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27-Mar-2014

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The angular control of light



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.
[Image courtesy of Weishun Xu and Yuhao Zhang]

Scientists trying to control light have made progress, a new study in the 28 March issue of the journal Science reports.

Light is energy carried in the shape of waves, and it has several distinct properties including intensity (or brightness), propagation direction (the direction in which it travels), and a specific frequency, or a specific wavelength. (Visible light, for example, has a wavelength between 400 and 700 nanometres.)

Scientists can select light with different characteristics for different applications, ranging from eyeglasses to fiber-optic cables.

But, while the selection of light based on characteristics like frequency has been well-studied, light selection based on propagation direction has not. Being able to select light based on this property would help scientists better control light for various applications, including privacy protection screens used on computers. (These screens allow the person looking straight at a screen to see it, but block others, looking at the same screen from sideways angles, from seeing its contents.)

Scientists do think about propagation direction and angles in some cases where they manipulate light (for example, in splitting light into beams that ultimately travel in different directions), but light selection in this process usually depends also on the length of the light's waves (its wavelength).

Now, however, the Massachusetts Institute of Technology's Yichen Shen and colleagues show they can select light based on propagation angle in a totally wavelength-independent manner.

Using crystals as the surface on which to shine light, and the Brewster angle an angle at which light is perfectly transmitted through a substance, with no reflection Shen et al. demonstrated a simple approach that allowed them to select the angle light will travel in. Their crystal material acts as a mirror for all but a narrow range of viewing angles, where the crystal goes from being dense to transparent.

The authors suggest that angular selection will be very useful in applications like solar energy conversion and privacy protection systems.

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