Showing papers on "Ponderomotive energy published in 2012"

Field-driven photoemission from nanostructures quenches the quiver motion

Abstract: Strong-field physics, an extreme limit of light-matter interaction, is expanding into the realm of surfaces and nanostructures from its origin in atomic and molecular science. The attraction of nanostructures lies in two intimately connected features: local intensity enhancement and sub-wavelength confinement of optical fields. Local intensity enhancement facilitates access to the strong-field regime and has already sparked various applications, whereas spatial localization has the potential to generate strong-field dynamics exclusive to nanostructures. However, the observation of features unattainable in gaseous media is challenged by many-body effects and material damage, which arise under intense illumination of dense systems. Here, we non-destructively access this regime in the solid state by employing single plasmonic nanotips and few-cycle mid-infrared pulses, making use of the wavelength-dependence of the interaction, that is, the ponderomotive energy. We investigate strong-field photoelectron emission and acceleration from single nanostructures over a broad spectral range, and find kinetic energies of hundreds of electronvolts. We observe the transition to a new regime in strong-field dynamics, in which the electrons escape the nanolocalized field within a fraction of an optical half-cycle. The transition into this regime, characterized by a spatial adiabaticity parameter, would require relativistic electrons in the absence of nanostructures. These results establish new degrees of freedom for the manipulation and control of electron dynamics on femtosecond and attosecond timescales, combining optical near-fields and nanoscopic sources.

Electron-energy bunching in laser-driven soft recollisions.

Abstract: We introduce soft recollisions in laser-matter interaction. They are characterized by the electron missing the ion upon recollision in contrast with the well-known head-on collisions responsible for high-harmonic generation or above-threshold ionization. We demonstrate analytically that soft recollisions can cause a bunching of photoelectron energies through which a series of low-energy peaks emerges in the electron yield along the laser polarization axis. This peak sequence is universal, it does not depend on the binding potential, and is found below an excess energy of one tenth of the ponderomotive energy.

Circularly polarized attosecond pulses from molecular high-order harmonic generation by ultrashort intense bichromatic circularly and linearly polarized laser pulses

Abstract: We describe the generation of high-order elliptically and circularly polarized harmonic spectra in an aligned H+2 molecule ion by a combination of two-colour ultrashort intense laser fields from numerical solutions of the corresponding time-dependent Schrodinger equation (TDSE). In intense bichromatic circularly and linearly or circularly polarized laser pulses with intensity I0 and angular frequencies ω0 and 2ω0, it is found that maximum molecular high-order harmonic generation (MHOHG) energies are functions of the molecular internuclear distance. Based on a classical model of laser-induced electron collisions with neighbouring ions, the optimal values of the pulse relative carrier envelope phase , the molecular internuclear distance R and the angle of molecular alignment to the laser polarization axis are obtained for efficiently producing MHOHG spectra with the maximum harmonic energy Ip + 13.5Up, where Ip is the ionization potential of the molecule and Up = I0/4meω20 is the ponderomotive energy of the continuum electron at intensity I0 and frequency ω0 of the laser pulse. The results have been confirmed from corresponding TDSE nonperturbative numerical simulations. The polarization property of the generated harmonics is also presented. The mechanism of MHOHG is further characterized with a Gabor time frequency analysis. It is confirmed that a single collision trajectory of the continuum electron with neighbouring ions dominates in the MHOHG processes. The high efficiency of the proposed MHOHG scheme provides a possible source for production of elliptically and/or circularly polarized attosecond extreme ultraviolet pulses. Circularly polarized attosecond pulses can also be generated by using intense ultrashort circularly polarized laser pulses in combination with static electric fields of comparable intensity for H+2 at equilibrium. A time frequency analysis also confirms the role of single recollisions as the dominant mechanism of the generation of circularly polarized harmonics.

