Modelling laser cleaning of low-absorbing substrates: the effect of near-field focussing

A three-dimensional model for laser cleaning of spherical, transparent particles on low-absorbing substrates has been developed. It takes into account near-field focussing of the laser radiation by the particles. The intensity distribution under a particle was found using Mie theory together with th...

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Bibliographic Details
Published in:Applied physics. A, Materials science & processing Materials science & processing, 2004-09, Vol.79 (4-6), p.1595-1598
Main Authors: PLEASANTS, S, LUK'YANCHUK, B. S, KANE, D. M
Format: Article
Language:English
Subjects:
Cleaning
Cross-disciplinary physics: materials science
rheology
Exact sciences and technology
Fluence
Glass
Lasers
Materials science
Mathematical models
Physics
Silicon dioxide
Surface chemistry
Surface cleaning, etching, patterning
Surface treatments
Thresholds
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Summary:A three-dimensional model for laser cleaning of spherical, transparent particles on low-absorbing substrates has been developed. It takes into account near-field focussing of the laser radiation by the particles. The intensity distribution under a particle was found using Mie theory together with the geometrical optics approximation. This permits the estimation of the beam width at the substrate surface and the focal distance of the radiation coming from the spherical particle. These parameters are used to find the distribution of intensity within the low-absorbing substrate from the formula for a focussed Gaussian beam. This is in contrast with most other models of laser cleaning, which assume that all absorption occurs at the surface of the substrate. The energy criterion was used to calculate the threshold fluence. The model predicts threshold fluences of the order of 10 J/cm for silica spheres having a diameter of the order of a micron on silica substrates, assuming adhesion by van der Waals force. As this is well above the damage threshold for silica, it effectively predicts that laser cleaning of silica spheres from silica will be impossible. For glass slides the threshold fluence is predicted to be a factor of 10 times smaller than that for silica slides (about 0.1 J/cm). This is due to the much higher absorption of glass compared to that of silica at 248 nm.
ISSN:0947-8396
1432-0630
DOI:10.1007/s00339-004-2858-9