Nonlinear light absorption and luminescence in silicate glasses

The study of luminescence of high-purity multicomponent silicate glasses excited by radiation of a N₂ laser (λ=337 nm, P≈12kW) showed darkening of a sample and a monotone decrease in luminescence intensity and the transmitted radiation intensity from pulse to pulse. Additional illumination with radiation of an Ar⁺ laser (λ=514.5 nm) caused an increase in luminescence intensity. An increase in absorption and a decrease in luminescence intensity were found to be caused by two-photon absorption and electron-hole pair production.     

Femtosecond petawatt laser

The high‐power femtosecond laser has now become an excellent scientific tool for the study of not only relativistic laser–matter interactions but also scientific applications. The high‐power femtosecond laser depends on the Kerr‐lens modelocking (KLM) and chirped‐pulse amplification (CPA) technique. An all‐Ti:sapphire‐based 30‐fs PW CPA laser, which is called the PULSER (Petawatt Ultrashort Laser System for Extreme Science Research) has been recently constructed and is being used for accelerating the charged particles (electrons and protons) and generating ultrashort high‐energy photon (X‐ray and γ‐ray) sources. In this review, the world‐wide PW‐level femtosecond laser systems are first summarized, the output performances of the PULSER‐I & II are described, and the future upgrade plan of the PULSER to the multi‐PW level is also discussed. Then, several experimental results on particle (electron and proton) acceleration and X‐ray generation in the intensity range of mid‐10¹⁸ W/cm² to mid‐10²⁰ W/cm² are described. Experimental demonstrations for the newly proposed phenomena and the understanding of physical mechanisms in relativistic and ultrarelativistic regimes are highly expected as increasing the laser peak intensity up to over 10²² W/cm² ～10²³ W/cm².     

On optical dispersion in transparent molecular systems

The inelastic optical scattering process leading to the generation of the recently reported new molecular state (being stable at 100 K) of sodium nitroprusside is investigated. The process is a strong function of the phase coherence (Glauber state) of the incident light.-With laser light of 20–200 mW/cm² the yield of new molecular states rapidly reaches saturation, about 50% of all molecules of an extended target. The generation process is a linear function of the product: light intensity × illumination time. The coherent yield function follows a very simple law which, however, is different from predictions of linear or non-linear optics. It is a strong function of the laser frequency (range at least (4900±400) Å) and of the optical polarization relative to the oriented molecules. — The yield obtained with chaotic light is one order of magnitude smaller and weaker dependent upon frequency and polarization. The chaotic yield function depends exponentially upon the light intensity and illumination time in a way as to be expected from linear optics. — The coherently generated new molecular states can be depopulated by chaotic light (or by thermal agitation at elevated temperatures) and coherently regenerated arbitrarily often. There is further evidence that the recently reported properties of the new state are predominantly determined by the internal molecular structure. The lifetimeτ?10⁷ s at 100 K.     

Neutron production in a picosecond laser plasma at a radiation intensity of 3×1017W/cm2

Neutron production as a result of the reaction ²H(d, n)³He in a picosecond laser plasma is reported. A considerable neutron yield of 5×10⁴ per pulse is obtained for the first time in a picosecond laser plasma on the surface of a solid deuterated target at laser radiation intensity of 3×10¹⁷ W/cm².     

Stabilization of porous silicon surface by low temperature photoassisted reaction with acetylene

A hybrid approach of forming a stabilizing layer of bonded carbon on porous silicon (PSi) surface by photoassisted reaction of acetylene gas at low temperatures is described. The PSi samples were made by anodization in a HF/H₂O₂ electrolyte at a current density of 80 mA/cm². Samples show a strong luminescence with a peak at 644±4 nm. The photoluminescence (PL) intensity shows a very strong quenching under the influence of continuous laser illumination (∼0.25 W/cm², 488 nm). The PSi samples were subjected to flowing acetylene under optical illumination from quartz halogen lamp (20 mW/cm²). The PL intensity is initially quenched to very low values (less than 5% of initial value) and then recovers on further exposure to acetylene to a final value ∼30% of the initial value. No quenching is observed on further exposure to laser illumination in ambient air instead an improvement of 15–25% in PL intensity is observed. This behaviour is a good indicator of the formation of a practically stable PSi surface.     

