Peak Laser Intensity Research Articles

Theory of Laser Induced Fusion in Compound Boron‐Hydrogen Microdroplets

AbstractThe yield of alpha‐particles in nuclear fusion reaction between 11B nuclei and protons in solid microdroplets induced by linearly polarized picosecond laser pulses with high laser contrast is derived analytically. The vacuum (Brunel) heating is the main mechanism for electron heating in overdense plasma. Induced inverse bremsstrahlung can be neglected in the electron heating. Nuclei acquire their kinetic energy due to ambipolar diffusion together with heated electrons at the plasma hydrodynamic expansion with the speed of the ionic sound. The yield of alpha‐particles per one microdroplet of 1¹m size is » 103 for Nd:glass laser with the peak laser intensity of 2 ¢ 1018 W/cm2 and the pulse duration of 1:5 ps. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dependence upon the molecular and atomic ground state of higher-order harmonic generation in the few-optical-cycle regime

High-order harmonic generation from molecules (${\mathrm{O}}_{2}$, ${\mathrm{N}}_{2}$, ${\mathrm{H}}_{2}$, and $\mathrm{C}{\mathrm{O}}_{2}$) and atoms (Xe, Ar, and Kr) has been studied in the few optical cycle domain. Two laser peak intensities; $2\ifmmode\times\else\texttimes\fi{}{10}^{14}$ and $6\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.3em}{0ex}}\mathrm{W}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, were compared. At the lower intensity spectra were approximately the same for molecules and atoms with the same ionization potential, at higher laser intensity the cutoff of ${\mathrm{O}}_{2}$ and $\mathrm{C}{\mathrm{O}}_{2}$ extends far beyond the cutoff of Xe and Kr, respectively, in contrast with ${\mathrm{N}}_{2}$ and ${\mathrm{H}}_{2}$ which exhibit cutoffs very close to that of Ar. This behavior is well explained by adopting an atomlike approximation for the molecule response in the high-field regime and employing the Lewenstein's model, properly modified in order to account for the nonlinear dipole moment of a randomly oriented molecule ensemble.

Coherent control of atomic quantum states by single frequency-chirped laser pulses

We present a scheme of population transfer between two metastable (ground) states of the $\ensuremath{\Lambda}$ atom without considerable excitation of the atom using single frequency-chirped laser pulses. The physics of the process is generation of the ``trapped'' superposition of the ground states by the laser pulse at sufficiently high laser peak intensity. The main conditions for realization of this regime are the following: The width of the transform-limited laser pulse envelope frequency spectrum (without chirp) must be smaller and the peak Rabi frequency of the pulse must be larger than the frequency interval between the two ground states of the $\ensuremath{\Lambda}$ atom. During the frequency chirp, the laser pulse must first come into resonance with the transition from the initially occupied ground state to the excited state and after that with the transition between the excited and second initially empty ground states. In the case when the envelope frequency spectrum width (without chirp) of the pulse exceeds the frequency interval between the two ground states, we show a possibility of controllable generation of superposition of the ground states with a controllable excitation of the $\ensuremath{\Lambda}$ atom.

Laser-peak-intensity calibration using recoil-ion momentum imaging

We exploit a method to precisely calibrate the peak intensity of intense laser pulses by measuring the momentum transferred to the ion in single ionization by circularly polarized light. Using this approach, we show how the experimental data can be used to get information about the absolute ionization rate at a specific local laser-peak intensity within the focal volume. The results of this method are compared to other results obtained with different methods of laser-peak-intensity calibration described in the literature.

