Toward optical chips – thin, bright and flexible display technologies

Dotted lines at left outline different layers of molybdenum disulfide, whose relative orientation determines the wavelength of the light that they emit.
Courtesy of the researchers

A promising light source for optoelectronic chips can be tuned to different frequencies.

Chips that use light, rather than electricity, to move data would consume much less power — and energy efficiency is a growing concern as chips’ transistor counts rise.

Of the three chief components of optical circuits — light emitters, modulators, and detectors — emitters are the toughest to build. One promising light source for optical chips is molybdenum disulfide (MoS₂), which has excellent optical properties when deposited as a single, atom-thick layer. Other experimental on-chip light emitters have more-complex three-dimensional geometries and use rarer materials, which would make them more difficult and costly to manufacture.

In the next issue of the journal Nano Letters, researchers from MIT’s departments of Physics and of Electrical Engineering and Computer Science will describe a new technique for building MoS₂ light emitters tuned to different frequencies, an essential requirement for optoelectronic chips. Since thin films of material can also be patterned onto sheets of plastic, the same work could point toward thin, flexible, bright, color displays.

The researchers also provide a theoretical characterization of the physical phenomena that explain the emitters’ tunability, which could aid in the search for even better candidate materials. Molybdenum is one of several elements, clustered together on the periodic table, known as transition metals. “There’s a whole family of transition metals,” says Institute Professor Emeritus Mildred Dresselhaus, the corresponding author on the new paper. “If you find it in one, then it gives you some incentive to look at it in the whole family.”

Joining Dresselhaus on the paper are joint first authors Shengxi Huang, a graduate student in electrical engineering and computer science, and Xi Ling, a postdoc in the Research Laboratory of Electronics; associate professor of electrical engineering and computer science Jing Kong; and Liangbo Liang, Humberto Terrones, and Vincent Meunier of Rensselaer Polytechnic Institute.

Monolayer — with a twist

Most optical communications systems — such as the fiber-optic networks that provide many people with Internet and TV service — maximize bandwidth by encoding different data at different optical frequencies. So tunability is crucial to realizing the full potential of optoelectronic chips.

The MIT researchers tuned their emitters by depositing two layers of MoS₂ on a silicon substrate. The top layers were rotated relative to the lower layers, and the degree of rotation determined the wavelength of the emitted light.

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