Determination of the radiative lifetime of the d3Πg state of the C2 molecule by the laser-pyrolysis technique. A feasibility study

The feasibility is studied of using the laser-pyrolysis technique for determination of the radiative lifetimes of molecules.Measurements of time-resolved spectral emission were made on spatially resolved sections of the luminous plume produced by laser illumination of a graphite target in a vacuum. The nature of certain emission continua was investigated. The effects on C₂ Swan band emission of focal spot size, illumination power density, height and width of the plasma section observed and its distance from the target were studied. Conditions were determined for measuring physically significant radiative lifetimes for the d³Π_g, v=0 and v=1 levels, of C₂. In particular, the laser flux used was 3.8×10⁸W-cm^?2 while observations were made of a plume section from 0.5 to 0.7 mm from the target. The results, τ(0,0)=235±20 ns, τ(0,1)=250±20 ns, τ(1,0)=255±20 ns, are compared with those obtained by other methods. They are in good agreement with results obtained by pulsed electron excitation. Analysis of the experimental conditions in previous experiments using the laser-pyrolysis technique for measuring τ(d³Π_g of C₂ shows that they could not have lead to meaningful results of purely molecular properties.     

Scaling of x-ray emission and ion velocity in laser produced Cu plasmas

The x-ray emission from slab targets of copper irradiated by Nd:glass laser (1.054 μm, 5 and 15 ns) at intensities between 10¹² and 10¹¹W/cm² has been studied. The x-ray emissions were monitored with the help of high quantum efficiency x-ray silicon photo diodes and vacuum photo diodes, all covered with aluminium filters of different thickness. The x-ray intensity vs the laser intensity has a scaling factor of (1.2–1.92). The relative x-ray conversion efficiency follows an empirical relationship which is in close agreement with the one reported by Babonneau et al. The ion velocities were monitored using Langmuir probes placed at different angles and radial distances from the target position. The variation of the ion velocity with the laser intensity follows a scaling of the form Φ ^β where β ∼0.22 which is in good agreement with the reported scaling factor values. The results on the x-ray emission from Cu plasma are reported.     

Experimental study of nuclear processes in the presence of superstrong fields of a picosecond laser plasma

Nuclear processes in the presence of the superstrong laser fields of a picosecond laser plasma are experimentally studied at a radiation intensity of 2 × 10¹⁸ W/cm² on a Neodim laser setup with a power of 10 TW. Experimental data regarding neutron generation on the surface of a deuterated target (CD₂)_n owing to the thermonuclear fusion ²H(d,n)³He and the neutron generation on the Be target due to the photonuclear reaction ⁹Be(γ,n)2α are presented. Neutron yields Y _n of 10⁶ and 10³ per 4π sr per laser pulse are obtained for the (CD₂)_n and Be targets, respectively. The alpha-particle yield is measured for the first time in the neutron-free thermonuclear reactions ¹¹B + H → 3⁴He in the laser plasma on the surface of the composite B + (CH₂)_n targets. The alpha-particle yield is 10³ per 4π sr per laser pulse.     

Nuclear fusion induced by a super-intense ultrashort laser pulse in a deuterated glass aerogel

A SiO₂ aerogel with absorbed deuterium is proposed as a target for the fusion reaction d + d → He³ + n induced by a superintense ultrashort laser pulse. The multiple inner ionization of oxygen and silicon atoms in the aerogel skeleton occurs in the superintense laser field. All the formed free electrons are heated and removed from the aerogel skeleton by the laser field at the front edge of the laser pulse. The subsequent Coulomb explosion of the deuterated charged aerogel skeleton propels the deuterium ions up to kinetic energies of ten keV and higher. The neutron yield is estimated at up to 10⁵ neutrons per laser pulse for ～200–500 ps if the peak intensity is 10¹⁸ W/cm² and the pulse duration is 35 fs.     

