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This chapter discusses the properties of surface excitons and polaritons. It presents a discussion of surface polaritons at the sharp interface between media, the observation of exciton surface polaritons at room temperature, the surface excitons in the presence of a transition layer, and nonlinear surface polaritons. The two-dimensional site shift surface excitons in outermost monolayers of molecular crystals and their properties are also considered. It is shown that in quasi-one dimensional crystals with molecules having the strong mixing of Frenkel- and CT exciton states at the edge of even perfect chain exciton states can be formed localized at edge analogous to Tamm states in perfect semiconductors.

This chapter deals with Bose-Einstein condensation of excitons. After a historical survey of searching for Bose-Einstein condensation of excitons, the chapter focuses on long-lived indirect excitons in coupled quantum wells: Bose-Einstein condensation of noninteracting bosons in a two-dimensional parabolic trap is outlined, and the ground-state wavefunction of indirect excitons is discussed in detail. Recent theoretical results on the concentration-dependent blue shift and broadening of the photoluminescence line associated with indirect excitons are discussed.

This chapter starts from the Lorentz oscillator model then discusses the optical transitions in semiconductors. After an introduction of Wannier-Mott and Frenkel excitons, the Hopfield equations are used to describe exciton-polaritons in bulk crystals. The non-local dielectric response theory is used to describe the optical response of semiconductor quantum wells, wires, and dots. Finally, the dispersion of exciton-polaritons in microcavities is derived, and the threshold between weak and strong light-matter coupling regime is discussed.

This chapter presents a second quantized theory of Frenkel excitons. An energy operator for a molecular crystal in the second quantization is introduced, and excitonic states for crystals with one and more molecules per unit cell are considered. The paulions are introduced in consideration of the two-level molecular model, and exact transformation from paulions to bosons is demonstrated. The mechanism of self-trapping of Frenkel excitons and spectra and transport of self-trapped excitons are discussed. The interaction of excitons and elementary lattice excitations is considered. The results of calculations of exciton states for some molecular crystals (anthracene, naphthalene, tetracene and pentacene) are presented.

This chapter reviews attempts to realize Bose-Einstein condensation of excitons in bulk semiconductors and semiconductor nanostructures, emphasizing on indirect (interwell) excitons in GaAs/AlGaAs coupled double quantum wells. Recent experiments with indirect excitons confined in a circular electric-field-induced in-plane trap are discussed. The observed spatially-ordered patterns in the photoluminescence signal and large coherence length are interpreted in terms of Bose-Einstein condensation of indirect excitons.

This introductory chapter defines the main topics of the book. It recalls the notions of molecular crystals and of Frenkel-and Wannier-Mott excitons, giving basic properties and constitutive equations. The relation between excitonic absorption spectrum and spectrum of excitonic luminescence is also given.

This chapter discusses Frenkel excitonic states in Heitler-London approximation. The Coulomb and mechanical excitons in molecular crystals are introduced using the crystal Hamiltonian. The excitons symmetry properties are considered via group-theoretical methods. Applications to crystals of naphthalene type are given.

This chapter discusses the properties of excitons molecular structures containing several monolayers, and its spectra and nonlinear effects at interfaces. The different so-called gas-condensed matter shift at surfaces and interfaces can be used for engineering of exciton spectra in multilayer structures with maximal or minimal energy of excitons at interface. The Fermi resonance for modes propagating along the interfaces and a bistable energy transmission across the interface with Fermi resonance under influence of pumping of molecular vibration from one side of interface is considered.

This chapter reviews the physics of conceptually new hybrid organic-inorganic semiconductor nanostructures. The resonant coupling between the Frenkel and Wannier-Mott exciton states in two different materials of the nanostructure can lead to fast and efficient non-contact and non-radiative energy transfer. Furthermore, the exciton-mediated nonlinearities are strongly enhanced in these artificial structures. A new family of light emitting devices based on the hybrid organic-inorganic semiconductor nanostructures is also discussed.