Characterization of Nonlinear Optical Properties of Silver Oxide Super Resolution Near-Field Structures by Z-Scan Measurements

Femtosecond laser Z-Scan measurements are used to characterize the nonlinear optical properties of AgOx super resolution near-field structures (super-RENS). Open aperture Z-Scan measurements reveal a complex transient nonlinear optical absorption response of AgOx super-RENS under intensive laser irradiation. This is attributed to the thermal decomposition of AgOx into Ag nanoparticles in the near-field optical mask layer. A bi-compositional model is proposed to explain this complex nonlinear optical response. The threshold laser peak intensity for the phase transition and the nonlinear optical absorption coefficient of an Ag nanoparticle are determined to be Ith∼0.3 GW/cm2 and β=(-5.6±0.5)×103 cm/GW at λ=800 nm, respectively.

Electron and nuclear dynamics of molecular clusters in ultraintense laser fields. III. Coulomb explosion of deuterium clusters.

In this paper we present a theoretical and computational study of the energetics and temporal dynamics of Coulomb explosion of molecular clusters of deuterium (D2)n/2 (n = 480 - 7.6 x 10(4), cluster radius R0 = 13.1 - 70 A) in ultraintense laser fields (laser peak intensity I = 10(15) - 10(20)W cm(-2)). The energetics of Coulomb explosion was inferred from the dependence of the maximal energy EM and the average energy Eav of the product D+ ions on the laser intensity, the laser pulse shape, the cluster radius, and the laser frequency. Electron dynamics of outer cluster ionization and nuclear dynamics of Coulomb explosion were investigated by molecular dynamics simulations. Several distinct laser pulse shape envelopes, involving a rectangular field, a Gaussian field, and a truncated Gaussian field, were employed to determine the validity range of the cluster vertical ionization (CVI) approximation. The CVI predicts that Eav, EM proportional to R0(2) and that the energy distribution is P(E) proportional to E1/2. For a rectangular laser pulse the CVI conditions are satisfied when complete outer ionization is obtained, with the outer ionization time toi being shorter than both the pulse width and the cluster radius doubling time tau2. By increasing toi, due to the increase of R0 or the decrease of I, we have shown that the deviation of Eav from the corresponding CVI value (Eav(CVI)) is (Eav(CVI) - Eav)/Eav(CVI) approximately (toi/2.91tau2)2. The Gaussian pulses trigger outer ionization induced by adiabatic following of the laser field and of the cluster size, providing a pseudo-CVI behavior at sufficiently large laser fields. The energetics manifest the existence of a finite range of CVI size dependence, with the validity range for the applicability of the CVI being R0 < or = (R0)I, with (R0)I representing an intensity dependent boundary radius. Relating electron dynamics of outer ionization to nuclear dynamics for Coulomb explosion induced by a Gaussian pulse, the boundary radius (R0)I and the corresponding ion average energy (Eav)I were inferred from simulations and described in terms of an electrostatic model. Two independent estimates of (R0)I, which involve the cluster size where the CVI relation breaks down and the cluster size for the attainment of complete outer ionization, are in good agreement with each other, as well as with the electrostatic model for cluster barrier suppression. The relation (Eav)I proportional to (R0)I(2) provides the validity range of the pseudo-CVI domain for the cluster sizes and laser intensities, where the energetics of D+ ions produced by Coulomb explosion of (D)n clusters is optimized. The currently available experimental data [Madison et al., Phys. Plasmas 11, 1 (2004)] for the energetics of Coulomb explosion of (D)n clusters (Eav = 5 - 7 keV at I = 2 x 10(18) W cm(-2)), together with our simulation data, lead to the estimates of R0 = 51 - 60 A, which exceed the experimental estimate of R0 = 45 A. The predicted anisotropy of the D+ ion energies in the Coulomb explosion at I = 10(18) W cm(-2) is in accord with experiment. We also explored the laser frequency dependence of the energetics of Coulomb explosion in the range nu = 0.1 - 2.1 fs(-1) (lambda = 3000 - 140 nm), which can be rationalized in terms of the electrostatic model.

