Femtosecond laser-induced breakdown in water-like tissues: Influence of wavelength and pulse duration on electron dynamics

A comprehensive model is developed for focused pulse propagation in water. The model incorporates self-focusing, group velocity dispersion, and laser-induced breakdown in which an electron plasma is generated via cascade and multiphoton ionization processes. The laser-induced breakdown is studied first without considering self-focusing to give a breakdown threshold of the light intensity, which compares favorably with existing experimental results. The simple study also yields the threshold dependence on pulse duration and input spot size, thus providing a framework to view the results of numerical simulations of the full model. The simulations establish the breakdown threshold in input power and reveal qualitatively different behavior for picoand femto-second pulses. For longer pulses, the cascade process provides the breakdown mechanism, while for shorter pulses the cooperation between the self-focusing and the multiphoton plasma generation dominates the breakdown threshold.

We have examined laser extinguishment of CH4-N2/Air counterflow diffusion flame. Laser extinguishment is to blow off flames with a laser-induced breakdown. Q-switched Nd : YAG laser [wavelength ; 532 nm, energy ; 200 mJ/pulse, pulse duration (FWHM); 6 ns] and focal lens (focal length ; 100 mm) were used for the laser-induced air breakdown. Probabilities of the laser extinguishment were examined by varying the fuel concentration and the distance from the breakdown point to the counterflow diffusion flame. As a result, the two concentration limits of the laser extinguishment were clarified. One was the laser extinguishable limit, below which the counterflow diffusion flame could be extinguished perfectly by the laser-induced breakdown. The other was the laser inextinguishable limit, over which any flame could not be extinguished by the laser-induced break-down. Moreover, we could consider that determination of the laser extinguishable limit was mainly influenced by the laser-induced blast wave. On the other hand, it could be thought that not only blast wave but also hot gas and turbulent mixing produced by the laser-induced breakdown influenced the determination of the laser inextinguishable limit.

The spatio-temporal evolution of the shock wave generated by a laser induced breakdown is often investigated and interpreted in the framework of the theory of shock similarity solutions. This work is a discussion about the choice of the most relevant geometry (spherical or cylindrical) to be used in the Jones modelling to track the intermediate-strength shockwave trajectory coming from a laser-induced non-resonant breakdown in argon. Laser incident energies ranging from 10 to 200 mJ with initial pressure of argon from 250 to 2500 mbar are investigated using a Q-switched Nd:YAG laser operating at a wavelength of 532 nm with a 6 ns pulse duration. Experimental results show that using the radial component rb of the ellipsoidal shape of the shockwave with a cylindrical geometry best describes the shockwave trajectory over time. Moreover, the deduced characteristic length r0 allows us to observe a shockwave shape similarity for all tested laser energies at a given pressure.

Direct measurements were made of the time-dependent electron density around the current-zero period in a scaled-down air blast breaker with an interruption current level of a few kiloamperes, by means of the Thomson scattering of ruby laser light. The effects of interruption parameters such as the electrode material (copper and graphite) and the gas flow rate on the magnitude of the electron density in AC arcs were quantitatively investigated. The results showed that, in an arc current of 1.5 kA, the electron density at current zero in a copper electrode arc was significantly enhanced by a factor of 4-5 compared with that in a graphite electrode arc. Spectroscopy measurements have shown that such an increase in the electron density is brought about by the admixture of copper vapour with a concentration of 1.5% into high-temperature air at current zero. The electron density, however, was suppressed to around half the magnitude by the increase in the forced convection of air from 100 to 250 l min-1. The inter-correlation between arc interruption performances of the test circuit breaker and the electron density at current zero is discussed. Statistical analysis showed that the success rate of arc interruption of the circuit breaker decreases almost straightforward with increase in the current-zero electron density on the normal probability chart.

