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Date of Award

9-2004

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Engineering Physics

Supervisor

Dr. Harold K. Haugen

Abstract

This thesis details the results of femtosecond laser ablation and micromachining of indium phosphide (InP). The experimental results presented consist of six sets of investigations divided into two categories: I) single and multiple pulse ablation of stationary samples; 2) laser micromachining and analysis of grooves cut in InP. The first series of experiments dealt with the analysis of the final state of InP after single and multiple pulse irradiation. The experiments were perfonned with femtosecond pulses, 60 -175 fs in duration, centered around wavelengths of 400, 800, 660, 800, 1300 and 2100 nm. In the first set of investigations, single pulse laser ablation craters on InP and GaAs were studied via scanning and plan-view transmission electron microscopy. The final state of the material near the laser-ablated region following femtosecond ablation was characterized in detail for three selected laser fluences. In the second set of investigations, single pulse ablation threshold measurements were performed in the wavelength range from 400 - 2100 nm, covering the photon energy above and below the bandgap of InP. The ablation thresholds detennined from depth and volume measurements varied from 87 mJ/cm2 at 400 nm to 250 mJ/cm2 at 2050 nm. The measurements were performed with optical microscopy, atomic force and scanning electron microscopy. In addition, sharp onsets of the measured depths versus laser fluence were observed at the ablation thresholds. In the third set of investigations, laser induced periodic surface stnlctures were investigated on the surfaces of InP, GaP, GaAs, InAs, Si, Ge and sapphire after multiple pulse femtosecond laser irradiation in the wavelength range from 800 - 2100 nm. High spatial frequency periodic stnlctures were observed on surfaces of InP, GaP, GaAs and sapphire. The periods of the structures were 4.2 - 5.1 times smaller than the free space wavelength of the incident radiation. Conditions required for formation of these ripple stnlctures were identified. The second series of experiments dealt with the analysis of the final state of the material after the cutting of grooves in InP under conditions potentially encountered in practical applications. The experiments can be grouped into three sets of investigations. In the fourth set of investigations, the ablation rate for grooves micromachined with ~ 150 fs pulses centered around 800 nm was investigated as a function of pulse energy, feed rate, number of passes over the same groove, and the light polarization relative to the cutting direction. A logarithmic dependence of the groove depth on the laser Iluence was observed with two regimes characterized by different ablation rates and different thresholds. The groove depth was found to be inversely proportional to the feed rate or equivalently, linearly proportional to the effective number of pulses delivered. With multiple passes over the same groove the depth was found to increase linearly up to approximately 20 consecutive passes. Above 20 passes the ablation rate decreased until a depth limit was asymptotically approached. The best results in terms of groove geometry and depth limit were obtained for grooves cut with the polarization of the beam perpendicular to the cutting direction. In the fifth set of investigations, the residual strain fields resulting from laser micromachining of grooves in InP with femtosecond and nanosecond pulses centered around 800 nm were analyzed using a spatially resolved degree-of-polarization photoluminescence technique. Significant differences in the geometry of the strain patterns were observed in grooves machined in the two temporal domains. The experimental data were compared with results from a finite element model. In the sixth set of investigations, grooves micromachined in InP with femtosecond and nanosecond pulses were investigated by cross- ectional transmission electron microscopy. Substantial densities of defects, extending over a few microns in depth, were observed beneath the grooves machined with femtosecond pulses. The high peak power density and the stress confinement caused by irradiation with femtosecond pulses, along with incubation effects, were identified as the major factors leading to the observed plastic deformations.

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