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.

Hot excitons in semiconductors

Typically, the light-emission of semiconductors always occurs from thermalized electrons, as electrons excited above the bandgap energy relax quickly. In contrast, non-thermalized excitonic light emission has now been observed in nanowires using resonant plasmonic nanocavities. The much higher radiative light-emission rates of the hot excitons suggest their use for ultrafast nanophotonic devices.

Tailoring hot-exciton emission and lifetimes in semiconducting nanowires via whispering-gallery nanocavity plasmons

The emission spectra of CdS1–xSex solid solutions (x ≦ 0.3) at helium temperatures are studied under monochromatic excitation in different regions of the absorption spectrum. In all samples very intense luminescence due to excitons localized by composition fluctuations is observed. High intensity of luminescence and complete absence of free exciton emission indicate a high probability of exciton localization. A model for exciton localization is proposed. It is assumed that only the hole component of the exciton is localized by the potential fluctuations. The electron is bound to this hole by Coulomb attraction. The energy migration between the localized states is discussed. Three different regions can be distinguished in the excitation spectrum of localized excitons. The first region corresponds to the excitation of free excitons which are trapped to localization sites in the process of energy migration. There is no energy migration in the third spectral region and all the emission is caused by directly photo-excited states. In the intermediate region II a limited energy transfer between the localized states takes place via phonon assisted tunneling.



Die Emissionsspektren von CdS1–xSex-Mischkristallen (x ≦ 0,3) werden bei Heliumtemperaturen unter monochromatischer Anregung in verschiedenen Bereichen des Absorptionsspektrums unter- sucht. In allen Proben wird sehr intensive Lumineszenz von Exzitonen beobachtet, die an Zusam- mensetzungsfluktuationen lokalisiert sind. Die hohe Intensitat der Lumineszenz und das vollstandige Fehlen der Emission freier Exzitonen weist auf eine hohe Wahrscheinlichkeit von Exzitonlokalisierung hin. Ein Modell fur die Exzitonlokalisierung wird vorgeschlagen. Es wird angenommen, ds nur der Lochanteil des Exzitons durch die Potentialfluktuationen lokalisiert ist. Das Elektron ist an diesem Loch durch Coulombanziehung gebunden. Die Energieubertragung zwischen den lokalisierten Zustanden wird diskutiert. Drei unterschiedliche Bereiche lassen sich im Anregungsspektrum der lokalisierten Exzitonen unterscheiden. Der erste Bereich entspricht der Anregung freier Exzitonen, die an Lokalisierungsplatzen wahrend der Energieubertragung getrapt werden. Im dritten Spektralbereich existiert keine Energieubertragung und die gesamte Emission wird durch direkt photo-angeregte Zustande verursacht. Im Zwischenbereich II findet ein begrenzter Energieubergang zwischen den lokalisierten Zustanden uber phononassistierte Tunnelung statt.

Localized Excitons in CdS1−xSex Solid Solutions

GaN epitaxial layers on sapphire substrates were grown by the rotating disk metal organic chemical vapor deposition technique. Excitonic transitions from conduction band to spin-orbit split valence bands were observed. At 12 K we observed donor bound exciton and a very weak acceptor bound exciton. The temperature dependence of luminescence peak positions of free-excitons A and B were fitted to the Varshni’s equation to study the variation of the band gap with temperature. The linewidth of the free exciton (A) was studied as a function of temperature and was explained by theoretical model considering the scattering of excitons with acoustic phonons and longitudinal optical phonons. In the 12 K spectrum we also observed phonon-assisted excitonic transitions. The activation energy of the free exciton (A) was found to be 26 meV, while that of the donor bound exciton was 7 meV. The binding energy of the donor was estimated as 35 meV and that of the acceptor as 250 meV. The band gap of GaN was found to be 3.505 eV at 12 K and 3.437 at room temperature. All the parameters obtained in the present investigation are compared with those reported in the literature.

Photoluminescence studies of excitonic transitions in GaN epitaxial layers

Hot excitons and exciton excitation spectra

Polariton emission from crystals

The emission spectrum of the zero-phonon n = 1 A exciton line in CdS crystals has been investigated for temperatures in the range 2 to 25 K with high resolution and compared with the polariton dispersion curve which has heen measured experimentally. It has been found that the resonant exciton luminescence arises from the emission of polaritons of the lower branch of the polariton dispersion curve. The emission line consists of two maxima, one being connected with the “knee” of the polariton dispersion curve and the other arising from the anomaly in the crystal boundary transparency.

E. Gross

Papers

Hot excitons and exciton excitation spectra

Polariton emission from crystals

The Resonant Luminescence of Free Excitons in Crystals