Giant Laser Research Articles

Absorption of laser radiation in a surface layer of a transparent dielectric

A theoretical analysis is made of an avalanche-like multiplication of electrons in a surface layer of a transparent dielectric which occurs in a strong optical-frequency field. The influence of this process on the surface absorption of the energy of a giant laser pulse is considered. It is found that during the initial cold stage of the damage of the surface by such a pulse (i.e., before melting and formation of a plasma on the entry face) the absorption process is dominated by the avalanche-like multiplication of electrons. It is shown that pulses of ~5108 W/cm2 power density and ~30 nsec duration heat a surface layer ~1 μ deep. The proportion of the energy absorbed during this stage is 1%.

Supersonic crack propagation in blocks of polymethyl methacrylate

It is shown that a giant laser pulse can cause supersonic crack propagation in PMMA. The results of the experiments and a microstructural analysis of the fracture surface are presented. A study of the microstructure shows that supersonic crack propagation is associated with the propagation of a shock wave in the focal region.

Band-to-band Auger Recombination in Silicon and Germanium

High concentrations of electrons and holes in silicon and germanium have been achieved by optical injection by a giant pulse ruby laser. The kinetics and spectral distribution of the recombination radiation during and after the injection pulse have been analyzed by a theoretical model which takes into account the heating of the sample by the laser pulse as well as the decrease of the band gap which is due to the high carrier concentration. The decay of the recombination radiation signal shows that a third-order recombination process (band-to-band impact or Auger recombination) is predominant at high concentrations. The value of the Auger recombination coefficient γ3 ≃ 10-31 - 4 × 10-30 cm6/s is obtained for silicon in the temperature interval 300 K-800 K.

Changes in the Characteristics of Giant Laser Radiation Pulses and of Luminous Plasma during Formation of Damage Regions on the Surface or in the Bulk of Transparent Dielectrics

An investigation was made of changes in the characteristics of gaint laser pulses and of luminous plasma in the course of formation of damage regions on the surfaces or in the bulk of transparent K-8, LK-5, and TF-5 glasses and of NaCl, KCl, and KBr crystals. For pulses of 15–40 nsec duration the decisive factor in the formation of the damage regions was the incident radiation power, whereas the ambient atmosphere affected the damage process only at pressures exceeding 50 mm Hg.

Theory of Propagation of Giant Laser Pulses in a Nonlinear Medium

Theory of Propagation of Giant Laser Pulses in a Nonlinear Medium

Spectrum of a giant laser pulse under frequency self-modulation conditions

A theoretical analysis is given of the broadening of the spectrum of a giant laser pulse due to the dependence of the refractive index of the active medium on the light intensity. A calculation is given of the integrated effect during the whole of a giant pulse on the assumption that initially the emission is a Gaussian random process. The theoretical estimates are in agreement with specially carried out experiments.

Population Inversion in Laser Plasmas

We define the conditions for which a population inversion with respect to the ground state, due to recombination at high electron densities, can appear in cold laser plasmas. We point out the interest of giant and short laser pulses for producing such plasmas.

Schlieren-Cinematographic and Holographic Diagnostic of a Laser-Produced Plasma in Xenon

We have studied extensively a gas breakdown in xenon produced by a giant pulse ruby laser with a power <100 MW. Detailed information concerning the structure of the laser plasma formation and of the following expansion has been obtained by different optical methods, including schlieren techniques (single frames and ultra high speed cinematography) and holography. The high quality of the holographic pictures was achieved by the use of a monomode laser. With this we were able to visualize the plasma history and to determine the velocities of the boundary layer and of the blast wave with utmost accuracy. Investigations of the electron density and electron temperature showed that a relaxation time of about 10 nsec is necessary to establish local thermodynamic equilibrium states. After this relaxation time it is then possible to carry out thermodynamic calculations, applying the shock-wave theory, to relate the optically measured expansion velocity with the plasma parameters involved. The mean specific internal energy epsilon , for instance, attained values in excess of 10(12) erg/g which decayed rapidly during the first 100 nsec to about 5. 10(10) erg/g. By comparing the results to theoretical calculations of we obtained a first estimate of the temperature, taking into consideration the partial densities n(j) as well as the partition functions Z(j)((i)) of the xenon atoms, the single-charged ions, and the double-charged ones. Furthermore, a two-step iteration computer program was used to give more detailed and more accurate results on the variations of the pressure, temperature, partial densities, and enthalpy as a function of time.

