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The role of non-trivial quantum mechanical effects in biology has been the subject of intense scrutiny over the past decade. Much of the focus on potential “quantum biology” has been on energy transfer processes in photosynthetic light harvesting systems. Ultrafast laser spectroscopy of several light harvesting proteins has uncovered coherent oscillations dubbed “quantum beats” that persist for hundreds of femtoseconds and are putative signatures for quantum transport phenomena. This review describes the language and basic quantum mechanical phenomena that underpin quantum transport in open systems such as light harvesting and photosynthetic proteins, including the photosystem reaction centre. Coherent effects are discussed in detail, separating various meanings of the term, from delocalized excitations, or excitons, to entangled states and coherent transport. In particular, we focus on the time, energy and length scales of energy transport processes, as these are critical in understanding whether or not coherent processes are important. The role played by the protein in maintaining chromophore systems is analysed. Finally, the spectroscopic techniques that are used to probe energy transfer dynamics and that have uncovered the quantum beats are described with reference to coherent phenomena in light harvesting.

Selected impurities are frequently introduced into semiconductors to make them n–type or p–type. The introduction of impurities into a crystal lattice not only shifts the Fermi level, but also results in a perturbation to the periodic potential, giving rise to bound impurity levels which often occur in the band gap of the semiconductor.

In this chapter, we summarize recent accomplishments in the area of high-pressure luminescence spectroscopy of phosphor materials. The effect of pressure on the luminescence related to f-f, d-d, and d-f transitions is discussed. Several recent examples from the literature are presented to illustrate the influence of pressure on luminescence energy, intensity, lineshape, luminescence kinetics, and luminescence efficiency. Especially, the unique ability of pressure to investigate the influence of impurity-trapped exciton states, which are created after ionization and charge-transfer transitions, on the luminescence of TM and RE ions in solids and energy-transfer processes are presented.

We used a variety of optical probe techniques including broadband femtosecond transient and continuous wave (cw) photomodulation spectroscopies and electroabsorption for studying the photophysics in two typical π-conjugated polymers, namely regio-regular poly (3-hexyl thiophene) (RR-P3HT) with self-organized π-stacked two-dimensional lamellae, and 2-methoxy-5-(2′-ethylhexyloxy) poly(p-para-phenylene-vinylene) (MEH-PPV) with amorphous nanomorphology; both polymers in pristine and blend with fullerene molecules. In the pristine forms we identified singlet excitons as the primary photoexcitations, having typical photoinduced absorption (PA) band that is correlated with stimulated emission. In contrast, in polymer/fullerene blends the photogenerated excitons quickly decay giving rise to a novel photoexcitation species, and the stimulated emission is absent. For the blends we provide strong evidence for the existence of charge transfer complex (CTC) manifold that is formed inside the optical gap of the polymer and fullerene constituents, which is clearly revealed in the electro-absorption spectrum. Because the lowest energy CTC lies below the optical gap then it is possible to directly generate polarons in the blends without involving intrachain excitons in the polymer phase, when using below gap pump excitation. When using excellent quality materials we present evidence that the CTC states are populated in RR-P3HT/fullerene blend within 20 ps following exciton photogeneration in the polymer chains; but no charge polarons are generated on their expense up to ~2 ns. Interestingly the CTC states are photogenerated much faster in D-A blends having smaller domain size such as in regio-random P3HT/PCBM and MEH-PPV/C60; however the CTCs do not easily dissociate in these blends because of the large binding energy. Our findings indicate that the CTC state and film morphology play a crucial role in carrier photogeneration in donor-acceptor blends. More thorough investigation of the CTC and its interaction with free polaron excitations may improve the power conversion efficiency of organic solar cells and drive the development of novel photoactive materials.