This work investigates Titanium Monoxide (TiO) in ablation-plasma by employing laser-induced breakdown spectroscopy (LIBS) with 1 to 10 TW/cm2 irradiance, pulsed, 13 nanosecond, Q-switched Nd:YAG laser radiation at the fundamental wavelength of 1064 nm. The analysis of TiO is based on our first accurate determination of transition line strengths for selected TiO A-X, B-X, and E-X transitions, particularly TiO A-X γ and B-X γ′ bands. Electric dipole line strengths for the A3Φ-X3δ and B3Π-X3δ bands of TiO are computed. The molecular TiO spectra are observed subsequent to laser-induced breakdown (LIB). We discuss analysis of diatomic molecular spectra that may occur simultaneously with spectra originating from atomic species. Gated detection is applied to investigate the development in time of the emission spectra following LIB. Collected emission spectra allow one to infer micro-plasma parameters such as temperature and electron density. Insight into the state of the micro-plasma is gained by comparing measurements with predictions of atomic and molecular spectra. Nonlinear fitting of recorded and computed diatomic spectra provides the basis for molecular diagnostics, while atomic species may overlap and are simultaneously identified. Molecular diagnostic approaches similar to TiO have been performed for diatomic molecules such as AlO, C2, CN, CH, N2, NH, NO and OH.

In the scientific literature there is little information that describes the fundamental physical processes of laser induced breakdown (LIB) in transparent liquids. Our goal is to characterize these fundamental properties, which are critical to the understanding of retinal and other ophthalmic damage produced by ultrashort laser pulses. Laser pulses of 5.0 nanoseconds (ns) at less than 5.0 milli-Joules (mJ) per pulse and pulses of 80 picoseconds (ps) at 0.5 to 1.5 mJ per pulse from a Nd:YAG regenerative amplifier were used to produce LIB in a variety of aqueous media. These include physiological saline solution, triple-distilled water, and tap water. The resulting luminescent plasmas were analyzed using integrated light spectroscopy from a Chromex 0.25 meter (m) spectrograph. Plasmas were recorded in the wavelength region from 300 to 900 nm. Each spectrum obtained was analyzed using a Planck distribution for blackbody emission. The surface temperatures of the plasmas for the two pulse durations were computed to be in the 5000 K to 10,000 K range, depending on the pulse duration and energy. Also, the spectrographs from the saline solution included distinct spectral lines of emission over the broad band spectra, such as the 589 nm atomic emission line of sodium. We will discuss the time-integrated spectroscopy of LIB in various solution, and how LIB might mediate retinal damage induced by ultrashort laser pulses.

The measurement and characterization of laser induced breakdown (LIB) in ocular media for ultrashort (< 1 ns) laser pulses is important in understanding both eye damage mechanisms and various ophthalmic applications. In particular, the American National Standards Institute laser safety standards (ANSI Z136.1-1993) have included only guidance but no definitive safety limits due to lack of both experimental data and quantitative understanding of the damage processed induced by ultrashort pulses. Moreover, LIB needs to be understood fully for the growing number of ophthalmic applications which employ LIB in beneficial ways, such as in capsulotomies and iridotomies. The threshold for gas bubble creation from a plasma induced by 100 fs, 400 fs, and 2.4 ps laser pulses at 0.58 micrometers was determined for various ocular media. Bubble creation was used as the endpoint for indication of LIB at these pulse durations due to the absence of broadband visible light emission (plasma spark) that is normally the indication of LIB at longer pulse durations. In addition, light emitted from the focal region was shown to come from gas breakdown within the bubbles produced by previous pulses when the laser was fired at 10 Hz. The difference in endpoints observed for ultrashort pulses and endpoints observed for longer pulses (> 30 ps) may result from aberrations in the optical setup, in particular the focusing optics. However, the nonlinear phenomena involved may play an important role in the observation of a different type of plasma. The cause and reduction of aberrations and the endpoints for LIB threshold studies are discussed.