Теория распространения гигантских лазерных импульсов в нелинейной среде

Теория распространения гигантских лазерных импульсов в нелинейной среде

Laser plasma generation from a deuterium ice pellet freely falling within a superconducting magnetic bottle

An experimental system is described for the production of an isolated plasma blob from a freely falling, isolated deuterium ice pellet by means of a high power giant laser pulse. A falling pellet which has dimensions ranging from 0.1 to 0.4 mm (with 5*1016-3*1018 atoms), is detached from the freezer head by gravity and is captured by the focused high power laser beam pulse after falling freely through about 10 cm. The plasma thus produced in a vacuum chamber is confined in a magnetic cusp or mirror field which is produced by a pair of superconducting coils.

Optical Bleaching and Deviations from Beer—Lambert's Law of Solutions Illuminated by a Ruby Laser. I. Cryptocyanine Solutions

The deviations from Beer—Lambert's law of solutions of cryptocyanine were measured when the solutions were illuminated by a giant ruby laser pulse. The transmission-intensity curves show quite unambiguously that a second absorption from an excited state is needed to explain the experimental results. It is also shown that under steady state conditions it is the number of absorptions and not the detailed level scheme that governs the behavior of the system. The transmitted pulse was shown to be narrower than the incident one in an amount depending on incident intensity. Using the two absorption steady state mechanism, the change of shape could be explained and calculated. The decay rates and absorption cross sections were calculated for two models—the one which assumes absorption from an excited singlet, and the other which assumes absorption from the lowest triplet state. The blue fluorescence of cryptocyanine is explained in terms of our results. A level scheme based on the known spectrum fits with the suggested model.

Investigation of non-linear absorption of light in solutions of cryptocyanine

Non-linear absorption of light in solutions of cryptocyanine in glycerol and methanol at light intensities between 2 × 10 5W/cm 2 and 5 × 10 9W/cm 2 was investigated theoretically and experimentally. Absorption cross sections and lifetimes of the excited states involved in the non-linear absorption process are estimated. Experimental evidence for the stationarity of the process in case of giant laser pulse excitation is given.

Schlieren cinematographic and holographic diagnostic of a laser-produced plasma in xenon

We have studied extensively a gas breakdown in xenon produced by a giant pulse ruby laser with a power <100 MW. Detailed information concerning the structure of the laser plasma formation and of the following expansion has been obtained by different optical methods, including schlieren techniques (single frames and ultra high speed cinematography) and holography. The high quality of the holographic pictures was achieved by the use of a monomode laser. With this we were able to visualize the plasma history and to determine the velocities of the boundary layer and of the blast wave with utmost accuracy. Investigations of the electron density and electron temperature showed that a relaxation time of about 10 nsec is necessary to establish local thermodynamic equilibrium states. After this relaxation time it is then possible to carry out thermodynamic calculations, applying the shock-wave theory, to relate the optically measured expansion velocity with the plasma parameters involved. The mean specific internal energy ∊¯, for instance, attained values in excess of 1012 erg/g which decayed rapidly during the first 100 nsec to about 5 · 1010 erg/g. By comparing the results to theoretical calculations of ∊¯ we obtained a first estimate of the temperature, taking into consideration the partial densities nj as well as the partition functions Zj(i) of the xenon atoms, the single-charged ions, and the double-charged ones. Furthermore, a two-step iteration computer program was used to give more detailed and more accurate results on the variations of the pressure, temperature, partial densities, and enthalpy as a function of time.

Motion and Collision of Plasma Blobs Produced by Giant Pulse Laser in a Transverse Magnetic Field

The properties of plasma created by the 200 MW giant pulse ruby laser irradiation on the lithium plate were investigated. Measurements of the variation of the shape and size of the plasma blob with and without the existence of the uniform magnetic field, its density, temperature, pressure, velocity of the motion and the electric field due to the charge separation were made. The phenomena occurring at the head-on collision of two blobs across the uniform magnetic field were also investigated and confirmed that the electric field could be canceled out at the colliding region.