The consideration of the screening dynamics prevents the formulation of a Bethe-Salpeter equation for the macroscopic polarization function that only depends on one frequency. This is only possible within approximate schemes, for instance the Shindo approximation. Due to the incomplete dynamical screening excitonic effects are increased. Including dynamical lattice screening the question which dielectric constant, the static electronic or the total one, has to be used to screen a Wannier-Mott exciton is discussed. Another consequence of the dynamically screened electron-hole attraction is an additional loss mechanism, the interference terms, which modify the spectral strength and the satellite structures over and above the intrinsic and extrinsic losses known from the single-quasiparticle description. It is shown that the combination of intrinsic, interference, and extrinsic effects leads to strong reduction of the satellite strengths, in particular for vanishing energies of an electron emitted in photoemission. The screening in a non-metal is significantly modified in the presence of free carriers. The band filling gives rise to a tendency for a Mott transition, a gap shrinkage, and a Burstein-Moss shift. However, even for large carrier densities Coulomb effects survive resulting e.g. in Mahan excitons.

Self-assembled nanofibers of organic dye thiacyanine (TC) with lengths of up to \( {\sim} 250\,\upmu {\text{m}} \) function as efficient active waveguides that propagate fluorescence (FL) over their entire lengths along the fiber axis. A spectroscopic investigation of the active waveguiding properties revealed that the FL strongly couples with molecular excitons and propagates in the form of exciton-polaritons. Such long-range propagation of exciton-polaritons at room temperature is rarely observed in inorganic materials. The high stability of the exciton-polaritons in the organic dye nanofibers is attributed to the large longitudinal transverse exciton splitting energy and exciton binding energy with respect to thermal energy. Unlike light propagating in conventional waveguides, exciton-polaritons can pass through bends in nanofibers with micron-scale radii of curvature. Utilizing this property, we fabricated miniaturized photonic circuit components using nanofiber building blocks. The fabricated components, including Mach–Zehnder interferometers and microring resonators, exhibit considerably high performance for their micron-scale dimensions. In addition to such photonic device applications, the organic dye nanofibers are ideal systems for studying the physics underlying strong light–matter interactions. In particular, the highly stable nature of the exciton-polaritons at relatively high temperature offers the possibility of a representative novel quantum phenomenon in their Bose–Einstein condensation (BEC). A theoretical analysis of this exciton-polariton BEC in the nanofiber system is presented in this chapter.

The photophysics of planar microcavities which employ organic materials as the optically resonant medium to achieve the strong-coupling regime is discussed. While as a result of the light–matter coupling, cavity polariton branches appear which are analogous to those observed in inorganic microcavities, many properties of organic-based microcavities are qualitatively and quantitatively different. The electronic excitations involved are molecular Frenkel excitons, rather than large radius Wannier excitons, which lead to large Rabi splitting values. The effects of disorder are typically much more pronounced as well as the exciton-phonon coupling, possibly leading to vibronic replicas. As a consequence, polariton relaxation and polariton-polariton scattering mechanisms also show features specific to the organic material employed. The field of organic-based microcavities is attracting an increasing interest as high excitation density phenomena such as polariton lasing have recently been reported. In view of their experimental relevance, two different kinds of organic microcavities, disordered J-aggregate-based microcavities and crystalline anthracene microcavities, are considered.

This chapter describes the optical properties of graphene quantum dots. It discusses the size, shape and edge dependence of the energy gap, optical joint density of states, excitons, charged excitons, optical spin blockade and optical control of the magnetic moment in triangular graphene quantum dots with zigzag edges. The electronic and optical properties of colloidal graphene quantum dots, and in particular the spectrum of band-edge excitons is described.

The dynamics of spin singlet and triplet excitons in p-conjugated polymers define their performance as optically active layer in organic light-emitting diodes (OLEDs) and organic photovoltaic (OPV) cells. As an example, if both triplet and singlet excitons can be used in OLEDs to convert electrical energy to electroluminescence (EL) emission, then the fraction of excitons that potentially can emit light may reach 100 % [1]. Similarly in OPV based on donor/acceptor (D–A) blends, the photogenerated singlet exciton in the polymer donor domains may recombine before reaching the D–A interface, because of its relatively short lifetime (~100 ps). In contrast, because of the much longer lifetime (~5 ms), triplet excitons could reach the D–A interface with larger probability and thus could potentially be the answer to this loss mechanism [2]. Therefore, both OLED and OPV technologies may substantially benefit from the proper use of the spin triplet states. Alas, because the spin-orbit coupling (SOC) in polymers is typically very weak (

