It's a well-known phenomenon in electronics: Shining light on a semiconductor, such as the silicon used in computer chips and solar cells, will make it more conductive. But now researchers have discovered that in a special semiconductor, light can have the opposite effect, making the material less conductive instead.

The phenomenon was discovered in an exotic two-dimensional semiconductor—a single layer of molybdenum disulfide (MoS2) just three atoms thick. The finding is reported in a paper in Physical Review Letters by MIT postdoc Joshua Lui; Nuh Gedik, the Lawrence C. and Sarah W. Biedenharn Career Development Associate Professor of Physics; and six others at MIT, Harvard University, and in Taiwan.

The researchers found that when illuminated by intense laser pulses, single-layer MoS2 is reduced to approximately one-third of its initial conductivity. The team used optical laser pulses to generate the effect and time-delayed terahertz pulses to detect the conductive response of the material.

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