Optical Breakdown In Water Research Articles

Micro-dynamics of optical breakdown in water induced by nanosecond laser pulses of 1064 nm wavelength

Optical breakdown in water induced by 1064 nm Nd:YAG laser was visualized and measured by applying shadow and Schlieren optical diagnostic techniques. The research was focused on the earlier stages of the breakdown process, just after the end of the heating laser pulse. The spatial characteristics of laser spark columns were measured and compared to numerical analysis of cavities arising over inclusions. Anomalously long laser spark columns were observed. The distribution of cavity dimensions and energies absorbed in the spark were measured along the laser spark column. A novel non-probability based criterion for laser breakdown threshold was suggested, namely, the pulse energy which generates a spark of negligibly small length.

Microscale nanosecond laser-induced optical breakdown in water

Microscale optical breakdown induced in bulk pure water by high-power nanosecond KrF laser pulses was studied using optical transmission and contact broadband photoacoustic techniques. The breakdown has been identified as a sharp transmission drop coinciding with the appearance of unipolar compressive acoustic pulses, both indicating a thresholdlike rise of local intrinsic absorption in the micrometer-scale laser focal volume. The acoustic pulses, which are much broader than the exciting laser pulse and show a strongly reduced far-field diffraction effect, result from breakdown-induced millimeter-sized steam bubbles. The acoustic pulse amplitudes exhibit a sub-linear ( proportional, variantI(3/4)) pressure dependence on the laser intensity I characteristic of subcritical electron-ion plasma and demonstrating the avalanche enhancement of two-photon ionization above the breakdown threshold until the appearance of the critical plasma. In the critical plasma regime, where the transmission and the acoustic signals slowly vary as a function of laser intensity, the main acoustic pulse is preceded by nanosecond and sub- micros prepulses, where the first one represents a GPa-level plasma-driven shock wave and the second one adjacent to the main pulse appears due to weak submillimeter-long heating of water surrounding the hot plasma by its bremsstrahlung radiation, indicating significant dissociation of water molecules in the plasma.

Focusing of shock waves induced by optical breakdown in water

The focusing of laser-generated shock waves by a truncated ellipsoidal reflector was experimentally and numerically investigated. Pressure waveform and distribution around the first (F(1)) and second foci (F(2)) of the ellipsoidal reflector were measured. A neodymium doped yttrium aluminum garnet laser of 1046 nm wavelength and 5 ns pulse duration was used to create an optical breakdown at F(1), which generates a spherically diverging shock wave with a peak pressure of 2.1-5.9 MPa at 1.1 mm stand-off distance and a pulse width at half maximum of 36-65 ns. Upon reflection, a converging shock wave is produced which, upon arriving at F(2), has a leading compressive wave with a peak pressure of 26 MPa and a zero-crossing pulse duration of 0.1 mus, followed by a trailing tensile wave of -3.3 MPa peak pressure and 0.2 mus pulse duration. The -6 dB beam size of the focused shock wave field is 1.6 x 0.2 mm(2) along and transverse to the shock wave propagation direction. Formation of elongated plasmas at high laser energy levels limits the increase in the peak pressure at F(2). General features in the waveform profile of the converging shock wave are in qualitative agreement with numerical simulations based on the Hamilton model.

Femtosecond-Laser-Induced Nanocavitation in Water: Implications for Optical Breakdown Threshold and Cell Surgery

We determined the bubble radius R_(max) for femtosecond optical breakdown in water at 347, 520, and 1040 nm with an unprecedented accuracy (+/-10 nm). At threshold, R_(max) was smaller than the diffraction-limited focus radius and ranged from 190 nm to 320 nm. The increase of R_(max) with laser energy E_(L) is slowest at 347 nm, providing optimum control of cell surgery. Experimental results agree with a model of bubble formation in heated and thermoelastically stretched liquids. Theory predicts a threshold temperature T_(th) approximately equal to 168 degrees C. For T>300 degrees C, a phase explosion sets in, and R_(max) increases rapidly with E_(L).