Introducing phosphorescent emitters into polymers to harvest both singlet and triplet excitons in the EL process has been proved to be an effective approach to enhance the device efficiency [1, 2, 3, 4, 5, 6]. To ensure that all the triplet excitons are confined effectively on the phosphorescent emitters, triplet energy back transfer (TEBT) from phosphors to polymer hosts should be inhibited (see Sect. 1.3 in Chap. 1)

We report on a magneto-photoluminescence study of single neutral excitons confined in single self-assembled semiconductor quantum rings. Oscillations in the exciton radiative recombination energy and in the emission intensity are observed under an applied magnetic field. Special emphasis is placed on the manipulation of this Aharonov-Bohm-type oscillations with a vertical electric field. We observe that both the exciton oscillator strength and the periodicity of the oscillation can be tuned. This tunability is explained by calculating the single particle wave function in both unstrained and strained semiconductor quantum rings in the presence of external electrical fields.

The chapter describes the principles of and reviews recent progress on the Optical Aharonov-Bohm effect in quantum dots focusing on type-II quantum dot structures. Type-II quantum dots have been predicted to display the Optical Aharonov-Bohm effect (OABE) due to the strong polarization of an exciton that results from the spatial separation of the electrons and holes in such systems. The Aharonov-Bohm effect for type-II II–VI Zn(Mn)Te/ZnSe quantum dots is the principal topic of this chapter, with additional discussion of InGaAs/GaAs, InP/GaAs and GeSi type-II quantum dots, in which these effects have also been observed.

Quantum interference patterns predicted by theory due to the finite structure of neutral excitons in InAs quantum rings are corroborated experimentally in the magneto-photoluminescence spectra of these nanostructures. The effects associated to built in electric fields and to the temperature on these Aharonov-Bohm-like oscillations are described and confirmed by complementary experimental procedures.

Ultrasonic and hypersonic waves interact with semiconductor lattice vibrations, as well as with various impurity centers, defects, free carriers, and excitons existing in crystals. It was shown that the acoustic effects on excitons at sufficiently low concentrations are one to three orders of magnitude higher than the lattice absorption of hypersound.

Organic light emitting devices are self-emitting light source based on radiative decay of exciton generated by the carrier recombination on an organic molecule, and utilized for various applications. The device efficiency has been improved drastically in recent a couple of decades. This chapter deals with the mechanism and materials of the devices and the improvement in luminance efficiency.

As a result of Coulomb binding of electrons and holes into excitons, a system can resonantly absorb energy from a microwave electromagnetic field. In addition to information about the structure of the conduction and valence bands, obtained by means of spin resonance of the conduction electrons (holes), exciton paramagnetic resonance can yield information about the exciton energy spectrum. This chapter discusses the contribution to the exciton spin–lattice relaxation of the dynamic contact hyperfine interaction between electron components of free excitons (s electrons of the conduction band, which are coupled with holes from the valence band during the formation of free excitons) with the nuclear spins of a crystal lattice. Despite the complete averaging of the hyperfine interaction during the motion of the individual exciton, the hyperfine interaction with the nuclear spin ensemble makes a nonzero contribution to the linewidth of the exciton paramagnetic resonance.

Microcavity exciton-polaritons are quantum quasi-particles arising from the strong light-matter coupling. They have exhibited rich quantum dynamics rooted from bosonic nature and inherent non-equilibrium condition. Dynamical condensation in microcavity exciton-polaritons has been observed at much elevated temperatures in comparison to ultracold atom condensates. Recently, we have investigated the behavior of exciton-polariton condensates in artificial trap and lattice geometries in zero-dimension, one-dimension (1D) and two-dimension (2D). Coherent π-state with p-wave order in a 1D condensate array and d-orbital state in a 2D square lattice are observed. We anticipate that the preparation of high-orbital condensates can be further extended to probe dynamical quantum phase transition in a controlled manner as quantum emulation applications.

