A torsional resonator is basically a rectangular paddle-shaped device with two thin support rods connecting it to the rest of a chip structure. When a force is applied to the paddle, it vibrates at a certain, characteristic frequency. There are two main ways in which the resonator vibrates: the so-called torsional mode (where the paddle twists back and forth along the rod axis) and the flexural mode (where the paddle vibrates up and down like a trampoline perpendicular to the rod axis).

A team led by Michael Roukes has now made thermal sensors from such resonator structures. The researchers began by coating the surface of a micropaddle, made of silicon, with a material that absorbs infrared radiation – they used titanium nitride. Next, they applied a time-varying voltage between the paddle and the substrate it was on. This generates a force that sets the resonator in motion.

Resonance frequency shifts

As mentioned, the paddle has a characteristic resonance frequency at which it vibrates the most. When the device is exposed to infrared radiation, it heats up and this shifts its resonance frequency. “By tracking this change in frequency, we are able to determine the intensity of radiation that hit the device,” explained team member Edward Myers.

Thanks to the tiny size of the nanorod supports (which are made of silicon nanowires and measure just 1 µm long and 50–100 nm in diameter), the device is extremely well isolated from its environment. This means that only a small amount of infrared radiation is needed to heat the device by a measurable amount.

The device was made using standard semiconductor and metal materials and with foundry-compatible techniques, so there is nothing to stop the California team from making large arrays of these paddles for use as thermal imagers.

“Ultimately, we believe that these sensors can perform as well as certain standard infrared sensors that require cooling to cryogenic temperatures,” Myers told nanotechweb.org. “Our devices can operate without cooling, which makes them potentially useful for portable applications, such as night-vision goggles, home surveillance and perhaps even the next generation of smartphone cameras.”

The researchers now plan to continue scaling down their device and improving the materials used to make it – something that should further improve its thermal characteristics. “We are also looking at scaling up from one device to an array of devices,” revealed Myers. “As part of this plan we hope to integrate these sensors with on-chip CMOS electronics, which will make for easier control and readout of many thousands of elements at once.”

The current work is detailed in Nano Letters.