New system could provide first method for filtering light waves based on direction

March 27, 2014 by David Chandler, Massachusetts Institute of Technology
A photograph of the angular selective sample. The sample is the rectangular area where the beam is projected. The white beam propagates through the sample as if the sample is a transparent glass. The red beam incident at a different angle is reflected as if the sample is a mirror. The three additional lines that only exist on the sample are the reflection images of the incident and reflected beams. The whole setup is immersed in the liquid filled with light-scattering nanoparticles so that the researchers can trace the ray. Credit: Weishun Xu and Yuhao Zhang

Light waves can be defined by three fundamental characteristics: their color (or wavelength), polarization, and direction. While it has long been possible to selectively filter light according to its color or polarization, selectivity based on the direction of propagation has remained elusive.

But now, for the first time, MIT researchers have produced a system that allows of any color to pass through only if it is coming from one specific angle; the technique reflects all light coming from other directions. This new approach could ultimately lead to advances in solar photovoltaics, detectors for telescopes and microscopes, and privacy filters for display screens.

The work is described in a paper appearing this week in the journal Science, written by MIT graduate student Yichen Shen, professor of physics Marin Soljačić, and four others. "We are excited about this," Soljačić says, "because it is a very fundamental building block in our ability to control light."

The new structure consists of a stack of of two alternating materials where the thickness of each layer is precisely controlled. "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."

This narrated video demonstrates the process of the experiment. The sample rotates 90°, as in the experimental setup mentioned in the manuscript. Credit: Yichen Shen

While the amount of light reflected at each of these interfaces is small, by combining many layers with the same properties, most of the light can be reflected away—except for that coming in at precisely the right angle and .

Using a stack of about 80 alternating layers of precise thickness, Shen 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."

Previous work had demonstrated ways of selectively reflecting light except for one precise angle, but those approaches were limited to a narrow range of colors of light. The new system's breadth could open up many potential applications, the team says.

A photograph of the angular selective sample (top view). Two white beams incident at different angles are either transmitted or reflected. The whole setup is immersed in the liquid filled with light-scattering nanoparticles so that the researchers can trace the ray. Credit: Weishun Xu and Yuhao Zhang

Shen says, "This could have great applications in energy, and especially in solar thermophotovoltaics"—harnessing 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.

The findings could also prove 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 could also be applied to display screens on phones or computers, so only those viewing from directly in front could see them.

In principle, the angular selectivity can be made narrower simply by adding more layers to the stack, the researchers say. 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.

While these experiments were done using layers of glass and tantalum oxide, Shen says that in principle any two materials with different refractive indices could be used.

Explore further: Ultra-thin light detectors

More information: "Optical Broadband Angular Selectivity," by Y. Shen et al. Science, 2014.

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5 / 5 (3) Mar 27, 2014
There's an easier approach to filter based on the direction using a reflective spherical ball with two small apertures on opposite sides. Light coming in through one aperture is internally reflected and appears at the other aperture. Putting in a film of liquid crystals which can be shuttered can be used to select which direction (or directions) of light are filtered.

That doesn't sound like a better way to me. In the article's approach, light incident on any point of the surface would be allowed through if it fell into the right range of angles. In your approach, only light that hit the small aperture would be allowed through. The rest of the light would be reflected away and lost.
not rated yet Mar 27, 2014
we will see optical computers in our lifetime at the consumer level... quantum computers at the consumer level are probably still 1 generation (30 - 50 yrs) aWAY

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