Researchers in the US have developed a new type of thermal sensor by combining micron-sized "paddles" with nanometre-sized support rods. The device is made using standard silicon-fabrication techniques and is extremely sensitive to infrared radiation – without the need for cooling. As a result, the team claims that it could be used as a thermal and infrared imager that is integrated into electronic devices.

A torsional resonator is 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 resonant frequency. There are two main ways in which the resonator vibrates. These are the 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.

Michael Roukes and colleagues at the California Institute of Technology have now made thermal sensors from such resonators. The researchers began by coating the surface of a micron-sized silicon paddle with titanium nitride – a material that absorbs infrared radiation. Next, they applied a time-varying voltage between the paddle and the substrate it was on. This generates a force that sets the paddle vibrating at its resonant frequency.

Resonance-frequency shifts

When the paddle is exposed to infrared radiation, it heats up and this shifts its resonant frequency. "By tracking this change in frequency, we are able to determine the intensity of radiation that hits the device," explains team member Edward Myers. This change is measured using an optical interference set-up that involves reflecting laser light from the surface of the paddle (see figure).

The paddle supports are made of silicon nanowires and measure just 1 µm long and 50–100 nm in diameter. Thanks to these tiny supports, the paddle 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 sensor was made from standard semiconductor and metal materials using standard industrial techniques – so in principle, Roukes and colleagues could make large arrays of these paddles for use as thermal imagers.

Portable applications

"Ultimately, we believe that these sensors can perform as well as certain standard infrared sensors that require cooling to cryogenic temperatures," Myers says. "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 work on making the device smaller 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," reveals 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 sensor is described in Nano Letters.