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

The dependence of the strength of the electron-phonon coupling and the electron heat capacity on the electron temperature is investigated for eight representative metals, Al, Cu, Ag, Au, Ni, Pt, W, and Ti, for the conditions of strong electron-phonon nonequilibrium. These conditions are characteristic of metal targets subjected to energetic ion bombardment or short-pulse laser irradiation. Computational analysis based on first-principles electronic structure calculations of the electron density of states predicts large deviations (up to an order of magnitude) from the commonly used approximations of linear temperature dependence of the electron heat capacity and a constant electron-phonon coupling. These thermophysical properties are found to be very sensitive to details of the electronic structure of the material. The strength of the electron-phonon coupling can either increase (Al, Au, Ag, Cu, and W), decrease (Ni and Pt), or exhibit nonmonotonic changes (Ti) with increasing electron temperature. The electron heat capacity can exhibit either positive (Au, Ag, Cu, and W) or negative (Ni and Pt) deviations from the linear temperature dependence. The large variations of the thermophysical properties, revealed in this work for the range of electron temperatures typically realized in femtosecond laser material processing applications, have important implications for quantitative computational analysis of ultrafast processes associated with laser interaction with metals.

Electron-phonon coupling and electron heat capacity of metals under conditions of strong electron-phonon nonequilibrium

We used 600-femtosecond electron pulses to study the structural evolution of aluminum as it underwent an ultrafast laser–induced solid-liquid phase transition. Real-time observations showed the loss of long-range order that was present in the crystalline phase and the emergence of the liquid structure where only short-range atomic correlations were present; this transition occurred in 3.5picoseconds for thin-film aluminum with an excitation fluence of 70 millijoules per square centimeter. The sensitivity and time resolution were sufficient to capture the time-dependent pair correlation function as the system evolved from the solid to the liquid state. These observations provide an atomic-level description of the melting process, in which the dynamics are best understood as a thermal phase transition under strongly driven conditions.

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An Atomic-Level View of Melting Using Femtosecond Electron Diffraction

Nonlinear optical phenomena are crucial for a broad range of applications, such as microscopy, all-optical data processing, and quantum information. However, materials usually exhibit a weak optical nonlinearity even under intense coherent illumination. We report that indium tin oxide can acquire an ultrafast and large intensity-dependent refractive index in the region of the spectrum where the real part of its permittivity vanishes. We observe a change in the real part of the refractive index of 0.72 ± 0.025, corresponding to 170% of the linear refractive index. This change in refractive index is reversible with a recovery time of about 360 femtoseconds. Our results offer the possibility of designing material structures with large ultrafast nonlinearity for applications in nanophotonics.

https://science.sciencemag.org/content/sci/352/6287/795.full.pdf

Large optical nonlinearity of indium tin oxide in its epsilon-near-zero region

Femtosecond-laser micromachining (also known as inscription or writing) has been developed as one of the most efficient techniques for direct three-dimensional microfabrication of transparent optical materials. In integrated photonics, by using direct writing of femtosecond/ultrafast laser pulses, optical waveguides can be produced in a wide variety of optical materials. With diverse parameters, the formed waveguides may possess different configurations. This paper focuses on crystalline dielectric materials, and is a review of the state-of-the-art in the fabrication, characterization and applications of femtosecond-laser micromachined waveguiding structures in optical crystals and ceramics. A brief outlook is presented by focusing on a few potential spotlights.

