Surface magnetic properties of the pseudomorphic Fe(110) monolayer on a W(110) substrate are investigated from first principles as a function of the substrate thickness (up to eight layers). Analyzing the magnetocrystalline anisotropy energies, we find stable (with respect to the number of substrate layers) in-plane easy and hard axes of magnetization along the [1[overline 1]0] and [001] directions, respectively, reaching a value in good agreement with experiment for thick substrates. Additionally, the changes to the magnetic spin moments and the density of the Fe d states are analyzed with respect to the number of substrate layers as well as with respect to the direction of magnetization. With respect to the number of W(110) substrate layers beneath the Fe(110) surface, we find that the first four substrate layers have a large influence on the electronic and magnetic properties of the surface. Beyond the fourth layer, the substrate has only marginal influence on the surface properties.

First-principles calculations, based on the full-potential linearized augmented plane wave (FLAPW) method, including spin-orbit coupling (SOC) as second variational treatment within a self-consistent procedure, were carried out to obtain the nonlinear optical (NLO) response from the fcc multilayer structure [Ni(1 ML)/Cu(1ML)](2); grown on Cu(001). Within the framework of a supercell approach we obtain the wave functions and the bandstructure for further calculation of the spectra of the nonlinear optical susceptibility tensor chi((2))(ijk): The reflected electric field at the frequency of the second harmonic (SHG) 2 omega is calculated on the basis of the surface-sheet model and the resulting SHG intensities, azimuthal dependence for main experimental geometries and Kerr angles are presented for the cases of the in-plane and out-of-plane magnetization of the multilayer, which correspond to longitudinal and polar magneto-optical configurations (MOC) respectively. We compare current results with previously obtained ones for the single fcc Ni/Cu(001) bilayer. It is shown, that in the case of the out-of-plane magnetization the magnetic tensor elements of the chi((2))(ijk) tensor are smaller than the nonmagnetic ones by only one order of magnitude. In the case of in-plane magnetization the magnetically induced components of chi((2))(ijk) become even comparable in magnitude with the nonmagnetic ones, confirming the important role of magnetic properties in the NLO response.

A novel effect of the electron spin in nonlinear optics is predicted. Starting from the Pauli Hamiltonian, we present the influence of the electron spin on the nonlinear optical response of a homogeneous electron gas in the low temperature limit. It is shown that the longitudinal second-harmonic response which results from a transverse incoming field differs significantly from the usual paramagnetic response when spin effects are included. The calculation is done both with and without screening.

A theoretical model of linear and nonlinear optics (second-harmonic generation, SHG) beyond the local electric dipole model is presented, and a simple calculation using a homogeneous electron gas is performed to show that linear and nonlinear optics are both heavily influenced by the presence of diamagnetism and screening. Also shown in the calculation is, that with the present model, the SHG response from centrosymmetric materials can be described. In the local electric dipole approximation, this response is zero for symmetry reasons, but experiments show it should be nonzero.

We present a local-field theory for spin and diamagnetism in linear and nonlinear optics. We examine all the processes contained in the Pauli Hamiltonian and its corresponding microscopic current density, including the terms depending on the electron spin. The resulting general real-space conductivities are presented and discussed. To quantify the implications of including the spin, we study the linear and nonlinear optical properties of free-electron metals, represented by the screened homogeneous electron gas. The real-space formalism is transformed into Fourier space, and the symmetries of the linear and nonlinear optical conductivities in a homogeneous electron gas are discussed. Numerical results are presented for the homogeneous electron gas, in which we treat omega and q as independent variables, thereby opening the theory to near-field optics and the study of evanescent waves. We show that in regions of the omega-q spectrum, the presence of diamagnetism and spin dynamics significantly alters the response in comparison to considering only the paramagnetic response. Additionally, we discuss the effects of screening, and we finish our treatment by a discussion of how to connect the present theory to existing methods in ab initio solid-state physics.

We compute the nonlinear optical response of an Fe monolayer placed on top of 1 to 4 monolayers of Cu(001). Our calculation is based on ab initio eigenstates of the slab, which are obtained within the full-potential linearized augmented plane-wave method. The ground-state spin-polarized electronic structure is converged self-consistently to an accuracy better than 0.1 mRy. Subsequently, we take the spin-orbit interaction into account within a second variational treatment. The new set of eigenstates allows us to calculate the magneto-optical transition matrix elements. The second-harmonic response is determined in the reflection geometry with magnetization perpendicular to the surface (the so-called polar configuration) using the surface-sheet model. Adding layers of a noble metal (Cu) to the Fe monolayer gives a new degree of freedom for the inclusion of nonmagnetic Cu d bands to the nonlinear magneto-optical response of the slab, and the energy bands show that such an addition converges essentially to an addition of d states and a small broadening of the d band with growing number of Cu layers. The screened nonlinear optical susceptibility is calculated and converges quite well with a growing number of Cu layers. Our first-principles results confirm that the magnetic tensor elements of the nonlinear optical response tensor are roughly of the same order of magnitude as the nonmagnetic ones (in contrast to linear optics, where the magnetic response is only a minor correction).

