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Physicists Detect Radio Waves With Light

By bouncing light off a vibrating nanomembrane, researchers can now detect radio waves in an entirely new way

The detection of weak radio signals is a ubiquitous problem in the modern world. Everything from NMR imaging and radio astronomy to navigation and communication depends on picking up faint radio signals that would have been undetectable just a few decades ago.

That’s why many groups are racing to find better ways to spot these signals and to process them using state-of-the-art techniques.

Today, Tolga Bagci at the University of Copenhagen in Denmark and a group of pals demonstrate a device that detects ultra-weak radio waves in an entirely new way. Their new box of tricks converts radio waves into light signals, which can then be transmitted and analysed using standard optical tools. “Our work introduces an entirely new approach to all-optical, ultralow-noise detection of classical electronic signals,” they say.

The new approach is simple in principle. Their device consists of a thin membrane of silicon nitride coated with a mirror-like layer of aluminium. This nanomembrane is suspended above an electrode forming a capacitor which is itself part of a standard LC-circuit that picks up radio waves at its resonant frequency.

When this happens, the resonating circuit causes the nanomembrane to vibrate.

The trick that Bagci and co have pulled off is to bounce a laser beam off the nanomembrane causing an optical phase shift that they then measure using standard optical techniques.

The result is that the nanomembrane converts the faint radio waves it picks up into optical signals.

This approach has significant advantages over traditional radio receivers. The big problem with current methods for detecting faint radio waves is that noise generated by heat can swamp the signal. The only way to get around this is by cooling the detection equipment, a process that significantly increases the complexity, size and cost of the job.

The big advantage of converting the radio signals into a resonant mechanical vibration is that the random effect of heat becomes negligible. That’s the beauty of resonant systems. So the reflected light picks out the radio signal with little of the noise that swamps conventional radio receivers.

The numbers are impressive. The new device has a room temperature sensitivity of 5 picoVolts per (Hz)^1/2 at a frequency of 1 Mhz. In other words, it does the same job at room temperature that physicists could only dream of doing at the temperature of liquid helium.

And this is only a proof of principle device. It has the potential to get even better with a little optimisation

That’s likely to have a significant impact in a number of areas that rely on cooled amplifiers to pick up faint radio signals. For example, nuclear magnetic resonance imaging relies on the detection of faint radio signals generated by protons precessing in a magnetic field. And radio astronomers rely on cooled amplifiers to pick up the faintest radio signals in the cosmos. “The usually required cryogenically cooled pre-amplifiers might be replaced by our transducer,” say Bagci and co.

That should significantly simplify this kind of work. Looking further ahead, there’s no reason why this kind of approach might not have even broader application, perhaps for ordinary mobile phone communication and for navigation. The ability to detect fainter signals could make these devices smaller and less power hungry.

And who doesn’t need smaller, less power-hungry gear?

Ref: arxiv.org/abs/1307.3467: Optical Detection Of Radio Waves Through A Nanomechanical Transducer

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