Optical waveguides in crystalline dielectric materials produced by femtosecond-laser micromachining

Transparent solids may show strong absorption when irradiated by a high-intensity laser pulse. Such laser induced breakdown is due to the formation of a free-electron gas. We investigate theoretically the role of ionization processes in a defect-free crystal, including in our model two competing processes: strong-electric-field ionization and electron impact ionization. Free-electron heating is described in terms of electron-phonon-photon collisions. Relaxation of the free electron gas occurs through electron-electron collisions and electron-phonon collisions. The latter are also responsible for energy transfer from the free-electron gas to the phonon gas. We solve numerically a system of time dependent Boltzmann equations, where each considered process is included by its corresponding collision integral. Our results show formation, excitation, and relaxation of the electron gas in the conduction band. We find that strong-electric-field ionization is mainly responsible for free-electron generation. No avalanche occurs at femtosecond laser irradiation. The electron density and the internal energies of the subsystems are calculated. Critical fluences obtained using various criteria for damage threshold are in good agreement with recent experiments.

Microscopic processes in dielectrics under irradiation by subpicosecond laser pulses

Irradiation of a metal with an ultrashort laser pulse leads to a disturbance of the free-electron gas out of thermal equilibrium. We investigate theoretically the transient evolution of the distribution function of the electron gas in a metal during and after irradiation with a subpicosecond laser pulse of moderate intensity. We consider absorption by inverse bremsstrahlung, electron-electron thermalization, and electron-phonon coupling. Each interaction process is described by a full Boltzmann collision integral without using any relaxation-time approach. Our model is free of phenomenological parameters. We solve numerically a system of time- and energy-dependent integro-differential equations. For the case of irradiation of aluminum, the results show the transient excitation and relaxation of the free-electron gas as well as the energy exchange between electrons and phonons. We find that laser absorption by free electrons in a metal is well described by a plasmalike absorption term. We obtain a good agreement of calculated absorption characteristics with values experimentally found. For laser excitations near damage threshold, we find that the energy exchange between electrons and lattice can be described with the two-temperature model, in spite of the nonequilibrium distribution function of the electron gas. In contrast, the nonequilibrium distribution leads at low excitations to a delayed cooling of the electron gas. The cooling time of laser-heated electron gas depends thus on excitation parameters and may be longer than the characteristic relaxation time of a Fermi-distributed electron gas depending on internal energy only. We propose a definition of the thermalization time as the time after which the collective behavior of laser-excited electrons equals the thermalized limit.

http://www.ilp.physik.uni-essen.de/vonderLinde/Publikationen/Rethfeld02PRB65_214303.pdf

Ultrafast dynamics of nonequilibrium electrons in metals under femtosecond laser irradiation

Free oscillations of the keyhole in penetration laser beam welding are studied theoretically with regard to characteristic frequencies, damping rates and stability at large amplitudes. The normal modes form a discrete set which may be characterized by axial and azimuthal numbers. Due to viscous damping, only the lowest modes survive many oscillation periods, which yields a limited range of frequencies for the dynamic response of the keyhole to fluctuations of external welding parameters.

https://iopscience.iop.org/article/10.1088/0022-3727/27/10/006/pdf

Oscillations of the keyhole in penetration laser beam welding

Electron transport processes in reflection electron energy loss spectroscopy (REELS) and X-ray photoelectron spectroscopy (XPS)

A dynamic model of melt ejection by a gas jet in laser cutting is presented. The molten material is removed due to friction forces and the pressure gradient of the gas flow. The solution of the stationary equations yields the thickness of the molten layer and its velocity of flow, dependent on cutting speed, gas jet formation and the viscosities and densities of the melt and the gas. A stability analysis of the stationary flow shows instabilities for a pressure gradient controlled melt removal. It is argued that these instabilities correlate with ripple formation on the cutting surface.

https://iopscience.iop.org/article/10.1088/0022-3727/20/1/021/pdf

M. Vicanek

Papers

Microscopic processes in dielectrics under irradiation by subpicosecond laser pulses

Ultrafast dynamics of nonequilibrium electrons in metals under femtosecond laser irradiation

Oscillations of the keyhole in penetration laser beam welding

Electron transport processes in reflection electron energy loss spectroscopy (REELS) and X-ray photoelectron spectroscopy (XPS)

Hydrodynamical instability of melt flow in laser cutting