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Shock waves generated by intense femtosecond lasers Ao, Tommy

Abstract

The advent of intense femtosecond lasers has created the exciting possibility of accessing regimes of extreme high pressure using a relatively small laser system. This stems from the lack of significant hydrodynamic expansion during the process of laser deposition in a solid via skin-depth absorption, which leads to extremely high energy densities in the irradiated sample. After the short-pulse laser energy has been absorbed, the laser-heated material begins to be released which drives a shock wave into the sample. However, unlike previous long-pulse laser driven shock waves, the shock wave driven by a intense short-pulse laser rapidly decays as it propagates through the sample. Before adopting such a shock wave as a new approach in the study of high density plasmas, its unique characteristics must be understood. A one-dimensional hydrodynamic code which is coupled to an electromagnetic wave solver is used to elucidate the basic properties of shock waves generated by intense femtosecond lasers. Using a unique experimental scheme, the electrical conductivity of silicon in the dense, plasma state can also be studied. Calculations were performed in which a shock wave was driven. into a silcion sample by a pump laser with a wavelength of 400 nm, pulse length of 120 fs (FWHM) and irradiances ranging from 10¹⁴ — 10¹⁵W/cm², while rear-side optical measurements were made by a 800 nm, 120 fs probe laser.

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