Controlling light with electric fields

Researchers have discovered a technique for electrically manipulating light via atomically-thin semiconductors.

A quick look at today’s electronics marketplace highlights the fact that we’ve never understood more about controlling electrons. Photons, on the other hand, are more challenging. They are electrically-neutral, so they cannot be directly manipulated by electric fields. But recent work from a team at North Carolina State University (NCSU) suggest a nanolayer-enabled workaround that could herald a new era of photonics.

If you’ve seen a straight drinking straw ‘bend’ in a glass of water, you’ll know of at least one reliable way to control light – by choosing a material with a specific refractive index. And if you can actively tune the refractive index, you can indirectly affect the path of the photons within that material. So in recent years, the literature has focused on doing just that – controlling photons via light-matter interactions, such as reflection, transmission, absorption, and scattering.
There has been limited success in electrically tuning refractive index for mid-IR or visible light though, with some studies achieving a change of between 0.1 and 1%. But in their latest NanoLetters paper [DOI: 10.1021/acs.nanolett.7b00768], NCSU researchers report that they’ve tuned the refractive index of transition metal dichacolgenide (TMDC) monolayers by more than 60%.

This observed ‘giant gating tunability’ has been attributed to the dominance of excitonic effects that resulted from the injection of charge carriers into the monolayer. The authors highlighted two key mechanisms – the interconversion of the neutral and charged excitons, and broadening of the exciton resonance peaks. Other effects, such as changes in exciton binding energy, were found to have negligible role to play in the results.

In addition, the team produced a simple device – a GaN structure on aluminium oxide, with monolayer of WS2 (the TMDC material) on the top, and a silver mirror at the bottom. With this, they demonstrated that electrically tuning the refractive index of the monolayer also produce significant modulation of light reflection and absorption (40−80%).
These results could get us a step closer to field-effect photonic devices that can be controlled in a similar way to CMOS circuits. For paper co-author, Assistant Prof Linyou Cao, the implications are profound, “With this new discovery, light may be controlled (by an electric field) to be strong or weak, spread or focused…. Just as computers have changed our way of thinking, this new technique will likely change our way of watching…. And may find applications in goggle-free virtual reality lenses and projectors, the movie industry or (in) camouflage.”

This story was commissioned by Materials Today, and can be found here:

The original research paper (£) is: Yiling Yu, Yifei Yu, Lujun Huang, Haowei Peng, Liwei Xiong, and Linyou Cao, “Giant Gating Tunability of Optical Refractive Index in Transition Metal Dichalcogenide Monolayers” NanoLetters, Article ASAP, DOI: 10.1021/acs.nanolett.7b00768
OpenGraph description (100 chars): Photons may be one step closer to coming under electric control, thanks to work from US scientists