Spatial coherence of the radiation emitted by a Q-switched ruby laser

The conditions for spatial coherence or noncoherence of a giant laser pulse are considered bearing in mind how the oscillation modes are affected by inhomogeneities in the permittivity or by changes in the permittivity due to the burning of holes in the population inversion. These conditions depend on the initial transverse distribution of the population inversion in the active rod and on the variation in the Q factor during the emission of laser radiation. The conditions were checked experimentally for a laser with a saturable filter excited under soft conditions.

Damage to transparent polymers caused by laser pulses lasting 10?3 and 10?8 sec

A study has been made of the internal damage produced in transparent polymers by laser pulses lasting 10−3 sec (free-generation mode) and 10−8 sec (giant-pulse mode). It has been established that in these cases different mechanisms are responsible for the damage. The action of giant laser pulses on transparent polymers evidently provides the first and so far only means of studying fracture effects associated with the truly instantaneous application of load, i. e., when the load application time is much less than the fracture time.

Heating of Laser Produced Plasmas Generated at Plane Solid Targets I. Theory

Abstract A hydrodynamic model for the heating of a plasma generated by the interaction of an intense giant laser pulse with a plane, solid target is developed. It is shown that in the case of nanosecond light pulses and a finite focal spot diameter, the plasma production may be considered as a steady state problem. Expressions for the electron temperature, the expansion energy of the ions, and the total particle number in the plasma as a function of the incoming light intensity are derived. An estimate of the ion temperature is discussed.

Acousto-optic method of producing giant laser pulses: Editorial Engineers Digest, Vol 30, No 8 (August 1969) p37 (2325)

Acousto-optic method of producing giant laser pulses: Editorial Engineers Digest, Vol 30, No 8 (August 1969) p37 (2325)

Chemical kinetics at the micro level: A laser micro temperature jump apparatus for relaxation studies in micro samples

The kinetics of chemical reactions at equilibrium can be investigated in samples of very small size with a giant laser pulse shifting the state of equilibrium by a temperature jump. The output of a Q-switched neodymium laser is fired into a microcavity of 5 μl. The solvent, which absorbs part of the laser energy, is heated instantaneously and the kinetics of the chemical processes leading to the new state of equilibrium can be followed up. The principles of the present design are based on the particular properties of a giant laser pulse—high energy and short duration, small beam divergence, emission at wavelengths where many solvents already absorb—and permits thereby (a) a very high time resolution, (b) a very small sample size (⩾ 5 μl), and (c) the use of polar as well as apolar solvents, since heating occurs by means of electromagnetic energy absorption instead of Joule-heating due to ionic conduction. The design of the instrument is such that reaction kinetics even in such integrated biological structures as single cells or cell organelles can be studied.

Experimental Investigation of Plasma Production by Irradiating Solid Hydrogen Foils with an Intense Pulse Laser

Abstract Foils of solid hydrogen were irradiated with a giant pulse ruby laser. The formation and ex-pansion of the plasma produced this way was investigated by time-resolved absorption measure-ments, by streak and framing photography, by holographic interferometry and by time-off-flight and charge collection measurements. The measurements yield a fairly complete picture of this process: After the initial breakdown in the solid a plasma is produced at the surface of the foil facing the laser. Owing to the high pressure of the plasma the solid hydrogen is compressed by a shock wave. This allows the layer in which the plasma is produced to penetrate into the interior of the foil with a velocity of 3 × 106 cm/sec. Thus under the action of the focused beam foils up to 1 mm thickness are pierced by the light before the end of the pulse. The plasma cloud, ejected towards the laser, contains 4 × 1016 ions with a mean kinetic energy of 200 eV this corresponding to about half of the absorbed energy. The observations are compared with the well known results obtained in a plane, one-dimensional geometry. If allowance is made for the influence of the focusing of the laser beam on the structure of the shock front, the observations are readily understood.