Photoacoustic studies and selective ablation of vascular tissue using a pulsed dye laser

The optical attenuation coefficient for human aorta has been measured in the 440–500 nm spectral region using the transient photoacoustic (thermoelastic) response produced by an excimer-laser-pumped dye laser. It is confirmed that diseased tissue has a significantly higher attenuation than normal samples in this spectral region. The maximum difference occurs at 480 nm and there is a lowering of the ablation threshold fluence for diseased tissue by a factor of 1.5 for dye laser ablation at this wavelength.     

Lifetime measurements using stepwise excitation by two pulsed dye lasers

The 4²D level of sodium was populated in a stepwise excitation process involving two pulsed dye lasers. The lifetime of the 4²D level was derived from the decay of the fluorescence intensity.     

Studies of laser-induced cavitation and tissue ablation in saline using a fibre-delivered pulsed HF laser

We describe studies of the interaction of 400 ns duration, fibre delivered, multiline (2.6–3.0 m) HF-laser pulses with cornea and retina samples in saline. In this wavelength region water exhibits strong absorption (beam penetration depth 1.6 m) and laser heating leads to the creation of a hot, high pressure, vapour cavity (laser-induced cavitation) at the fibre tip. The dynamics of vapour cavity growth have been investigated experimentally using the laser shadowgraph technique and theoretically by employing an equivalent spherical cavity model for an incompressible liquid. Measurements of ablation rates and transient acoustic pressures for cornea ablation in saline, together with scanning electron microscope evaluation of irradiated samples are used to assess the damage range and mechanisms for this mid-IR laser in a strongly absorbing fluid.     

Excitation dependence of the bending mode temperature in pulsed TE CO2 lasers

The observation of “steps” which appear in the gain immediately after excitation in pulsed TE CO₂ discharges is reported. It is shown that the measurement of the step size relative to the peak gain serves to determine the population of the lower laser level and thereby, the initial vibrational temperature (T₁) of the bending mode. This technique is applied to a study of T₁ as a function of the discharge input energy.     

Wavelength tunability of L-band fiber ring lasers using mechanically induced long-period fiber gratings

We report on oscillation wavelength control in erbium-doped fiber ring lasers by adjusting the period of a mechanically induced long-period fiber grating (LPFG) inserted into the fiber ring resonator. Pump light is provided by a 974 nm laser diode (LD), the emission of which is coupled into the fiber ring resonator through a wavelength-division multiplexing coupler. Laser oscillation occurs with a threshold pump LD current of 40 mA, corresponding to a threshold pump power of 5 mW. When a periodic pressure of 0.81 N/mm is applied to form the LPFG, the fiber ring laser exhibits the tunable range of 40.9 nm, i.e., from 1563.1 to 1604 nm, by changing the grating period.     

Wavelength beam combining of ytterbium fiber lasers

Wavelength beam combining of five ytterbium fiber lasers is demonstrated in a master-oscillator power-amplifier configuration at combined powers up to 6 W. The combined beam profile has an M2 value of 1.14, which is equal to that of an individual fiber. Beam steering in one dimension over 140 resolvable spots is also demonstrated.     

Thermo-mechanical laser ablation of soft biological tissue: modeling the micro-explosions

Characteristics of thermo-mechanical laser ablation process are investigated using an original numerical model. In contrast with previous models, it is based on a microscopic physical model of the micro-explosion process, which combines thermodynamic behavior of tissue water with elastic response of the solid tissue components. Diffusion of dissipated heat is treated in one dimension, and the amount of thermal damage is assessed using the Arrhenius model of the protein denaturation kinetics. Influence of the pulse fluence and duration on temperature profile development, ablation threshold, and depth of thermal damage is analyzed for the case of Er:YAG laser irradiation of human skin. Influence of mechanical properties on the ablation threshold of soft tissue is predicted theoretically for the first time. In addition, feasibility of deep tissue coagulation with a repetitively pulsed Er:YAG laser is indicated from the model. Received: 9 July 1998 / Revised version: 26 February 1999 / Published online: 26 May 1999     

Wavelength-versatile subpicosecond pulsed lasers using Raman gain in figure-of-eight fiber geometries

A family of compact pulsed fiber lasers is described that employs broadband, wavelength-flexible Raman scattering in passively mode-locked figure-of-eight fiber laser geometries. Specifically, sources at 1.57, 1.33, and 1.41 microm with respective soliton durations of 440, 500, and 860 fs are reported. Operation is possible at other wavelengths with a suitable pump source, gain fiber, and components.     

