Showing papers on "Ponderomotive energy published in 2013"

Scaling of the low-energy structure in above-threshold ionization in the tunneling regime: theory and experiment

Abstract: A calculation of the second-order (rescattering) term in the S-matrix expansion of above-threshold ionization is presented for the case when the binding potential is the unscreened Coulomb potential. Technical problems related to the divergence of the Coulomb scattering amplitude are avoided in the theory by considering the depletion of the atomic ground state due to the applied laser field, which is well defined and does not require the introduction of a screening constant. We focus on the low-energy structure, which was observed in recent experiments with a midinfrared wavelength laser field. Both the spectra and, in particular, the observed scaling versus the Keldysh parameter and the ponderomotive energy are reproduced. The theory provides evidence that the origin of the structure lies in the long-range Coulomb interaction.

Semiconductor quantum well excitons in strong, narrowband terahertz fields

Abstract: Optical transitions between exciton states in semiconductors—intraexcitonic transitions—usually fall into the terahertz (THz) range and can be resonantly excited with narrowband, intense THz radiation as provided by a free-electron laser. We investigate this situation for two different quantum well structures by probing the near-infrared excitonic absorption spectrum near the band edge. We observe the dynamical Stark—or Autler–Townes—splitting of the 1s exciton ground state and follow its evolution for various THz photon energies and field strengths. The behavior is considerably more complex as compared to the atomic systems. At the highest field strengths, where the Rabi energy is of the same order of magnitude as the exciton level separation, the system cannot be described within the standard framework of a two-level system in rotating wave approximation. When the ponderomotive energy approaches the exciton binding energy, signatures of exciton field ionization are observed.

Effects of excitation frequency on high-order terahertz sideband generation in semiconductors

Abstract: We theoretically investigate the effects of the excitation frequency on the plateau of high-order terahertz sideband generation (HSG) in semiconductors driven by intense terahertz (THz) fields. We find that the plateau of the sideband spectrum strongly depends on the detuning between the near-infrared laser field and the band gap. We use the quantum trajectory theory (three-step model) to understand the HSG. In the three-step model, an electron?hole pair is first excited by a weak laser, then driven by the strong THz field, and finally recombined to emit a photon with energy gain. When the laser is tuned below the band gap (negative detuning), the electron?hole generation is a virtual process that requires quantum tunneling to occur. When the energy gained by the electron?hole pair from the THz field is less than 3.17 times the ponderomotive energy (Up), the electron and the hole can be driven to the same position and recombined without quantum tunneling, so that the HSG will have large probability amplitude. This leads to a plateau feature of the HSG spectrum with a high-frequency cutoff at about 3.17Up above the band gap. Such a plateau feature is similar to the case of high-order harmonics generation in atoms where electrons have to overcome the binding energy to escape the atomic core. A particularly interesting excitation condition in HSG is that the laser can be tuned above the band gap (positive detuning), corresponding to the unphysical ?negative? binding energy in atoms for high-order harmonic generation. Now the electron?hole pair is generated by real excitation, but the recombination process can be real or virtual depending on the energy gained from the THz field, which determines the plateau feature in HSG. Both the numerical calculation and the quantum trajectory analysis reveal that for positive detuning, the HSG plateau cutoff depends on the frequency of the excitation laser. In particular, when the laser is tuned more than 3.17Up above the band gap, the HSG spectrum presents no plateau feature but instead sharp peaks near the band edge and near the excitation frequency.

Dynamic polarizability of Rydberg atoms: Applicability of the near-free-electron approximation, gauge invariance, and the Dirac sea

Abstract: Ponderomotive energy shifts experienced by Rydberg atoms in optical fields are known to be well approximated by the classical quiver energy of a free electron. We examine such energy shifts quantum mechanically and elucidate how they relate to the ponderomotive shift of a free electron in off-resonant fields. We derive and evaluate corrections to the ponderomotive free electron polarizability in the length and velocity (transverse or Coulomb) gauges, which agree exactly as mandated by the gauge invariance. We also show how the free electron value emerges from the Dirac equation through summation over the Dirac sea states. We find that the free-electron AC Stark shift comes as an expectation value of a term proportional to the square of the vector potential in the velocity gauge. On the other hand, the same dominant contribution can be obtained to first order via a series expansion of the exact energy shift from the second order perturbation theory in the length gauge. Finally, we numerically examine the validity of the free-electron approximation. The correction to the free-electron value becomes smaller with increasing principal quantum number, and it is well below a per cent for 60s states of Rb and Sr away from the resonances.

