Difference Frequency Generation Measurements of Phase Transitions in Gallium

We recently measured the vibrational excitation spectra of solid and liquid gallium with ultrafast terahertz difference frequency generation (DFG) spectroscopy. The two phases had clearly different DFG spectra, with a 250 cm$^{-1}$ phonon feature visible in the solid phase and a 50 cm$^{-1}$ excitation feature seen in the liquid phase. Prospects for using this technique to measure phase transitions of shocked systems \textit{in situ} will be discussed.