This is a review of spin-dependent (polarization) properties of multicomponent exciton-polariton condensates in conditions when quasi-equilibrium mean-field Gross-Pitaevskii description can be applied. Mainly two-component (spin states ±1) polariton condensates are addressed, but some properties of four-component exciton condensates, having both the bright (spin ±1) and the dark (spin ±2) components, are discussed. Change of polarization state of the condensate and phase transitions in applied Zeeman field are described. The properties of fractional vortices are given, in particular, I present recent results on the warping of the field around half-vortices in the presence of longitudinal-transverse splitting of bare polariton bands, and discuss the geometrical features of warped half-vortices (in the framework of the lemon, monstar, and star classification).

Excitons in crystals may affect the NMR spectra in all cases when hyperfine interaction of the charge carriers forming excitons with the nuclear spins takes place. The modulation of this interaction by the lattice vibrations influences the nuclear spin relaxation rate through the excitons. The local magnetic field of the excitons also shifts the energy of the nuclear spin levels and, correspondingly, the NMR frequencies (the exciton Knight shift). These shifts essentially depend on the type of statistics of the exciton gas. This fact may be used practically for the purpose of diagnostics of the exciton gas. Specifically, the exciton Knight shift exhibits a sharp increase at the phase transition of the exciton gas to the Bose–Einstein condensate state. The influence of excitons on NMR in semiconductors is considered in Sects. 5.1–5.3.

This chapter introduces the essential physics of excitons. In Sects. 1.1 and 1.2, and partially in Sect. 1.5, the individual properties of excitons are considered. The basis of the microscopic theory and the theoretical group classification of exciton states is given. The characteristic properties of excitons are discussed in terms of the relative electron–hole and translational motion. The spin structure and the longitudinal–transverse splitting of excitons due to long-range Coulomb interactions are considered in the framework of a simple two-band model for a semiconductor. The most impressive results are achieved in the physics of high-density excitons, a field that continues to develop rapidly (see Chap. 4). The main results in this domain are summarized in Sect. 1.3. The Lenard–Dyson theorem is an important basis for understanding the stability of the ground state of a system of particles interacting through Coulomb forces. In Sect. 1.5, we discuss the effect of impurities capturing one or two excitons. Examples include the capture of excitons by isoelectron traps, resulting in the formation of bound excitons or localized exciton molecules.

Novel excitonic properties of single-walled carbon nanotubes are studied by single-nanotube spectroscopy and time-resolved spectroscopy. Due to the enhanced Coulomb interaction, the optically generated electron-hole pair forms a strongly “bound exciton” state, analogous to the hydrogen-like state in the carbon nanotubes. The exciton properties and dynamics dominate their optical properties. The striking features of excitons in the carbon nanotube, multiple-exciton states, charged exciton (trion) formation, and exciton-multiplication are described in this chapter.

In the presence of a strong resonant electromagnetic (or hypersonic) field, the energy spectrum of an exciton changes its structure. The quasi-energy spectrum of the exciton–photon system was studied early on for the case when the frequency of the laser radiation is in resonance with the transitions between the degenerate exciton levels. However, neither in these nor in other studies has the time-reversal symmetry been discussed for quasi-energy states.

Carrier scattering at low fields involve a large variety of scattering centers. Types of these scattering centeres are intrinsic lattice defects with acoustic or optical phonons, intrinsic point defects, alloys, extrinsic point defects with charged or neutral impurities; line defects; surface defects at grain boundaries, outer surfaces; metal/semiconductor interfaces. Three dimensional defects as atomic clusters or micro crystalline or colloidal inclusions; and secondary defects such as electron–electron scattering, electron–hole scattering or electron–plasmon scattering. Matthiessen rule is given. Intervalley and intravalley scattering and; warped surface effects are described. Quasi-particles as polarons or exciton interaction is discussed. Elastic and inelastic scattering is evaluated. Each of these scattering mechanisms are theoretically described. Phonon generation and annihilation is introduced. Longitudinal acoustic scattering are analyzed. Deformation potential table is given. Acoustic scattering with piezo electric interaction is shown. Optical phonon scattering in polar and non polar semiconductors are enumerated. Scattering by intrinsic point defects and by neutral defects, as well as by ionic defects are evaluated. Coulomb scattering in anisotropic semiconductors is discussed. Quantum correction for ion scattering is introduced. Carrier–carrier scattering is identified.