Impact of subthreshold laser defect accumulation on the optical stability of crystals

Optoacoustic means are used to study the kinetics of laser damage accumulation in potassium chloride crystals. This approach allows us to observe the accumulation of changes in KCl crystals when it is irradiated by a series of laser pulses of subthreshold intensity in a variety of actions (alloying, applying an external electric field, UV illumination). Number of laser pulses N _cr required to destroy a specimen at the same density of radiation power depends on density power I as N ~ I ^?4. Results are explained by the formation and accumulation of structural defects during photochemical reactions under the action of laser radiation.     

Optimization of C<Superscript>5+</Superscript> Balmer-α line intensity at 182 Å from laser-produced carbon plasma

Parametric dependence of the intensity of 182 Å Balmer-α line (C⁵⁺; n = 3 → 2), relevant to xuv soft X-ray lasing schemes, from laser-produced carbon plasma is studied in circular spot focusing geometry using a flat field grating spectrograph. The maximum spectral intensity for this line in space integrated mode occurred at a laser intensity of 1.2 × 10¹³ W cm^?2. At this laser intensity, the space resolved measurements show that the spectral intensity of this line peaks at ～1.5 mm from the target surface indicating the maximum population of C⁵⁺ ions (n = 3), at this distance. From a comparison of spatial intensity variation of this line with that of C⁵⁺ Ly-α (n = 2 → 1) line, it is inferred that n = 3 state of C⁵⁺ ions is predominantly populated through three-body recombination pumping of C⁶⁺ ions of the expanding plasma consistent with quantitative estimates on recombination rates of different processes.     

Neutron production in picosecond laser-generated plasma on a be target

Experimental data on neutron production in a plasma generated on a Be target by a picosecond laser of intensity 2 × 10¹⁸ W/cm² are presented. In contrast to previous measurements, a Ta converter is not used in this study to generate γ rays. The neutron yield is equal to 2 × 10³ over a solid angle of 4π steradians per laser pulse. A simultaneous measurement of the maximum energy of hard x rays gave E _γmax ～ 6 MeV, the number of these photons being 5 × 10⁸ over an angle of 4π steradians per laser pulse. The energy distributions of fast electrons and photons are estimated theoretically.     

Generation of characteristic x rays by a terawatt femtosecond chromium-forsterite laser

Characteristic K _α x rays arising when a metallic target is irradiated by femtosecond infrared pulses that are generated by a terawatt chromium-forsterite laser system (1240 nm, 90 mJ, 80 fs) are studied. The absolute yield (up to 3 × 10⁸ photons/sr pulse) and the coefficient of the transformation of laser radiation to K _α radiation (maximum value ≈0.03%) are measured for an iron target. The dependence of the radiation intensity on the angle of incidence of p polarized laser radiation is analyzed. The mechanisms of the production of fast electrons responsible for generating characteristic x rays are discussed.     

Energetic ion generation by Coulomb explosion in cluster gas and a low-density plastic foam with an intense femtosecond laser

The energy distributions of protons emitted from the Coulomb explosion of hydrogen clusters by an intense femtosecond laser have been experimentally obtained. Ten thousand hydrogen clusters were exploded, emitting 8.1-keV protons under laser irradiation of intensity 6 × 10¹⁶W/cm². The energy distributions are interpreted well by a spherical uniform cluster analytical model. The maximum energy of the emitted protons can be characterized by cluster size and laser intensity. The laser intensity scale for the maximum proton energy, given by a spherical cluster Coulomb explosion model, is in fairly good agreement with the experimental results obtained at a laser intensity of 10¹⁶–10¹⁷ W/cm² and also when extrapolated with the results of three-dimensional particle simulations at 10²⁰–10²¹ W/cm². Energetic proton generation in low-density plastic (C₅H₁₀) foam by intense femtosecond laser pulse irradiation has been studied experimentally and numerically. Plastic foam was successfully produced by a sol-gel method, achieving an average density of 10 mg/cm³. The foam target was irradiated by 100-fs pulses of a laser with intensity 1 × 10¹⁸ W/cm². A plateau structure extending up to 200 keV was observed in the energy distribution of protons generated from the foam target, with the plateau shape explained well by Coulomb explosion of lamella in the foam. The laser-foam interaction and ion generation were studied qualitatively by two-dimensional particle-in-cell simulations, which indicated that energetic protons are mainly generated by the Coulomb explosion. From the results, the efficiency of energetic ion generation in a low-density foam target by Coulomb explosion is expected to be higher than in a gas-cluster target.     

