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John O | May 2019

Researchers design thermomechanical device that detects radiation in the terahertz range

By Josh Perry, Editor


The terhertz (THz) region of the electromagnetic spectrum, between microwaves and infrared light, has been largely untapped because of the difficulty in building devices to detect it, but scientists at the University of Tokyo (Japan) demonstrated a new thermomechanical device that quickly detects THz radiation without requiring cryogenic temperatures.


Researchers created an ultra-sensitive sensor for detecting terahertz radiation.
(University of Tokyo)


According to a report from the university, the researchers utilized the heat generated by THz radiation to detect it.


“They created a device featuring a tiny beam suspended across a gap, which they then coated with a resistive metal film [nickel-chromium (NiCr) in this case],” the report explained. “This metal film has the ability to absorb THz radiation, which in turn transfers heat to the beam as a whole. This increase in temperature causes the beam to expand very slightly, which can be detected as a change in the frequency at which the beam resonantly vibrates.”


The beam was made of gallium arsenide. The device detected THz radiation 100 times faster than conventional sensors and was sensitive enough to detect radiation that caused temperature changes as low as one-millionth of a degree. Its ability to function at room temperature makes it viable for real-world applications.


The research was recently published in the Journal of Applied Physics. The abstract stated:


“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.”

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