This chapter deals with the exchange interaction between the electron and hole forming a free exciton with paramagnetic centers (PCs). During this interaction the reorientation of the PC spin is accompanied by transitions between exciton states.

Traditional spintronics relies on the spin transport by charge carriers, such as electrons in semiconductor crystals. This brings several complications: the Pauli principle prevents the carriers from moving with the same speed, Coulomb repulsion leads to rapid dephasing of electron flows. Spin-optronics is a valuable alternative to the traditional spintronics. In spin-optronic devices the spin currents are carried by electrically neutral bosonic quasi-particles: excitons or exciton-polaritons. They can form highly coherent quantum liquids and carry spins over macroscopic distances. The price to pay is a finite and usually very short life-time of the bosonic spin carriers, which breaks the flow conservation rule. In this chapter we present the theory of exciton spin transport which may be applied to a range of systems where remarkable observations of bosonic spin transport have been reported, in particular, to indirect excitons in coupled GaAs/AlGaAs quantum wells and exciton polaritons in planar microcavities. We describe the effect of spin-orbit interaction of electrons and holes on the exciton spin, account for the Zeeman effect induced by external magnetic fields, long range and short range exchange splittings of the exciton resonances. We also consider the exciton transport in the non-linear regime and discuss the definitions of exciton spin current, polarization current and spin conductivity. We address the perspective of observation of dissipationless exciton spin currents sometimes referred to as “spin superfluidity”.

Calculations based on (occupation constrained) density functional theory using local as well as hybrid functionals to describe the electron-electron exchange and correlation are combined with many-body perturbation theory in order to determine the electronic and optical excitation properties of 5-(pentafluorophenyl)pyrimidin-2-amine, 5-(4-methoxy-2,3,5,6-tetrafluorophenyl)pyrimidin-2-amine, and 5-(4-(dimethylamino)-2,3,5,6-tetrafluorophenyl)pyrimidin-2-amine. Large quasiparticle shifts and exciton binding energies of about 4 eV are found. They cancel each other partially and thus allow for a meaningful description of the molecular optical response within the independent-particle approximation. We find a surprisingly strong influence of local-field effects as well as resonant-nonresonant coupling terms in the electron-hole Hamiltonian on the optical properties.

Optical absorption (ABS), steady-state photoluminescence (PL), resonant Raman, and photoabsorption (PA) spectroscopies are employed to study quantum-size effects in II–VI semiconductor quantum dots (QDs) grown in glass samples. We observe a size-dependent shift in the energetic position of the first exciton peak and have examined the photoinduced evolution of the differential absorption spectra. The Raman shifts of the phonon modes are employed to monitor stoichiometric changes in the composition of the QDs during growth. Two sets of glass samples were prepared from color filters doped with CdS x Se1 − x and Zn x Cd1 − x Te. We analyze the optical properties of QDs through the ABS, PL, resonant Raman, and PA spectroscopies. The glass samples were prepared from commercially available semiconductor doped filters by a two-step thermal treatment. The average size of QDs is estimated from the energetic position of the first exciton peak in the ABS spectrum. A calculation based on a quantized-state effective mass model in the strong confinement regime predicts that the average radius of QDs in the glass samples ranges from 2.9 to 4.9 nm for CdTe and from 2.2 to 9.3 nm for CdS0. 08Se0. 92. We have also studied the nonlinear optical properties of QDs by reviewing the results of size-dependent photoinduced modulations in the first exciton band of CdTe QDs studied by PA spectroscopy.

