Contents lists available at ScienceDirect Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng Studies on surface pitting during laser assisted removal of translucent ellipsoidal particulates from metallic substrates Bijoy Sugathan a,b , J.Padma Nilaya a , V.P.Mahadevan Pillai b , D.J. Biswas a, ⁎ a Laser and Plasma Technology Division, BARC, Mumbai 400085, India b Department of Optoelectronics, University of Kerala, Karyavattom, Thiruvananthapuram 695581, India ARTICLE INFO Keywords: Laser assisted cleaning Field enhanced surface absorption Ellipsoidal particulates Pitting depth Surface topography ABSTRACT We report on the manifestation of field enhanced surface absorption during laser assisted removal of translucent particulates of ellipsoidal geometry from a metallic substrate surface. The surface pitting caused due to this effect has been experimentally probed as a function of the ratio of minor to major axis of the ellipsoid and the behavioral trend has been theoretically interpreted by invoking the principle of geometrical optics. The study also includes the effect of fluence and wavelength of the incident coherent radiation on the surface pitting. Probing of the surface topography has helped gain insight into the formation of multiple pits by a single particulate following its removal post laser exposure. 1. Introduction Laser assisted surface cleaning has decided advantage over con- ventional methods [1,2] as it is a dry process generating very little secondary waste, can be performed in a remote manner, and can be very well controlled to remove the contamination selectively without altering the substrate surface properties [3]. No wonder that laser cleaning has found wide application in areas pertaining to semicon- ductor industry [4], art restoration [5], nuclear [6] and aerospace [7] industry etc. Further, the controlled manner in which surface contam- ination layers can be removed by judicious choice of laser parameters [8] makes this technique more versatile. The duration of the laser pulse, in particular, needs to be carefully chosen depending on the nature of the contamination. In case of fixed contamination, the exposure of the surface to ultrafast pulses results in swift transforma- tion of the thin contamination layer into hot and dense plasma [9] often termed as cold ablation as there is barely any time for the transfer of the absorbed laser energy from the extremely hot electrons to the lattice. In the case of loose surface contamination, for which the laser induced thermal stress is responsible for the generation of cleaning force [3], the pulse duration is long enough for the rapid transfer of the absorbed laser energy into the bulk. Consideration of various relaxation time constants involved generally identifies microsecond to nanosec pulses suitable for removal of loose particulates and picosecond to sub picosec pulses suitable for removal of fixed contamination. We have chosen a coherent source capable of delivering pulses ranging from 1.5 to 0.3 ns for the present study of laser assisted removal of loose contamination. There are however instances wherein even exposure by microsecond [10] and nanosec [11] pulses have led to removal of fixed contamination, in particular, when a compromise has been made between the ablation rate and quality of cleaning [12]. It is important to note here, in the context of laser cleaning of loose particulates, that transparent/semi-transparent contaminant particulates can act as focusing media and enhance the intensity of the incident radiation underneath the particulates. The increased intensity leads to enhanced absorption by the substrate [13] causing surface pitting as a result of ablation even when the incident laser intensity is well below the ablation threshold [13–15]. While the substrate ablation due to optical field enhancement is an undesired effect in the cleaning process, it can be gainfully employed in diverse areas e.g., surface nano-patterning [16–18], optical trap assisted nano-patterning [19] nanophotonics [20] and biomedicine [21]. In our previous works on this subject, we provided experimental signature of increased surface cleaning effi- ciency as a result of field enhanced surface absorption [13] and studied surface pitting as a function of both the fluence and wavelength of the incident coherent radiation [22]. In this communication, we have extended the characterization domain of pitting by way of including the size of the particulates and the corresponding depth of pitting in addition to the dependence on laser fluence and wavelength. Further, the research to this end until now has been confined to spherical or hemispherical particulates [16,17], and our work on particulates predominantly of ellipsoidal geometry represents an additional con- tribution to this emerging area of laser assisted surface patterning. http://dx.doi.org/10.1016/j.optlaseng.2016.11.009 Received 3 August 2016; Received in revised form 29 October 2016; Accepted 7 November 2016 ⁎ Corresponding author. E-mail address: dhruba8biswas@gmail.com (D.J. Biswas). Optics and Lasers in Engineering 91 (2017) 24–29 0143-8166/ © 2016 Elsevier Ltd. All rights reserved. crossmark