Surface modifications induced by ns and sub-ps excimer laser pulses on titanium implant material

Medical implants used in oral and orthopaedic surgery are mainly produced from titanium. Their biological behaviour, e.g. osseointegration, essentially depends on both the chemical composition and the morphology of the surface. Modifications achieved by excimer laser irradiation of titanium samples were investigated in order to improve their surface characteristics so as to facilitate biointegration. To enlarge the effective interfacial area of bone-implant contact, holes were ablated by laser pulses of ns or sub-ps length. During ns ablation, crown-like projecting rims formed around the borders of the holes. Ultra-short (0.5ps) KrF excimer laser pulses were successfully applied to avoid these undesirable formations. Since a smooth dental implant surface is necessary to maintain a healthy connection with the soft tissues, laser polishing of samples was investigated, too. Irradiation with a series of ns laser pulses resulted in effective smoothing, as measured with atomic force microscope. X-ray photoelectron spectroscopy analysis of the laser-polished titanium surface revealed that laser treatment led to a decrease of the surface contamination and in thickening of the oxide layer. X-ray diffraction measurements demonstrated that the original alpha-titanium crystal structure was preserved.

Regeneration of rabbit cornea following excimer laser photorefractive keratectomy: a study on gap junctions, epithelial junctions and epidermal growth factor receptor expression in correlation with cell proliferation

Corneal wound repair was investigated in rabbits following excimer laser ablation of a 6 mm diameter and 90 microm deep disc. In the healing process particular attention was focused on the epithelium where gap junction expression and the rearrangement of desmosomes and hemidesmosomes were correlated with cell proliferation and epidermal growth factor receptor expression. Immunofluorescence-based confocal laser scanning microscopy, semithin resin section morphology and electron microscopy were utilized. In resting cornea two isotypes of gap junctions, confined to different regions in the same basal epithelial cells, were detected. Particulate connexin43 (alpha1) immunostaining was concentrated on the apical while the connexin26 type (beta2) in the baso-lateral cell membranes. This is the first report of connexin26 in the cornea. Connexin43 was found also in corneal keratocytes and endothelial cell. Since the two connexins do not form functioning heteromeric channels and have selective permeabilities they may serve alternative pathways for direct cell-cell communication in the basal cell layer. During regeneration both connexins were expressed throughout the corneal epithelium including the migrating cells. They also showed transient up-regulation 24 hr after wounding in the form of overlapping relocation to the upper cell layers. At this time, basal epithelial cells at the limbal region, adjacent to the wound and those migrating over the wounded area all expressed membrane bound epidermal growth factor receptor and they were highly proliferating. In conclusion, like in other stratified epithelia connexin26 is also expressed in the cornea. Transient up-regulation and relocation of connexins within the regenerating epithelium may reflect the involvement of direct cell-cell communication in corneal wound healing. Mitotic activity in the migrating corneal epithelial cells is also a novel finding which is probably the sign of the excessive demand for new epithelial cells in larger wounds not met alone by the proliferating limbal stock.

Diffraction of short pulses with boundary diffraction wave theory

The diffraction of short pulses is studied on the basis of the Miyamoto-Wolf theory of the boundary diffraction wave, which is a mathematical formulation of Young's idea about the nature of diffraction. It is pointed out that the diffracted field is given by the superposition of the boundary wave pulse (formed by interference of the elementary boundary diffraction waves) and the geometric (direct) pulse (governed by the laws of geometrical optics). The case of a circular aperture is treated in details. The diffracted field on the optical axis is calculated analytically (without any approximation) for an arbitrary temporal pulse shape. Because of the short pulse duration and the path difference the geometric and the boundary wave pulses appear separately, i.e., the boundary waves are manifested in themselves in the illuminated region (in the sense of geometrical optics). The properties of the boundary wave pulse is discussed. Its radial intensity distribution can be approximated by the Bessel function of zero order if the observation points are in the illuminated region and far from the plane of the aperture and close to the optical axis. Although the boundary wave pulse propagates on the optical axis at a speed exceeding c, it does not contradict the theory of relativity.

Enhanced optical microlithography with a Fabry-Perot-based spatial filtering technique

A coherent multiple imaging technique for use in optical microlithography was studied. The technique involves placing a thin Fabry-Perot etalon between the mask and the projection lens of an optical stepper. An optical lithographic computer simulation tool, Prolith/2, was used to evaluate the aerial image profile obtained for extended mask structures such as typical contact hole arrays and line-space patterns used in integrated circuit fabrication. Additionally, a set of experimental studies were performed to validate the simulation results. Enhancement of both resolution and depth of focus can be obtained simultaneously with appropriate etalon parameters.

Atomic force microscopic study of the human cornea following excimer laser keratectomy

The aim of this study was to examine the corneal surface structures with a new investigative method, the atomic force microscope following 193 nm excimer laser photoablation. Fresh human corneas were irradiated in vitro with an increasing number of impulses emitted by a 193 nm ArF laboratory excimer laser in order to produce either smooth flat surfaces or stair-like formations within the cornea. The corneas were investigated in a Topometrix(R) atomic force microscope in their native state. For comparison, three corneas were fixed with glutaraldehyde and processed for scanning electron microscopy. Atomic force microscopy and scanning electron microscopy revealed the same surface characteristics of photoablated corneas, though the preparation for scanning electron microscopy induced considerable shrinkage of the tissues. The layers of the cornea could be distinguished from each other and deeper ablations of the stroma produced a rougher surface. On the lateral walls of ablated stairs small droplets of ejected material could be seen with scanning electron microscope. Atomic force microscope produces three-dimensional images of the scanned native corneal surfaces and it could be a valuable tool to investigate the corneal smoothness. Our investigations have provided similar results as those obtained with scanning electron microscopy showing that the laser-ablated corneal surface remains relatively smooth. We suggest that the formation of condense droplets of ejected materials is based on hydrodynamic motions induced by boiling water solutions.