The impact of a random disorder potential on the dynamical properties of Bose–Einstein condensates is a very wide research field. In microcavities, these studies are even more crucial than in the condensates of cold atoms, since random disorder is naturally present in the semiconductor structures. In this chapter, we consider a stable condensate, defined by a chemical potential, propagating in a random disorder potential, like a liquid flowing through a capillary. We analyze the interplay between the kinetic energy, the localization energy, and the interaction between particles in 1D and 2D polariton condensates. The finite lifetime of polaritons is taken into account as well. In the first part, we remind the results of [Malpuech et al., Phys. Rev. Lett. 98, 206402 (2007)] where we considered the case of a static condensate. In that case, the condensate forms either a glassy insulating phase at low polariton density (strong localization), or a superfluid phase above the percolation threshold. We also show the calculation of the first-order spatial coherence of the condensate versus the condensate density. In the second part, we consider the case of a propagating noninteracting condensate which is always localized because of Anderson localization. The localization length is calculated in the Born approximation. The impact of the finite polariton lifetime is taken into account as well. In the last section, we consider the case of a propagating interacting condensate where the three regimes of strong localization, Anderson localization, and superfluid behavior are accessible. The localization length is calculated versus the system parameters. The localization length is strongly modified with respect to the noninteracting case. It is infinite in the superfluid regime, whereas it is strongly reduced if the fluid flows with a supersonic velocity (Cerenkov regime).

Next Generation solar cells based onMultiple Exciton Generation (MEG) in semiconductorquantum dots (QDs) are described. This application of QDs depends upon efficient MEG in QDs incorporated into PV cells, followed by efficient exciton splitting into free electrons and holes and their efficient separation and collection in the cell contacts to produce multiple free carriers per absorbed photon. Using time-resolved transient absorption, bleaching, photoluminescence and THz spectroscopy, MEG has been initially confirmed in several Group IV-VI, III-V, II-VI, and IV colloidal semiconductor QDs. Some controversy using these techniques have now been attributed to effects of the variable of the QD surface chemisty and under certain conditions to artifacts arising from long-lived trapping of photoinduced charge; in our opinion these controversies have been resolved and are discussed here. Furthermore, various photovoltaic cell architectures utilizing QDs have recently been constructed and the photocurrent and photovoltage characterisitics have been studied. These photocurrent measurements provide a more direct measurement of MEG since the photogenerated carriers are counted directly via the current, and they are very consistent with the QYs of MEG reported using the proper spectroscopic techniques; thus, these new photocurrent measurements confirm the existence of enhanced exciton and carrier multiplication in QDs. The past work and prognosis for QD-based Next Generation PV cells based on MEG are discussed.

The optical properties of matter are determined by the coupling of various types of oscillators in matter to the electromagnetic radiation field. In other words, an incident electromagnetic field will cause these oscillators to perform driven or forced oscillations. The amplitude of these driven oscillations depends on the angular frequency of the incident field, on the eigenfrequency0 of the oscillators, on the coupling strength f between electromagnetic field and oscillator, and on its damping. In semiconductors the main intrinsic oscillators or resonances are optical phonons, excitons including their ionisation continuum and higher band-to-band transitions or plasmons.

In Chap. 8 we defined the bandstructure for electrons and holes as the solutions to the (N ± 1)-particle problem and later we saw that the number of electrons in a band can be increased or decreased by donors and acceptors, respectively (Sect. 8.14).

In this chapter we return briefly to the concept of a Fabry–Perot resonatorin the form of a (micro) cavityand then proceed to the cavity polaritons as a mixed state between a resonance in a solid (these are generally exciton resonances in quantum wells, wires or dots) and a cavity resonance. From there we reach, via different paths, the presently very active and potentially application-relevant field of photonic crystals with a subspecies known as photonic band gap materials.

Having treated the optical properties of phonons, plasmons and magnons, we come in this and the following chapters to the essence of semiconductor optics, namely the optical properties of excitons.

