Physical basis for Descartes ring scattering in laser-irradiated microdroplets.

Stimulated Raman scattering from laser-irradiated microdroplets is observed from two distinct spatial regions, the droplet rim and a ringlike region encircling the laser beam axis on the droplet shadow face (the Descartes ring). With the use of two pulsed laser beams, a perturbing beam and a delayed stimulated-Raman-scattering pump beam, the physical mechanism for Descartes ring scattering is investigated. Evidence is obtained of a persistent mechanism that continues even after the perturbing laser pulse is turned off. This mechanism is tentatively identified as electrostriction. The possible existence of an additional prompt mechanism that requires overlap between perturbing and pump laser pulses is also discussed.