Electron-Hole Droplet Formation in Direct-Gap Semiconductors Observed by Mid-Infrared Pump-Probe Spectroscopy

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 $ħ{\omega }_p\approx 0.5\mathrm{eV}$ 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 $ħ{\omega }_p/\sqrt{3}$ allows us to obtain the carrier density in the metastable electron-hole droplets of $2\times {10}^{20}{\mathrm{cm}}^{-3\mathrm{}}$.