Dual role of longitudinal optical phonons for generation of coherent oscillations in gallium arsenide under optical pumping

The authors propose here a simple picture of the generation dynamics of coherent optical phonons and phonon-plasmon coupled modes by ultrafast infrared pulses in gallium arsenide. This picture can explain the spectral coexistence of the bare phonons and the phonon-plasmon coupled modes by their being in different subspaces of Hilbert space (electronic ground and excited states), rather than separated in real space. The quantum mechanical calculations reproduce well the coherent oscillations obtained in transient reflectivity measurements.