Intense Short Pulse Research Articles

Phenomenological quantum description of the ultrafast response of arrayed waveguide gratings

This work presents a detailed and quantitative quantum description of the ultrafast response of arrayed waveguide gratings (AWG) illuminated with relatively intense short pulses of light. This is achieved with no more mathematical or conceptual complexities than that required by a classical description. The presented approach is based on the phenomenological interpretation of the photon, that is, a photon is what produces a “click” in a photodetector. This phenomenological approach was combined with the application of Feynman’s rules for describing interference and Bohr’s correspondence principle, i.e., quantum theory should somehow converge in the limit with the classical description of the interference phenomena. This basic approach reveals that, in apparent opposition to wide-held beliefs, especially designed AWGs can be used to produce interference in conditions where the “which-path” information is available.

Ablation of bone cells by electroporation

Short intense electrical pulses transiently increase the permeability of the cell membrane, an effect known as electroporation. This can be combined with antiblastic drugs for ablation of tumours of the skin and subcutaneous tissue. The aim of this study was to test the efficacy of electroporation when applied to bone and to understand whether the presence of mineralised trabeculae would affect the capability of the electric field to porate the membrane of bone cells. Different levels of electrical field were applied to the femoral bone of rabbits. The field distribution and modelling were simulated by computer. Specimens of bone from treated and control rabbits were obtained for histology, histomorphometry and biomechanical testing. After seven days, the area of ablation had increased in line with the number of pulses and/or with the amplitude of the electrical field applied. The osteogenic activity in the ablated area had recovered by 30 days. Biomechanical testing showed structural integrity of the bone at both times. Electroporation using the appropriate combination of voltage and pulses induced ablation of bone cells without affecting the recovery of osteogenic activity. It can be an effective treatment in bone and when used in combination with drugs, an option for the treatment of metastases.

Self-compression controlled by the chirp of the input pulse

Self-compressed (SC) pulses have been achieved through the filamentation process in air without any additional dispersion compensation, using the input pulse chirp as the control parameter. For any studied input pulse energy (3-5 mJ), we have found two opposite sign input group-delay dispersion values for which SC pulses can be achieved systematically. In addition, we have observed that the energy coupled into the inner core of the filament is always of the order of 20% of the total input pulse energy, which opens the way to a scalable technique to obtain intense short pulses directly from the filamentation process.

Observation of Magnetized Soliton Remnants in the Wake of Intense Laser Pulse Propagation through Plasmas

Slowly evolving, regularly spaced patterns have been observed in proton projection images of plasma channels drilled by intense (≳10¹⁹ W cm⁻²) short (∼1 ps) laser pulses propagating in an ionized gas jet. The nature and geometry of the electromagnetic fields generating such patterns have been inferred by simulating the laser-plasma interaction and the following plasma evolution with a two-dimensional particle-in-cell code and the probe proton deflections by particle tracing. The analysis suggests the formation of rows of magnetized soliton remnants, with a quasistatic magnetic field associated with vortexlike electron currents resembling those of magnetic vortices.

Two-color ionization of hydrogen by short intense pulses

Photoelectron energy spectra resulting by the interaction of hydrogen with two short pulses having carrier frequencies, respectively, in the range of the infrared and XUV regions have been calculated. The effects of the pulse duration and timing of the X-ray pulse on the photoelectron energy spectra are discussed. Analysis of the spectra obtained for very long pulses show that certain features may be explained in terms of quantum interferences in the time domain. It is found that, depending on the duration of the X-ray pulse, ripples in the energy spectra separated by the infrared photon energy may appear. Moreover, the temporal shape of the low frequency radiation field may be inferred by the breadth of the photoelectron energy spectra.

Photonuclear reactions induced by intense short laser pulses

A measurement of the decay in time of nuclei excited by an intense short laser pulse of energy E 0 yields the Fourier transform of the autocorrelation function of the associated scattering matrix. We determine the optimal length (in time) of the pulse and evaluate the time-decay function using random-matrix theory. That function is shown to contain information not otherwise available. We approximate that function in a manner that is useful for the analysis of data. For E 0 below the threshold energy E n of the first neutron channel, the time-decay function is exponential in time t while it is the product of an exponential and a power in t for E 0 > E n . The comparison of the measured decay functions in both energy domains yields an unambiguous and novel test of random-matrix theory in nuclei.

Elliptical Polarization and Probability of Double Ionization

The degree of elliptical polarization of intense short laser pulses is shown to be related to the timing of strong-field nonsequential double ionization. Higher ellipticity is predicted to force the initiation of double ionization into a narrower time window, and this "pins" the ionizing field strength in an unexpected way, leading to the first experimentally testable formula for double ionization probability as a function of ellipticity.

