Diagnostics of fs Laser-Induced Plasmas in Solid Dielectrics

The formation of dense plasmas inside dielectric materials by ultrashort laser pulses has many applications ranging from refractive-index modifications to the formation of channels and voids. Furthermore, such plasmas enable the fundamental investigation of ultrafast non-equilibrium dynamics in highly excited materials. The present paper provides an overview of current experimental approaches to investigating such plasmas. Much information about the plasma relaxation is obtained by measuring the spatial and temporal evolution of the dielectric properties of the excited material through time-resolved absorption and phase-shift measurements. In order to investigate and resolve the individual stages of plasma formation, experimental approaches with a temporal resolution beyond the capabilities of traditional optical pump-probe studies are required. Recent examples for schemes that may enable the investigation of the plasma formation with sub-cycle time resolution are thus reviewed. These include recent results from time-resolved high-harmonic generation as well as the two-color pump-probe analysis of non-perturbative low-order wave mixing for the tracking of strong-field excitation dynamics. Alternative approaches employ attosecond transient absorption spectroscopy, attosecond polarization spectroscopy and nonlinear photoconductive sampling for resolving the temporal evolution of the carrier dynamics down to sub-optical-cycle timescales. In solid dielectrics, the formation of an electron-hole plasma produced by irradiation with an intense, ultrashort electromagnetic field plays a central role in such diverse areas as high-harmonic generation, laser processing, and optical damage. In this paper, experimental techniques are reviewed for investigating the formation and relaxation of such a confined, microscopic plasma in bulk transparent solids. image