Exciton–polaritons in microcavities form an unusual gas of weakly interacting bosons. It has no direct analogy in cold atomic gases, superfluids or superconductors due to its two-component spin structure: in typical planar microcavities the polaritons have two allowed spin projections to the structure axis. This is why the order parameter of a polariton condensate is a complex spinor. The magnitude and, possibly, sign of polariton–polariton interaction constant depends on the spin state of polaritons. The energy of an exciton–polariton condensate is also spin-dependent. These specific features make polariton condensates a unique laboratory for studies of spin effects in interacting Bose gases. Several new spin-dependent effects in polariton condensates have been theoretically predicted and experimentally observed during the recent decade. This review chapter addresses some of these effects: polarisation multistability, spin switching, spin rings and spin Meissner effect. In the last section we address the perspective of observation of spin superfluidity in microcavities.

Having introduced the basic idea of the electron-hole plasma (EHP) in Sect. 19.2, we now give details of some of its properties, e.g., the density at which the transition from an exciton gas to an EHP has to be expected in Sect. 21.1, the renormalization of the band gap in the EHP and its thermodynamic properties in Sect. 21.2. Then we present results for indirect and direct-gap bulk semiconductors showing some characteristic differences in these properties.

A technique which reveals fascinating new phenomena as well as providing a powerful tool to detect and probe the properties of excitons is the application of external fields. For a rather early treatment from the beginning of the last century.

The results of experimental studies are presented of the polariton system in a semiconductor microcavity excited resonantly at various wave vectors by ns-long pulse laser with various light polarizations along with a theoretical description of the nonlinear effects in the polariton system. The interplay between the parametric scattering and self-instability of the driven mode results in a rich variety of scattering scenarios (or cavity dynamics) sensitive to variation of both the intensity and polarization state of the external pump. The observed instabilities and hysteresis effects in a scalar polariton system excited with circularly polarized pulses can be qualitatively reproduced within the semiclassical model of dynamically self-organized optical parametric oscillator (OPO), based on the resonant approximation of cavity electrodynamics and the Ginzburg-Landau-Gross-Pitaevskii-type equation for coherent excitonic interband polarization. However, this model fails to explain the polarization instabilities in the effectively spinor system excited with elliptically polarized pulses. The dynamics of such a system is strongly affected by the long-lived exciton reservoir (excited due to polariton scattering) which brings about additional blueshift of both components of bright excitons and results in the qualitative changes in the development of the polarization instabilities in the driven mode and in the OPO signal. Those transitions are phenomenologically introduced into the modified semiclassical model. In spite of some limitations, this model provides a self-consistent approach to description of intracavity field dynamics under both pulse and continuous wave excitation conditions and gives a good qualitative description of the observed polarization instabilities and hysteresis effects in the dynamics of both the driven mode and OPO signal.

In the following sections we present selected examples from the intermediate density regime where excitons biexcitons and trions are still good quasiparticles. We cite here [63H1, 85E1, 88Q1, 89S1, 96S2, 99K1, 01C1,03K4, 03O1, 04E1, 07B1, 10D1] (apart from the textbooks, conference proceedings and journal issues dedicated completely or partly to nonlinear optics mentioned already in Chap. 1 [81A1, 84H1, 84S1, 86L1, 88Z1, 89L1, 93H1, 95M1, 96H1, 96S2, 97W1, 02S1, 04O1], in Chap. 9 e.g. [77H2, 94E1] or in Chap. 19 e.g. [65B2, 81K1, 84S1, 86E1, 89N1, 90B1, 90H1, 95M1, 99N1, 02S2, 04W2]), where [88Q1, 89S1, 03O1] concentrate on nonlinear optics in quantum wells (QW) and superlattices (SL), [07B1] in photonic crystals and [99K1, 10D1] in micro cavities.

In the previous chapter we discussed mainly the optical properties of intrinsic excitons. Here we consider the optical properties of defect and localized states in bulk materials, but mention that many of these aspects are also relevant for the structures of reduced dimensionality presented in the next chapter.