Transport of hot electron currents in solid targets irradiated by high intensity short laser pulses

We have analyzed the transport of hot electrons generated in the interaction between a short-pulse, ultra-high intensity laser beam (pulse duration τ<ps, intensity Iλ > 1018 W.cm 2.um2) and a solid or dense target through the use of proton emission imaging. We used targets of different material (Cu, Al, Au) with a regularly modulated rear target surface in order to compare the electron transport in different conditions. As result, we see that the electron transport depends on the target material and on the interaction conditions.

Control of proton energy in ultra-high intensity laser-matter interaction

Recent breakthroughs in short pulse laser technology resulted in (i) generation of ultra-high intensity (2×1022 W/cm2) and (ii) ultra-high contrast (10−11) short pulses at the Hercules facility of the University of Michigan, which has created the possibility of exploring a new regime of ion acceleration – the regime of Directed Coulomb Explosion (DCE). In this regime of sufficiently high laser intensities and target thicknesses approaching the relativistic plasma skin depth it is possible to expel electrons from the target focal volume by the laser's ponderomotive force allowing for direct laser ion acceleration combined with a Coulomb explosion. That results in greater than 100 MeV protons with a quasi-monoenergetic energy spectrum. The utilization of beam shaping, namely, the use of flat-top beams, leads to more efficient proton acceleration due to the increase of the longitudinal field. According to the results of 2D PIC simulations a 500 TW laser pulse with a super-Gaussian beam profile interacting with 0.1 micron aluminium-hydrogen foil is able to produce monoenergetic protons with the energy up to 240 MeV.

Proposal for a Raman X-ray free electron laser

A scheme for an X-ray free electron laser is proposed, based on a Raman process occurring during the interaction between a moderately relativistic bunch of free electrons, and twin intense short pulse lasers interfering to form a transverse standing wave along the electron trajectories. In the high intensity regime of the Kapitza-Dirac effect, the laser ponderomotive potential forces the electrons into a lateral oscillatory motion, resulting in a Raman scattering process. I show how a parametric process is triggered, resulting in the amplification of the Stokes component of the Raman-scattered photons. Experimental operating parameters and implementations, based both on LINAC and Laser Wakefield Acceleration techniques, are discussed.

Editorial on Dissipative Optical Solitons

The fundamental mechanism that defines the concept of an optical soliton is the exact balance between two opposite mechanisms, one allowing for the spreading, the other for the concentration of a localized distribution of light. Historically, this was found in conservative systems and can be described by the nonlinear Shrodinger equation and other related equations. This situation applies both to the case of temporal solitons where group velocity dispersion and self-phase modulation counteract one another in the longitudinal dimension of light propagation and to the case of spatial solitons where diffraction spreading and Kerr lensing compensate, leading to a stable distribution of light with a constant beam waist. Solitons have been known as an experimental evidence since the early observation by John Scott Russel of a water solitary wave in 1844. The study of temporal optical solitons more than a century later has benefited from the very rapid development during the 70’s of optical fiber performance whose residual Kerr effect could be used to propagate short intense pulses across distances that can reach now thousands of kilometers. From the 1990’s a new branch of soliton studies has developed in dissipative optical systems defined as lossy systems fed with an external input of energy. Therein, it has been progressively shown through the pioneering theoretical work of Lugiato and Lefever [1], Rosanov [2], Mandel [3], and Firth [4] that nonlinear optical cavities or feedback systems can reach various levels of self-organization among which patterned and localized bistable states are expected. These predictions were rapidly confirmed by their observation in various systems including liquid crystal light valves [5], sodium vapors [6], photorefractive materials [7], and most recently semiconductors [8,9]. These localized states, when exhibiting bistability, were called cavity solitons and appeared to display new properties. The term “soliton”, though not strictly matching the requirement of being an exact solution of a conservative non linear propagation equation, conveys however the concept that an equilibrium is reached between counteracting mechanisms, one of them at least being nonlinear. Indeed, in addition to obtaining

Stimulated Raman scattering of laser in heavily doped semiconductors

An intense short pulse laser propagating through a heavily doped degenerate semiconductor, parametrically excites a free carrier space charge mode and back scattered electromagnetic mode via free carrier nonlinearity. Landau damping and collisional damping of the decay waves determine the threshold for the instability. Above the threshold, the growth rate scales as γ∼(v0/2c)(εLωω0)1/2, where v0 is the free carrier oscillatory velocity, c is the velocity of light in vacuum, εL is the lattice permittivity, ω is the frequency of the space charge mode, and ω0 is the pump frequency.