飞秒激光诱导水光学击穿的椭球体模型

飞秒激光诱导水光学击穿的椭球体模型

Numerical modeling of transient progression of plasma formation in biological tissues induced by short laser pulses

A theoretical model based on the rate equation for free electron density is proposed to investigate transient progression of plasma formation in soft biological tissues during laser shock processing. The laser focusing region around the focus point is considered to be one-dimensional along the direction of the incident beam, and is discretized into numerous thin control volumes. In simulation of the transient plasma progression, the laser intensity distribution and the temporal evolution of the free electron density are calculated sequentially for each control volume using a fourth-order Runge–Kutta method with adaptive time step control. The rate-equation formalism is first validated with previously published theoretical and experimental results. Simulation of the dynamics of plasma formation is then performed. The results include temporal evolution and spatial distribution of the free electron density as well as the growth of the plasma. It is shown that the threshold laser intensity for optical breakdown in water and the maximum length of the resulting plasma obtained from the present model are in good agreement with existing experimental data.

Optodynamic Characterization of Laser-Induced Bubbles

Laser-induced bubbles can be caused by an optical breakdown in water. They are a result of the optodynamical process where the energy of a high intensity laser pulse is converted into the mechanical energy through an optodynamic conversion. At this process the absorbed optical energy causes plasma expansion that in turn initiates dynamic phenomena: spreading of a shock wave and the development of a cavitation bubble. When the cavitation bubble reaches its maximum radius it starts to collapse due to the pressure of the surrounding liquid. This collapse in turn initiates a new bubble growth and bubble collapse. The process therefore repeats itself, resulting in so-called cavitation-bubble oscillations, with a new shock wave being emitted after every collapse. We present an optodynamic characterization of cavitation bubble’s oscillations based on a laser beam-de∞ection probe. Employed setup enabled us one- or two-dimensional scanning with de∞ections of a laser probe beam. De∞ections were detected with a fast quadrant photodiode.

Stress wave emission and cavitation bubble dynamics by nanosecond optical breakdown in a tissue phantom

Stress wave emission and cavitation bubble dynamics after optical breakdown in water and a tissue phantom with Nd: YAG laser pulses of 6 ns duration were investigated both experimentally and numerically to obtain a better understanding of the physical mechanisms involved in plasma-mediated laser surgery. Experimental tools were high-speed photography with 50000 frames s, and acoustic measurements. The tissue phantom consisted of a transparent polyacrylamide (PAA) gel, the elastic properties of which can be controlled by modifying the water content. Breakdown in water produced a purely compressive stress wave. By contrast, in stiff PAA samples and for sufficiently large pulse energies, the compression wave was followed by an intense tensile wave, similar to the behaviour previously observed in cornea. The elastic/plastic response of the medium led to a significant decrease of the maximum size of the cavitation bubble and to a shortening of its oscillation period which was found to be related to the generation of the tensile stress wave upon breakdown. For increasing elastic modulus of the PAA, both the amplitudes of the bubble oscillation and of the stress wave emitted during bubble collapse decreased until the bubble oscillation was so strongly damped that no collapse stress wave was emitted. Numerical simulations were performed using a spherical model of bubble dynamics which includes the compressibility and elastic/plastic behaviour of the medium, viscosity, density and surface tension. The calculations revealed that consideration of the elastic/plastic behaviour of the medium surrounding the bubble is essential to describe the experimentally observed bipolar shape of the stress wave emitted upon optical breakdown. Water is a poor tissue model because the shape of the emitted stress waves and the bubble dynamics differ strongly for both materials. The mechanical properties of PAA were also found to be quite different from those of tissues. Experimental and numerical results provided evidence that the dynamic mechanical properties relevant for optical breakdown in PAA and tissue differ by as much as two orders of magnitude from the static values. The discovery of a tensile stress wave after optical breakdown in tissue-like media is of great importance for the assessment of collateral damage in laser surgery because biological tissues are much more susceptible to tensile stress than to compressive stress.

Role of a dissolved gas in the optical breakdown of water

It is shown experimentally that the optical breakdown in water produced by nanosecond pulses is initiated by a gas dissolved in it due to the formation of bubston clusters playing the role of heterogeneous seeds of the breakdown. By using the method proposed in the paper, a completely degassed water is obtained, which is optically stable to the seed mechanism of breakdown both away and near the boiling point. The existence of long-lived hydrates of a dissolved gas in water is established and a new effect of water purification from such hydrate traps by 'washing' it with helium is discovered.

