Energy Density Of Laser Radiation Research Articles

Neutralisation of parasitic photorefraction in a laser with electrooptic negative feedback

The boundaries of the stable operation region were determined experimentally for an yttrium laser with active negative feedback and a photorefractive electrooptic control component in the resonator. The relationship was established between the boundaries of the stability region and photorefractive parameters of the electrooptic cell, feedback coefficient, integration constant of the signal in the feedback loop, and the laser radiation power. When the integration constant exceeded 2×10-8 s, tuning to single-pulse lasing was of resonant nature, i.e. the single-pulse output power was a function of the negative feedback coefficient, confirming the results of analytic predictions. The conditions were found under which single-pulse lasing occurred in a wide range of pump intensities (the maximum experimentally determined energy density of laser radiation inside the resonator was 30 J cm-2). The effectiveness of electronic tuning of the duration of single pulses was demonstrated. The experiments were carried out on a commercial Kvant-15 laser and the result could be used in the modernisation of technological laser equipment.

LASER ABLATION OF POLYMERS, CAUSED BY MECHANICAL STRESSES

Ablation of polymethylmethacrylate (PMMA) runs effectively when the polymer is affected with pulsed laser radiation in ultra-violet range of electron absorption at the wavelength λ shorter than 260 nm. This effect only takes place when the energy density of laser radiation Φ exceeds a certain threshold. The model of ablation of PMMA proposed in this work takes into account the processes which run under UV irradiation of polymer: photochemical breaking of bonds, heating of polymer film by laser radiation, as well as the change of absorption coefficient (self-darkening of PMMA) and the sharp increase of volume of molecules due to the photochemical reaction. The increase of volume of polymer molecules induces mechanical stresses. Its temporal dependence is rather complicated, namely, in addition to the wave of loading-unloading the quasi-static mechanical stress appears in the film. This component of mechanical stress is the main factor that leads to breaking of bonds of PMMA macromolecules, thus causing ablation. We have obtained the calculated dependence of threshold energy density Φ ° on wavelength λ which agrees well with literature experimental data and those we have obtained. The model developed in this work was used to determine the range of parameters (in the scale of energy density versus wavelentgh) for which the effect of self-darkening is essential Φ c (λ). During a pulse of excimer laser, the polymer gets heated to the depth of penetration of radiation. Then this region cools down due to thermal conductivity. Since the time of cooling is rather long, the polymer melting Φ m (λ) is lower than Φ ° (λ). Accordidng to the estimates, melting is only essential for radiation with λ > 235 nm. Really, radiation of ArF-laser (λ=193 nm) leaves clean surface of PMMA after ablation, while KrF-laser (λ=248 nm) leaves traces of melting. The model of ablation of PMMA allowed us to consider the question about the ultimate spatial resolution which can be achieved no matter what optical system is used to shape the laser beam. It was shown that the limit of spatial resolution of ablation of PMMA is △≃γ -1 /π, where γ is the coefficient of light absorption. The dependence △=△(λ) was analyzed.

Infrared laser driven degradation of polytetrafluoroethene

CO 2 laser induced degradation of polytetrafluoroethene yields gaseous tetrafluoroethene, hexafluoropropene and octafluorocyclobutane along with solid polytetrafluoroethene deposited on reactor surface. All the products except octafluorocyclobutane are suggested to arise from primary cleavage of the polymer and the relative amounts depend on incident energy density of laser radiation and added gases. The characterization of solid deposit by scanning electron microscopy and adsorption measurements is presented.

Inverse electronic relaxation in the case of multiphoton excitation of molecules by infrared laser radiation

The first systematic account is presented of the current status of the problem of inverse electronic relaxation under the conditions of multiphoton excitation with infrared laser radiation, which converts a strong vibrational excitation of a molecule into an electronic excitation and gives rise to ultraviolet or visible radiation. This review deals with the fundamental principles of the appearance of inverse electronic relaxation in isolated molecules. An analysis is made of the characteristics of visible radiation generated by this relaxation process. Current ideas are used in a detailed investigation of those characteristics of infrared multiphoton excitation which determine inverse electronic relaxation. An account is given of the recently developed experimental methods for attribution of the observed radiation to a specific particle (molecular fragment). These methods include time-of-flight spectroscopy and separation of successive stages of the emission process in accordance with the laser radiation energy density. A critical analysis is made of the numerous experimental observations of collisionless visible radiation resulting from multiphoton excitation by infrared laser radiation. Some of the most thoroughly investigated molecules are used to consider the cases of inverse electronic relaxation resulting from multiphoton excitation of specific molecules and radicals, particularly SO2 and OsO4. Apart from inverse electronic relaxation, attention is given to another nonadiabatic process occurring in the case of infrared multiphoton excitation, which is the detachment of electrons from molecular ions induced by CO2 laser radiation. The review covers the work published up to September 1986.

