New Method Of Filtering Light Has Potential Applications In Microscopes And Phones

By Shweta Iyer on March 31, 2014 11:27 AM EDT

light
Researchers from MIT have created a device that allows light of any wavelength to pass through only if it is coming from a specific angle. (Photo: Photo courtesy of Shutterstock)

All light waves can be defined by three basic properties: their wavelength (color), polarization, and direction.  It is possible to filter light based on its wavelength and polarization, a technique used in photography and display technologies but so far filtering light based on direction of propagation had not yet been achieved. But now researchers from MIT (Massachusetts Institute of Technology) have created a device that allows light of any wavelength to pass through only if it is coming from a specific angle, according to a press release. Light coming from all other angles is reflected. This property of light can be harnessed in solar photovoltaics, detectors for telescopes and microscopes, and privacy filters for display screens.

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The experiment was the brainchild of MIT graduate student Yichen Shen, professor of physics Marin Soljačić, and four others. Their paper has been published this week in Science. "We are excited about this," Soljačić says, "because it is a very fundamental building block in our ability to control light."

The structure that they created for the experiment consists of a stack of ultrathin layers of two alternating materials with precise thickness."When you have two materials, then generally at the interface between them you will have some reflections," Soljačić explains. But at these interfaces, "there is this magical angle called the Brewster angle, and when you come in at exactly that angle and the appropriate polarization, there is no reflection at all."

The amount of light reflected at each of these thin layers is quite small but when many layers with the same properties are combined together, most of the light can be reflected away while the light coming in at precisely the right angle and polarization passes through.

Shen and his team used a stack of about 80 alternating layers of glass and tantalum oxide of precise thickness. He says, "We are able to reflect light at most of the angles, over a very broad band [of colors]: the entire visible range of frequencies."

Although there have been experiments that showed ways to selectively reflect light except that coming from a precise angle, they were limited to narrow wavelengths of light.

The work done by Shen and his team could be exploited in a number of applications."This could have great applications in energy, and especially in solar thermophotovoltaics", says Shen.

The system can be used to harness solar energy by using it to heat a material, which in turn radiates light of a particular color. That light emission can then be harnessed using a photovoltaic cell tuned to make maximum use of that color of light. But for this approach to work, it is essential to limit the heat and light lost to reflections, and re-emission, so the ability to selectively control those reflections could improve efficiency.

This principle can also be useful in optical systems, such as microscopes and telescopes for viewing faint objects that are close to brighter objects, for example a faint planet next to a bright star. By using a system that receives light only from a certain angle, such devices could have an improved ability to detect faint targets. The filtering gives an added security when used in display screens on phones or computers, since only those viewing from directly in front can see the screen. 

According to the researchers, the angular selectivity of the material can be made narrower simply by adding more layers to the stack. For the experiments performed so far, the angle of selectivity was about 10 degrees; roughly 90 percent of the light coming in within that angle was allowed to pass through.

Shen says that any two materials with different refractive indices could also be used for the stacks.

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