Electron-hole liquid formation by exciton and biexciton resonant excitation in CuCl

The dynamics of a cold photoexcited electron-hole system created by resonant excitation with picosecond optical pulses are studied by time-resolved emission and pump-probe measurements. A rapid build-up of strong metallic reflection in the mid-infrared region was observed, which indicates the creation of electron-hole plasma via the exciton or biexciton Mott transition. Transient reflection spectra clearly show the transformation of high-density electron-hole plasma into a metallic colloid-like state within 10 ps. In the meantime, broad plasma emission turns to a narrow emission band, in which the spectral profile does not change within several tens of picoseconds. These features in the emission spectra disappear when switched to the band-to-band excitation. An analysis of the reflection and emission spectra reveals that the density of metastable electron-hole liquid is as high as 1020 cm-3. This indicates that the plasma formation via the Mott transition is crucial to eliminate excess heating for reaching the low-temperature states of an electron-hole ensemble