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

A review of some properties of chalcogenide glasses and the current status of their applications is given. Techniques to characterize the linear and non-linear properties of these glasses are introduced and used to measure the optical constants of chalcogenide glasses in the form of bulk, thin film and fiber. Different techniques for the fabrication of gratings and waveguides in these glasses are described. Achievable efficiencies of gratings, as well as propagation losses of fabricated waveguides, are presented. The possibilities of fabricating active devices, such as fiber amplifiers and lasers, are presented. Finally, a novel application of chalcogenide glasses, namely all-optical switching for the fabrication of efficient femtosecond switches, is introduced.

Optical properties and applications of chalcogenide glasses: a review

We have synthesized a series of chalcogenide glasses from the As–S–Se system that is designed to have strong nonlinearities. Measurements reveal that many of these glasses offer optical Kerr nonlinearities greater than 400 times that of fused silica at 1.25 and 1.55 µm and figures of merit for all-optical switching greater than 5 at 1.55 µm.

Highly nonlinear As-S-Se glasses for all-optical switching.

Using in situ Raman scattering in a confocal microscopy setup, we have observed changes in the network structure of fused silica after modifying regions inside the glass with tightly focused 800-nm 130-fs laser pulses at fluences of 5–200 J cm-2 The Raman spectra show a large increase in the peaks at 490 and 605 cm-1, owing to 4- and 3-membered ring structures in the silica network, indicating that densification occurs after exposure to the femtosecond laser pulses The results are consistent with the formation of a localized plasma by the laser pulse and a subsequent microexplosion inside the glass

Structural changes in fused silica after exposure to focused femtosecond laser pulses

Combining femtosecond pump–probe techniques with optical microscopy, we have studied laser-induced optical breakdown in optically transparent solids with high temporal and spatial resolution. The threshold of plasma formation has been determined from measurements of the changes of the optical reflectivity associated with the developing plasma. It is shown that plasma generation occurs at the surface. We have observed a remarkable resistance to optical breakdown and material damage in the interaction of femtosecond laser pulses with bulk optical materials.

http://www.ilp.physik.uni-essen.de/vonderLinde/Publikationen/vonderLinde96JOSAB13_216.pdf

Breakdown threshold and plasma formation in femtosecond laser-solid interaction

The optical properties of silicate glasses under high-power, 850-nm femtosecond laser irradiation have been studied. Photoinduced processes occurred at irradiances well below the threshold for laser-induce damage. Laser spectral line broadening leading to supercontinuum generation in the visible and UV spectral regions was observed in all the glasses studied. Color-center generation and intrinsic luminescence were found in boro-silicate and alkali silicate glasses. It is believed that these processes result from linear and two-photon absorption of the short-wavelength component of the supercontinuum, causing ionization of the glass matrix. No color-center absorption in the visible region was observed in fused silica at irradiances up to the laser-damage threshold.

Color-center generation in silicate glasses exposed to infrared femtosecond pulses

Copyright (c) 1997 Elsevier Science B.V. All rights reserved. The optical properties of commercial and experimental arsenic sulfide glasses were studied during and after exposure to pulsed Nd:YAG laser radiation at 1.064 and 1.318 μm. Photo-induced losses (absorption and scattering) are produced in chalcogenide glasses by high-power laser radiation, the with photon energy hn<E g . This phenomena is only observed if the irradiance is greater than a value of about 10 8 W/cm 2 . Such exposure results in both reversible and irreversible structural transformations of these glasses in the exposed region. The reversible darkening is similar to the known effect of photo-structural transformations. It is connected with non-linear excitation to upper intrinsic levels of glass when it exposed to photons with energy less than glass bandgap. The irreversible contribution is due to the photo-induced localized destruction of bulk material, which is not completely erased by annealing near the glass transition temperature. Damage sites are responsible for absorption and scattering in this case. It is believed that a process of laser damage is caused by the thermal instability of structural irregularities randomly distributed in glass, which absorb laser radiation.

Photo-structural transformation of chalcogenide glasses under non-linear absorption of laser radiation

The dependence of laser-induced damage threshold of K8 (Russia) and BK7 (USA) glasses on laser pulse duration in the wide region from 4 (DOT) 10-11 to 3 (DOT) 10-8 s has measured. Strong attention is paid to methods of measurements. It is indicated to strong influence of laser radiation statistics on the results of the threshold measurements. It is shown that intrinsic laser-induced damage threshold of glasses does not depend on pulse duration when there is not self-focusing. However the time dependence appears when spot size has become more than the wavelength of laser radiation. It is determined by Kerr nonlinearity in the region of pulse durations less than 10-9 s. For longer pulses the other types of nonlinearity are important as well. It is established that the critical power of the self-focusing is not constant for the same glass, and one depends on the concrete focusing conditions. Besides it is shown that the vector character of electric field strength of laser radiation must take into account under calculation of this value in the focal region under tight focusing conditions.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Laser-induced damage of glasses by pulsed radiation in nano-picosecond region

A study was made of the mechanism of the formation of colour centres by the exposure of lead silicate glasses to high-power laser radiation. The long-wavelength mobility edge (>5.8 eV) of charge carriers in these glasses was located well above the fundamental absorption edge (3.5 eV), so that coloration was observed only when the glass matrix experienced multiphoton ionisation. The newly formed colour centres make the main contribution to the change in the transmission directly during the action of nanosecond laser pulses. An estimate was obtained of the three-photon absorption coefficient of TF10 glass at the wavelength of 527 nm.

Multiphoton coloration of lead silicate glasses by high-power laser radiation

In this work, the processes resulting in optical parameters change of glasses was investigated during the beforethreshold laser irradiation. Residual changes of refractive index resulting in beam contraction and intensity growth of laser radiation in the caustic center were found to be conditioned by the production of mechanical stresses in the sample bulk. Under multiple laser radiation with h(nu)© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Oleg M. Efimov

Papers

Color-center generation in silicate glasses exposed to infrared femtosecond pulses

Photo-structural transformation of chalcogenide glasses under non-linear absorption of laser radiation

Laser-induced damage of glasses by pulsed radiation in nano-picosecond region

Multiphoton coloration of lead silicate glasses by high-power laser radiation

Investigation of optical parameters of silicate glasses in case of before-threshold effect of laser radiation