Breakdown threshold and localized electron density in water induced by ultrashort laser pulses

Optical breakdown by ultrashort laser pulses in dielectrics presents an efficient method to deposit laser energy into materials that otherwise exhibit minimal absorption at low laser intensities. During optical breakdown, a high density of free electrons is formed in the material, which dominates energy absorption, and, in turn, the material removal rate during ultrafast laser-material processing. Classical models assume a spatially uniform electron population and constant laser intensity in the focal region, which results in time-dependent expressions only, i.e., the rate equations, to predict electron evolution induced by nanosecond and picosecond pulses. For femtosecond pulses, however, the small spatial extent of the pulse requires that the pulse propagation be considered, which results in an inhomogeneous plasma and localized electron formation during optical breakdown. In this work, a femtosecond breakdown model is combined with the classical rate equations to determine both time- and position-dependent electron density during femtosecond optical breakdown in water. The model exhibits good agreement when compared with experimental results. For other transparent or moderately absorbing dielectric media, the model also shows promise for determining the time- and position-dependent electron evolution induced by ultrashort laser pulses. Another interesting result is that the maximum electron density formed during femtosecond-laser-induced optical breakdown may exceed the conventional limit imposed by the plasma frequency.

Role of laser-induced plasma formation in pulsed cellular microsurgery and micromanipulation.

We investigate experimentally the physical processes underlying pulsed cellular microsurgery and micromanipulation using nanosecond 532- and 1064-nm laser pulses focused at high numerical aperture. We find that the laser parameters employed for many microirradiation techniques are congruent with those leading to optical breakdown in water. We determine the size and shape of the laser-induced plasma, pressure of the emitted shock wave, and size and energy of the cavitation bubble formed by the expanding plasma. We discuss implications of the results for biophysical microirradiation procedures.

Energy balance of optical breakdown in water at nanosecond to femtosecond time scales

During optical breakdown, the energy delivered to the sample is either transmitted, reflected, scattered, or ab- sorbed. Pathways for the division of the absorbed energy are the evaporation of the focal volume, the plasma radiation, and the mechanical effects such as shock wave emission and cav- itation. The partition of laser energy between these channels during breakdown in water was investigated for four selected laser parameters typical for intraocular microsurgery ( 6-ns pulses of 1 and 10 mJ focused at an angle of 22 ,a nd30-ps pulses of 50 mJ and 1m Jfocused at 14 ,a ll at1064 nm). Scattering and reflection were found to be small compared to transmission and absorption during optical breakdown. The ratio of the shock wave energy and cavitation bubble energy was approximately constant (between 1.5:1 and 2:1). These results allowed us to perform a more comprehensive study of the influence of pulse duration ( 100 fs- 76 ns )a nd focus- ing angle (4- 32) on the energy partition by measuring only the plasma transmission and the cavitation bubble energy. The bubble energy was used as an indicator for the total amount of mechanical energy. We found that the absorption at the breakdown site first decreases strongly with decreasing pulse duration, but increases again for < 3p s. The conversion of the absorbed energy into mechanical energy is 90% with ns pulses at large focusing angles. It decreases both with de- creasing focusing angle and pulse duration (to < 15 %f or fs pulses). The disruptive character of plasma-mediated laser effects is therefore strongly reduced when ultrashort laser pulses are used.

Shock wave emission and cavitation bubble generation by picosecond and nanosecond optical breakdown in water

