Experimental and theoretical studies of picosecond laser interactions with electronic materials-laser ablation
- Univ. of California, Berkeley, CA (United States)
Lasers having picosecond and shorter pulse duration are receiving much attention due to their capabilities for direct-write micromachining on many materials with minimal substrate damage. Substantial progress has been made in the understanding of laser ablation processes, particularly the creation of plasmas that often shield the target and reduce the material processing efficiency at nanosecond time scales. However, a considerable challenge that still remains is the understanding of the underlying mechanisms during picosecond laser interactions with electronic solids. In this work we first study picosecond laser-induced electron emission from semiconductor surfaces. A theoretical model was set up based on carrier transport inside the semiconductor material during picosecond laser-semiconductor interactions. We demonstrate that nonequilibrium carrier dynamics plays a significant role for picosecond, as well as short nanosecond, laser induced electron emission from semiconductors. Photoelectric effect is found to be responsible for electron emission at low incident laser fluences, whereas thermionic emission is dominant at higher fluences. We have also performed experimental and theoretical studies on the formation and subsequent evolution of plasmas during laser-metal interactions at the picosecond time scale. Using picosecond time-resolved shadowgrams ahd interferograms, a novel type of plasma is observed, which has an electron density on the order of 1020cm-3.The origin of this picosecond plasma is attributed to gas breakdown, which is caused by laser-induced electron emission fi-om the target surface. After the laser pulse is completed, the longitudinal expansion of the plasma is suppressed. This suppression is found to result from an electric field above the target that prevents, after laser irradiation, fbrther movement of the electrons inside the plasma. Measurements of lateral plasma expansion indicate that the picosecond plasma may absorb substantial amount of incident laser energy during laser irradiation. This result is consistent with the measurements of laser ablation efficiency, which levels off when the laser fluence exceeds a certain threshold. The information provided in this work would be essential for precise control of laser energy coupling with materials, particularly for machining at microscale depth.
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), Basic Energy Sciences (BES)
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 764398
- Report Number(s):
- LBNL-46001; R&D Project: 478101; TRN: AH200102%%144
- Resource Relation:
- Other Information: TH: Thesis (Ph.D.); Submitted to University of California at Berkeley, CA (US); PBD: 1 May 2000
- Country of Publication:
- United States
- Language:
- English
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