COLLOIDAL CRYSTAL PHOTOTHERMAL DYNAMICS

Localized heating of a colloidal crystal by absorption of high intensity laser radiation (> 25 W/cm2) results in compression of the hot region. The time dependent variations in the lattice spacing of the crystal due to this local heating has been studied by monitoring the transmission profile of a weak laser beam incident on the crystal at an angle close to the Bragg angle. The dynamics of the compression phenomenon and its subsequent relaxation after removing the pump laser beam are analyzed using the screened Coulomb pair potential formulation. A simple linear model is developed to explain the deformation of the crystal during both the heating and cooling processes. Using this model we estimate the time dependent temperature rise in the crystal for any given pump power. During the heating process and the early stages of cooling the change in the crystal lattice spacing lags behind the rate of temperature change, while during the later stages of cooling it follows the rate of cooling. We demonstrate the necessity for the renormalization of the surface charge of colloidal particles when using the screened Coulomb pair potential.