Boron isotope separation by multiphoton dissociation of BCl3in a two-frequency infrared laser field

A determination was made of the spectral dependences of the yield and selectivity of multiphoton dissociation of BCl3 in a two-frequency infrared laser field. The high yield of multiphoton dissociation (~20% measured in terms of 10B with a selectivity α = 4.5) was achieved for a considerable reduction in the total energy density of laser radiation compared with the one-frequency dissociation process.

Influence of the laser pulse shape on plasma formation near absorbing targets

An experimental justification is given for an expression relating the energy density of laser radiation interacting with a target to the time of the initial formation of plasma, and a simple formula following from it is proposed for estimating this time as a function of the shape of the leading edge of the laser pulse and of the maximum density of the high-power laser radiation. The proposed formula enables a detailed analysis to be made of the results of measurements, obtained under differing experimental conditions, of the formation time of a laser plasma.

Variation of the absorption cross section of high-power infrared laser radiation in homologous series of CnH2n+1OH molecules

An experimental investigation was made of variation of the characteristics of infrared multiphoton absorption in a homologous series of CnH2n+1OH alcohols (n = 1–5) excited with CO2 laser pulses. The dependences of the energy absorbed by the molecules on the frequency and energy density of laser radiation were determined by the optoacoustic method. It was found that the multiphoton absorption cross section decreases on increase in the radiation energy density at a rate which becomes slower on increase in the molecular size. A model is proposed for multiphoton excitation of molecules in a homologous series. This model is based on an analysis of a resonant mode interacting with the infrared radiation field and coupled to a reservoir of modes that do not interact with the field. The model predicts correctly the change in the multiphoton absorption cross section on increase in the number of the degrees of freedom of a molecule.

Resonant multistage ionization of the iodine molecule

A scheme for resonant three-stage ionization of the I2 molecule was proposed and realized. A tunable dye (rhodamine 6G) laser and a nitrogen laser were used. The dependences of the ion yield on the dye laser wavelength and on the laser radiation energy density were determined.

Interaction of a laser-breakdown plasma with a charged metallic target

A study was made of the influence of a laser plasma on the potential of an insulated conducting target. It was discovered that the target potential changed stepwise on illumination with a high-power TEA CO2 laser. A step, ΔU, in the target potential was observed and the dependences of ΔU on the initial potential, laser radiation energy density, and geometrical dimensions of the illuminated region were determined. There was an optimal pressure of the surrounding air for which ΔU had the maximum value. The dependence of ΔU on the pressure was determined, when illuminating a target in air and in nitrogen. The temporal characteristics of the variation in ΔU were correlated with the time variation of the visible and ultraviolet luminescence from the plasma. A mechanism was proposed to explain the potential step accompanying the interaction of a laser-produced plasma with a charged metallic target.

Multiphoton absorption and dissociation of hexafluoroacetone in a strong infrared CO2laser field

An investigation was made of the multiphoton absorption and dissociation of the hexafluoroacetone (COC2F6) molecule excited by a TEA CO2 laser. The dissociation threshold was determined as 0.7 J/cm2 and the dissociation products were identified. The dependence of the dissociation probability and the average number of absorbed photons per single molecule on the laser radiation energy density were determined. It was found that the multiphoton absorption and dissociation maximum was shifted toward the red by approximately 15 cm–1 from the linear absorption maximum. The prospects for using hexafluoroacetone for separation of carbon isotopes is discussed.

Grain size dependence in a self-implanted silicon layer on laser irradiation energy density

The transformation of amorphous Si layers to polycrystalline material induced by Q-switched ruby laser single pulses of 20 and 50 nsec duration has been investigated. The analysis has been performed by transmission electron microscopy and by channeling measurements using 2.0-MeV He+ Rutherford backscattering. The average grain size of the polycrystalline layers increases with the incident energy density of the laser pulse in the range 0.6–1.7 J/cm2. A transition to single-crystal layers is found for incident energy densities around 2.0 J/cm2. The grain size correlates with incident energy density (J/cm2) rather than incident power density (MW/cm2) for these pulse durations.

Optical breakdown of transparent media with random inhomogeneities

A theoretical analysis is given of the influence of the overlap of temperature fields created by isolated inhomogeneities on the probability of optical breakdown of a transparent medium as a result of thermal instability. It is shown that the required probability has a number of universal properties governed by the average number of inhomogeneities present in the focal region and by the asymptotic behavior of the size distribution of the larger inhomogeneities. General expressions are obtained for determining in each concrete case the explicit dependence of the breakdown probability on the laser radiation energy density. It is shown that, in general, inhomogeneities cannot be replaced by an effective continuous background even when their concentration is high because the breakdown probability decreases slowly (as a power law) in the range of low laser radiation intensities.

Device for measuring the energy density of laser radiation with the aid of a ferromagnetic element

Device for measuring the energy density of laser radiation with the aid of a ferromagnetic element