Creation of electron-positron plasma with superstrong laser field

We present a short review of recent progress in studying QED effects within the interaction of ultra-relativistic laser pulses with vacuum and e ^? e ⁺ plasma. Current development in laser technologies promises very rapid growth of laser intensities in the near future. Two exawatt class facilities (ELI and XCELS, Russia) in Europe are already in the planning stage. Realization of these projects will make available a laser intensity of ～ 10²⁶?W/cm² or even higher. Therefore, discussion of nonlinear optical effects in vacuum are becoming compelling for experimentalists and are currently gaining much attention. We show that, in spite of the fact that the expected field strength is still essentially less than E _S = m ² c ³/e? = 1.32 · 10¹⁶?V/cm, the nonlinear vacuum effects will be accessible for observation at the ELI and XCELS facilities. The most promissory effect for observation is pair creation by a laser pulse in vacuum. It is shown, that at intensities ～ 5 · 10²⁵?W/cm², creation even of a single pair is accompanied by the development of an avalanche QED cascade. There exists a distinctive feature of the laser-induced cascades, as compared with the air showers arising due primarily to cosmic rays entering the atmosphere. In our case the laser field plays not only the role of a target (similar to a nucleus in the case of air showers) but is also responsible for the acceleration of slow particles. It is shown that the effect of pair creation imposes a natural limit for the attainable laser intensity and, apparently, the field strength E ～ E _S is not accessible for a pair-creating electromagnetic field at all.     

Reflectance measurements on laser-produced plasmas at 0.26 μm

Total and specular reflection from planar Al-targets was measured with a frequency-quadrupled (λ = 0.26 μm) Nd-YAG laser. The intensity on target was 4 × 10¹³ W/cm² with pulse duration 20 ps. Total absorption for near normal incidence was 80%, very little dependence on intensity and pulse duration was found. By varying the polarization and angle of incidence (0°–80°) the characteristic behaviour of resonance absorption was observed.     

Overheated plasma at the surface of a target with a periodic structure induced by femtosecond laser radiation

A periodic structure is induced at the surface of a metal target exposed to a series of p-polarized 200-femtosecond laser pulses with intensity close to the melting threshold of the target material. The period of the structure is determined by the interference between the incident pump wave and the surface electromagnetic wave. Exposure of the obtained structure to the same laser pulse, but with an intensity of ～10¹⁶ W/cm², provides resonant excitation of the surface electromagnetic waves at the plasma-vacuum interface. This leads to an increase in the X-ray output and the temperature of plasma hot electrons.     

Ordered nano-scale domains in lithium niobate single crystals via phase-mask assisted all-optical poling

We report the formation of directionally ordered nano-scale surface domains on the +z face of undoped congruent lithium niobate single crystals by using UV illumination through a phase mask of sub-micron periodicity with an energy fluence between ∼90 mJ/cm² and 150 mJ/cm² at λ = 266 nm. We clearly show here that the UV-induced surface ferroelectric domains only nucleate at and propagate along maxima of laser intensity. Although the domain line separation varies and is greater than 2 μm for this set of experimental conditions, this enables a degree of control over the all-optical poling process.     

Neutral Dissociation of Superexcited Nitric Oxide Induced by Intense Laser Fields

利用波长800 nm、脉宽60 fs、强度0.2 PW/cm²的强激光激发NO分子,随后通过荧光光谱检测到了处于激发态的N和O的中性碎片. 说明强激光导致的多光子激发同样可以引起NO分子的超激发,并且发生中性解离. 采用离子实解离模型,建立了NO分子的超激发态势能曲线. 提出了直接解离和预解离两种解离过程,分别对应生成N^*+O或O^*+N两个通道.     

Nonlinear absorption at 10.6 μm in Hg1−xCdxTe

Near band-edge illumination of Hg_1?xCd_xTe (x ≈ 0.2) by short bursts of CO₂ laser radiation has been shown to lead to highly nonlinear absorption and saturation. For sufficiently high infrared intensities the transmission through an absorber is enhanced more than three orders of magnitude. The effect should be useful in temporal shaping of high intensity CO₂ laser pulses.