Christianus M.J. Wijers

Exact solution of the optical response of thick slabs in the discrete dipole approach

The recently developed double cell technique, which describes the optical response of an arbitrary semi-infinite dielectric crystal taking into account internal field effects, is extended to include the response of thick slabs. The surface sensitivity of the first technique is fully retained. The implications of the internal field effects on the microscopy of these thick slabs are examined for three simple model systems. Further, we investigated under which conditions deviations from classical Fresnel-behaviour are to be expected and how important these corrections are.

Second harmonic generation from thin slabs in the discrete dipole approach

The nonlinear optical response of thin Si slabs is calculated using a discrete dipole approach. The s-polarized second harmonic response as a function of the angle of incidence appears to be in reasonable agreement with experimental results. The p-polarized SHG shows a high sensitivity for the shape of the polarizability profile.

Ab initio calculation of the reflectance anisotropy of GaAs(110): the role of nonlocal polarizability and local fields

We demonstrate that the description of the optical reflectance anisotropy of GaAs(110) requires a complete microscopic treatment of both surface and bulk, which is feasible in the discrete cellular method. This method is an extension of standard discrete dipole calculations and accounts for non-locality in the electro-dynamical and quantum-mechanical interactions through the use of both real space local fields and ab-initio nonlocal polarizabilities. The results of our calculations are in excellent agreement with experiment and we show that the anisotropy is surface induced.

Full microscopic treatment of the optical response of the Si(100)2x1 surface

The optical reflection from the Si(100) 2 × 1 surface has been calculated, using the discrete dipole model and local polarizabilities obtained from quantum mechanical cluster calculations. Results have been compared with experimental differential reflectance (Si) and optical anisotropy measurements (Ge).

Change of the surface induced optical anisotropy of the clean Si(110) surface by oxidation

Normal incidence ellipsometry has been used to measure the change in the complex anisotropic reflectance ratio ? upon oxidation of the clean Si(110)16 × 2 surface. The spectroscopic change in the amplitude of ? (tan(?)) shows a broad maximum of height 1.4 × 10?3 in the high energy region above 2.5 eV. No phase shift difference for the reflectance coefficients belonging to the surface principal optical axes has been measured. A Kramers-Kronig transformation of the amplitude ratio showed that a change in the phase is not expected. The change in tan(?) indicates that the change in reflection upon oxidation in the optical region ismainly in the (10) direction.

Microscopic treatment of the IR spectroscopy of CO physisorbed onNaCl(100)

The infrared spectrum of a monolayer of CO physisorbed on the NaCl(100) surface has been calculated using the discrete dipole approach. The corresponding description is rigorously microscopic and accounts exactly for the occurring local fields in both the substrate and the adsorbed monolayer. The model does not impose restrictions upon the configuration and links smoothly the microscopy of the system to the externally observable macroscopic response. Using established values for the geometry and polarizabilities of the NaCl, we have calculated the integrated absorbance of the system. We find that, in order to arrive at an agreement between theory and experiment, the usual values for the polarizability and/or the height above the surface of the CO molecule should be decreased. No unique answer as to the value of these two parameters can be obtained from interpreting IR absorption experiments.

Reconstructions of the Ge(0 0 1) surface

We have performed dipole calculations of energies of the Ge(0 0 1) surface to compare the ground states of b(2 × 1), c(4 × 2), p(2 × 2) and p(4 × 1) symmetry dimer reconstructions. We have found that p(2 × 2) is the lowest energy reconstruction at zero temperature.

Nonlocality and optics of inhomogeneous systems : The role of quantum induction

Nonlocal interactions play a prominent role in the optics of inhomogeneous systems. Classical discrete dipole descriptions take into account only electro-magnetic nonlocality. This is insufficient to describe correctly the inhomogeneous optical response (e.g., reflectance anisotropy) for covalently bonded systems like semiconductor surfaces. For those systems also a prominent quantum mechanical nonlocality exists. In a cellular description this can be understood easily from the behavior of the wave function. For strongly bonded systems the wave function extends across cell boundaries and when cells are polarized, neighboring cells get polarized as well. This quantum induction introduces nonlocal polarizabilities in the description. The technical details how discrete dipole models have to be adapted to use nonlocal polarizabilities in finite systems and crystalline slabs and surfaces are given in this paper. The modified method is called discrete cellular method.

Effect of Linear Polarisability and Local-Fields on Surface Shg

A discrete dipole model has been developed to describe Surface Second Harmonic Generation by centrosymmetric semiconductors. The double cell method, which enables the linear reflection problem to be solved numerically for semi-infinite systems, has been extended for the nonlinear case. It is shown that a single layer of nonlinear electric dipoles at the surface and nonlocal effects allows to describe the angle of incidence dependent anisotropic SHG obtained from oxidised Si(001) wafers. The influence of the linear response, turns out to be essential to understand the anisotropic SHG-process.

The Local Field and What It Means

The easy link between the dielectric constant of isotropic media and the molecular polarizability of the constituents, is traditionally given by the Lorentz-Lorenz (LL) relation. The local field concept however upon which this relation is based, needs to be re-examined for highly inhomogeneous systems, like surfaces and interfaces. Local fields belong inherently to discrete dipole models, but for the comparison with continuum approaches only descriptions based on average fields, are the more suited ones. This holds particularly for the comparison with the Fourier-type solution of the continuum problem, upon which the classical Adler and Wiser papers [1, 2] are based.