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TECHNICAL PAPERS

# Nanostructuring Borosilicate Glass With Near-Field Enhanced Energy Using a Femtosecond Laser Pulse

[+] Author and Article Information
Alex Heltzel, Arvind Battula, J. R. Howell, Shaochen Chen

Department of Mechanical Engineering,  The University of Texas at Austin, Austin, TX 78712

J. Heat Transfer 129(1), 53-59 (May 26, 2006) (7 pages) doi:10.1115/1.2360595 History: Received January 24, 2006; Revised May 26, 2006

## Abstract

A model based on the evolution of electron density derived from the Fokker-Planck equation has been built to describe ablation of dielectrics during femtosecond laser pulses. The model is verified against an experimental investigation of borosilicate glass with a $200fs$ laser pulse centered at $780nm$ wavelength in a range of laser energies. The ablation mechanisms in dielectrics include multi-photon ionization (MPI) and avalanche ionization. MPI dominates the ionization process during the first stages of the laser pulse, contributing seed electrons which supply avalanche ionization. The avalanche process initiates and becomes responsible for the majority of free-electron generation. The overall material removal is shown to be highly dependent upon the optical response of the dielectric as plasma is formed. The ablation model is employed to predict the response of borosilicate glass to an enhanced electromagnetic field due to the presence of microspheres on the substrate surface. It is shown that the diffraction limit can be broken, creating nanoscale surface modification. An experimental study accompanies the model, with AFM and SEM characterizations that are consistent with the predicted surface modifications.

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## Figures

Figure 1

Single shot ablated crater, Epulse=13.8μJ

Figure 3

Surface reflectivity at r=0 during 200fs pulse

Figure 4

Absorption coefficient at r=0, z=0 during 200fs pulse

Figure 5

Near-field enhanced laser intensity, λ=800nm, r=880nm

Figure 6

Predicted ablation craters from near-field enhanced laser energy at three fluences

Figure 7

Schematic of (a) experimental setup, (b) irradiation of the silica spheres on borosilicate glass substrate

Figure 8

SEM image of the monolayer of silica spheres with a diameter of 1.76μm

Figure 9

SEM micrograph of the features formed on the glass substrate using microspheres with a single laser pulse irradiation (λ=800nm and FWHM=100fs) having laser fluence of (a) 330mJ∕cm2, (b) 550mJ∕cm2, and (c) 765mJ∕cm2

Figure 10

AFM profiles of the features formed on the glass substrate using microspheres with a single laser pulse irradiation (λ=800nm and FWHM=100fs) having laser fluence of (a) 230mJ∕cm2 and (b) 405mJ∕cm2

Figure 11

AFM cross-section profiles of the features formed on the glass substrate using microspheres with a single laser pulse irradiation (λ=800nm and FWHM=100fs) having laser fluence of (a) 230mJ∕cm2, (b) 550mJ∕cm2, and (c) 950mJ∕cm2

Figure 2

Single shot ablated crater, Epulse=18.8μJ

## Errata

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