Optical-to-acoustic energy conversion efficiency upon interaction of pulsed laser radiation with a liquid: II. Conversion efficiency measurement by holographic interferometry upon acoustooptic interaction

The efficiency of conversion of pulsed CO₂ laser energy to acoustic energy is measured with holographic interferometry. The method suggested is simple and does not require that fringe shifts in the interferogram be converted to local pressure values in the acoustic wave. The efficiency measured for the thermal mechanism of energy conversion is in good agreement with analytical calculations [1] where the temperature dependence of the volume thermal expansion coefficient of water is taken into account.     

Radiation damping effects on the interaction of ultraintense laser pulses with an overdense plasma

A strong effect of radiation damping on the interaction of an ultraintense laser pulse with an overdense plasma slab is found and studied via a relativistic particle-in-cell simulation including ionization. Hot electrons generated by the irradiation of a laser pulse with a radiance of I lambda(2)>10(22) W microm(2)/cm(2) and duration of 20 fs can convert more than 35% of the laser energy to radiation. This incoherent x-ray emission lasts for only the pulse duration and can be intense. The radiation efficiency is shown to increase nonlinearly with laser intensity. Similar to cyclotron radiation, the radiation damping may restrain the maximal energy of relativistic electrons in ultraintense-laser-produced plasmas.     

Intermolecular vibrational relaxation upon multiphoton excitation of triplet molecules by a CO2 laser

Intermolecular vibrational relaxation is studied in mixtures of polyatomic molecules (benzophenone and fluorene) with bath gases after multiphoton excitation of the triplet molecules by CO₂ laser radiation. The dependences of the decay rate and the intensity of laser-induced delayed fluorescence on the laser energy density E _CO2 and pressure P _fg of bath gases are analyzed. They are found to be different for the fast and slow components of delayed fluorescence, which decays nonexponentially. It is shown that a change in the decay rate of the fast fluorescence component with increasing pressure P _fg is governed by the properties of vibration-translation relaxation. The efficiency β of this process is estimated in a broad range of vibrational energies. It is found that β weakly changes with increasing E _vib upon excitation of molecules to high vibrational levels. The features of intermolecular vibrational relaxation at high densities of anharmonically coupled vibrational states are discussed.     

Spectroscopy and energy balance of the Nd<Subscript>2</Subscript>O<Subscript>3</Subscript> selective emission upon the laser thermal excitation

The laser thermal melting of powders is used to fabricate selective emitters (SEs) that represent Nd₂O₃ and Y₂O₃-Nd₂O₃ polycrystals on quartz holders. The SEs are stable under atmospheric conditions upon multiple heating by laser radiation up to the melting point. The spectral shape and integral intensity of the selective heat radiation (SHR) of the Nd₂O₃ microcrystalline powder and the Nd₂O₃ and Y₂O₃-Nd₂O₃ polycrystals are experimentally studied in the near-IR and visible spectral ranges versus the intensity of the laser thermal excitation at a wavelength of 10.6 μm in comparison with the absorption and luminescence spectra of the YAG:Nd³⁺ and YAlO₃:Nd³⁺ single crystals. The SHR spectra are determined by the vibronic transitions between the electronic states ² G _7/2-⁴F_3/2 ⁴I_11/2 and ⁴I_9/2 of the Nd³⁺ ions that are thermally excited due to the multiphonon transitions from the ground state. The energy balance of the SE laser thermal heating is experimentally investigated. The coefficient of the laser energy conversion to the Nd³⁺ SHR is measured, and the emissivity of the SEs that can be used for the study of the thermophotovoltaic generators and the optical excitation of the laser-active media in the near-IR spectral range is estimated.     

Explosive boiling of water induced by the pulsed HF-laser radiation

The surface evaporation and explosive boiling of water induced by the radiation of a nonchain pulsed HF laser are studied using piezoelectric acoustic pressure transducers. The evolution of pressure signals is studied and the relative contributions of thermal (photoacoustic) and evaporation mechanisms to these signals are determined for a wide range of the laser energy densities. A threshold of bulk explosive boiling with respect to laser pulse energy density (W ₀ = 0.23 J/cm²) is determined.     

