Alfred Vogel

Shielding properties of laser-induced breakdown in water for pulse durations from 5 ns to 125 fs.

The shielding effectiveness of laser-induced breakdown from focused, visible laser pulses from 5 ns to 125 fs is determined from measurements of transmission of energy through the focal volume. The shielding efficiency decreases as a function of pulse duration from 5 ns to 300 fs and increases from 300 fs to 125 fs. The results are compared with past studies at similar pulse durations. The results of the measurements support laser-induced breakdown models and may lead to an optimization of laser-induced breakdown in ophthalmic surgery by reduction of collateral effects.

Ultrashort pulse laser induced bubble creation thresholds in ocular media

The measurement and characterization of laser induced breakdown (LIB) in ocular media for ultrashort (< 1 ns) laser pulses is important in understanding both eye damage mechanisms and various ophthalmic applications. In particular, the American National Standards Institute laser safety standards (ANSI Z136.1-1993) have included only guidance but no definitive safety limits due to lack of both experimental data and quantitative understanding of the damage processed induced by ultrashort pulses. Moreover, LIB needs to be understood fully for the growing number of ophthalmic applications which employ LIB in beneficial ways, such as in capsulotomies and iridotomies. The threshold for gas bubble creation from a plasma induced by 100 fs, 400 fs, and 2.4 ps laser pulses at 0.58 micrometers was determined for various ocular media. Bubble creation was used as the endpoint for indication of LIB at these pulse durations due to the absence of broadband visible light emission (plasma spark) that is normally the indication of LIB at longer pulse durations. In addition, light emitted from the focal region was shown to come from gas breakdown within the bubbles produced by previous pulses when the laser was fired at 10 Hz. The difference in endpoints observed for ultrashort pulses and endpoints observed for longer pulses (> 30 ps) may result from aberrations in the optical setup, in particular the focusing optics. However, the nonlinear phenomena involved may play an important role in the observation of a different type of plasma. The cause and reduction of aberrations and the endpoints for LIB threshold studies are discussed.

Numerical simulations of optical breakdown for cellular surgery at nanosecond to femtosecond time scales

We have shown by experimental investigations that cellular surgery (microdissection, optoporation, and optoinjection) with Nd:YAG laser pulses of 1064 nm and 532 nm wavelength relies on nonlinear absorption leading to optical breakdown and plasma formation at the laser focus. The present study explores possibilities of refining the breakdown effects by employing shorter pulse durations and irradiances that generate plasmas below the threshold for shock wave and bubble formation. Optical breakdown in water at NA=0.9 and NA=1.3 was simulated numerically for wavelengths of 1064 nm and 532 nm and 355 nm, and pulse durations of 6 ns, 30 ps and 100 fs. We used a rate equation model that allows the calculation of the temporal evolution of the free electron density (rho) during breakdown. (rho) (t) could be followed separately for the free electrons generated by multi photon ionization and avalanche ionization. We obtained excellent agreement between the calculated and measured threshold values for breakdown with 6-ns pulses. The simulations predict that the energy threshold for cellular surgery can be reduced by a factor of 350-2600 (depending on wavelength) when the pulse duration is reduced from 6 ns to 100 fs. The calculated breakdown energies for 100 fs pulses focused by an objective with NA=1.3 are 0.6 nJ at 355 nm, 1.6 nJ at 532 nm, and 3.9 nJ at 1064 nm. With ns-pulses at 1064 nm, the breakdown threshold is very sharp, i.e. there is either no effect at all, or a dense plasma is formed causing a micro- explosion. With shorter wavelengths and pulse durations, the threshold is smoother, and electron densities may be produced that stay below the threshold for explosive evaporation and bubble formation. This creates the possibility of achieving highly localized plasma-mediated chemical or thermal changes in the cell. We conclude that both the reduced energy threshold and the smoother breakdown process with fs pulses bear a large potential for the refinement of intracellular surgery.

Shock-wave energy and acoustic energy dissipation after laser-induced breakdown

We investigated the spatial distribution of energy dissipation during propagation of the shock front arising from optical breakdown in water, because it is related to the stress- induced cellular changes in plasma-mediated laser surgery. The dissipation can be calculated from the shock wave velocity (mu) s by a relation derived from the Rankine-Hugoniot equation. (mu) s was measured as a function of time and space for various laser parameters. With a 1 mJ/6-ns pulse, 64% of the absorbed light energy are converted into acoustic energy, but the largest part of this energy are converted into heat already within the first 200 micrometer of shock front propagation. Afterwards, the dissipation occurs at a much slower rate. Only approximately 10% of the acoustic energy reaches a distance of 10 mm. Far-field measurements can thus be very misleading for an energy balance. The energy dissipation at the shock front leads to a temperature rise of the medium. At 10 mJ pulse energy, the temperature close to the plasma exceeds the critical point of water. This means that the shock wave passage goes along with an enlargement of the cavitation bubble. High-pressure-induced bubble formation can also occur at locations further away from the laser plasma where shock waves from adjacent plasmas interfere. We have thus demonstrated a mechanism of stress wave induced cavitation which does not rely on tensiel stress, but on very high overpressures. Since most of the dissipation takes place within the first 200 micrometer, the shock wave effects are mostly covered by the effects of the cavitation bubble which reaches a radius of 800 micrometer in water at the same laser parameters. Acoustic tissue effects are, nevertheless important, because the bubble is smaller in tissue than in water, the weakening of the tissue structure by the shock wave passage probably contributes to the cavitation-induced damage, and the range for acoustic damage is larger in nonspherical geometries.

