A. T. Georges

Two-photon-resonant third-harmonic generation in cesium vapor

Abstract It is shown that two-photon-resonant tripling of the ruby laser frequency in cesium vapor is limited by ionization. The laser induced Stark shift is found to be favorable to the process in that it counteracts saturation of the resonant transition.

Calculation of surface electromagnetic fields in laser–metal surface interaction

Abstract The semi-classical infinite barrier (SCIB) model for the calculation of surface electromagnetic fields in jellium models for metals is extended to handle smooth interfaces. In the extended model, the phenomenological nonlocal susceptibility of a semi-infinite free-electron gas is equal to the product of the nonlocal bulk susceptibility and the Fourier transform of the derivative of the normalized electron density profile. For a step function electron density profile the extended model reduces to the SCIB model, while for diffuse electron density profiles it predicts stronger surface electromagnetic fields and single-electron absorption than the SCIB model, in qualitative accord with fully numerical microscopic models. Numerical results pertinent to laser–metal surface interactions are presented for infrared, visible, and ultraviolet laser wavelengths.

Theory of stimulated Raman scattering in a chaotic incoherent pump field

Abstract A novel theory of forward and backward Raman amplification in a dispersive medium pumped by a markovian chaotic field of arbitrary bandwidth is reported. Calculations are presented for the average intensity, spectrum and two-time intensity correlation of the Stokes output.

Dependence of broadband Raman amplification in dispersive media on the pump–Stokes input correlation

A statistical theory is presented of the dependence of Raman amplification in a dispersive medium on the cross correlation of the pump and the Stokes input. Both forward and backward amplification of broadband Stokes radiation by a broadband pump are considered. It is shown that group-velocity dispersion reduces the gain enhancement that is observed when the pump and the Stokes input are correlated. The curve for the gain enhancement versus the relative time delay (advance) of the two waves becomes asymmetric and broader than it would in the absence of dispersion. For typical parameters the lack of perfect correlation between the Stokes output from a Raman generator and the pump reduces the gain enhancement in a cascaded Raman amplifier by only a few percent.

Intensity correlations in resonance fluorescence excited by intense incoherent fields

Abstract A general rigorous theory of the intensity correlations in resonance fluorescence excited by markovian incoherent fields of arbitrary bandwidth and average intensity is presented. Both single and many independent two-level atom fluorescence are considered.

Model for ultrafast harmonic generation from a gold surface: extraction of dephasing times for continuum-continuum transitions

A model is presented for describing ultrafast interferometric second- and third-harmonic generation (SHG and THG) from a gold surface. The model uses an effective four-level density matrix system that accounts for the stepwise resonant excitation channels with finite dephasing and energy relaxation times, and the fast nonresonant channel through the energy continua. By fitting recent experimental results for SHG and THG from a polycrystalline gold surface irradiated with 18 fs Ti:sapphire laser pulses, we have extracted a T2=7.9 fs dephasing time at 1.56 eV above the Fermi energy, compared with a T1~30 fs energy relaxation time at the same energy level. The difference indicates a strong contribution to the dephasing of the polarization from elastic processes. Using these values, we present calculations for other laser pulse durations, from 30 fs down to one optical cycle (2.65 fs). It is shown that, in particular, interferometric THG from a gold surface can be used to measure and characterize with high accuracy laser pulses as short as one optical cycle.

High-gain free-electron laser pulse amplifiers with variable wigglers

Abstract Calculations are presented for pulse amplification in high-gain free-electron lasers with variable wiggler magnets. Better than 90% peak electron trapping and overall peak energy extraction efficiencies of over 60% are demonstrated for 1 μm radiation. Pulse reshaping and phase modulation effects are investigated.

Comments on the short-time behaviour of multiphoton ionisation

Some questions regarding the applicability of theoretical results (1977, 1978) for the short-time behaviour of the multiphoton ionisation to the experiments are discussed. It is argued that the relevant conditions could not have been satisfied in the experiments concerned.

Parametric compensation of power losses in surface plasmon polaritons

A theoretical study is presented for parametric compensation of power losses in surface plasmon polaritons (SPPs) along single metal–dielectric interfaces. An optical or near-infrared signal SPP with subwavelength transverse confinement and an idler SPP with 2–3 times longer wavelength are amplified parametrically by a broadside laser pump. Despite the fact that Drude losses are much higher than parametric gain, there is more than tenfold enhancement of the signal SPP propagation length, albeit at a reduced but detectable signal power level. Numerical results are presented for a silver surface interfaced with a polymer/dye or porous silicon layer that allow phase matching of the noncollinear parametric process.

Saturation and Stark-Splitting of Resonant Transitions in Stochastically Fluctuating Laser Fields of Arbitrary Bandwidth

The interaction of strong electromagnetic radiation with matter usually leads to nonlinear processes [1–23]. In fact, the presence of such nonlinearities may usefully be taken as the definition of “strong” radiation, at least in the sense the term is used in this paper. Examples are: Multi-photon excitation or ionization, harmonic generation or wave mixing, as well as the saturation of a bound-bound transition, even if a single-photon transition, as it is the saturation that introduces the non-linearity. A process that depends on the field non-linearly cannot be described in terms of the average intensity and spectrum of the radiation (first order correlation function) because in general higher order correlation functions are also involved [24]. The electron responds to more than just the total radiation; it “sees” the fluctuations of the field.