Optical second harmonic imaging: a versatile tool to investigate semiconductor surfaces and interfaces

Optical second harmonic (SH) imaging using femtosecond laser pulses (782 nm, 80fs) is applied to investigate the structural quality of SiC and ZnO thin films grown by chemical vapour deposition (CVD) on Si(100) substrates. We find a spatially uniform SH response from the SiC sample as well as a regular 4-fold symmetry (3C-SiC-(001)) in the rotational SH anisotropy. The ZnO film is poly-crystalline as indicated by a strongly position dependent SH response and the missing regularity in the rotational SH anisotropy. The estimated grain size is ∼ 30-50 μm. Furthermore our time dependent SHG measurements on native Si/SiO 2 for laser peak intensities up to 100 GW/cm 2 indicate that hole injection and trapping into the ultrathin SiO 2 layer contributes to the SH signal above > 45 GW/cm 2 . The SHG images of defects irreversibly induced in Si/SiO 2 under laser irradiation are reproduced by scanning electron microscopy (SEM).

Charge-carrier dynamics and trap generation in nativeSi/SiO2interfaces probed by optical second-harmonic generation

Employing femtosecond laser pulses [1.59 eV, $(80\ifmmode\pm\else\textpm\fi{}5)\mathrm{fs},$ 80 MHz], we perform time-dependent second-harmonic (SH) measurements to probe the dynamics of charge-carrier transfer processes across native ${\mathrm{S}\mathrm{i}/\mathrm{S}\mathrm{i}\mathrm{O}}_{2}$ interfaces in a new peak intensity regime up to $100{\mathrm{G}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}.$ The SH signal increases steadily during irradiation up to $45{\mathrm{G}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ laser peak intensity, but changes its temporal evolution above $45{\mathrm{G}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}:$ as a different phenomenon, the SH signal surpasses a maximum, decreases slowly, and appears to approach a steady state level. We assign this observed signal decrease to the light induced injection of holes into the ${\mathrm{SiO}}_{2}$ thin layers involving a four-photon process, which opposes the electron contributions (three-photon process). Furthermore, we investigate the SH signal development after irradiation interrupts and find that preirradiated samples show a drastically accelerated SH response. Both, the electron and the hole injection processes appear to be about two orders of magnitude faster in preirradiated than in ``virgin'' samples. We assign this acceleration to the laser induced generation of permanent defects, which affect electron as well as hole trapping in the ${\mathrm{SiO}}_{2}$ layers. In addition the SH signal amplitude is shown to be a function of the dark period duration, and the typical time constant for the depopulation of hole trap sites under dark conditions has been extracted and amounts to $(110\ifmmode\pm\else\textpm\fi{}15)\mathrm{s}.$ We present an empirical model involving four exponential functions, taking into account the electron and hole contributions to the time-dependent SH response. This model fully reproduces the recorded experimental data. It supports our interpretation of opposing three- and four-photon processes being involved in electron and hole injection, respectively. Finally, we employ SH imaging in comparison with scanning electron microscopy to visualize sample areas with laser defects.

Relativistic electron drift in overdense plasma produced by a superintense femtosecond laser pulse.

The general peculiarities of electron motion in the skin layer at the irradiation of overdense plasma by a superintense linearly polarized laser pulse of femtosecond duration are considered. The quiver electron energy is assumed to be a relativistic quantity. Relativistic electron drift along the propagation of laser radiation produced by a magnetic part of a laser field remains after the end of the laser pulse, unlike the relativistic drift of a free electron in underdense plasma. As a result, the penetration depth is much larger than the classical skin depth. The conclusion has been made that the drift velocity is a nonrelativistic quantity even at the peak laser intensity of 10(21) W/cm(2). The time at which an electron penetrates into field-free matter from the skin layer is much less than the pulse duration.

