Measurement of energy penetration depth of subpicosecond laser energy into solid density matter

The energy penetration depth characteristic of the interaction of intense subpicosecond (∼600 fs) ultraviolet (248 nm) laser radiation with solid density material has been experimentally determined. This was accomplished by using a series of ultraviolet transmitting targets consisting of a fused silica (SiO₂) substrate coated with an 80-600 nm layer of MgF₂. The measurement of He-like and H-like Si and Mg lines, as a function of MgF ₂ thickness, enabled the determination of the energy penetration depth. It was found that this depth falls in the range of 250-300 nm for a laser intensity of ∼3×10¹⁶ W/cm². Based on numerical simulations, it is estimated that solid density material to a depth of ∼250 nm is heated to an electron temperature of ∼500 eV.