A backward shock wave comes forward

Hank Hogan, hank@hankhogan.com

A group of researchers has demonstrated a reverse photonic shock wave. As a result, high-energy physics could have a new particle detector, and proposed invisibility cloaks already may have been pierced.

The speed of light is the ultimate in space but not in air, water or other materials. In such media, charged particles can move faster than light itself. When they do, they give rise to Cerenkov radiation, an example being the blue glow seen near an underwater nuclear reactor.

To date, all Cerenkov radiation has been in the forward direction because all known natural materials are right-handed and have a positive index of refraction. But the development of left-handed metamaterials – which are engineered to have a negative index at specific wavelength bands and for particular electromagnetic modes – gave professor Min Chen, a physics professor at MIT in Cambridge, an idea a few years ago.

“We should build a backward-emitting Cerenkov detector so that the radiation could be readily separated from the emitting particles,” he said.

A backward photonic shock wave could lead to new particle detectors. Simulated charged particles traveling faster than the speed of light through a left-handed material (latticework on left) give rise to backward traveling microwave radiation (trace on right). Courtesy of Hongsheng Chen, Zhejiang University, and Min Chen, MIT.

Following Chen’s idea, graduate student Sheng Xi, associate professor Hongsheng Chen and professor Lixin Ran, all of Zhejiang University, implemented the concept, as outlined in the November 2009 issue of Physical Review Letters. In doing so, they overcame several problems.

The first is that optical frequency metamaterials not only are difficult to build but also suffer high losses. So the researchers chose to use microwaves, where a low-loss left-handed metamaterial is easier to design and build.

That decision led to another hurdle, however. Cerenkov radiation power is strongly dependent on frequency, so microwave emissions would be much weaker than optical ones.

The researchers solved this problem by not using particles. Instead, they showed mathematically that they could mimic moving particles, at least as far as Cerenkov radiation is concerned, by using a rectangular waveguide with an array of slots. In fabricating their demonstration device, they printed 17.5-µm-thick copper in a rod-and-split-ring structure with dimensions of a few tens of a millimeter. They repeated the structure every 3 mm over a polytetrafluoroethylene substrate 33 µm thick.

With this easily fabricated structure, they achieved negative refraction at a frequency from 8.1 to 9.5 GHz. They confirmed backward Cerenkov radiation at these frequencies.

Because the radiation and the particle travel in opposite directions, this could give rise to better detectors. Reversed Cerenkov emission also could be a new radiation source. The next step will be verification with actual charged particles moving through a metamaterial. One goal will be to do this at optical frequencies, which the recent development of bulk optical left-handed metamaterials may make possible.

As for invisibility cloaks, one proposed implementation uses left-handed metamaterials to guide light around an object and make it invisible. According to new research, that invisibility can be punctured, Chen said.

“Backward Cerenkov radiation is the only electromagnetic method known to us to detect the above cloak by shooting an electron beam through the left-handed shell.”