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This chapter is devoted to a series of studies on a peculiar kind of pulse laser ablation which is found to take place at the interface of two transparent materials. Transparent media treated here include air (gas), liquid and solid medium. This research started from the discovery of appreciable reduction of laser ablation threshold for roughened solid surface by our group. (Nagayama et al, 2005) As will be explained later in detail, precise study on this phenomenon revealed that i. peculiar effects are observed for the interface of transparent and roughened solid surface, ii. the phenomena have no appreciable dependence on the surface roughness, iii. the phenomena depend strongly on the direction of the incident laser beam, i.e., whether the incident beam comes from the material of higher refractive index to that of lower one. iv. this effect is not a microscopic one, but a macroscopic one, v. since the phenomenon can be qualitatively described by optics equations. Appreciable enhancement of laser absorption or the reduction of ablation threshold occurs only when the laser beam is incident from the material of higher refractive index to that of lower index. In other words, the phenomena are represented by the difference in refractive index of adjacent media in both sides of the material interface, and not by the mechanical properties of media. This chapter consists of three topics, which are (i) the enhancement of laser beam absorption by roughened surface, and its evidence through high speed imaging of the ground glass surface ablation, and its unique application to the initiation of high explosive powder (Nagayama et al, 2005, 2007, 2007a), (ii) after a simple theoretical discussions on physics involved in the phenomena, pressure wave generation from material interface of glass or PMMA surface in air or in water, and its high speed imaging, (Nakahara et al, 2008) are described and finally, (iii) liquid jet formation from liquid-air surface will be described. (Utsunomiya et al, 2009, Kajiwara et al, 2009, Utsunomiya et al, 2010) Key issue of the physics involved in all the phenomena treated in this chapter is the difference in refractive index of a transparent material from that of the adjacent medium.

Pulse Laser Ablation by Reflection of Laser Pulse at Interface of Transparent Materials