Mechanism of heating of pre-formed plasma electrons in relativistic laser-matter interaction

Abstract: The role of the longitudinal ambipolar electric field, present inside a pre-formed plasma, in electron heating and beam generation is investigated by analyzing single electron motion in the presence of one electromagnetic plane wave and “V” shaped potential well (constant electric field) in a one dimensional slab approximation. It is shown that for the electron confined in an infinite potential well, its motion becomes stochastic when the ratio of normalized laser electric field a0, to normalized longitudinal electric field Ez, exceeds unity, i.e., a0/Ez≳1. For a more realistic potential well of finite depth, present inside the pre-formed plasma, the condition for stochastic heating of electrons gets modified to 1≲a0/Ez≲L, where L is the normalized length of the potential well. The energy of electron beam leaving such a potential well and entering the solid scales ∼a02/Ez, which can exceed the laser ponderomotive energy (∼a0) in the stochastic regime.

Multipeak envelope of the above-threshold-ionization energy spectrum in a strong circularly polarized laser field

Abstract: We develop a well-known strong-field approximation in the length gauge for a two-dimensional model atom in a strong circularly polarized laser field. For excited initial states (with the principal quantum number $n=2$) outgoing photoelectrons usually show energy spectra with an envelope having two or three peaks: one always below and one always above the ponderomotive energy ${U}_{P}$. In contrast, for the ground state (with $n=1$) there is always a single peak near ${U}_{P}$. Our numerical calculations show that the multipeak effect appears for sufficiently high laser intensities $I$ and is more pronounced for higher laser frequencies $\ensuremath{\omega}$. We believe that the field parameters $\ensuremath{\omega}$ and $I$, for which the multipeak envelope in the energy spectrum of above-threshold ionization exists, will enable its experimental observation in the near future.

The effects of pre-formed plasma on the generation and transport of fast electrons in relativistic laser-solid interactions

Abstract: In this thesis we present the dynamics of relativistic fast electrons produced in the laser-solid interactions at the intensities greater than 10¹⁸ W/cm². In particular, the effects of pre-formed plasma in front of a solid target on the generation and transport of these fast electrons is studied. The presence of such a pre-formed plasma is ubiquitous in almost all the present short pulse high intensity laser-solid interaction experiments. First, the generation of fast electrons in the presence of pre- formed plasma of varying density scale-lengths is studied with the help of Particle In Cell (PIC) simulations. It is shown that the fast electrons energy increases with the increasing pre-formed plasma, consistent with the experimental observations. The possible mechanism of generation of such energetic electrons is studied. It is proposed that the interaction of plasma electrons with the laser in the presence of ambipolar electric field, generated due to the laser heating, can result in the electron acceleration beyond laser ponderomotive energy. The analytical and numerical studies of this heating mechanism are presented. In the second part of thesis, the influence of pre-formed plasma on the fast electrons transport is studied. Especially the physics of refluxing of these fast electrons due to the excitation of electrostatic sheath fields inside the pre-formed plasma is investigated. It is shown that this refluxing is responsible for the annular ring shaped' copper K[alpha] x -ray emission observed in the recent high intensity laser- solid experiments

Strong-Field Effects and Attosecond Control of Electrons in Photoemission from a Nanoscale Metal Tip

Abstract: We focus few-cycle Titanium:sapphire oscillator pulses on a sharp tungsten tip and measure spectra of the emitted electrons. We observe above-threshold photoemission with a photon order of up to nine, peak shifting and peak suppression, and carrier-envelope phase sensitive photoemission. In a first attempt to understand the underlying physics we model the behavior with theory models borrowed from atomic physics, namely the Three-Step Model and an integration of the one-dimensional time-dependent Schrodinger equation. The models match the high-energy part of the spectra surprisingly well, and we infer that re-scattering and much of the well-known underlying physics of the Three-Step Model takes also place here, even though the parameters are such that this is almost surprising; for instance, the classical excursion of the electrons is only slightly larger than one atomic diameter.