Enhanced absorption of intense short-pulse laser light by subwavelength nanolayered target

Two-dimensional particle-in-cell simulation shows that a target with subwavelength nanolayered front can reduce the reflection and increase the absorption of the energy of an intense short laser pulse. The electrons within the skin depth on the surfaces of the nanolayers are accelerated by J×B heating to relativistic velocities and ejected into the narrow vacuum spaces between the layers. They then propagate forward with most of the absorbed laser energy along the surfaces of the layers. Conversion of the laser energy into electron energy can be enhanced by optimizing the vacuum spacing between the nanolayers since the phase structure of the laser field in the target is modified. The effects of the layer width, length, and spacing on the energy conversion efficiency are investigated.

Dependence on pulse duration and foil thickness in high-contrast-laser proton acceleration

Experimental measurements of proton acceleration with high intensity and high-contrast short laser pulses have been carried out over an order of magnitude range in target thickness and laser pulse duration. The dependence of the maximum proton energy with these parameters is qualitatively supported by two-dimensional particle-in-cell simulations. They evidence that two regimes of proton acceleration can take place, depending on the ratio between the density gradient and the hot electron Debye length at the rear target surface. As this ratio can be affected by the target thickness, a complex interplay between pulse duration and target thickness is observed. Measurements and simulations support unexpected variations in the laser absorption and hot electron temperature with the pulse duration and laser intensity, for which density profile modification at the target front surface is the controlling parameter.

Experimental study on multiphoton ionization of Ar with high intensity short laser pulses

Experimental study on multiphoton ionization of Ar with high intensity short laser pulses

超短强激光场中Ar原子多光子电离实验研究

We present high resolution photoelectron energy spectra from multiphoton ionization (MPI) of Ar subject to laser pulses with wavelength. of 400 mn, pulse duration of 35 fs, and maximum intensity of 5x10(13) W/cm(2) Ionizations into (2)P(3/2) and (2)P(1/2) channels are observed and distinct resonance structures are found in both ionization channels The intensity dependence of the resonance structures is explained in terms of the mechanism of Freeman resonance, i.e transient resonances of alternating current, (AC) Stark-shifted Rydberg states at specific intensities within the laser pulse.

The time-dependent Dirac equation for atoms exposed to short intense electromagnetic pulses

The time dependent Dirac equation for hydrogen-like systems exposed to intense laser pulses, is solved numerically and compared to the corresponding prediction from the non-relativistic Schrödinger equation. It is found that virtual electron-positron pairs are instrumental for a correct description of effects beyond the dipole approximation. Relativistic effects in the ionization dynamics of highly charged systems are studied

Photodissociation of H2+ by intense chirped pulses – beyond the effect of pulse duration and peak power

H2+ photodissociation, induced by intense short laser pulses, was measured by a full 3D imaging system. We have conducted a series of experiments, in which we systematically changed the linear chirp, using a pulse shaper, and observed the kinetic energy release spectra(KER). Distinct differences in the KER spectra are observed both in peak positions and angular distribution for laser pulses with similar duration and intensity but opposite chirp sign.

Effects of multiple electronic shells on strong-field multiphoton ionization and high-order harmonic generation of diatomic molecules with arbitrary orientation: An all-electron time-dependent density-functional approach

We present a time-dependent density-functional theory approach with proper long-range potential for an ab initio study of the effect of correlated multielectron responses on the multiphoton ionization (MPI) and high-order harmonic generation (HHG) of diatomic molecules ${\text{N}}_{2}$ and ${\text{F}}_{2}$ in intense short laser pulse fields with arbitrary molecular orientation. We show that the contributions of inner molecular orbitals to the total MPI probability can be sufficiently large or even dominant over the highest-occupied molecular orbital, depending on detailed electronic structure and symmetry, laser field intensity, and orientation angle. The multielectron effects in HHG are also very important. They are responsible for enhanced HHG at some orientations of the molecular axis. Even strongly bound electrons may have a significant influence on the HHG process.

Resonant third harmonic generation of a short pulse laser in plasma by applying a wiggler magnetic field

We examine the effect of wiggler magnetic field on pulse slippage of short pulse laser-induced third harmonic generation in plasma. The process of third harmonic generation of an intense short pulse laser in plasma is resonantly enhanced by the application of a magnetic wiggler. The laser exerts a ponderomotive force at second harmonic driving density oscillations. The second harmonic oscillations coupled with electron velocity at the laser frequency, produces a non-linear current, driving the third harmonic. Third harmonic pulse generates in the fundamental pulse domain. However, the group velocity of the third harmonic wave is greater than the fundamental wave. Hence, the third harmonic pulse saturates strongly and moves forward from the fundamental pulse at shorter distance than the second harmonic pulse.