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Date of Award

2009

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Physics

Supervisor

H. Haugen

Co-Supervisor

G. Botton

Language

English

Abstract

This thesis reports observations of modification to single crystal CVD grown diamond irradiated with ultrashort (~150 fs), infrared (800 nm) laser pulses. The first set of experiments reports on modifications when the pulses were focused on the surface of single crystal diamond. A second set of experiments was done where pulses were focused in the bulk to study the formation of internal structures. Diamond is a dielectric material that is normally transparent to infrared radiation. However , the high intensity of femtosecond laser pulses allows for nonlinear absorpt ion of laser radiation. The surface features that formed on diamond were studied using optical microscopy, scanning electron microscopy, and atomic force microscopy. Thin cross sections of diamond were also made using a focused ion beam (FIB) machine. These thin membranes were analyzed in a transmission electron microscope (TEM) to study modifications in the crystal structure directly below the surface of areas irradiated with ultrashort laser pulses. A FIB was also used to prepare TEM samples from areas in the bulk that had been irradiated by femtosecond laser pulses.

Surface craters were made with single femtosecond laser pulses. A ring around the edge of the single pulse craters was observed for craters made with higher pulse energies. Atomic force microscope measurements of this ring revealed it was substantially deeper than the area in the center of the crater and therefore more ablation occurred in the outer ring than in the center. This left a protrusion in the center of the crater. The formation of this ring feature was initially unexpected because the laser pulse has a Gaussian profile. More ablation might be expected to occur in the center of the crater which received a higher intensity and fluence. TEM cross sections also revealed a modified layer of amorphous carbon or polycrystalline diamond on the surface of the crater which had a very uniform thickness inside the ring or central protrusion. This modified layer was thicker underneath the deeper ring feature.

Surface craters were also made with multiple laser pulses and surface lines were made by translating the sample as it was irradiated. In these experiments ripples were observed. Two different types of ripples formed . The first type had a spacing of 550-600 nm which was slightly less than the wavelength of the 800 nm laser pulses. Another type of ripples, with a spacing of 200-250 nm, was also observed. These higher spatial frequency ripples formed in areas which received a lower fiuence/pulse such as around the edge of craters made 'with multiple pulses or lines made with low energy pulses. Both kinds of ripples formed perpendicular to the direction of the laser polarization. This sections of t he ripples viewed in a TEM revealed the high and low spatial frequency ripples had a similar structure. The center of each ripple was unmodified diamond that was part of the main single crystal lattice. The surface of the ripples was covered with a thin, uniform layer of polycrystalline diamond, amorphous carbon or graphite. The ripples had a very smooth, sinusoidal profile instead of forming more angular grooves.

Areas inside the diamond samples were irradiated by focusing the laser pulses in the bulk of the single crystal diamond samples. The sample was translated while being irradiated to make lines within the bulle These lines appeared as darkened areas which could be seen in an optical microscope. TEM sections revealed that cracks had formed in the bulk along the cleavage planes of diamond. These cracks contained polycrystalline material. However, no periodic structures were observed inside diamond. Other dielectrics irradiated with femtosecond laser pulses have revealed periodic structures formed in the bulk.

The results of a TEM analysis on a specimen of laser irradiated single crystal InP are shown in Appendix A. The TEM sample was made by taking a section of a crater formed with a single laser pulse. The laser pulse had a central wavelength of 800 nm and a duration of 10 fs. The irradiation was performed at the Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie in Berlin, Germany. This research was in collaboration with Jorn Bonse (Bundesanstalt für Materialforschung und-prüfung, Berlin, Germany) et al. The crater had a central protrusion feature similar to the feature seen in diamond craters made with single laser pulses. The crater was covered in a laser modified layer made up of polycrystalline and amorphous InP. This layer was much thicker in the area under the central protrusion. There was a very sharp boundary between the laser modified layer and the unmodified InP.

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