Wavelength dependence of transient photovoltage polarity

Using pulsed laser with different wavelengths, transient photovoltage (TPV) is investigated for the sandwich device of indium tin oxide (ITO)/poly (2-methoxy-5-(2′-ethylhexyloxy)-1, 4-phenylenevinylene) (MEH-PPV) (500 nm)/Al. At laser wavelength of 500 nm, a 0.1 millisecond negative TPV signal appears followed by a positive one lasting 40-80 milliseconds. With increasing laser wavelength, the negative signal becomes weak and disappears when wavelengths λ?560 nm. This work demonstrates the process of exciton dissociation at the interface between ITO and MEH-PPV. A new method measuring the relative dissociation rate at the interface between polymer and electrode is introduced. It is estimated that the dissociation rate at Al interface is 4-8 times of that at the ITO interface.     

Spectral purity of pulsed dye lasers

High-spectral purity of pulsed dye lasers is required by many experiments. Unfortunately, the dye laser output is usually accompanied by a spectrally broadband background of spontaneous emission (ASE). We have carried out detailed studies of the ASE background using a dye laser operating with an oscillator, pre- and main amplifier.Dedicated to Prof. Dr. Herbert Welling on the occasion of his 60th birthday     

Transient behaviour of pulsed dye lasers

Some aspects of the transient behaviour of pulsed dye lasers are investigated both theoretically and experimentally. Relaxation oscillations induced in the output from a dye cell by an external reflector have been observed and explained, using a rate equation approximation to the physical situation in the dye cell. These oscillations are shown to play an important part in the generation of short laser pulses from long cavity dye lasers. Finally, it is proposed that 10 ps pulses can be obtained from conventional dye lasers pumped by 100 ps pulses from a high-pressure nitrogen laser.     

Carbon nanocrystals produced by pulsed laser ablation of carbon

Plasma laser ablation experiments were performed irradiating glassy-carbon targets placed in vacuum through a pulsed Nd:YAG laser operating at the second harmonic (532 nm), 9 ns pulse width and 10⁹ W/cm² density power.

Thin films of ablated carbon were deposited on silicon oxide substrates placed at different distances and angles with respect to the target.

The analysis of the deposited material was carried out by using surface profiler, scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) and Raman spectroscopy.

Results show the evidence of carbon nanocrystals and nanostructures with dimension of the order of 100 nm deposited on the substrates together with a large amount of amorphous phase. The spectroscopic investigations and the SEM images indicate the formation of nanodiamond seeds as a nucleation process induced on the substrate surface. Nanostructures were investigated as a function of the laser intensity and angle distribution. Experimental results were compared with the literature data coming from nanodiamonds growth with different techniques.

Experiments performed at Instituto Nazionale di Fisica Nucleare-Laboratori Nazionali del Sud (INFN-LNS) of Catania (Italy) and data analysis conducted at Dipartimento di Fisica and DFMTA of the Università of Messina (Italy), CNR-ITIS of Messina and ST-Microelectronics of Catania will be presented and discussed.     

Non-equilibrium plasma production by pulsed laser ablation of gold

A gold target has been irradiated with a Q-switched Nd:Yag laser having 1064?nm wavelength, 9?ns pulse width, 900?mJ maximum pulse energy and a maximum power density of the order of 10¹⁰?W/cm². The laser–target interaction produces a strong gold etching with production of a plasma in front of the target. The plasma contains neutrals and ions having a high charge state. Time-of-flight (TOF) measurements are presented for the analysis of the ion production and ion velocity. A cylindrical electrostatic deflection ion analyzer permits measurement of the yield of the emitted ions, their charge state and their ion energy distribution. Measurements indicate that the ion charge state reaches 6⁺ and 10⁺ at a laser fluence of 100?J/cm² and 160?J/cm², respectively. The maximum ion energy reaches about 2?keV and 8?keV at these low and high laser fluences, respectively. Experimental ion energy distributions are given as a function of the ion charge state. Obtained results indicate that electrical fields, produced in the plume, along the normal to the plane of the target surface, exist in the unstable plasma. The electrical fields induce ion acceleration away from the target with a final velocity dependent on the ion charge state. The ion velocity distributions follow a “shifted Maxwellian distribution”, which the authors have corrected for the Coulomb interactions occurring inside the plasma.     

