Scientists reveal the insanely perfect trick to trap light

It doesn't involve magic but mirrors and lenses.
Rupendra Brahambhatt

Energy can be trapped in the form of electric charge and heat, but until now, it has been impossible to absorb it in the form of light using traditional methods.

Now a team of researchers from the Hebrew University of Jerusalem and Vienna University of Technology (TU Wien) claims to have developed the perfect setup to trap light, according to a press release published by EurekAlert on Thursday.

Although this isn’t the first time scientists have come up with a way to absorb light energy, it is probably the only “light trap” method using which light energy can be absorbed even by very thin and weak mediums.

“In our work, we show how this process can be implemented very efficiently; specifically, we demonstrate that laser light of any shape can be fully absorbed even by a very weakly absorbing medium such as a thin film or a weakly tainted piece of glass,” Stefan Rotter, one of the authors of the study and a physics professor at TU Wien, told Interesting Engineering (IE).

The researchers build a carefully designed cavity around the absorbing medium that does not let light escape. So the light gets trapped in the cavity, where it passes through the absorbing medium multiple times until it is completely absorbed and nothing is left of it.

What is the need for a light trap?

Scientists reveal the insanely perfect trick to trap light
Light bulbs.

Before going deep into how the light trap functions, you need to understand the significance of trapping light energy. There are still fewer known ways to directly store light energy feasibly and efficiently and therefore, it has to be converted into other forms of energy. “From the absorption of radiation by plants to the detection of light in your cell phone camera, the energy carried by light waves or “photons” needs to be converted into other forms of energy to be exploitable,” said Professor Rotter.

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For instance, the light that you experience on your smartphone display first gets stored in the form of chemical energy in the battery. The circuit board inside the phone allows it to further convert into electrical energy, and finally, it becomes the light that lit the LCD or LED screen of your phone.

Direct absorption of light can bring great improvements in both the design and technology of the devices we use on a daily basis. The researchers believe that trapping or “harvesting” light is at the heart of many important processes in science, engineering, and nature. It has the potential to enhance the performance of spectrally selective detectors (detectors capable of absorbing light rays of different frequencies) and future light-powered devices.

How does the proposed “perfect light trap” work?

Scientists reveal the insanely perfect trick to trap light
Light trap setup.

The researchers designed a cavity in which multiple mirrors and lenses surrounded a thin light-absorbing medium. They arranged the mirrors and lenses in such a way (as shown in the figure above) that when a light ray entered the cavity, it started to move in a circular fashion eventually, blocking its own path. In the end, the light beam is left with no choice but to get absorbed by the thin medium.

Apart from the absorbing medium, this light trapping arrangement incorporated a partially transparent mirror, a reflecting mirror, and two convex lenses. According to the researchers, the first mirror in the light trap is kept partially transparent so that light can enter the cavity. However, light could also escape via the same mirror.

Scientists reveal the insanely perfect trick to trap light
Diagram showing the path of a light beam in the trap.

So to prevent this, they made use of wave interference, a property of light that leads to the cancellation of back reflections by increasing (or decreasing) the overall amplitude of the light waves.

As a laser beam falls on the partially transparent mirror, it gets divided into two parts. After hitting the lenses, absorbing medium, and the reflecting mirror, the parts finally superimpose on each other, blocking the entire light beam in a position from where it could go nowhere but get absorbed by the thin medium. The researchers claim that this technique is so perfect that it does not get affected by even frequent changes in temperature or air pressure.

When asked about the limitations of this light trap mechanism, Professor Stefan said, “Our device only works at a single frequency of the incoming light. We are currently working on an extension to a more broad-band design.”

The study is published in the journal Science.

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