Generation of a variable linear array of phase-coherent supercontinuum sources

We investigate the coherence properties of a linear array of white-light sources produced in bulk media by ultrashort laser pulses. The array is generated out of the spatial interference pattern between two laser pump pulses, so that the number of supercontinuum sources and their separations can be easily manipulated by varying the geometry of the laser beam interaction. We find that all the secondary white-light sources which arise from the generation of filaments in the optical medium are well phase-locked and are thus able to generate stable and high-visibility multiple-beam interference patterns in the far-field. Observations are compared to the results of a simple model which takes into account a clamping of the peak laser intensity inside the filaments and includes intensity-dependent phase shifts among the different sources.

Electron bunch acceleration and trapping by the ponderomotive force of an intense short-pulse laser

By utilizing a pulsed laser beam of TEM(1,0)+TEM(0,1) mode, it was found numerically for the first time that an electron bunch can be effectively trapped by the transverse ponderomotive force in the transverse direction and at the same time accelerated by the longitudinal ponderomotive force to about 378 MeV at the laser peak intensity of I∼5.48×1018 W/cm2. In addition, the electron bunch size is preferably small: at this laser intensity the electron bunch thickness is ∼10λ in the longitudinal direction and the bunch radius is about 625λ in the transverse direction.

Analysis of resonance structure in the above-threshold ionization photoelectron spectra of magnesium

Photoelectron spectra �PES� finally, have been used quite exten­sively over the past 20 years as a tool to investigate what happens when matter is irradiated with high-intensity light. Multiphoton ionization �MPI� was first observed in 1965 by Voronov and Delone �1�. As peak laser intensities increased with the advancement of laser technology, the propensity for an atom to absorb more photons than the minimum required for ionization became non-negligible, and above-threshold ionization �ATI � was reported by Agostini et al. in 1979 �2�. In the earlier days the pulse widths of lasers were long with respect to the time taken by the ionized electron to leave the interaction region. PES for this long pulse regime yielded structureless peaks separated by a photon energy, revealing little information about the ionization dynamics other than the number of photons the electron absorbed and the relative probabilities for each order �3�. In 1987, Freeman et al. re­ported that as the laser pulse width decreased, the kinetic energy of each ATI order shifted to lower energy and broke apart into structured sub-peaks �3�. The peaks in the PES corresponded to population of specific atomic states during the ionization process and became known as Freeman reso­nances. The ability of shorter laser pulses to observe atomic transitions during the ionization process opened the door for a new field of study in high-intensity laser-atom interactions for peak laser intensities up to 10

X-Ray and Extreme Ultraviolet Emission from Small-Sized KrClusters Irradiated by 150-fs Laser Pulses

x-ray and extreme ultraviolet (EUV) emission from Kr clustersirradiated by 150-fs laser pulses at the peak laser intensity of5×1015W/cm2 was experimentally investigated. Strongtransitions (10 nm-13 nm) from Kr X and Kr IX were observed and somespectral lines from Kr XIII and Kr XIV, which have beenpredicted to be not produced by optical-field-ionization at the laser intensity used, alsoappeared. The laser energy absorption and the intensity of x-rayemission started to grow remarkably above the backing pressure of0.5 MPa and to decrease at the backing pressure of 3 MPa. It is suggestedthat an optimum backing pressure may exist for Kr clusters heated by150 fs laser pulses at a certain laser intensity to produce x-rayemission.

Model for bichromatic laser emission from a laser dye with nanoparticle scatterers

In this work, we present model for bichromatic laser emission from laser dye solutions containing randomly distributed scattering particles. We suggest that bichromatic emission is produced by two fluorescent aggregates: monomers and dimers. The shorter-wavelength laser peak was attributed to the monomer emission and the secondary longer-wavelength peak was attributed to the dimer emission. From neat dye solutions, using absorption and emission spectroscopies, the monomer and dimer absorption and emission cross sections were obtained. The partial overlap between the dimer absorption cross section and the monomer emission cross section indicates a unidirectional radiative and nonradiative energy transfers from excited monomers to fundamental dimers. The dynamics of the laser emission was modeled by a set of rate equations, whose solutions agree with our experimental data. It is also shown that nonradiative energy transfer plays a very important role in the dimer laser process, and helps to explain the relatively strong dimer laser peak intensities.

