Controlling Light with Electric Fields

Technical scheme of a new technique to electrically manipulate light through interaction with an atomically thin semiconductor. (Source: NCSU)

Researchers from North Carolina State Uni­versity have discovered a technique for control­ling light with electric fields. “Our method is similar to the technique used to provide the computing capa­bilities of computers,” says Linyou Cao, an assistant pro­fessor of materials science and engi­neering at NC State. “In computers, an electric field is used to turn electric current on or off, which corresponds to logic 1 and logic 0, the basis of binary code. With this new dis­covery, a light may be controlled to be strong or weak, spread or focused, pointing one direction or others by an electric field.

We think that, just as computers have changed our way of thinking, this new tech­nique will likely change our way of watching. For instance, it may shape a light into arbi­trary patterns, which may find appli­cations in goggle-free virtual reality lenses and projectors, the animation movie industry or camou­flage.” Control­ling light with electric fields is difficult. Photons usually do not respond to electric fields. Instead, light may be controlled by tuning the refrac­tive index of materials. Refractive index refers to the way materials reflect, transmit, scatter and absorb light. The more one can control a material’s refrac­tive index, the more control you have over the light that interacts with that material.

“Unfor­tunately, it is very difficult to tune refrac­tive index with electric fields,” Cao says. “Previous tech­niques could only change the index for visible light by between 0.1 and 1 percent at the maximum.” Cao and his colla­borators have developed a technique that allows them to change the refrac­tive index for visible light in some semi­conductor materials by 60 percent – two orders of magnitude better than previous results. The researchers worked with a class of atomically thin semi­conductor materials, transition metal dichal­cogenide mono­layers. Specifically, they worked with thin films of molyb­denum sulfide, tungsten sulfide and tungsten selenide.

“We changed the refrac­tive index by applying charge to two-dimen­sional semi­conductor materials in the same way one would apply charge to transistors in a computer chip,” Cao says. “Using this technique, we achieved signi­ficant, tunable changes in the index within the red range of the visible spectrum.” Currently, the greater the voltage applied to the material, the greater will be the degree of change in the refrac­tive index. And, because the researchers are using the same techniques found in existing computa­tional transistor techno­logies, these changes are dynamic and can be made billions of times per second.

“This technique may provide capa­bilities to control the amplitude and phase of light pixel by pixel in a way as fast as modern computers,” says Yiling Yu, a recent graduate of NC State. “This is only a first step,” Cao says. “We think we can optimize the technique to achieve even larger changes in the refrac­tive index. And we also plan to explore whether this could work at other wave­lengths in the visual spectrum.” Cao and his team are also looking for industry partners to develop new appli­cations for the dis­covery. (Source: NCSU)

Reference: Y. Yu et al.: Giant Gating Tunability of Optical Refractive Index in Transition Metal Dichalcogenide Monolayers, Nano Lett., online 15 May 2017; DOI: 10.1021/acs.nanolett.7b00768

Link: Nanoscale Photophysics group (L. Cao), Dept. of Physics, North Carolina State University, Raleigh, USA

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