T.H.R. Crawford

High-resolution observations of an amorphous layer and subsurface damage formed by femtosecond laser irradiation of silicon

Using transmission electron microscopy (TEM), we observed the micro- and nanostructures of silicon after irradiation by ∼150fs duration pulses centered at 800nm wavelength. Specimens irradiated with a single pulse of 11J∕cm2 fluence and with five pulses, each with a fluence of 1.3J∕cm2, exhibited various structures which included amorphous phases. The amorphous phases were pure silicon, as was revealed by high-resolution TEM imaging, nanobeam diffraction patterns, high-angle annular dark-field images, conventional diffraction images, and energy-dispersive x-ray spectra. Irradiation with a single pulse of 1.5J∕cm2 produced neither amorphous material nor lattice defects. Single-pulse irradiation at a fluence of 33J∕cm2 and irradiation by four pulses at 11J∕cm2 led to substantial subsurface damage around the center of the laser spot. It is concluded that multiple-pulse irradiation produces crystallographic damage more readily than a single pulse.

Cross-sectional study of periodic surface structures on gallium phosphide induced by ultrashort laser pulse irradiation

We present cross-sectional transmission electron microscopy studies of laser induced periodic surface structures (LIPSS) formed on gallium phosphide after irradiation with ultrashort laser pulses with a pulse duration of 150fs and centered at a wavelength of 800nm. LIPSS with spatial periods (∼165nm) substantially smaller than the incident wavelength are found to have vertical profiles as high as ∼960nm measured from the crest to the trough. The present observations provide important insights into the formation mechanisms of subwavelength LIPSS.

Periodic surface structures on gallium phosphide after irradiation with 150fs–7ns laser pulses at 800nm

Gallium phosphide is irradiated with 150fs–7ns duration laser pulses at a wavelength of 800nm. After irradiation with 150fs pulses, periodic surface structures (ripples) are observed on the GaP surface, exhibiting near-wavelength and substantially subwavelength spatial periods depending on irradiation conditions. As the pulse duration increases, near-wavelength ripples become a more dominant feature, completely replacing subwavelength ripples for pulse lengths beyond 80–130ps. Overall the results show that subwavelength ripples can be generated for a wide range of pulse durations, and their formation is quite insensitive to pulse intensity.

Porcine cortical bone ablation by ultrashort pulsed laser irradiation.

Ultrashort pulsed lasers in bone ablation show promise for many orthopedic applications. To minimize collateral tissue damage and control the ablation process, the ablation threshold fluence must be well characterized. Using an amplified femtosecond laser (170 fs, 800 nm, 1 kHz), the ablation threshold on unaltered porcine cortical bone was measured using the D(2) method at multiple incident pulse numbers ranging from 25 to 1000 pulses per spot. The lowered threshold at greater pulse numbers indicated an incubation effect. Using a power law model, the incubation coefficient of unaltered porcine cortical bone was found to be 0.89 ± 0.03. Through extrapolation, the single-pulse ablation threshold was found to be 3.29 ± 0.14 J/cm(2).