Study of opto-mechanical characteristics of interaction of ultrashort laser pulses with polymer materials

The opto-mechanical characteristics, such as the specific mechanical recoil momentum, the specific impulse, and the energy efficiency, of the laser ablation of flat polymer targets ((C₂F₄) _n , (CH₂O) _n ) have been determined experimentally for the first time for the case of excitation with femtosecond pulses (τ ∼ 45–70 fs) of UV-IR (λ ∼ 266, 400, 800 nm) laser radiation (I ₀ up to 10¹⁵ W/cm²) under normal atmospheric and vacuum (p ∼ 10⁻⁴ mbar) conditions. The efficiency of mechanical recoil momentum generation is analyzed for various regimes of the laser irradiation.     

Mechanisms for second harmonic generation in the interaction of a superintense ultrashort laser pulse with cluster plasma

The effects of the interaction of an intense femtosecond laser pulse with large atomic clusters are considered. The pulse intensity is of the order of 10¹⁸ W cm^?2. New effects appear when the magnetic component of the Lorentz force is taken into account. The second harmonic of laser radiation is generated. The second harmonic generation (SHG) efficiency is proportional to the square of the number of atoms in a cluster and the square of the laser radiation intensity. The resonance increase in the SHG efficiency at the Mie frequencies (both at the second harmonic frequency and fundamental frequency) proved to be insignificant because of the fast passage through the resonance during cluster expansion. The mechanisms of the expansion and accumulation of energy by electrons and ions in the cluster are discussed in detail. The energy accumulation by electrons mainly occurs due to stimulated inverse bremsstrahlung upon elastic reflection of the electrons from the cluster surface. The equations describing the cluster expansion take into account both the hydrodynamic pressure of heated electrons and the Coulomb explosion of the ionized cluster caused by outer shell ionization. It is assumed that both inner shell and outer shell ionization is described by the over barrier mechanism. It is shown that atomic clusters are more attractive for the generation of even harmonics than compared to solid and gas targets.     

High precision materials processing using a novel Q-switched CO2 laser

Holes with diameters of about 400 µm have been laser trepanned in Ti6Al4V and carbon fibre reinforced polymer (CFRP) thin sheets with a thickness of 0.5 mm. A commercial CO₂ laser (SM1500E, FEHA LaserTec, Germany) and a novel Q-switched CO₂ laser (µ-storm, IAI, Netherlands) were used as radiation sources. Optical microscopy, scanning electron microscopy and replicas of the processed holes were used to investigate the influence of the CO₂ laser pulse parameters (e.g. pulse energy, duration and peak power) on the processing quality. It was shown that melt formation and high temperature oxidation reactions of Ti6Al4V during thermal laser processing were reduced significantly by using short and high intense Q-switched CO₂ laser pulses. During trepanning of CFRP heat affected zones resulting from the extremely different thermal properties (melting and vaporisation temperature, heat conduction) of the reinforcing carbon fibres and the polymer matrix were reduced significantly by using the Q-switched CO₂ laser. The results demonstrate that Ti6Al4V and CFRP can be processed very precisely with CO₂ laser radiation and air as processing gas without melt formation and thermal damage.     

Photostimulated detection of radiation defects produced by VUV light in BaF2

Production of radiation defects in a widely used scintillation material BaF₂ has been studied by means of a combination spectroscopy of synchrotron radiation (SR) and laser, in which defects produced by SR irradiation are sensitively detected by observing the luminescence stimulated by laser light. The photostimulated luminescence arises from the recombination of self-trapped holes (V_K centers) with electrons released from trapped centers by laser light. The obtained result reveals that the production efficiency of radiation defects is drastically dependent on the excitation photon energy of valence or core excitons.     

Nonlinear diffraction in a quantum-dot system with allowance for the Hubbard interaction

The propagation of monochromatic laser radiation in a volume system of quantum dots (QDs) that are tunnel-coupled along one axis is considered. The electron energy spectrum of the QD system is modeled in the tight-binding approximation with allowance for the Coulomb interaction of electrons in the Hubbard model. The electromagnetic field of laser radiation in a QD system is described quasi-classically by Maxwell equations; as applied to this problem, they are reduced to a non-one-dimensional wave equation for the vector potential. As a result of the analysis of the wave equation in the approximation of varying amplitudes and phases, an effective equation describing the electromagnetic field in a QD system is obtained and numerically solved. The influence of the parameters of the system and the amplitude and frequency of the field of incident laser radiation on the character of its propagation is investigated. Nonmonotonic dependences of the factor characterizing the laser beam diffraction spread on the parameters of the electron energy spectrum of the system are obtained.     

