Charged-particle emission upon ruby laser irradiation of transparent dielectric materials

Positive and negative charged particle currents induced in vacuo by high-power laser irradiation of transparent dielectric materials below the threshold for physical damage have been investigated. Soft glass, Pyrex, fused quartz, Supracil quartz, CaF₂, LiF, and sapphire all show similar behavior. Currents as large as 10¹⁰ charged particles per pulse are obtained using ruby laser radiation with normal mode power levels of about 200 kW. For negative particle currents, two distinct signals have been time resolved using Q-switched pulses. The emission can be correlated qualitatively with that from laser irradiated metal surfaces. A phenomenological model is presented to account for the observed emission pulses. For the dielectric materials, currents are ascribed to laser heating of surface contaminants with subsequent production of high-energy photons and thermionic emission of electrons and ions. The emission of photons, electrons, and ions can be reduced, but not completely eliminated, by careful precleaning and repeated lasing on the dielectric surface. Much smaller emissions have been obtained from a "clean" surface than were previously considered to be intrinsic to the dielectric material.