Femtosecond filamentation in transparent media

We review recent advances in laser cell surgery, and investigate the working mechanisms of femtosecond laser nanoprocessing in biomaterials with oscillator pulses of 80-MHz repetition rate and with amplified pulses of 1-kHz repetition rate. Plasma formation in water, the evolution of the temperature distribution, thermoelastic stress generation, and stress-induced bubble formation are numerically simulated for NA=1.3, and the outcome is compared to experimental results. Mechanisms and the spatial resolution of femtosecond laser surgery are then compared to the features of continuous-wave (cw) microbeams. We find that free electrons are produced in a fairly large irradiance range below the optical breakdown threshold, with a deterministic relationship between free-electron density and irradiance. This provides a large ‘tuning range’ for the creation of spatially extremely confined chemical, thermal, and mechanical effects via free-electron generation. Dissection at 80-MHz repetition rate is performed in the low-density plasma regime at pulse energies well below the optical breakdown threshold and only slightly higher than used for nonlinear imaging. It is mediated by free-electron-induced chemical decomposition (bond breaking) in conjunction with multiphoton-induced chemistry, and hardly related to heating or thermoelastic stresses. When the energy is raised, accumulative heating occurs and long-lasting bubbles are produced by tissue dissociation into volatile fragments, which is usually unwanted. By contrast, dissection at 1-kHz repetition rate is performed using more than 10-fold larger pulse energies and relies on thermoelastically induced formation of minute transient cavities with lifetimes <100 ns. Both modes of femtosecond laser nanoprocessing can achieve a 2–3 fold better precision than cell surgery using cw irradiation, and enable manipulation at arbitrary locations.

Mechanisms of femtosecond laser nanosurgery of cells and tissues

Applied Numerical Analysis. By C.F. Gerald. Pp. xii, 340. £3·60. 1970. (Addison-Wesley.)

For pt. I see ibid., vol.31, no.12, p.2241-9 (1995). An analytic, first-order model has been developed to calculate irradiance thresholds for laser-induced breakdown (LIB) in condensed media, including ocular and aqueous media. A complete derivation and description of the model was given in a previous paper (Part I). The model has been incorporated into a computer code and code results have been compared to experimentally measured irradiance thresholds for breakdown of ocular media, saline, and water by nanosecond, picosecond, and femtosecond laser pulses in the visible and near-infrared. The comparison included both breakdown data from the literature and from our own measurements. Theoretical values match experiment to within a factor of 2 or better, over a range of pulsewidths spanning five orders of magnitude.

A first-order model for computation of laser-induced breakdown thresholds in ocular and aqueous media: Part II-Comparison to experiment

We describe an analytical method for element-specific in situ investigations of biological samples with high spatial resolution, using laser-induced-breakdown spectroscopy (LIBS). The prerequisites for spatially highly resolved LIBS are an appropriate analytical performance in combination with precise micro-ablation conditions, both laterally and axially. In order to identify the best suitable laser source, the analytical performance and the ablation process were studied for ns-LIBS and fs-LIBS. The analytical performance was studied on an aqueous solution of CaCl2, where water served as a first substitute for biological material. The ablation process was investigated in the outer epidermal wall of a sunflower seedling stem. Besides direct measurements, the physiology of the sunflower seedling can be used in order to estimate the ablation depth. Our results show that analytical measurements with high spatial resolution can be performed on biological samples using fs-LIBS. This was demonstrated by in situ measurements of wall-associated calcium ion (Ca2+) distributions within the peripheral cell wall of the sunflower seedling (Helianthus annuus L.) stem. In this biological sample an axial resolution of about 100 nm was achieved.

Femtosecond laser-induced-breakdown spectrometry for Ca2+ analysis of biological samples with high spatial resolution

Optical breakdown by ultrashort laser pulses in dielectrics presents an efficient method to deposit laser energy into materials that otherwise exhibit minimal absorption at low laser intensities. During optical breakdown, a high density of free electrons is formed in the material, which dominates energy absorption, and, in turn, the material removal rate during ultrafast laser-material processing. Classical models assume a spatially uniform electron population and constant laser intensity in the focal region, which results in time-dependent expressions only, i.e., the rate equations, to predict electron evolution induced by nanosecond and picosecond pulses. For femtosecond pulses, however, the small spatial extent of the pulse requires that the pulse propagation be considered, which results in an inhomogeneous plasma and localized electron formation during optical breakdown. In this work, a femtosecond breakdown model is combined with the classical rate equations to determine both time- and position-dependent electron density during femtosecond optical breakdown in water. The model exhibits good agreement when compared with experimental results. For other transparent or moderately absorbing dielectric media, the model also shows promise for determining the time- and position-dependent electron evolution induced by ultrashort laser pulses. Another interesting result is that the maximum electron density formed during femtosecond-laser-induced optical breakdown may exceed the conventional limit imposed by the plasma frequency.

C. H. Fan

Papers

Breakdown threshold and localized electron density in water induced by ultrashort laser pulses