Near QED regime of laser interaction with overdense plasmas

Interaction of laser plulses with intensities up to 10²⁵?W/cm² with overdense plasma targets is investigated via three-dimensional particle-in-cell simulations. At these intensities, radiation of electrons in the laser field becomes important. Electrons transfer a significant fraction of their energy to γ-photons and obtain strong feedbacks due to radiation reaction (RR) force. The RR effect on the distribution of laser energies among three main species: electrons, ions and photons is studied. The RR and electron-positron pair creation are implemented by a QED model. As the laser intensity inreases, the ratio of laser energy coupled to electrons drops while the one for γ-photons reaches up to 35%. Two distinctive plasma density regimes of the high-density carbon target and low-density solid hydrogen target are identified from the laser energy partitions and angular distributions of photons. The power-laws of absorption efficiency versus laser intensity and the transition of photon divergence are revealed. These show enhanced generation of γ-photon beams with improved collimation in the relativistically transparent regime. A new effect of transverse trapping of electrons inside the laser field caused by the RR force is observed: electrons can be unexpectedly confined by the intense laser field when the RR force is comparable to the Lorentz force. Finally, the RR effect and different regions of photon emission in laser-foil interactions are clarified.     

Generation of high-energy negative hydrogen ions upon the interaction of superintense femtosecond laser radiation with a solid target

The formation of a high-energy (～35 keV) beam of negative hydrogen ions was observed in the expanding femtosecond laser plasma produced at the surface of a solid target by radiation with an intensity of up to 2× 10¹⁶ W/cm². The energy spectra of the H⁺ and H^?-ions show a high degree of correlation.     

Simulation of THz emission from laser interaction with plasmas

One-dimensional particle-in-cell (PIC) program is used to simulate the generation of high power terahertz (THz) emission from the interaction of an ultrashort intense laser pulse with underdense plasma. The spectra of THz radiation are discussed under different laser intensity, pulse width, incident angle and density scale length. High-amplitude electron plasma wave driven by a laser wakefield can produce powerful THz emission through linear mode conversion under certain conditions. With incident laser intensity of 10¹⁸ W/cm², the generated emission is computed to be of the order of several MV/cm field and tens of MW level power. The corresponding energy conversion efficiency is several ten thousandths, which is higher then the efficiency of other THz source and suitable for the studies of THz nonlinear physics.     

A Model for Extreme Ultraviolet Radiation Conversion Efficiency From Laser Produced Mass-Limited Tin-Based Droplet Target Plasmas

Simple arguments are used to construct a model to explain the extreme ultraviolet radiation conversion efficiency(EUV-CE) of a tin-based droplet target laser produced plasmas by calculating the laser absorption efficiency,radiation efficiency,and spectral efficiency.The dependence of drive laser pulse duration and laser intensity on EUV-CE is investigated.The results show that at some appropriate laser intensity,where the sum energy of the thermal conduction,out-off band radiation and plasma plume kinetic losses is at a minimum,the EUV-CE should reach a maximum.The EUV-CE predicted by the present simple model is also compared with the available experimental and simulation data and a fair agreement between them is found.     

The influence of vibrational excitation on the burning voltage of a pulsed electric discharge in HF laser working media

The burning voltages of an intermediate pressure self-sustained volume discharge (SSVD) in SF₆ and SF₆-C₂H₆ mixtures irradiated by a 10.6 μm pulse TEA CO₂ laser, have been measured on varying the laser fluences over a wide range. The delay between the voltage application and the laser pulse onset is 4 μs, and the laser pulse lasts ∼3 μs. The considerable rise observed in the discharge voltages with increasing absorbed specific laser radiation energy, is due to electron attachment to vibrationally excited molecules of SF₆. Different processes of relaxation of the vibrational energy stored in SF₆ molecules are analyzed and the relevant characteristic times are numerically assessed. The gas heating process owing to vibration-translation energy exchange is qualitatively described in terms of the “thermal explosion”. The relation between the “explosion” and delay times determines the peculiarities of electron attachment to vibrationally excited SF₆ molecules. The burning voltages of a submicrosecond non-irradiated SSVD in the above-mentioned media versus the specific electric energy deposited are also measured. They are compared to those of a laser-illuminated SSVD at commensurable specific laser energy depositions. It is concluded that electron attachment to the discharge-produced vibrationally excited SF₆ molecules is not capable of noticeably affecting the discharge voltages of a submicrosecond non-irradiated SSVD. PACS 42.55 Ks; 52.80     