This chapter reviews electric-field-induced Raman scattering of the light field, which resonates with the interband transition in a semiconductor. The role played by the Franz-Keldysh effect in surface space-charge electric-field-induced Raman scattering is emphasized. A strong enhancement of the efficiency of ‘forbidden’ exciton-mediated Raman scattering by applying a static electric field, the effect predicted and observed by the author and his colleagues, are also discussed.

This chapter reviews recent achievements in the experimental realization of a cold but still dense gas of long-lived indirect excitons in GaAs-based coupled quantum well structures. The unusual dynamics (photoluminescence jump) and pattern formation (inner and external rings, as well as fragmentation of the external ring in circular-shaped spots), which were detected in time-resolved and spatially-resolved photoluminescence associated with indirect excitons, are described in detail.

This chapter discusses a collective coherent ground state in ‘exciton-like’ many-body systems (excitons, microcavity polaritons, quantum Hall bilayers, dimer spin systems, ultracold atomic fermi gases, etc.) in terms of Bose-Einstein condensation. With both cases, the high-density limit (BCS limit) and low-density limit (BEC limit) are described in a unified way. The decoherence effects which are relevant to a thermodynamically open system such as short-lived microcavity polaritons are outlined.

This chapter describes metal-insulator transitions in disorder-free crystals. It reviews excitonic insulators, electron-hole liquids, and metal-insulator transitions due to band crossing. The realization of exciton (Bose-Einstein) condensate in the high-density limit is illustrated by experiments on a quantum Hall bilayer. In the second part of the chapter, the Mott transition from a metal to a Coulomb localized insulator is discussed in terms of the one band Hubbard model.

This chapter reviews an electron-hole liquid. This very interesting phase — observed mainly in bulk indirect semiconductors, Ge and Si — was thoroughly investigated in the late 1960s to mid 1980s. The first finding of the electron-hole liquid with the following study of its many unusual properties was initiated by L. V. Keldysh.

This chapter reviews the recently proposed concept of resonant acousto-optics, when interaction between the light and acoustic waves is mediated and strongly enhanced by the polarization field associated with either excitons (i.e., visible spectral band) or TO-phonons (THz band). For the resonant acousto-optics, both dominant interactions — the polarization-light coupling (polariton effect) and the interaction of the polarization wave with the acoustically-induced grating — are treated non-perturbatively (strong coupling regime) and on an equal basis.

Luminescence of excitons’ treats in a comprehensive way a series of luminescence manifestations of bound electron–hole pairs, i.e. excitons. First, the concept of a free Wannier exciton is introduced and a shape of the pertinent absorption spectrum is described. Then multiple luminescence effects due to the free exciton radiative recombination are discussed in direct bandgap materials (exciton–polariton luminescence and LO-phonon assisted exciton annihilation) as well as in indirect bandgap materials (no-phonon luminescence, phonon replicas). Relevant selection rules are derived. Various kinds of excitons bound to impurities (neutral and/or ionized donors and acceptors, Haynes’ rule) are dealt with. Description of the luminescence of bound multiexciton complexes follows. The application of the bound exciton concept to quantitative impurity analysis in silicon is described. Finally, the cases of excitons bound to isoelectronic impurities and of self-trapped excitons are analyzed. Whenever meaningful, a particular characteristic emission lineshape that helps to decipher experimental spectrum is pointed out.

Excitons are the primary photo-excited states in conjugated polymers. This chapter develops effective-particle models of excitons in the weak and strong-coupling limits. Excitons are classified as being Mott-Wannier in the weak-coupling limit. This class includes Frenkel, charge-transfer, and Wannier excitons. An exciton wavefunction mapping is introduced that transforms configuration interaction amplitudes into real-space wavefunctions. Excitons are classified as being Mott-Hubbard in the strong-coupling limit. Finally, numerical calculations of the intermediate coupling regime are described.