Laser induced breakdown (LIB) in air has been explained by different ways along pulse width τ. Cascade ionization (CI) is considered as the mechanism for long pulse widths of over 1 ns. On the other hand, laser filamentation is alternative mechanism of air-breakdown for fs pulses, because CI processes based on collision cannot play a role in fs pulses. However, for intermediate ps pulse region, the mechanism is unclear because of ambiguity of contribution of multi-photon ionization (MPI) to air-breakdown. Since we cannot have access to a pulse width gap between one nanosecond to a few tens picosecond for a long time [1], there is a lack of knowledge about air-breakdown based on CI in sub-ns region. Wang et al. reported that breakdown threshold of fluence Fth has ∼τ scaling for a long pulse width of over than ∼1 ns [2]. On the other hand, Van Stryland et al. reported that Fth is almost constant in 30–140 ps region [3, 4]. Nevertheless, there are still open questions about the cause of the scaling and the degree of contribution of MPI because of lack of data between the pulse width gap.

Recent developments in high-energy regenerative amplifiers and broadly tunable optical parametric amplifiers (OPA) opened new spectral windows to study the impact of ultrashort laser pulses on biological tissues. These sources can generate extraordinarily high peak power capable of causing laser-induced breakdown. However, current laser safety standards (ANSI Z136.1-2014) do not provide guidance on maximum permissible exposure (MPE) values for the skin with pulse durations less than one nanosecond. This study measured damage thresholds in excised porcine skin in the mid-infrared (MIR) region of the electromagnetic spectrum. The laser system, comprised of a high-energy regenerative amplifier and OPA, was tuned to wavelengths between 4000-6000 nm to coincide with heightened absorption for both water and collagen. The laser operated at a fundamental repetition rate of 1 kHz and a nominal pulse width of 150 fs. The beam was focused at the sample surface with a 36X aluminum reflective objective and scanned over a 4 mm2 area for each exposure condition. Spectral domain optical coherence tomography (SD-OCT) imaging of the tissue provided a volumetric assessment of tissue morphology and identified changes in the backscattering profile within the laser-exposed regions. The determination of laser damage thresholds in the MIR for ultrafast lasers will guide safety standards and establish the appropriate MPE levels for exposure to sensitive biological tissue. These data will help guide the safe use of ultrafast MIR lasers in emerging applications across a multitude of industries and operational environments.

An interferometric analysis was performed to investigate the influence of argon (Ar) buffer gas on the characteristics of laser-induced aluminum (Al) plasma at atmospheric pressure. The plasma was produced by focusing a Q-switched Nd:YAG laser pulse (λ=1064 nm, pulse duration ∼5 ns, E=6.0 mJ) onto an Al target. The interference patterns were constructed using a Nomarski interferometer incorporated with a frequency-doubled, Q-switched Nd:YAG laser (λ=532 nm, pulse duration ∼10 ns) that generates an interferometric probe beam. The interferometric measurements were carried out as a function of the elapsed time after the onset of breakdown under the conditions of open air and an Ar gas jet flow (5 l/min). With the injection of an Ar buffer gas jet in the ablation process, an increase in electron density and a preferential axial plasma expansion of the plasma plume were observed during the early stages of plasma formation as a consequence of increased inverse-Bremsstrahlung (IB) absorption efficiency.

The spatial and temporal evolution of plasma generated via laser-induced breakdown in air was investigated. The plasma was produced in air using a focused Nd:YAG Q-switched laser (λ = 1064 nm; pulse duration ∼5 ns; pulse energy ∼60 mJ). The interference patterns of the resulting plasma were measured as a function of time using a Nomarski interferometer. The elapsed times were in the range 59–232 ns. A frequency-doubled Q-switched Nd:YAG laser (λ = 532 nm; pulse duration ∼10 ns; pulse energy ∼10.5 mJ) was coupled with the Nomarski interferometer to form an interferometric probe beam. The electron density was inferred from Abel inversion and fast Fourier transformation analysis of the recorded interference patterns. The measured electron densities were on the order of ∼1018 cm−3. Using the Saha equation, assuming that the plasma is in local thermodynamic equilibrium, and that the ionization reaction is (N → N+ + e), the electron temperatures were estimated to be in the range 17600–15500 K.