Bulk ablation of soft tissue with intense ultrasound: modeling and experiments

Methods for the bulk ablation of soft tissue using intense ultrasound, with potential applications in the thermal treatment of focal tumors, are presented. An approximate analytic model for bulk ablation predicts the progress of ablation based on tissue properties, spatially averaged ultrasonic heat deposition, and perfusion. The approximate model allows the prediction of threshold acoustic powers required for ablation in vivo as well as the comparison of cases with different starting temperatures and perfusion characteristics, such as typical in vivo and ex vivo experiments. In a full three-dimensional numerical model, heat deposition from array transducers is computed using the Fresnel approximation and heat transfer in tissue is computed by finite differences, accounting for heating changes caused by boiling and thermal dose-dependent absorption. Similar ablation trends due to perfusion effects are predicted by both the simple analytic model and the full numerical model. Comparisons with experimental results show the efficacy of both models in predicting tissue ablation effects. Phenomena illustrated by the simulations and experiments include power thresholds for in vivo ablation, differences between in vivo and ex vivo lesioning for comparable source conditions, the effect of tissue boiling and absorption changes on ablation depth, and the performance of a continuous rotational scanning method suitable for interstitial bulk ablation of soft tissue.     

Deposition of NbC thin films by pulsed laser ablation

Niobium carbide thin films were prepared by pulsed laser ablation of a stoichiometric NbC target. XeCl (308 nm, 30 ns) and Nd:YAG (266 nm, 5 ns) lasers operating at a repetition rate of 10 Hz were used. Films were deposited on Si (100) substrates at room temperature either in vacuum or in an argon atmosphere (2᎒^-1 mbar). Different laser fluences (2, 4 and 6 J/cm²) and different numbers of pulses (1᎒⁴, 2᎒⁴ and 4᎒⁴) were tested. For the first time, NbC films were prepared through a clean procedure without the addition of a hydrocarbon atmosphere. The phase constitution of the films, unit cell size, mean crystallite dimensions and preferred orientation are determined as a function of deposition conditions by X-ray diffraction. Complementary morphological and structural analysis of the films were performed by scanning electron microscopy, atomic force microscopy and Rutherford backscattering spectroscopy.     

Plasma dynamics and determination of ablation parameters using the near-target magnified imaging during pulsed laser ablation

A comparative study of the ultrafast dynamics on ZnO thin films deposited on flat Si substrates and on Si micro-cones following ultrashort laser excitation has been carried out using time-resolved optical spectroscopy. By monitoring the transient band gap renormalization induced by nonlinearly excited carriers it is found that fast electron scattering and trapping occurs more efficiently in the micro-cones as compared to the flat films. This enhanced trapping efficiency is attributed to the defects and imperfections that are introduced by the increased surface roughness due to the conical shape.     

Non-equilibrium plasma production by pulsed laser ablation of gold

A gold target has been irradiated with a Q-switched Nd:Yag laser having 1064\,\hbox{nm} wavelength, 9\,\hbox{ns} pulse width, 900\,\hbox{mJ} maximum pulse energy and a maximum power density of the order of 10^{10}\,\hbox{W}/\hbox{cm}^2 . The laser-target interaction produces a strong gold etching with a production of a plasma in front of the target. The plasma contains neutrals and ions having high charge state. Time-of-flight measurements are presented for the analysis of the ion production and ion velocity. A cylindrical electrostatic deflection ion analyzer permits to measure the yield of the emitted ions, their charge state and their ion energy distribution. Measurements indicate that the ion charge state reaches 6^+ and 10^+ at a laser fluence of 100\,\rm{J/cm}^2 and 160\,\rm{J/cm}^2 , respectively. The maximum ion energy reaches about 2\,\hbox{keV} and 8\,\hbox{keV} at these low and at high laser fluence, respectively. Experimental ion energy distributions are given as a function of the ion charge state. Obtained results indicate that electrical fields, produced in the plume, along the normal to the plane of the target surface, exist in the unstable plasma. The electrical fields induce ion acceleration away from the target with a final velocity dependent on the ion charge state. The ion velocity distributions follow a "shifted Maxwellian distribution", which the authors have corrected for the Coulomb interactions occurring inside the plasma.