Sebastian Freidank

Sensitive high-resolution white-light Schlieren technique with a large dynamic range for the investigation of ablation dynamics

We developed a modified Hoffman contrast technique with a 12 ns pulsed incoherent extended white-light source that enables an easily interpretable visualization of ablation plumes with high resolution, a large dynamic range, and color information. By comparison, a conventional dark-field setup with a slitlike laser light source provides large sensitivity but a small dynamic range, and it is difficult to interpret the filtered images.

A New Nanosecond UV Laser at 355 nm: Early Results of Corneal Flap Cutting in a Rabbit Model

PURPOSE A new 355 nm UV laser was used for corneal flap cutting in an animal model and tested for clinical and morphologic alterations. METHODS Corneal flaps were created (Chinchilla Bastards; n = 25) with an UV nanosecond laser at 355 nm (150 kHz, pulse duration 850 ps, spot-size 1 μm, spot spacing 6 × 6 μm, side cut Δz 1 μm; cutting depth 130 μm) and pulse energies of 2.2 or 2.5 μJ, respectively. Following slit-lamp examination, animals were killed at 6, 12, and 24 hours after treatment. Corneas were prepared for histology (hematoxylin and eosin [HE], TUNEL-assay) and evaluated statistically, followed by ultrastructural investigations. RESULTS Laser treatment was tolerated well, flap lift was easier at 2.5 μJ compared with 2.2 μJ. Standard HE at 24 hours revealed intact epithelium in the horizontal cut, with similar increase in corneal thickness at both energies. Irrespective of energy levels, TUNEL assay revealed comparable numbers of apoptotic cells in the horizontal and vertical cut at 6, 12, and 24 hours, becoming detectable in the horizontal cut as an acellular stromal band at 24 hours. Ultrastructural analysis revealed regular morphology in the epi- and endothelium, while in the stroma, disorganized collagen lamellae were detectable representing the horizontal cut, again irrespective of energy levels applied. CONCLUSIONS This new UV laser revealed no epi- nor endothelial damage at energies feasible for corneal flap cutting. Observed corneal swelling was lower compared with existing UV laser studies, albeit total energy applied here was much higher. Observed loss of stromal keratinocytes is comparable with available laser systems. Therefore, this new laser is suitable for refractive surgery, awaiting its test in a chronic environment.

Probing the immune and healing response of murine intestinal mucosa by time-lapse 2-photon microscopy of laser-induced lesions with real-time dosimetry

Gut mucosa is an important interface between body and environment. Immune response and healing processes of murine small intestinal mucosa were investigated by intravital time-lapse two-photon excited autofluorescence microscopy of the response to localized laser-induced damage. Epithelial lesions were created by 355-nm, 500-ps pulses from a microchip laser that produced minute cavitation bubbles. Size and dynamics of these bubbles were monitored using a novel interferometric backscattering technique with 80 nm resolution. Small bubbles (< 2.5 µm maximum radius) merely resulted in autofluorescence loss of the target cell. Larger bubbles (7-25 µm) affected several cells and provoked immigration of immune cells (polymorphonuclear leucocytes). Damaged cells were expelled into the lumen, and the epithelium healed within 2 hours by stretching and migration of adjacent epithelial cells.

Controlled Nonlinear Energy Deposition In Transparent Materials: Experiments And Theory

Both femtosecond and UV/VIS nanosecond pulses can create low‐density plasmas in transparent dielectrics suitable for nano‐cell surgery and modification of glasses. Controlled nonlinear energy deposition is thus possible in a large part of the wavelength/pulse duration parameter space. Tunable energy deposition is demonstrated on cell and tissue surgery and in glass, and a model with high predictive value is presented.

[Alternatives to femtosecond laser technology: subnanosecond UV pulse and ring foci for creation of LASIK flaps].

BACKGROUND In refractive corneal surgery femtosecond (fs) lasers are used for creating LASIK flaps, dissecting lenticules and for astigmatism correction by limbal incisions. OBJECTIVES Femtosecond laser systems are complex and expensive and cutting precision is compromised by the large focal length associated with the commonly used infrared (IR) wavelengths. Based on investigations of the cutting dynamics, novel approaches for corneal dissection using ultraviolet A (UVA) picosecond (ps) pulses and ring foci from vortex beams are presented. METHODS Laser-induced bubble formation in corneal stroma was investigated by high-speed photography at 1-50 million frames/s. Using Gaussian and vortex beams of UVA pulses with durations between 200 and 850 ps the laser energy needed for easy removal of flaps created in porcine corneas was determined and the quality of the cuts by scanning electron microscopy was documented. Cutting parameters for 850 ps are reported also for rabbit eyes. The UV-induced and mechanical stress were evaluated for Gaussian and vortex beams. RESULTS The results show that UVA picosecond lasers provide better cutting precision than IR femtosecond lasers, with similar processing times. Cutting energy decreases by >50 % when the laser pulse duration is reduced to 200 ps. Vortex beams produce a short, donut-shaped focus allowing efficient and precise dissection along the corneal lamellae which results in a dramatic reduction of the absorbed energy needed for cutting and of mechanical side effects as well as in less bubble formation in the cutting plane. CONCLUSION A combination of novel approaches for corneal dissection provides the option to replace femtosecond lasers by compact UVA microchip laser technology. Ring foci are also of interest for femtosecond laser surgery, especially for improved lenticule excision.

