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Optical Pulse Can Change Metal to Semiconductor

30 October 2013

Researchers at the Massachusetts Institute of Technology (MIT) have demonstrated that they can control the band structure of a material by subjecting it to an intense optical pulse. This coupling of photons and electrons on the surface of a topological insulator has been predicted by theorists, but never observed.

The finding could lead to the creation of materials whose electronic properties could be "tuned" in real time simply by shining precise laser beams at them. It would be possible, for example, to change the material from a conductor to a semiconductor just by changing the laser beam's polarization.

Altering the bandgap in materials used in chips and solar cells by shining a polarized laser beam at the material could change it from a metal to a semiconductor.

The MIT researchers carried out the experiments by shooting femtosecond laser pulses in mid-infrared light range at a sample of material and observing the results with an electron spectrometer.

They demonstrated the existence of a quantum-mechanical mixture of electrons and photons, known as a Floquet-Bloch state, in a crystalline solid. Photons interacting with matter leads to Floquet states. "Entangling" electrons with photons in a coherent manner generates the Floquet-Bloch state, which is periodic both in time and space.

Normally, to produce dramatic changes in a material's properties, "you have to do something violent to it," said lead researcher Nuh Gedik. "But in this case, it may be possible to do this just by shining light on it. That actually modifies how electrons move in this system. And when we do this, the light does not even get absorbed."

Gedik explained that in this experiment the light's energy is below the absorption threshold, which enables switching a material's behavior back and forth without inducing other effects, such as heating.

The researchers used bismuth selenide crystals, a basic topological insulator, but other materials such as graphene, could be used as well, according to Gedik.

The work "opens up a new avenue for optical manipulation of quantum states of matter," said Gedik, the Lawrence C. and Sarah W. Biedenharn Associate Professor of Physics and senior author of the paper published this week in Science.

The work was supported by the U.S. Department of Energy and the Army Research Office, and made use of shared facilities at the MIT Center for Materials Science.

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