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Phys Rev Lett. 2001 Jun 18;86(25):5795-8.

Electron-hole droplet formation in direct-gap semiconductors observed by mid-infrared pump-probe spectroscopy.

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Department of Applied Physics, the University of Tokyo, and Cooperative Excitation Project ERATO, Japan Science and Technology Corporation (JST), 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.


Mid-infrared pump-probe measurements with subpicosecond time resolution reveal the existence of a metastable condensed phase of the electron-hole ensemble in a direct-gap semiconductor CuCl. High-density electrons and holes are directly created in a low-temperature state by the resonant femtosecond excitation of excitons above the Mott transition density. Strong metallic reflection with a plasma frequency Planck's over 2pi(omega)p approximately 0.5 eV builds up within 0.3 ps. Within a few picoseconds, the mid-infrared reflection spectrum is transformed from metalliclike into colloidlike. The observed resonance feature at Planck's over 2pi(omega)p/sqrt[3] allows us to obtain the carrier density in the metastable electron-hole droplets of 2x10(20) cm(-3).

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