Shock wave emission and cavitation bubble expansion after optical breakdown in water with Nd:YAG laser pulses of 30-ps and 6-ns duration is investigated for energies between 50 μJ and 10 mJ which are often used for intraocular laser surgery. Time-resolved photography is applied to measure the position of the shock front and the bubble wall as a function of time. The photographs are used to determine the shock front and bubble wall velocity as well as the shock wave pressure as a function of time or position. Calculations of the bubble formation and shock wave emission are performed using the Gilmore model of cavitation bubble dynamics and the Kirkwood–Bethe hypothesis. The calculations are based on the laser pulse duration, the size of the plasma, and the maximally expanded cavitation bubble, i.e., on easily measurable parameters. They yield the dynamics of the bubble wall, the pressure evolution inside the bubble, and pressure profiles in the surrounding liquid at fixed times after the start of the laser pulse. The results of the calculations agree well with the experimental data. A large percentage of the laser pulse energy (up to 72%) is transformed into the mechanical energy ES and EB of the shock wave and cavitation bubble, whereby the partitioning between ES and EB is approximately equal. 65%–85% of ES is dissipated during the first 10 mm of shock wave propagation. The pressure at the plasma rim ranges from 1300 MPa (50 μJ, 30 ps) to 7150 MPa (10 mJ, 6 ns). The calculated initial shock wave duration has values between 20 and 58 ns, the duration measured 10 mm away from the plasma is between 43 and 148 ns. A formation phase of the shock front occurs after the ns pulses, but not after the ps pulses where the shock front exists already 100 ps after the start of the laser pulse. After shock front formation, the pressure decays approximately proportional to r−2, and at pressure values below 100 MPa proportional to r−1.06. The maximum bubble wall velocity ranges from 390 to 2450 m/s. The calculations of bubble and shock wave dynamics can cover a large parameter range and may thus serve as a tool for the optimization of laser parameters in medical laser applications.

Application systems for intracorporeal laser-induced shockwave lithotripsy using the Nd:YAG Q-switched laser.

For laser-induced shockwave lithotripsy, the electromagnetic energy of a laser light pulse is converted intracorporeally into the acoustic energy of a shockwave. The lithotriptor is based on a specially developed, Q-switched Nd:YAG laser whose high power light pulses (70 mJ, 25 nsec) are coupled into a flexible quartz fiber with a core diameter of 600 mum. Using focusing elements, energy densities higher than 6 x 10 5 J m -2 can be achieved, resulting in an optical breakdown in water followed by a shockwave. As a result of different absorption mechanisms, the breakdown threshold can be decreased by placing a metallic target into the laser beam. The different shockwave formations of such optomechanical transducers have been measured. First clinical applications have been performed.

Destruction of coarse and fine water drops by monopulses of a ruby laser

In addition to the known method of evaporating water drops in the intensive radiation field of a CO2 laser, papers have recently appeared wherein the destruction of water drops without the conversion of the light energy they absorbed into heat is investigated (surveys [1, 2], for example). Papers devoted to nonthermal methods of destroying a water aerosol, although still few in number, indicate the proposal of three methods of destroying the drops: optical breakdown in water, excitation of mechanical vibrations of the drops, and photochemical destruction of the water molecules [1, 2]. The optical breakdown phenomenon, when intense destructive shocks occur in a water drop subjected to a laser monopulse, has been investigated more fully than the other methods but also clearly insufficiently. Experiments on destroying millimeter- and micron-sized drops by ruby laser monopulses are described in this paper, values of the parameters characterizing this process are determined, and an approximate estimate of the energy and power of the laser pulses required to destroy a water aerosol in a track of definite length is also given.

Acoustooptic beam deflection for spatial frequency multiplexing in high speed holocinematography

Four holograms are recorded superimposed on the same plate at maximum repetition rates of about 10 kHz. A multiply Q-switched ruby laser produces the series of coherent light pulses for hologram exposure. Image separation of different holograms is achieved by spatial frequency multiplexing. The reference beam direction is altered by a unique acoustooptic beam splitter and deflector unit. The operating principle is a sound pulse-light pulse interception technique. Only one ultrasonic transducer is necessary. The quality of the holograms is demonstrated by the reconstructed images taken of laser produced cavitation bubbles following optical breakdown in water.

High speed holocinematography using spatial multiplexing for image separation

Up to eight holograms are successively recorded at maximum repetition rates of 20kHz. A multiply Q-switched ruby laser is used to produce the series of coherent light pulses for hologram exposure. All holograms are recorded on the same plate. Image separation of different holograms is achieved by spatial multiplexing using a rotating disk with apertures directly in front of the holographic plate. The performance of the technique is demonstrated by reconstructions of a hologram series taken of laser produced cavitation bubbles following optical breakdown in water.

Shock wave associated with optical breakdown in water

Shock wave associated with optical breakdown in water