Invited p aper Mechanisms of femtosecond laser nanosurgery of ...

VOGEL et al. Mechanisms of femtosecond laser nanosurgery of cells and tissues 1035lated using the measured spot size. However, in various studies,the quality of the laser beam and focusing optics were farfrom optimal and, nevertheless, the diffraction-limited spotsize has been used for the calculation of I th . In the presence ofaberrations, this leads to erroneously large threshold values.Even when the spot size is measured, aberrations will lead tofalse results because breakdown occurs in hot spots within thefocal region that are smaller than the measured spot size. Thethreshold values determined in this way are, therefore, smallerthan the actual values [184]. Many breakdown experimentswere performed by focusing a laser beam into a glass cellby means of external focusing optics. Even when this opticsis of high quality, the glass cell itself will introduce sphericalaberrations. This source of error can be avoided by usingwater-immersion microscope objectives built into the wall ofthe cell [3].For nanosecond pulses, longitudinal mode beating in thelaser resonator will usually produce intensity spikes of picosecondduration with amplitudes up to 4–5 times as large asthe averaged intensity. Such statistical fluctuations of the peakirradiance are a major cause of the wide statistical distributionof optical breakdown thresholds and substantially reducetheir absolute values [185, 186]. Therefore, single-frequencylasers providing a temporally Gaussian pulse shape shouldbe used for threshold measurements. However, to date allmeasurements of nanosecond optical breakdown thresholdsin water have been performed with longitudinally multimodelaser beams.To avoid a change of the focal spot size by nonlinearbeam propagation, sufficiently large focusing angles must beused [5, 15, 187]. While this is easy to achieve for nanosecondpulses, a numerical aperture NA ≥ 0.9 is required fora pulse duration of 100 fs to be sure to exclude a diminutionof the spot size by self-focusing and the corresponding apparentreduction of the breakdown threshold [5]. To date, nothreshold data obtained with such large NAs are available forultra-short laser-induced breakdown in water. We must concludethat the present experimental data basis is not reliableenough to perform a thorough comparison with theoreticalpredictions. Future measurements with aberration-free temporallyGaussian laser pulses focused at large NA will have tofill this gap.In addition, a better adjustment of the numerical breakdowncriterion to the experimental criterion of bubble formationis needed to enable a meaningful comparison of experimentaldata with model predictions. While bubble formationrequires an approximately constant energy density within thefocal volume for all laser pulse durations and wavelengths,the energy density associated with a fixed value of the freeelectrondensity, such as ϱ cr = 10 21 cm −3 , varies considerablywith pulse duration. Thus, the assumption of a constant freeelectrondensity as breakdown criterion is quite arbitrary, especiallyfor cases where the threshold is smooth, i.e. whereϱ max increases continuously with irradiance. In these casesit seems more reasonable to relate the critical free-electrondensity to the energy density within the medium that leadsto bubble formation. Equation (17) provides the required linkbetween electron and energy densities, and an analysis ofϱ max (I/I rate ) curves (Figs. 3 and 5) then yields the correspondingthreshold irradiance.The new threshold definition for the numerical model willyield a better match between the experimentally observedand numerically predicted wavelength dependence of breakdownthresholds than the values in Table 1. Experimentally,the thresholds for ns and ps pulses decrease with decreasingwavelength [3, 15], but values calculated with a constant ϱ crshow the opposite trend. Previous calculations revealed a relativelysmooth threshold for ns and ps breakdown with visibleand UV wavelengths, in contrast to the sharp threshold forIR wavelengths shown in Fig. 3c [91]. Calculated thresholdvalues based on the new definition will for IR wavelengthsthus be the same as those obtained with ϱ cr = 10 21 cm −3 ,butthey will be lower for visible and UV wavelengths, in accordwith experimental observations.6.4 Cavitation bubble dynamicsFigure 18 shows a two-dimensional representationof the evolution of the thermoelastic stress wave and of theregion in which the kinetic spinodal is surpassed (bubble nucleus)for a peak temperature of 200 ◦ C, slightly above thethreshold for bubble formation. The subsequent bubble dynamicscalculated for cases 1 and 2 described in Sect. 6.1 ispresented in Fig. 19. It turned out that for a vapor pressurethat changes adiabatically with bubble size (case 2 in Fig. 19b)the bubble motion is very similar to the case where the vaporpressure is not taken into account at all and only the negativepressurepulse drives the bubble expansion (not shown). Figure20 shows the maximum bubble radii for case 1 and case2 as a function of the maximum temperature achieved in thecenter of the focal volume, together with the radius of the nucleus,R 0 . Close to threshold, the actual bubble radius willprobably resemble case 1 in which the bubble nucleus is assumedto be much smaller than the focal region. For energieswell above threshold, the predictions obtained for case 2 becomemore realistic.The most prominent feature of the transient bubbles producedclose to the threshold of femtosecond optical breakdownis their small size and short lifetime. The bubble radiusamounts to only about 200 nm in water, and will be evensmaller in a visco-elastic medium such as the cytoplasm. Thismakes a dissection mechanism associated with bubble formationcompatible with intracellular nanosurgery, in contrastto nanosecond optical breakdown (τ L = 6ns,λ= 1064 nm)where the smallest bubble radius in water observed for NA =0.9 was R max = 45 µm [3]. The small bubble size correspondsto a small energy,E B = 4π 3 (p ∞ − p v )R 3 max , (21)of the expanded bubble. For the case presented in Fig. 19a, itamounts to 5.3 × 10 −15 J (5.3 femtojoules). The smallness ofthe bubble energy is largely due to the small energy content ofthe stress transient creating the bubble. This energy is equal tothe energy stored in thermoelastic compression of the heatedfluid volume [156],E TE = 12ϱ 0 c 2 0∫p 2 0 dV , (22)