Researchers at the University of Michigan have found a way to accurately detect electromagnetic waves in the terahertz range by first converting them into sound. The advance opens up new applications ranging from tighter airport security to safer medical imaging.

Engineers know very well how to manipulate a wide spectrum of electromagnetic waves to send and receive signals. However, waves with frequencies ranging from 0.1 to 10 THz have so far been a very difficult customer. Their frequencies are too high to be measured directly by current-gen electronics, so scientists must rely on measuring their physical properties, such as their energy and wavelength. We have ways to do just that, but they are highly inefficient and impractical because, among other things, such devices need to be cooled down to work properly.


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A team led by Prof. Jay Guo at the University of Michigan has designed and tested a transducer that fills this so-called "terahertz gap" by turning the very high-frequency terahertz waves into ultrasound, which we already know how to process. More importantly, the system does so in a reliable and convenient way, and can operate at room temperature.

Here's how it works. When waves in the terahertz range hit the transducer, their energy is absorbed by carbon nanotubes and turned into heat. The heat expands a fine network of spongy polydimethylsiloxane (PDMS) plastic, creating a pressure wave in the ultrasound range, one thousand times too high for human ears to pick up. Finally, a microring resonator only a few millimeters in diameter picks up and measures the ultrasound waves.

This device has a response speed of less than one millionth of a second, which is four orders of magnitude faster than similar devices. This allows it to detect individual pulses of terahertz waves, rather than just continuous streams.

Finding a practical way to interact with this part of the electromagnetic spectrum opens up exciting new possibilities. For one, every object a few degrees above absolute zero emits black-body radiation on this spectrum. Detecting those waves would allow us to accurately image objects at a distance, from drugs and weapons at the airport to interstellar dust in the far reaches of the Milky Way.

Moreover, because the waves in this spectrum are thought to be harmless to the human body (only causing a mild increase in temperature), they could also be used for safer and more effective medical imaging and diagnostics.

The advance is described in the latest issue of the journal Nature Photonics.

Source: University of Michigan

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