Ultrashort laser pulses can be used to create high precision incision in transparent and translucent tissue with minimal damage to adjacent tissue. These performance characteristics meet important surgical requirements in ophthalmology, where femtosecond laser flap creation is becoming a widely used refractive surgery procedure. We summarize clinical findings with femtosecond laser flaps as well as early experiments with other corneal surgical procedures such as corneal transplants. We also review laser-tissue interaction studies in the human sclera and their consequences for the treatment of glaucoma.
A brief review of commercial applications of femtosecond lasers in a clinical setting with emphasis on applications to corneal surgery is presented. The first clinical results of 208 procedures conducted from June to November 2000 is reported. The results show that femtosecond lasers may be safely used as keratome for use in LASIK procedures.
Shock waves and cavitation bubbles generated by optical breakdown may strongly influence the surgical effect of photodisruptive lasers. We have investigated the shock wave and cavitation bubble effects of femtosecond and picosecond laser pulses generated during photodisruption in corneal tissue and water. Laser pulses with 150 fs duration at approximately 620 nm wavelength have been focused into both corneal tissue and water to create optical breakdown. Pulses with durations of 20 ps have been applied for comparative studies. Time-resolved flash photography has been used to investigate the dynamics of the generated shock waves and cavitation bubbles. Femtosecond pulse engender rapidly decaying shock waves in both materials. The spatial range of shock waves induced by femtosecond laser pulses is considerably smaller than that of shock waves induced by picosecond optical breakdown. Cavitation bubbles excited by femtosecond pulses are observed to develop more rapidly and to reach smaller maximum diameter than those generated by longer pulses. In corneal tissue intrastromal cavitation bubbles generated by femtosecond pulses disappear within a few tens of seconds, notably faster than cavitation bubbles generated by picosecond pulses. The reduced shock wave and cavitation bubble effects of the femtosecond laser result in more localized tissue damage. Therefore, a more confined surgical effect should be expected from a femtosecond laser than that from picosecond (or nanosecond) lasers. This indicates a potential benefit from the application of femtosecond laser technology to intraocular microsurgery.
The transport of electronic carriers in gold films excited by femtosecond laser pulses is observed to contain both a ballistic and an interactive component and is, accordingly, well outside thermal equilibrium. The ballistic component is observed to traverse up to 400 nm through the film at near the Fermi velocity. The interactive component undergoes various scattering events and requires a correspondingly longer time to traverse the film. The carriers which originate through laser excitation at the 'front' surface of the film arrive at the 'back' surface and influence the reflectivity. The relative reflectivity, (Delta) R/R, at the back surface is measured with a time delayed probe beam which produces information on the temporal evolution of the coupled dynamics and transport of the carriers. Experimental results are compared to a theoretical basis in terms of the Fermi-liquid theory and 1D carrier transport. Agreement between experiment and theory is remarkably good. Electron transport is also influenced by the presence of multiple metallic layers, for example, the carrier arrival profile at the back surface in gold differs from that of a corresponding Au-Ti-Au multilayer. The difference permits observation of an 'image' of the gold and multilayer interface.