Reshaping of femtosecond pulses by the Gouy phase shift

It is shown that because of the phase anomaly a femtosecond pulse propagates on the optical axis with a velocity greater than c in the vicinity of the focus not only for large but also small values of the Fresnel number. The group velocity is calculated and its physical meaning discussed. Analytical expressions are derived for the electric field. The causality of the system is proved. The mechanism of the superluminality is a reshaping process caused by the interference. Contrary to other superluminal phenomena, the superluminal propagation occurs in a classically not forbidden region.

Plano-concave microcuvette for measuring the absorption coefficient of highly absorbing liquids

A powerful and simple method based on the use of a plano-concave microcuvette was investigated for measuring the absorption coefficient of highly absorbing liquids. A plano-convex lens put on a plane-parallel plate formed a microcuvette with small, continuously varying thicknesses. This microcuvette was filled with liquid and illuminated by a homogeneous beam. The parabolic variation of the liquid thickness generates a Gaussian spatial intensity distribution behind the cuvette. This Gaussian profile, detected by a CCD camera, was used to determine the absorption coefficient of the liquid. An absorption coefficient as high as 1.54 x 10(4) cm(-1) was measured by use of high-concentration malachite green dye solutions. A comparison of the results with data extrapolated from those of conventional methods showed good agreement.

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

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 approximately 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.

Group-delay measurement on laser mirrors by spectrally resolved white-light interferometry

The frequency-dependent group delay of dielectric mirrors was measured by spectrally resolved white-light interferometry. Chirped mirrors and thin-film Gires-Tournois interferometers designed for dispersion control in a femtosecond Ti:sapphire laser oscillator-amplifier system were tested with a group-delay resolution of +/-0.2 fs and a spectral resolution of ~1 nm over the spectral range of 670-870 nm.

Dynamics of shock waves and cavitation bubbles generated by picosecond laser pulses in corneal tissue and water

Time-resolved flash photography was used to investigate the dynamics of shock waves and cavitation bubbles generated by picosecond optical breakdown in bovine corneal tissue and water. A picosecond Nd:YLF laser was employed. A rapid decay of the shock waves was observed in both materials, with similar temporal characteristics, indicating that water serves as a good model for shock wave studies. In contrast, differences in the cavitation bubble dynamics were found between cornea and water, which are related to differences in the mechanical and thermal properties of the two media, suggesting that water should not be used to model cavitation dynamics in cornea. The experimental results also suggest that the efficiency of intrastromal ablation may be increased by using short pulses and moderate pulse energies in order to avoid the creation of large cavitation bubbles. The experiment indicates that the optimum laser repetition rate for intrastromal ablation is between 1 and 5 kHz.

Plume emission, shock wave and surface wave formation during excimer laser ablation of the cornea

Excimer lasers are now used for corneal surgery; however, the physical processes occurring during photoablation of the cornea are incompletely understood. High speed laser-based photographic arrangement was constructed. The temporal resolution was better than 1 ns. The setup could work as a Schlieren arrangement, which is sensitive to the refractive index change caused by the shock wave propagating in the air above the eye. With minor changes the setup was converted into a shadowgraph, which could detect the ablation plume and the waves propagating on the surface of the eye. Due to the impact of the excimer laser pulse onto the surface of the cornea, a shock wave was generated in the air. The shadowgraph clearly showed the ejection of the ablated cornea. The ejection velocity of the plume was found to be over 600 m/s. It was shown for the first time that the recoil forces of the plume are generating a wave on the surface of the eye. The laser-based high speed photographic arrangement is a powerful arrangement in the study of physical effects occurring during photoablation of the cornea.

Femtosecond-resolution pulse-front distortion measurement by time-of-flight interferometry

The pulse-front distortion occurring in lenses and lens systems has been measured by a Michelson interferometer. In this technique the plane pulse front from one arm of the interferometer is used as a temporal reference level to map contour lines of equal propagation time on the pulse-front surface. The experimental arrangement is capable of detecting pulse-front distortion with a resolution of 20 fsec, and this can be improved to approximately 1 fsec. The measured value of pulse-front distortion in a telescope (1.1 psec) is in good agreement with the calculated data.

Distortion of femtosecond laser pulses in lenses

Large temporal front distortion of femtosecond pulses occurs in lenses having chromatic aberration. The effect is due to the difference between the phase and group velocities. Equations describing the pulse-front delay in singlet lenses, achromats, and compound lenses are presented. The pulse-front delay is several orders of magnitude larger than the broadening caused by group-velocity dispersion in the lens material. Delays occurring in Fresnel-type zone plates are also described.

Phase-sensitive single-pulse autocorrelator for ultrashort laser pulses

A new single-shot phase-sensitive autocorrelator for picosecond and femtosecond measurements is presented. The experimental results show that this device is suitable for the study of chirp in single pulses.