Generation of hemispherical fast electron waves in the presence of preplasma in ultraintense laser-matter interaction

Abstract: Hemispherical electron plasma waves generated from ultraintense laser interacting with a solid target having a subcritical preplasma is studied using particle-in-cell simulation. As the laser pulse propagates inside the preplasma, it becomes self-focused due to the response of the plasma electrons to the ponderomotive force. The electrons are mainly heated via betatron resonance absorption and their thermal energy can become higher than the ponderomotive energy. The hot electrons easily penetrate through the thin solid target and appear behind it as periodic hemispherical shell-like layers separated by the laser wavelength.

Femtosecond Photodetachment of Silver Anions

Abstract: The two- and three-photon detachment of negative silver ions in a femtosecond infrared laser field is studied using photoelectron velocity map imaging methods. Photoelectron angular distributions (PADs) are obtained for these detachment channels; these PADs change dramatically when the laser wavelength and intensity are changed. Theoretical predictions, which are based on the adiabatic Keldysh-Faisal-Reiss saddle point method, are in good agreement with our experiment. The dependence of the PAD on the laser wavelengths and intensities is due to the interference between the different partial wave functions. The relative contributions of the different partial waves to the detachment amplitude are altered by changing the laser parameters and, as a result, the shape of the PAD. Close to the detachment threshold, the two-photon detachment process also follows the Wigner threshold law. Near the detachment threshold, the large differences between the calculated results and our experimental results indicate that the ponderomotive energy shifts caused by the femtosecond laser fields must be taken into account in the theoretical model. The three-photon detachment of Ag(-) is also observed and compared with theoretical calculations.

From Above-Threshold Photoemission to Attosecond Physics at Nanometric Tungsten Tips

Abstract: The interaction of few-cycle laser pulses with a nanometric metal tip is described. We find many effects that the strong-field physics community has discovered with atoms in the last 30 years, and describe them here in experiments with solid nanotips. Starting with a clear identification of several photon orders in above-threshold photoemission, via strong-field effects such as peak shifting and peak suppression, to the observation of a pronounced plateau in electron spectra, we show that we have reached the level of control necessary for attosecond physics experiments. In particular, we observe electronic wavepacket dynamics on the attosecond time scale. Namely, by variation of the carrier-envelope phase of the driving laser pulses, we observe a qualitative change in the electron spectra: For cosine pulses we obtain an almost flat plateau part, whereas for minus-cosine pulses the plateau part clearly shows photon orders. We interpret this change by the occurrence of a single or a double slit configuration in time causing electronic matter wave interference in the time-energy domain.

Nonlinear terahertz spectroscopy in solids with single-cycle terahertz pulses

Abstract: We present novel generation methods of intense terahertz single cycle pulses. The Cherenkov scheme with tilted wave-front technique in the LiNbO3 crystal gives us the maximum electric field larger than 1 MV/cm, which ponderomotive energy is as large as 10 eV. The ponderomotive energy is strong enough to ionize bound electronic states in solids such as donors and accepters and easy to induce nonlinear optical effects in solids.

Effects of excitation frequency on high-order terahertz sideband generation in semiconductors

Abstract: We theoretically investigate the effects of the excitation frequency on the plateau of high-order terahertz sideband generation (HSG) in semiconductors driven by intense terahertz (THz) fields. We find that the plateau of the sideband spectrum strongly depends on the detuning between the NIR laser field and the band gap. We use the quantum trajectory theory (three-step model) to understand the HSG. In the three-step model, an electron-hole pair is first excited by a weak laser, then driven by the strong THz field, and finally recombine to emit a photon with energy gain. When the laser is tuned below the band gap (negative detuning), the electron-hole generation is a virtual process that requires quantum tunneling to occur. When the energy gained by the electron-hole pair from the THz field is less than 3.2 times the ponderomotive energy, the electron and the hole can be driven to the same position and recombine without quantum tunneling, so the HSG will have large probability amplitude. This leads to a plateau feature of the HSG spectrum with a high-frequency cutoff at about 3.2 times the ponderomotive energy above the band gap. Such a plateau feature is similar to the case of high-order harmonics generation in atoms where electrons have to overcome the binding energy to escape the atomic core. A particularly interesting excitation condition in HSG is that the laser can be tuned above the band gap (positive detuning), corresponding to the unphysical "negative" binding energy in atoms for high-order harmonic generation. Now the electron-hole pair is generation by real excitation, but the recombination process can be real or virtual depending on the energy gained from the THz field, which determines the plateau feature in HSG.