Although energy can be stored as electric charge and heat, it has previously been impossible to absorb it as light using conventional methods.
In a press release released on Thursday by EurekAlert, a group of scientists from the Hebrew University of Jerusalem and Vienna University of Technology (TU Wien) claims that they have now created the perfect setup to trap light.
Although scientists have previously devised methods for absorbing light energy, this is likely the only “light trap” technique that allows light energy to be absorbed even by extremely thin and weak mediums.
Stefan Rotter, a physics professor at TU Wien and one of the study’s authors, told that his team’s work “shows 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”.
The researchers create a carefully designed cavity around the absorbing medium that prevents light from escaping. As a result, the light gets trapped in the cavity and passes through the absorbing medium multiple times until it is completely absorbed and nothing remains.
What is the need for a light trap?
Before delving into how the light trap works, you should understand the significance of capturing light energy. Because there are few known ways to directly store light energy in a feasible & efficient manner, it must be converted into other forms of energy. “From plant absorption of radiation to cell phone camera detection of light, the energy carried by light waves or “photons“ must be converted into other forms of energy to be exploitable,” said Professor Rotter.
For example, the light that you see on your smartphone display is first stored in the battery as chemical energy. The circuit board inside the phone converts it further into electrical energy, and it eventually becomes the light that illuminates your phone’s LCD or LED screen.
Direct light absorption has the potential to significantly improve the design & technology of everyday devices. Light trapping or “harvesting,” according to the researchers, is at the heart of many important processes in science, engineering, and nature. It has the potential to improve the performance of spectrally selective detectors (detectors capable of absorbing different frequencies of light rays) and future light-powered devices.
How does the proposed “perfect light trap” work?
The researchers created a cavity filled with multiple mirrors & lenses that surrounded a thin light-absorbing medium. They arranged the mirrors and lenses in such a way that when a light ray entered the cavity, it began to move in a circular fashion, eventually blocking its own path (as shown in the figure above). Finally, the light beam has no choice but to be absorbed by the thin medium.
This light trapping setup also included two convex lenses, a reflecting mirror, and a partially transparent mirror in addition to the absorbing medium. According to the researchers the first mirror in the light trap, is kept partially transparent to allow light to enter the cavity. However, the same mirror could also allow light to escape.
In order to avoid this, they used wave interference, a characteristic of light that causes back reflections to be cancelled by increasing (or decreasing) the overall amplitude of the light waves.
A laser beam splits in two as it strikes the partially transparent mirror. After hitting lenses, absorbing medium, and reflecting mirror Eventually, the parts superimpose on one another, blocking the entire light beam so that it could only be absorbed by the thin medium from the position from which it was blocked. According to the researchers, this method is so perfect that it is unaffected by even frequent changes in air pressure or temperature.
Professor Stefan responded when asked about the limitations of this light trap mechanism: “Our device only operates at a single frequency of the incoming light. We are currently developing an upgrade to a broad-band design.
The study is published in the journal Science.