Characterization of surface and sub-surface defects in optical materials using the near field evanescent wave
- LLNL
In this paper, we characterize surface and sub-surface defects in fused silica and optical coatings using the surface evanescent wave measured by NSOM implemented on a large-stage AFM. A laser irradiates the sample surface in a total internal reflection configuration. The evanescent wave from the surface is collected by an apertured fiber probe of the NSOM. The amplitude of the surface evanescent wave is proportional to the laser intensity at the surface and therefore sensitive to surface as well as sub-surface defects located in the near-field range (~ 100nm). This subsurface region is thought to contain the great majority of polishing-induced defects. The apertured near field fiber probe provides a spatial resolution of ~100nm and the large stage AFM makes it possible to locate defects in samples with diameters up to 6". We are thus able to map out surface- as well as near-surface optical defects in a large optic as a first step in understanding laser damage mechanisms. These observed defects will be exposed in situ to high fluence laser light to correlate with the initiation of laser damage. We have also used NSOM to measure in situ the effect of chemical etching on optical properties of defects. Post-processing such as chemical etching performed after mechanical polishing is frequently used, and sometimes increases laser damage thresholds. We have measured the amplitude variation of the near field evanescent wave around nodular defects and sub-surface inclusions in optical coatings and thus detected local laser field intensification. The observed intensity variation of the evanescent wave agrees with theoretical calculations of laser amplification around the inclusion. These findings support the theory that laser damage may be induced by local electrical field enhancement associated with micron and sub-micron defects.
- Research Organization:
- Lawrence Livermore National Laboratory (LLNL), Livermore, CA
- Sponsoring Organization:
- USDOE Office of Defense Programs (DP)
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 8061
- Report Number(s):
- UCRL-JC-131225; YN0100000; 97-ERD-013; ON: DE00008061
- Country of Publication:
- United States
- Language:
- English
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