Pulsed and cw laser performance of heavily doped Ti:sapphire crystals

Abstract

Laser ablation is a very useful technique for producing a variety of nanostructured materials. It can provide rapid prototyping of multielement, multilayer thin films. Various clusters, nanoparticles, and nanotubules can also be produced by laser ablation of either conducting or insulating targets. This paper reports our production and characterization of (i) nitrogen-doped fullerene clusters and (ii) boron-nitrogen nanoparticles and nanotubules.Nitrogen-doped fullerene clusters, in milligram quantity, were synthesized by ablation of a hot graphite rod under a nitrogen environment with use of a nanosecond pulsed Nd:YAG laser. Laser desorption/ionization time-of-flight and Fourier transform mass spectrometry measurements revealed presence of C,-,N+ (m = 60, 70, etc.) in addition to undoped fullerene ions CmC. X-ray photoemission spectroscopy (XPS) study showed the nitrogen 1s level at 400.7 eV. As suggested by Hummelen et al.' the C,,N+ ions observed in the mass spectrometry were likely due to (C,,N), dimers fragmented during the ionization process. Indeed, C,,,N+ and, at averylow intensity, C,,,N+, were observed in our mass spectrometry measurements. No dimers of larger doped fullerenes (m 2 70), however, were observed. The state of these species in the sample needs further investigation.In separate experiments, solid boronnitride targets were laser ablated, creating powder-like materials. High-resolution electron microscopy (HREM) study indicated that the powders were consisted of boron-nitrogen nanotubules, boron particles, and boronnitride flakes. The observed nanotubules had multiple wells and were filled to the center. A fraction of the boron particles was coated by multilayer shells of boron nitride. The boronnitrogen tubules and coated boron particles were separated from the uncoated boron particles and large boron-nitride flakes by a hydrogen-peroxide treatment. These boronnitrogen nanomaterials are similar to carbon nanotubules and nanoparticles, often produced by arc discharge. The laser ablation technique allows boron-nitride, an insulating material, to be used as the starting material, and opens a door for producing a wide variety of nanomaterials. "To understand the fine particle formation process by laser ablation, behaviors of atomic species and growing fine particles in the laserablated plume were visualized by laserinduced fluorescence imaging and Rayleigh scattering imaging techniques. The results on Si particle formation process are presented and discussed.The experimental setup is shown in Fig. 1. A Si target was ablated by a KrF laser. The ablation fluence was in the range from 4.0 J/cm2 to 10 J/cm2. For two-iimensional laser-induced fluorescence (2D-LIF), a sheet ultraviolet laser beam from a frequency-doubled optical parametric oscillator (OPO) was passed parallel to the target surface, and the fluorescence images were taken by a gated and image-intensified CCD camera. Rayleigh scattering (RS) images of condensed particles were obtained with the same probe laser system. To discriminate LIF ...