Optical control of the coherent dynamics of a bismuth lattice at liquid-helium temperature at low and high excitation levels

The fully symmetric A_1g phonons of bismuth have been investigated at liquid-helium temperature by the coherent control method for various crystal excitation levels. It has been found that large-amplitude coherent phonons exhibit the “rigidity” of the phase, which is absent at a small amplitude. The impossibility of changing the phase of phonon oscillations appears at the excitation levels at which their amplitude relaxation law changes from exponential to power. The modification of the phase properties and relaxation law of the excitations of the crystal lattice can be understood in terms of the concept of the condensation of phonons, which occurs with an increase in the crystal excitation level.