The influence of the pulse duration on the mechanical effects following laser-induced breakdown in water was studied at pulse durations between 100 fs and 100 ns. Breakdown was generated by focusing laser pulses into a cuvette containing distilled water. The pulse energy corresponded to 6-times breakdown threshold energy. Plasma formation and shock wave emission were studied photographically. The plasma photographs show a strong influence of self-focusing on the plasma geometry for femtosecond pulses. Streak photographic recording of the shock propagation in the immediate vicinity of the breakdown region allowed the measurement of the near-field shock pressure. At the plasma rim, shock pressures between 3 and 9 GPa were observed for most pulse durations. The shock pressure rapidly decays proportionally to r−(2⋯3) with increasing distance r from the optical axis. At a 6 mm distance of the shock pressure has dropped to (8.5±0.6) MPa for 76 ns and to <0.1 MPa for femtosecond pulses. The radius of the cavitation bubble is reduced from 2.5 mm (76 ns pulses) to less than 50 μm for femtosecond pulses. Mechanical effects such as shock wave emission and cavitation bubble expansion are greatly reduced for shorter laser pulses, because the energy required to produce breakdown decreases with decreasing pulse duration, and because a larger fraction of energy is required to overcome the heat of vaporization with femtosecond pulses.

Nonlinear absorption through laser-induced breakdown (LIB) offers the possibility of localized energy deposition in linearly transparent media and thus of non-invasive surgery inside the eye. The general sequence of events--plasma formation, stress wave emission, cavitation--is always the same, but the detailed characteristics of these processes depend strongly on the laser pulse duration. The various aspects of LIB are reviewed for pulse durations between 80 ns and 100 fs, and it is discussed, how their dependence on pulse duration can be used to control the efficacy of surgical procedures and the amount of collateral effects.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The laser bioeffects team at Brooks Air Force Base has extensively studied the mechanisms for retinal damage to laser exposures shorter than one nanosecond. This regime, called the ultrashort regime, has novel new mechanisms for retinal damage. In previous work in the nanosecond regime we have shown that the threshold for laser-induced breakdown is higher than the threshold for ophthalmoscopically visible retinal damage. But, as the pulse duration is reduced into the femtosecond regime, the laser-induced breakdown threshold and retinal damage threshold approach each other. We discuss the most recent data collected for sub-50 fs laser induced breakdown thresholds and retinal damage thresholds. With these short pulse durations, the chromatic dispersion effect on pulse chirp should be considered to gain a full understanding of the mechanisms for damage. We discuss the most likely damage mechanisms operative in this pulse width regime and discuss relevance to laser safety.The laser bioeffects team at Brooks Air Force Base has extensively studied the mechanisms for retinal damage to laser exposures shorter than one nanosecond. This regime, called the ultrashort regime, has novel new mechanisms for retinal damage. In previous work in the nanosecond regime we have shown that the threshold for laser-induced breakdown is higher than the threshold for ophthalmoscopically visible retinal damage. But, as the pulse duration is reduced into the femtosecond regime, the laser-induced breakdown threshold and retinal damage threshold approach each other. We discuss the most recent data collected for sub-50 fs laser induced breakdown thresholds and retinal damage thresholds. With these short pulse durations, the chromatic dispersion effect on pulse chirp should be considered to gain a full understanding of the mechanisms for damage. We discuss the most likely damage mechanisms operative in this pulse width regime and discuss relevance to laser safety.

The generation of plasmas in water by high-power laser pulses was investigated for pulse durations between 100 ns and 100 fs on the basis of a rate equation for the free electron density. The rate equation was numerically solved to calculate the evolution of the electron density during the laser pulse and to determine the absorption coefficient and energy density of the plasma. For nanosecond laser pulses, the generation of free electrons in distilled water is initiated by multiphoton ionization but then dominated by cascade ionization. For shorter laser pulses, multiphoton ionization gains ever more importance, and collision and recombination losses during breakdown diminish. The corresponding changes in the evolution of the free carrier density explain the reduction of the energy threshold for breakdown and of the plasma energy density observed with decreasing laser pulse duration. By solving the rate equation, we could also explain the complex pulse duration dependence of plasma transmission found in previous experiments. Good quantitative agreement was found between calculated and measured values for the breakdown threshold, plasma absorption coefficient, and plasma energy density.