In order to study the possible phase conjugation of optical near-fields, it is necessary to go beyond the slowly varying envelope- and electric dipole approximations that are normally applied in phase conjugation studies where spatially non-decaying (or at least slowly decaying) modes are mixed. In the present dissertation, the minimal coupling Hamiltonian is used to create a microscopic theoretical description of degenerate four-wave mixing. It is a semiclassical description where the electromagnetic field is treated as a classical quantity and the active medium is treated quantum mechanically. Numerical results are given for a single-level quantum well (exclusively intraband contributions) and for a two-level quantum well (mainly interband contribution). Focusing of a phase conjugated field is also discussed. (Full-length abstracts in Danish and English included).

The phase conjugated response from nonmagnetic multilevel metallic quantum wells is analyzed and an essentially complete analytical solution is presented and discussed. The description is based on a semiclassical local-field theory for degenerate four-wave mixing in mesoscopic interaction volumes of condensed media developed by the present authors [T. Andersen and O. Keller, Phys. Scr. 58, 132 (1998)]. The analytical solution is supplemented by a numerical analysis of the phase conjugated response from a two-level quantum well in the case where one level is below the Fermi level and the other level is above. This is the simplest configuration of a quantum-well phase conjugator in which the light-matter interaction can be tuned to resonance. The phase conjugated response is examined in the case where all the scattering takes place in one plane, and linearly polarized light is used in the mixing. In the numerical work we study a two-monolayer thick copper quantum well using the infinite barrier model potential. Our results show that the phase conjugated response from such a quantum-well system is highly dependent on the spatial dispersion of the matter response. The resonances showing up in the numerical results are analytically identified from the expressions for the linear and nonlinear response tensors. In addition to the general discussion of the phase conjugated response with varying frequency and parallel component of the wave vector, we present the phase conjugated response in the special case where the light is in resonance with the interband transition. [S0163-1829(99)03448-7].

A local-field theory describing optical phase conjugation in condensed media in the special case of degenerate four wave mixing is established. The aim of the theory is to form the framework for microscopic studies of optical phase conjugation (i) of evanescent fields in the context of near-field optics, (ii) in mesoscopic films and quantum wells, (iii) in small particles, and (iv) in lossy media where the field penetration depth is comparable to or (substantially) less than the vacuum wavelength of the driving field. The aforementioned goal makes it necessary to abandon both the slowly varying envelope- and the electric dipole approximations usually adopted in phase conjugation studies where spatially slowly decaying or modulated fields are mixed. By keeping in the interaction Hamiltonian the term of second order in the vector potential and in the current-density operator the term of first order in the vector potential new microscopic field-matter interaction processes of particular importance in the present context are included. The physics of the various nonlinear microscopic processes is analysed, and systematised by presenting in diagrammatic form the nonlocal electrodynamics hidden in the nonlinear constitutive relation. A new nonlocal conductivity tensor, enabling one to describe the degenerate four wave mixing process among the prevailing local fields, is presented and the eigensymmetries of its various parts are analysed. Starting from the general local-field theory a degenerate four wave mixing response tensor of relevance for media exhibiting two-dimensional translational invariance is established and discussed. In the last part of the paper an integral equation allowing one to obtain the phase conjugated local field inside and outside the nonlinear medium is established, discussed and formally solved, and it is pointed out that this equation for systems with two-dimensional translational invariance often can be analysed analytically and numerically using methods previously developed in theoretical studies of the linear local-field electrodynamics of mesoscopic films.

Using a recently developed local-field theory for optical phase conjugation we study the confinement of phase conjugated light in the near field regime. A dipole wire acting as the source of radiation is placed in vacuum outside a single-level metallic quantum well phase conjugator at a distance of less than a quarter of an optical wavelength. A single-level well is particularly effective in phase conjugating evanescent modes and with a properly oriented wire current a field compression substantially beyond the Rayleigh limit is obtained.

We present a calculation of the phase conjugated response from a single-level metallic quantum well. The description builds upon a recently developed local-field theory for degenerate four-wave mixing in mesoscopic interaction volumes of condensed media. The single-level quantum well represents the simplest possible configuration of a quantum well phase conjugator. Furthermore, the single-level quantum well is an interesting object, since the optical response contains no dipole terms. The discussion of the response is based on the use of linearly polarized light to excite the process. We demonstrate that the phase conjugation process is extremely efficient in the evanescent regime of the wave-vector spectrum. We also address the problem of plane-wave excitation of the high wave-number end of the evanescent regime. We end our discussion by suggesting the use of a broadband source to excite the process. One such broad angular band source is a quantum wire, and the phase conjugated angular spectrum from a quantum wire is presented and discussed.