Lasers with picosecond and shorter pulse duration are receiving much attention due to their capabilities for direct-write micro-machining on most materials with minimal lateral damage. Deposition of energy from lasers of large power density inevitably creates plasmas that often shield the target and reduce material processing efficiency. Nevertheless, there is little knowledge on the formation and subsequent evolution of plasmas during laser processing of materials at the picosecond time scale. Such information is essential for precise control of laser energy coupling with target materials, particularly for machining at microscale depth.

We have performed experiments by imaging the spatial and temporal development of plasmas during picosecond laser processing of electronic materials. The color plate (1064 nm laser irradiation of a copper target) shows a series of plasma images at times ranging from 5 to 2500 ps. For the first time, a cone-shaped plasma plume has been observed to grow during the early stage of picosecond laser-material interactions. The plasma front moves away from the target at 109 cm/s during the laser pulse (35 ps), but expands primarily in the radial direction after the pulse. The origin of this early- stage plasma has been attributed to gas breakdown assisted by laser-induced electron emission from the target. Beginning at about 200 ps, a second plume emerges from the target surface, which consists of vaporized target material and expands at 106 cm/s into the free space above the target. This vapor plume gradually reaches the front of the early-stage plasma at times about 2000 ps.