Streak-photographic investigation of shock wave emission after laser-induced plasma formation in water

The shock wave emission after dielectric breakdown in water was investigated to assess potential shock wave effects in plasma mediated tissue ablation and intraocular photodisruption. Of particular interest was the dependence of shock wave pressure as a function of distance from the plasma for different laser pulse energies. We have generated plasmas in water with a Nd:YAG laser system delivering pulses of 6 ns duration. The pulses, with energies between 0.4 and 36 mJ (approximately equals 180 times threshold), were focused into a cuvette containing distilled water. The shock wave was visualized with streak photography combined with a schlieren technique. An important advantage of this technique is that the shock position as a function of time can directly be obtained from a single streak and hence a single event. Other methods (e.g. flash photography or passage time measurements between fixed locations) in contrast rely on reproducible events. Using the shock wave speed obtained from the streak images, shock wave peak pressures were calculated providing detailed information on the propagation of the shock. The shock peak pressure as a function of distance r from the optical axis was found to decrease faster than 1/r2 in regions up to distances of 100-150 micrometers . For larger distances it was found to be roughly proportional to 1/r. The scaling law for maximum shock pressure p, at a given distance was found to be proportional to the square root of the laser pulse energy E for distances of 50-200 micrometers from the optical axis.

Shock waves and cavitation effects in aqueous media induced by ultrafast laser pulses

The peak pressure evolution during shock wave propagation after optical break-down in water with pulse durations between 80 ns and 100 fs was investigated by streak photography. The cavitation bubble size was measured through acoustic detection of the bubble oscillation time. The shock wave peak pressure decreased from ≈300 kbar at 6 ns to ≈10 kbar at 100 fs. The conversion efficiency of absorbed light energy into bubble energy decreased from > 20% at 6 ns to < 5% at 100 fs. The weaker mechanical action of fs-pulses, together with the much lower break-down threshold makes it possible to produce structural effects in cells and biological tissues on a micrometer and submicrometer level.

Acoustic online monitoring of IR laser ablation of burnt skin

In burn surgery necrotic tissue has to be removed prior to skin grafting. Tangential excision causes high blood loss and destruction of viable tissue. Pulsed IR laser ablation can overcome these problems because of its high precision and the superficial coagulation of the remaining tissue. We realized an acoustic on-line monitoring system for a selective removal of necrotic tissue that is based on the detection of the energy of the acoustic signal produced during ablation. We developed a PC based system for data acquisition and real-time data analysis running at laser repetition rates of more than 30 Hz, and studied free- running Er:YAG laser ablation of burned skin and stacked gelatin samples which served as reproducible tissue models. Spectral analysis of the ablation noise showed that the optimum tissue specificity of the acoustic energy can only be achieved if the bandwidth of the acoustic transducer range up to more than 300 kHz. We were able to detect the boundary between gelatin layers of different water content by applying a threshold criterion for the relative increase of the acoustic energy with respect to the first laser pulse at each ablation site. Healthy and burned parts of skin samples as well as necrotic and viable tissue layers in second degree burns could be discriminated, in agreement with the result of histologic examinations. Superficial vascular structures could be distinguished fro surrounding burned tissue with good spatial resolution.

All-Optical Characterization of Large Amplitude Pressure Transients in Water

A streak-photographic technique capable of measuring peak pressures of shock waves propaxad gating in aqueous media has been developed. Unlike all other methods, our technique allows non-invasive detection of the shock peak pressure at various locations from a single event. The key features of this method are examplified on laser generated shock waves similar to those encountered in intraocular photodisruption. Reproducibility and accuracy were found to be good considering the statistical nature of the shock-generating event. Shock peak pressures were found to decay proportional to the square of distance from the source. At a given the distance peak pressure increased with the square root of the laser pulse energy.

Plasma formation in water by picosecond and nanosecond Nd:YAG laser pulses: transmission, scattering, and reflection

We investigated the transmission, scattering and reflection of plasmas produced in water by Nd:YAG laser pulses of 6 ns and 30 ps duration. The transmission measurements comprise a large energy range at a wavelength of 1064 nm and various focusing angles between 1.7 degrees and 22 degrees. This parameter range covers the parameters used for intraocular microsurgery, but also allows to asses the influence of self-focusing on plasma shielding, which is only relevant at small focusing angles. We found that most of the laser light is either absorbed or transmitted; scattering and reflection amount to only a few percent of the incident laser energy. The transmission is considerably higher for ps pulses than for ns pulses, regardless of the focusing angle. The plasma transmission increases with decreasing focusing angle. Self- focussing, which occurs at focusing angles below 2, leads to a further increase of transmission. The efficacy of plasma- mediated intraocular laser surgery is higher with 6-ns pulses than with 30-ps pulses, because with the ns pulses nearly 50 percent of the laser pulse energy is absorbed already at threshold, whereas it is only 8 percent with the ps-pulses. The small fractional energy deposition with ps pulses together with a low energy threshold for breakdown can, however, be useful for the generation of very fine tissue effects. Structures beyond the laser focus are 2-6 times more effectively shielded from laser radiation by plasmas generated with ns pulses than by ps plasmas. The transmitted energy at equal normalized energy E/Eth is, nevertheless, always by more than a factor of 8 less than for ps-pulses because of their lower energy threshold for plasma formation.