs beam sample is about 3 times higher than that of the GaAs beam sample, demonstrating that the introduction of buckling is useful for achieving higher thermal sensitivity for the MEMS beam resonators.

The doubly clamped microelectromechanical system (MEMS) beam resonators exhibit extremely high sensitivity to tiny changes in the resonance frequency owing to their high quality (Q-) factors, even at room temperature. Such a sensitive frequency-shift scheme is very attractive for fast and highly sensitive terahertz (THz) detection. The MEMS resonator absorbs THz radiation and induces a temperature rise, leading to a shift in its resonance frequency. This frequency shift is proportional to the amount of THz radiation absorbed by the resonator and can be detected and quantified, thereby allowing the THz radiation to be measured. In this review, we present an overview of the THz bolometer based on the doubly clamped MEMS beam resonators in the aspects of working principle, readout, detection speed, sensitivity, and attempts at improving the performance. This allows one to have a comprehensive view of such a novel THz detector.

We report on a giant enhancement in the thermal responsivity of doubly clamped $\mathrm{Ga}\mathrm{As}$ microelectromechanical-system- (MEMS) beam resonators by using the internal-mode-coupling effect. This is achieved by coupling the fundamental bending mode with the fundamental torsional mode of the MEMS-beam resonators through the cubic Duffing nonlinearity. In the mode-coupling regime, we find that when the input heat to the MEMS resonators is modulated at a particular frequency, the resonance-frequency shift caused by heating can be enhanced by almost 2 orders of magnitude. The observed effect is promising for realization of high-sensitivity thermal sensing by use of MEMS resonators, such as ultrasensitive terahertz detection at room temperature.