Background and Objective

Photodisruption in ocular media with high power pulsed lasers working at non-absorbing frequencies have become a well established surgical tool since the late seventies Shock waves and cavitation bubbles generated by the optical breakdown may strongly influence the surgical effect of photodisruptive lasers We have investigated the shock wave and cavitation bubble effects of femtosecond laser pulses generated during photodisruption in corneal tissue and water The results are compared to those obtained with longer laser pulses



Study Design/Materials and Methods

Laser pulses with 150 fs duration at ∼620 nm wavelength have been focused into corneal tissue and water to create optical breakdown Time-resolved flash photography has been used to investigate the dynamics of the generated shock waves and cavitation bubbles



Results

A rapid decay of the shock waves is observed in both materials with similar temporal characteristics and with a spatial range considerably smaller than that of shock waves induced by picosecond (or nanosecond) optical breakdown Cavitation bubbles are observed to develop more rapidly and to reach smaller maximum diameter than those generated by longer pulses In corneal tissue, single intrastromal cavitation bubbles generated by femtosecond pulses disappear within a few tens of seconds, notably faster than cavitation bubbles generated by picosecond pulses



Conclusions

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 applications of femtosecond laser technology to intraocular microsurgery © 1996 Wiley-Liss, Inc

Time-resolved observations of shock waves and cavitation bubbles generated by femtosecond laser pulses in corneal tissue and water

Threshold values of laser-induced optical breakdown (LIOB) on the surface of human corneal tissues, human enamel, and bovine brain tissues are presented. The data are obtained by using a regeneratively amplified Nd:YLF laser and a multistage dye laser system, respectively. The measured decrease in threshold fluence at shorter pulse durations is in good agreement with the authors' theoretical model.

Laser-induced optical breakdown on hard and soft tissues and its dependence on the pulse duration: experiment and model

The behaviour of conduction heat transfer in a metallic film irradiated by a laser pulse with Gaussian temporal profile is investigated by using the thermal wave model. Calculations are performed to describe the propagation and reflection of a temperature wave and heat flux through the film. The temperature response at the front surface obtained from the thermal wave model is compared with that from the microscopic two-step conduction model by considering a real situation that a Cu film is irradiated by a picosecond laser pulse.

https://iopscience.iop.org/article/10.1088/0022-3727/29/10/001/pdf

The wave characteristics of thermal conduction in metallic films irradiated by ultra-short laser pulses

A femtosecond laser is used to generate and probe hot electrons in polycrystalline and single-crystalline thin gold films. Transient thermoreflectivity and thermotransmissivity for different heating-laser-pulse fluences are performed. Analysis of the transient thermotransmissivity and thermoreflectivity signals allows us to resolve the modulation to the real and the imaginary parts of the dielectric constant. The latter is found to be dominant at our probe wavelength. The hot-electron energy-loss lifetime is shown to be 1--3 ps and increases with the laser fluence. For film thickness comparable to the optical skin depth, the transient decay time of the reflectivity and of the transmissivity are equal, with the polycrystalline films showing a slightly faster decay time. For thicker films, hot-electron transport to the bulk of the film gives a faster transient-reflectivity decay. Electron transport is slower in the polycrystalline films.

Femtosecond time-resolved thermomodulation of thin gold films with different crystal structures

The dependence on the ambient temperature of the transient thermoreflectivity of single-crystal thin gold films has been determined experimentally. We assume the existence of an effective electron temperature throughout the experiment, and demonstrate that it is possible to transform the temporal evolution of the thermoreflectivity to the temporal evolution of the effective electron temperature, and to determine the dependence of the decay of the effective electron temperature on the ambient temperature

Time-resolved thermoreflectivity of thin gold films and its dependence on the ambient temperature.

Synchronized femtosecond and tunable picosecond laser pulses have been generated using a linear cavity configuration in both lasers. The long-term stability of the synchronization, as well as the frequency of the synchronously pumped femtosecond dye laser, is assured by active stabilization of the cavity length. Additionally, the picosecond and femtosecond pulses have been amplified in dual amplifier chains to pulse energies in the microjoule range at a 1 kHz repetition rate. The total peak intensity available for two-photon, nonlinear excitations is on the order of 10/sup 9/ times greater than that available from unamplified beams. >

Generation and kilohertz rate amplification of synchronized femtosecond and picosecond laser pulses

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Generation and kilohertz rate amplification of synchronized femtosecond and picosecond laser pulses