In this review, we discuss first some of the recent works to reveal properties of conventional ZnO/ZnMgO QWs grown c-axis oriented. This will include the properties of the quantum confined Stark effect (QCSE) that results from the internal electric field in the unit cell. We will then discuss various unconventional QW growths, including non-polar ZnO QWs, graded barrier QWs and double QWs. We finish with a review of current progress towards light emitting devices based on ZnO/ZnMgO QWs. ZnO has been a material of interest for over 50 years; however, the ability to grow high-quality epilayers of ZnO and ZnO-based ternary systems[1–4 has led to renewed interest over the past decade in ZnO for device applications. The demand for optoelectronic devices in the blue-UV region of the electromagnetic spectrum has been well established and ZnO possesses several properties that are superior to GaN for many applications [5, 6]. The large exciton binding energy of ∼ 60 meV suggests excitonic emission that is very efficient above room temperature, leading to great potential for light emitting devices. The large piezoelectric and pyroelectric coefficients suggest potential for applications as piezoelectric sensors or actuators. Other advantages of ZnO are its comparatively low growth temperatures [5], low optical power threshold for lasing [7], radiation hardness [8, 9] and biocompatibility [10]. ZnO may also find application as a transparent conductive oxide [11] to replace indium-tin-oxide (ITO) in photovoltaic applications because it remains transparent even when doped above the level of degeneracy [12]. This ability to grow high-quality epilayers and thin films rapidly led to the development of ZnO-based quantum wells (QWs). The two main attractions of developing QWs are the tunability offered for the transition energy, and the increased exciton binding energy, which typically leads to increased oscillator strength and greater efficiency for light emitting devices. The two most common types of ZnO-based QWs are Zno/ZnMgo and ZnO/ZnCdO-based systems, where alloying with Mg leads to an increase in the band gap and with Cd leads to a reduction in the band gap. In this chapter, we will focus on the more common ZnO/ZnMgO QWs. The band gap of Znx − 1Mg x O is given by 3. 37 + 2. 51x eV [13], which together with a conduction to valence band offset ratio of approximately 70:30 leads to type I confinement in \({\mathrm{ZnO/Zn}}_{x-1}{\mathrm{Mg}}_{x}\mathrm{O}\) QWs. The value of x is limited to less than 0.43 [13] in these systems as above this value phase separation tends to occur. Nevertheless, even with values significantly less than 0.43, strong confinement is obtained. In such ZnO/ZnMgO QWs, exciton binding energies up to 120 meV have been reported [14]. The biexciton binding energy is also enhanced in QWs going from 15 meV in bulk ZnO to ∼ 25 meV in ZnO/ZnMgO QWs [15–17]. With biexciton binding energies greater than the thermal energy, it is possible that biexcitons may play a major role in the optical properties at room temperature. Since the first ZnO epitaxial layers were grown, high-quality ZnO quantum wells have been grown by several different methods, including molecular beam epitaxy (MBE), metal-organic vapour phase epitaxy (MOVPE) and pulsed laser deposition (PLD). The growth in almost all cases is c-axis oriented despite growth on a range of single crystal substrates, including sapphire, ZnO and Si oriented along various crystal planes.

A review is given of the synthesis and characterization of high Al-content AlGaN thin films, including optical properties, bandgap bowing, exciton localization, and n- and p-type doping. A summary of energy levels for various acceptors in AlN is also given.

The deep centres are intrinsic or extrinsic complexes or isolated FAs with ground-state levels deep in the band gap, hence their name. They give rise to relatively high-energy transitions whose excited states cannot be described by donor or acceptor EMT. They can be found in some as-grown crystals, but they are also produced by irradiation with h-e particles or γ-rays, or associated with TMs introduced in the crystals. When their concentration is dominant, the resistivity of the material can reach the intrinsic resistivity and classical resistivity measurements are difficult to perform on such materials. These centres are characterized by the position(s) of their energy level(s) in the band gap, by their point-group symmetries, and by their isotopic distributions. In semiconductors, many high-energy absorption lines are also due to the creation of excitons bound to defects whose electronic properties are only roughly understood, but these lines can bring useful information on the nature of these defects, for instance by their electronic isotope shifts or splitting under a uniaxial stress.