Alternativen zur Femtosekundentechnologie

BACKGROUND In refractive corneal surgery femtosecond (fs) lasers are used for creating LASIK flaps, dissecting lenticules and for astigmatism correction by limbal incisions. OBJECTIVES Femtosecond laser systems are complex and expensive and cutting precision is compromised by the large focal length associated with the commonly used infrared (IR) wavelengths. Based on investigations of the cutting dynamics, novel approaches for corneal dissection using ultraviolet A (UVA) picosecond (ps) pulses and ring foci from vortex beams are presented. METHODS Laser-induced bubble formation in corneal stroma was investigated by high-speed photography at 1-50 million frames/s. Using Gaussian and vortex beams of UVA pulses with durations between 200 and 850 ps the laser energy needed for easy removal of flaps created in porcine corneas was determined and the quality of the cuts by scanning electron microscopy was documented. Cutting parameters for 850 ps are reported also for rabbit eyes. The UV-induced and mechanical stress were evaluated for Gaussian and vortex beams. RESULTS The results show that UVA picosecond lasers provide better cutting precision than IR femtosecond lasers, with similar processing times. Cutting energy decreases by >50 % when the laser pulse duration is reduced to 200 ps. Vortex beams produce a short, donut-shaped focus allowing efficient and precise dissection along the corneal lamellae which results in a dramatic reduction of the absorbed energy needed for cutting and of mechanical side effects as well as in less bubble formation in the cutting plane. CONCLUSION A combination of novel approaches for corneal dissection provides the option to replace femtosecond lasers by compact UVA microchip laser technology. Ring foci are also of interest for femtosecond laser surgery, especially for improved lenticule excision.

Femtosecond and nanosecond laser-induced nanoeffects for cell surgery and modification of glass

Both femtosecond and nanosecond pulses can create low-density plasmas in transparent dielectrics suitable for nano-cell surgery and modification of glasses. The variation of mechanisms with pulse repetition rate and duration is discussed.

Unified model of plasma formation, bubble generation and shock wave emission in water for fs to ns laser pulses (Conference Presentation)

We developed modeling tools for optical breakdown events in water that span various phases reaching from breakdown initiation via solvated electron generation, through laser induced-plasma formation and temperature evolution in the focal spot to the later phases of cavitation bubble dynamics and shock wave emission and applied them to a large parameter space of pulse durations, wavelengths, and pulse energies. The rate equation model considers the interplay of linear absorption, photoionization, avalanche ionization and recombination, traces thermalization and temperature evolution during the laser pulse, and portrays the role of thermal ionization that becomes relevant for T > 3000 K. Modeling of free-electron generation includes recent insights on breakdown initiation in water via multiphoton excitation of valence band electrons into a solvated state at Eini = 6.6 eV followed by up-conversion into the conduction band level that is located at 9.5 eV. The ability of tracing the temperature evolution enabled us to link the model of laser-induced plasma formation with a hydrodynamic model of plasma-induced pressure evolution and phase transitions that, in turn, traces bubble generation and dynamics as well as shock wave emission. This way, the amount of nonlinear energy deposition in transparent dielectrics and the resulting material modifications can be assessed as a function of incident laser energy. The unified model of plasma formation and bubble dynamics yields an excellent agreement with experimental results over the entire range of investigated pulse durations (femtosecond to nanosecond), wavelengths (UV to IR) and pulse energies.

Wavelength dependence of femtosecond laser-induced breakdown in water, and implications for laser surgery (Conference Presentation)

Studying the wavelength dependence of femtosecond optical breakdown in water helps resolving an ongoing controversy on the relative importance of multiphoton, tunneling and avalanche ionization. Measurements of the bubble formation threshold at 50 wavelengths from UV to near-IR revealed a continuous decrease of the irradiance threshold with increasing wavelength. This is indicative for a dominant role of avalanche ionization, which gains strength with wavelength whereas the multiphoton ionization rate decreases. Fitting data by a model considering breakdown initiation via a solvated electron state yielded an effective Drude electron collision time of 1 fs. Modeling predicts that the threshold continues to decrease up to 1.3 μm but levels out for longer wavelengths. It remains low in the mid IR because wavelength-independent tunneling ionization ensures a constant level of seed electrons for the ionization avalanche even though the influence of multiphoton ionization ceases. The low breakdown threshold opens promising perspectives for ultrashort-pulsed laser surgery at wavelengths around 1.3 μm and 1.7 μm, which are attractive due to a favorable combination of low scattering and moderate water absorption. The wavelength dependence of the irradiance threshold together with tissue optical data was used to estimate the wavelength dependence of the energy threshold at various cutting depths. For focusing depths up to 200 μm, pulse energies required for surgery are smallest for < 800 nm. However, the energy minimum shifts to wavelengths around 1350 nm for z = 500 μm, and to the region around 1700 nm for z = 1 mm.

Correction of hyperopia by intrastromal cutting and biocompatible filler injection (Conference Presentation)

For ametropic eyes, LASIK is a common surgical procedure to correct the refractive error. However, the correction of hyperopia is more difficult than that of myopia because the increase of the central corneal curvature by excimer ablation is only possible by intrastromal tissue removal within a ring-like zone in the corneal periphery. For high hyperopia, the ring-shaped indentation leads to problems with the stability and reproducibility of the correction due to epithelial regrowth. Recently, it was shown that the correction of hyperopia can be achieved by implanting intracorneal inlays into a laser-dissected intrastromal pocket. In this paper we demonstrate the feasibility of a new approach in which a transparent, and biocompatible liquid filler material is injected into a laser-dissected corneal pocket, and the refractive change is monitored via OCT. This technique allows for a precise and adjustable change of the corneal curvature. Precise cutting of the intrastromal pocket was achieved by focusing UV laser picosecond pulses from a microchip laser system into the cornea. After laser dissection, the transparent filler material was injected into the pocket. The increase of the refractive power by filler injection was evaluated by taking OCT images from the cornea. With this novel technique, it is possible to precisely correct hyperopia of up to 10 diopters. An astigmatism correction is also possible by using ellipsoidal intrastromal pockets.