Scientists at NC State have developed a new method to control light. To do this, they use electric fields to change the refractive index of materials. Researchers investigated thin films of semiconductor materials: molybdenum sulfide, tungsten sulfide and tungsten selenide. In some of these materials, the refractive index was changed by as much as 60 percent.

A press release can be found here.

“Giant Gating Tunability of Optical Refractive Index in Transition Metal Dichalcogenide Monolayers”

Authors: Yiling Yu, Yifei Yu, Lujun Huang and Linyou Cao, North Carolina State University; Haowei Peng, Temple University; and Liwei Xiong, Wuhan Institute of Technology

Published: May 15, 2017, Nano Letters

Abstract: We report that the refractive index of transition metal dichacolgenide (TMDC) monolayers, such as MoS₂, WS₂, and WSe₂, can be substantially tuned by > 60% in the imaginary part and > 20% in the real part around exciton resonances using CMOS-compatible electrical gating. This giant tunablility is rooted in the dominance of excitonic effects in the refractive index of the monolayers and the strong susceptibility of the excitons to the influence of injected charge carriers. The tunability mainly results from the effects of injected charge carriers to broaden the spectral width of excitonic interband transitions and to facilitate the interconversion of neutral and charged excitons. The other effects of the injected charge carriers, such as renormalizing bandgap and changing exciton binding energy, only play negligible roles. We also demonstrate that the atomically thin monolayers, when combined with photonic structures, can enable the efficiencies of optical absorption (reflection) tuned from 40% (60%) to 80% (20%) due to the giant tunability of refractive index. This work may pave the way towards the development of field-effect photonics in which the optical functionality can be controlled with CMOS circuits.