Optimized energetic particle emissions from Xe clusters in intense laser fields

We have probed the interaction of intense laser fields with Xe clusters by measuring the energy and charge state distributions of emitted ions and electrons using ultrafast 20-fs pulses. This work demonstrates that the energetic particle emissions from laser-cluster interactions can be optimized by manipulating the character of ultrafast laser pulses, i.e., pulse duration and sign of chirp. The mean and maximum ion energies as high as 0.1 and 2 MeV, respectively, are achieved when Xe clusters are irradiated by 500-fs pulses with negative chirp as an optimal condition under a laser peak intensity of $2\ifmmode\times\else\texttimes\fi{}{10}^{17}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}.$ The mean ion energy obtained from 500-fs pulses with negative chirp is found to be about 1.6 times larger than that with positive chirp.

Abnormal pulse duration dependence of the ionization probability of Na atoms inintense laser fields

We studied the ionization probability of Na atoms in an intense Ti–sapphire laserat 800 nm versus the laser pulse duration. The ionization probabilities for theNa(3s) and Na(3p) states were found to vary non-monotonically with the pulseduration at a constant peak laser intensity. The abnormal (non-monotonic) pulseduration dependence was traced to the competition between resonant andnon-resonant multiphoton ionization processes, which occurs when the frequencybroadening due to the finite pulse duration is compatible with the energydifference between the excitation energy and multiphoton energy.

Enhanced spatiotemporal laser-beam smoothing in gas-jet plasmas.

Spatiotemporal smoothing of large-scale laser intensity fluctuations is observed for a laser beam focused into underdense helium plasmas. This smoothing is found to be severely enhanced when focusing the laser beam into a helium gas jet. In contrast to other experiments with preformed plasmas, the average and the peak laser intensities are well below the threshold for ponderomotive self-focusing. The coherence characteristics of the transmitted light are measured for various electron densities, and the smoothing effect is explained by multiple scattering of laser light on self-induced density perturbations.

Xe(L) X-ray emission from laser–cluster interaction

We have measured quantitative L X-ray emission yields from (Xe)n clusters (with n between 104 and 106 atoms/cluster) irradiated by 60 fs 800 nm IR and 180 fs 400 nm UV laser pulses of 1015–1017 Wcm−2 peak intensity. Measurements have been performed as a function of cluster size (backing pressure) and laser peak intensity. Identification of spectroscopic features as well as X-ray emission yield variation with laser wavelength, intensity and cluster size are in strong contradiction with results and interpretation from previous studies.

Harmonic generation of ultraintense laser pulses in underdense plasma.

We propose a nonlinear theory of the generation of harmonic radiation from the interaction of ultraintense laser beams with plasma. The harmonic generation is related to the transition of the laser-plasma equilibrium state. By taking into account correlations among sidebands, we study many sidebands comprehensively. The harmonic generation is viewed as a redistribution of laser field over different frequencies because of the requirement of system stability. We introduce a system parameter S that is related to the sideband intensity spectrum and self-consistently calculate the value of S. Our numerical experiments reveal that the variations of controllable system parameters, plasma density, and laser peak intensity have a great effect on S.

Electron Energy Spectra from the Strong-Field Double-Ionization of Xenon

The electron energy spectra correlated to the strong-field double-ionization of xenon are presented at three laser intensities. The double-ionization electrons are on average more energetic than those generated from single-ionization. This difference in energy is not manifested as a simple scaling to higher electron energies, but rather as a change in the shape of the spectra. This trend is observed at all intensities. Most notably, the comparison between single- and double-ionization spectra is very similar at low and high peak laser intensities. This could imply that a sequential double-ionization process dominates at all intensities, even where the double ion yield is enhanced.