It is shown that the ordinary semiclassical theory of the absorption of light by exciton states is not completely satisfactory (in contrast to the case of absorption due to interband transitions). A more complete theory is developed. It is shown that excitons are approximate bosons, and, in interaction with the electromagnetic field, the exciton field plays the role of the classical polarization field. The eigenstates of the system of crystal and radiation field are mixtures of photons and excitons. The ordinary one-quantum optical lifetime of an excitation is infinite. Absorption occurs only when "three-body" processes are introduced. The theory includes "local field" effects, leading to the Lorentz local field correction when it is applicable. A Smakula equation for the oscillator strength in terms of the integrated absorption constant is derived.

a Quantum-Mechanical Theory of the Contribution of Excitons to the Complex Dielectric Constant of Crystals.

Theory of elementary excitations in quasiperiodic structures

Nonreciprocal surface waves

Plasmons and magnetoplasmons in semiconductor heterostructures

General dispersion equations for exchange, magnetostatic and retarded waves in infinite and semi-infinite ferromagnetic superlattices are derived by the 'transfer matrix' (T matrix) method. The elementary unit of the superlattices under consideration consists of N different ferromagnetic layers. The equations for bulk waves are expressed through the diagonal elements of the T matrix whereas those for surface waves are expressed through the T matrix and a column matrix D which describes surface boundary conditions. The explicit form of the T and D matrices is given for 'pure' exchange and 'pure' dipolar waves.

Spin waves in superlattices. I. General dispersion equations for exchange, magnetostatic and retarded modes

Spin waves in a magnetic superlattice

Etude de l'effet d'un champ magnetique externe sur des polaritons de plasmon se propageant dans des superreseaux n-i-p-i tronques. On utilise une methode de matrice de transfert pour simplifier les expressions algebriques. Analyse numerique des differences de spectre avec ou sans la presence d'un champ magnetique externe

Magnetoplasmon polaritons in finite n-i-p-i superlattices

The acoustic and optic modes of a one-dimensional model of a diatomic superlattice are considered. By employing a microscopic theory it is possible to explore the zone-folding characteristics. The treatment is based on linear lattice dynamics with nearest neighbour interactions. The equations of motion for the different atoms (on either side) at two superlattice interfaces replace the boundary conditions used in continuum theories. This microscopic approach has the advantage that both optic and acoustic branches are described within a single formulation and without recourse to the long wavelength limit. The algebra, which is otherwise quite involved, is simplified by the use of transfer matrix techniques. The spectrum contains a number of interesting features whose underlying physical principles are discussed.



Die akustischen und optischen Moden eines eindimensionalen Modells eines diatomaren Supergitters werden betrachtet. Durch Anwendung einer mikroskopischen Theorie ist es moglich, die Zonenfaltungscharakteristiken zu untersuchen. Das Verfahren beruht auf einer linearen Gitterdynamik mit Wechselwirkungen nachster Nachbarn. Bewegungsgleichungen fur die verschiedenen Atome (auf jeder Seite) an zwei Supergittergrenzflachen ersetzen die Randbedingungen, die in den Kontinuumstheorien benutzt werden. Dies mikroskopische Verfahren hat den Vorteil, das sowohl optische als auch akustische Zweige innerhalb einer einzigen Formel auch ohne Ruckgriff auf den Grenzfall langer Wellenlangen beschrieben werden. Die Algebra, die im anderen Fall ver wickelt ist, wird durch die Benutzung der Transfermatrixtechnik vereinfacht. Das Spektrum enthalt eine Anzahl von interessanten Einzelheiten, und die ihnen unterliegenden physikalischen Prinzipien werden diskutiert.

Lattice Dynamics for Superlattices

Theory of far-infrared ATR from surface polaritons in an n-i-p-i superlattice

The surface exciton polariton spectrum in superlattices made up of two alternating media A and B is reported, where one of the layers shows spatial dispersion in its frequency dependent dielectric function. A theoretical approach based on the effective medium theory to determine the surface exciton polariton dispersion relation is used. Also the attenuated total reflectivity (ATR) is calculated to probe the surface exciton polariton modes.

P. Fulco

Papers

Spin waves in a magnetic superlattice

Magnetoplasmon polaritons in finite n-i-p-i superlattices

Lattice Dynamics for Superlattices

Theory of far-infrared ATR from surface polaritons in an n-i-p-i superlattice

Far-infrared ATR from surface exciton polaritons