Jonathan B. Ashcom

Numerical aperture dependence of damage and white light generation from femtosecond laser pulses in bulk fused silica

The femtosecond laser has become an important tool in the micromachining of transparent materials. In particular, focusing at high numerical aperture enables structuring the bulk of materials. At low numerical aperture and comparable energy, focused femtosecond pulses result in white light or continuum generation. It has proven difficult to damage transparent materials in the bulk at low NA. We have measured the threshold energy for continuum generation and for bulk damage in fused silica for numerical apertures between 0.01 and 0.65. The threshold for continuum generation exhibits a minimum near 0.05 NA, and increases quickly near 0.1 NA. Greater than 0.25 NA, no continuum is observed. The extent of the anti-stokes pedestal in the continuum spectrum decreases strongly as the numerical aperture is increased to 0.1, emphasizing that slow focusing is important for the broadest white light spectrum. We use a sensitive light scattering technique to detect the onset of damage. We are able to produce bulk damage at all numerical apertures studied. At high numerical aperture, the damae threshold is well below the critical power for self-focusing, which allows the breakdown intensity to be determined. Below 0.25 NA, the numerical aperture dependence suggests a possible change in damage mechanism.

Submicron and nano-diameter silica wires for optical wave guiding

Based on the exact solutions of the Maxwells equations, we have theoretically studied basic properties of submicron and nano-diameter air-cladding silica-wire waveguides, the single-mode condition and the modal field of the fundamental modes have been obtained. Silica wires with diameters of 100-1000nm and lengths ranging from hundreds of microns to over 1 millimeter have been fabricated, SEM examination shows that they have uniform diameters and smooth surfaces, which are favorable for optical wave guiding. Light has been sent into these wires by optical coupling. Guiding light through a bent wire is also demonstrated. These wires are promising for assembling photonic devices on a micron or submicron scale.

Morphology and mechanisms of femtosecond laser-induced structural change in bulk transparent materials

Summary form only given. When an intense femtosecond laser pulse is tightly focused into a transparent material, energy is deposited in the focal volume by nonlinear absorption. If enough energy is deposited, permanent structural change results. Characterization of the morphology of structural changes and identification of the mechanisms producing these changes represent essential steps in refining the femtosecond laser as a micromachining tool. We present a systematic study of structural changes produced in bulk glass by tightly-focused femtosecond pulses. Using optical microscopy and scanning electron microscopy (SEM), we identify a transition from a structural change mechanism dominated by localized melting or bond-break to one dominated by an explosive expansion.