Improving the time-averaged thermal efficiency of a laser beam by acoustooptic correction of the directional pattern

Applicability of the acoustooptic method for raising the time-averaged thermal efficiency of laser radiation is substantiated theoretically and confirmed experimentally. The effect produced by laser radiation on materials being processed (laser cutting, welding, engraving, etc.) has a threshold in light intensity. Importantly, a beam with the most frequently used normal (Gaussian) angular distribution of intensity is not optimal from the technological viewpoint. A method proposed for its optimization is based on acoustooptic refraction, i.e., fast nonlinear scanning of the initial beam around its central position, which improves (at certain values of the parameters) the time-averaged angular distribution of the beam intensity. In the experiment, the thermal efficiency of laser radiation is raised by several times.     

Surface modification of the Cr-based coatings by the pulsed TEA CO2 laser

The interaction of a transversely excited atmospheric (TEA) CO₂ laser with chromium oxynitride (CrON) coating deposited on a AISI 304 steel substrate was considered. The results have shown that CrON was surface-modified by the laser beam of 45 J/cm² energy density. The energy absorbed from the TEA CO₂ laser beam was partially converted into thermal energy, which has generated a series of effects such as melting, vaporization of the molten material, and shock waves in the vapor and in the solid. Morphological manifestations on the CrON coating surface can be summarized as follows: non-uniform features with ablation and appearance of crater-like form (central zone of interaction); appearance of three damaged areas and presence of hydrodynamic effects with resolidified droplets (periphery zone of interaction). In case of applied energy density the interaction of laser radiation with CrON has been always followed by plasma creation in front of the coating. PACS 79.20.Ds; 61.80.Ba     

Energy of a shock wave generated in different metals under irradiation by a high-power laser pulse

The energies of a shock wave generated in different metals under irradiation by a high-power laser beam were determined experimentally. The experiments were performed with the use of targets prepared from a number of metals, such as aluminum, copper, silver and lead (which belong to different periods of the periodic table) under irradiation by pulses of the first and third harmonics of the PALS iodine laser at a radiation intensity of approximately 10¹⁴ W/cm². It was found that, for heavy metals, like for light solid materials, the fraction of laser radiation energy converted into the energy of a shock wave under irradiation by a laser pulse of the third harmonic considerably (by a factor of 2–3) exceeds the fraction of laser radiation energy converted under irradiation by a laser pulse of the first harmonic. The influence of radiation processes on the efficiency of conversion of the laser energy into the energy of the shock wave was analyzed.     

Interaction of an Optical Discharge with a Shock Wave

The results of an experimental study of the impact of the focused pulsed-periodic radiation from a CO₂ laser on a gas-dynamic structure in a supersonic jet are presented. The radiation of the CO₂ laser is propagated across the stream and focused by a lens on the axis of the supersonic jet. To register the flow structure, a shadow scheme with a slit and a flat knife located along the flow is used. The image is fixed by a speed camera with an exposure time of 1.5 μs and a frame rate of 1000 1/s. In the flow, the plasma initiated by the pulsedperiodic laser is visualized in order to identify and determine the period of plasma development, as well as the motion of the initial front of the shock wave. It is shown that at the transverse input of laser radiation into the stream the periodic structure of the thermal trace is created with the formation of an unsteady shock wave from the energy release zone. At small repetition rates of laser radiation pulses, the thermal spot interacts with the flow in the pulsed mode. It is shown that elliptic nonstationary shock waves are formed only at low subsonic flow velocities and in a stationary atmosphere. The process of nonstationary ignition by an optical discharge of a methane–air mixture during a subsonic outflow into a motionless atmosphere is shown experimentally. The results of optical visualization indicate burning in the trace behind the optical discharge region.