Narkis Tzoar

Propagation of nonlinear optical pulses in inhomogeneous media

We discuss the propagation of nonlinear optical pulses in dispersive media in the presence of inhomogeneities. Two kinds of inhomogeneities are considered, one arising due to radial variation of the refractive index of the medium and the other due to boundary conditions at the medium boundary, i.e., at the radius of an optical fiber. The wave equation is solved in the presence of these effects, taking an average of the radial dependence. It is shown that inhomogeneities have a remarkable influence on the dispersive properties of the medium, and even at normal dispersion, ’’light’’ pulses can be used for soliton transmission. Numerical results are obtained for a Nd : glass laser (λ=1.06 μm) propagating in SiO2 glass.

Theory of Nonlinear Beam Propagation in Optical Waveguides

We present an approximate theory of nonlinear pulse propagation in homogeneous and inhomogeneous waveguides. Our analysis takes account of physical effects arising from transverse confinement, dispersion, and nonlinearity. We find that both bright and dark solitons may be supported by typical waveguides under a variety of conditions. Moreover, it is possible to achieve a condition of “zero dispersion,” in which a soliton of arbitrarily small amplitude may be propagated, independent of pulsewidth. In the presence of weak longitudinal inhomogeneity, we find that solitions continue to propagate without a change of shape, but that their group velocity becomes time dependent.

Kinetic Theory Description of Electron Stimulated Desorption.

Abstract A first principles kinetic theory is developed, applicable to angular dependent emission in Electron Stimulated Desorption. Expressions for the ionic and neutral atom ESD cross sections are formulated and applied in a model calculation of O+ emission from W(111). Strong focussing of the outgoing ions was found, with the use of a model ion-solid potential in which the substrate was free of excitation. Off-axis spot groups were simulated. The peak ion energies obtained with this model are, however, small compared to experimental energies for the high coverage case. The need to introduce substrate excitations to describe this case is discussed.

The interaction of polar-optical phonons with a two-dimensional electron gas

Abstract The interaction Hamiltonian for a two-dimensional electron gas with optical phonons was obtained, for an electron gas on the interface of a polar semiconductor in a metal-insulator-semiconductor device. We found that the electron-phonon interaction is proportional to the inverse square root of the phonon wave number. The dependence of the electron-phonon coupling constant on the dielectric constant of the insulator is determined.

Radiation loss in tapered waveguides

The eikonal or local-normal-mode (LNM) approximation method is presented and applied to the investigation of the radiation loss of a TE guided mode that is due to a narrowing linear taper in slab geometry using a weak-coupling approximation. An exact expression for the total radiation loss is given within the limits of the theory, these limits being small refractive-index changes per wavelength. An approximation to the loss expression is evaluated numerically, indicating that the total radiative loss is roughly exponentially dependent on mode number and on taper step size, at least for weak tapers. The method developed here is not restricted to tapered structures but can be readily adapted to any type of longitudinal disturbance.

Theory of continuous-wave beam propagation in nonlinear optical waveguides

The paraxial approximation is applied to calculate the propagation characteristics of cw beams possessing Gaussian spatial profiles in nonlinear optical waveguides. For powers below the critical power for self-focusing in a homogeneous medium, we find that the waveguide effect dominates, i.e., the beam becomes trapped and the spot size varies sinusoidally. Above the critical power, the propagation is dominated by nonlinearity; the beam becomes unstable and displays self-focusing analogous to that in a homogeneous medium. Mode mixing is defined in terms of the nonlinearity-induced mixing of the initially excited modes at the guide face, and explicit expressions are obtained for the mode mixing for special cases. This mixing vanishes only if the spot size is a constant, a condition that can be satisfied only for special values of the parameters in longitudinally homogeneous, lossless guides.

Modulation of x-ray absorption cross-section by optical radiation☆

Abstract We consider the deep level absorption of X-rays in the presence of optical radiation. The optical field is considered to be small relative to atomic fields but strongly interacting with the outgoing electron. The parameters which justify this approach are derived and the general expression for the x-ray absorption cross-section as a function of the optical field is obtained.

X-ray photoelectric effect in the presence of laser radiation

Abstract We show here that the energy analyzed X-ray photoelectric current in the presence of laser radiation provides direct observation on multiphoton processes.

High-frequency high-temperature conductivity of metals

We calculate from first principles the high-frequency high-temperature conductivity of metals and find that to dominant order the temperature-dependent collision frequency is given by ν(T) = ν1(T/Tm) + ν2(T/Tm)2, where Tm represents the melting temperature and ν1 and ν2 are independent of T. The origin of the quadratic-temperature term arises from the contribution of the anharmonic ion potential near melting to the phonon spectra.

Four‐photon parametric amplification in semiconductors

A theoretical study of four‐photon parametric amplification in narrow‐band‐gap semiconductors is made. It is shown that phase matching is achievable in a linear geometry if a magnetic field is employed. Furthermore, a substantial cyclotron resonance enhancement occurs in the presence of a magnetic field. We calculate the growth rates and threshold fields associated with the parametric amplification and conclude that an efficient laser may be designed based on this process.