Laser-induced cavitation fascinates because it involves a sequence of nonlinear interactions (plasma formation, shock wave emission, and bubble dynamics) but the experimental investigation is challenging due to the large range of spatial and temporal scales. We imaged laser and bubble interactions with a solid target during laser ablation in liquids in side view using a water immersion microscope objective with high numerical aperture. For stroboscopic and high-speed imaging, femtosecond laser pulses were coupled into a multimode fiber. With optimized fiber length, spatial mode scrambling provides speckle-free illumination that enabled us to freeze bubble and shock wave dynamics at diffraction-limited resolution.
Laser-induced cavitation accompanies laser surgery in cells and transparent tissues and its engineering can improve surgical results. To elucidate the underlying mechanisms, plasma-mediated shock wave formation and bubble dynamics are investigated by stroboscopic and high-speed photography with ultrahigh spatiotemporal resolution. We developed a novel light source for speckle-free illumination at exposure times < 100 ps that is based on amplified spontaneous emission (ASE) and lasing in a femtosecond-laser-pumped Rhodamine dye cell. The contributions from ASE and lasing and their influence on pulse duration, divergence and coherence are investigated, and the emission characteristics are optimized for high repetition rates.
Photodamage in nonlinear microscopy of transparent tissues starts at irradiances 1.5 times above the autofluorescence imaging level. Although the free-electron density is low, their energy suffices to break bonds in water, DNA and the backbone and side chains of proteins. We explored photodamage kinetics using physical indicators (hyperfluorescence, plasma luminescence, bubble formation). By plotting threshold values in (irradiance/radiant exposure) space, we identified a “safe” region for microscopy. Thermomechanical effects become relevant in melanin-containing tissue. Two-photon excitation of retinal fluorophores allows monitoring metabolic transformations. We analyze the thermomechanical damage pathways in retinal imaging, and discuss strategies for mitigating such damage.
Laser-induced plasma generation by single and multiple femtosecond laser pulses is used surgically and constitutes a source of photodamage in nonlinear microscopy. The irradiance threshold at which transient vapor bubbles in water are produced is 20x higher than the irradiance used for nonlinear microscopy. However, photodamage in multiphoton microscopy already starts, when the irradiance is 1.5x above autofluorescence imaging. Thus, there is a huge realm of low-density plasma effects between the multi-pulse damage threshold and the single-pulse surgical regime, and the talk will provide a systematic overview over laser applications and the irradiance and radiant exposure dependence of these effects.
Femtosecond laser-induced plasma generation is used surgically and may also cause photodamage in nonlinear microscopy. Photodamage in multiphoton microscopy already starts at irradiances 1.5 times above the value used for autofluorescence imaging but the cavitation bubble threshold is 20 x higher. We explore the realm of low-density plasma effects between multi-pulse nonlinear imaging and single-pulse surgical regime. We characterize the transition from unchanged tissue (emitting autofluorescence) to slightly changed tissue (hyperfluorescence), drastically changed tissue (plasma luminescence) and disintegrated biomolecules (gas bubble formation). By plotting the threshold values in (irradiance, radiant exposure) space, we identified a “safe” region for nonlinear microscopy.
Understanding free-electron mediated effects of tightly focused femtosecond pulse series is essential for minimizing photodamage in nonlinear microscopy and opens new avenues for nanosurgery and intentional modifications of biomolecules. We tracked different stages of the photomodification kinetics (hyperfluorescence, plasma luminescence, bubble formation) by time-lapse 2-photon microscopy, fluorescence lifetime measurements, and bubble interferometry with nanometer resolution. Monitoring of bubble growth during pulse series enabled us to quantify chemical reaction rates leading to gas formation via molecular disintegration. Novel ways of data evaluation were used to create a comprehensive picture of the photomodification kinetics in the (irradiance/irradiation dose) parameter space.
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.
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.
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