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This introductory chapter considers the basic principles of operation of diode lasers. The condition of inverse population in semiconductors is derived from Fermi statistics. Light confinement in a laser cavity, including transverse and longitudinal optical modes, is discussed. Main device characteristics of a diode laser, such as light-current and current-voltage curves as well as power conversion efficiency are described in their relation with the internal parameters of the active region. Optical gain and its relationship with laser threshold are considered. Electronic structure of solid state and microscopic theory of optical gain are briefly summarized in the chapter. Size quantization in semiconductors caused by energy barriers at heterointerfaces is considered. Density of states for various types of size dimensionality is presented. Effect of the density of states in the laser active region on the gain characteristics is discussed with attention to the ideal quantum dot array.

Stimulated emission is treated as a ‘special case of luminescence’ that requires population inversion of energy levels. After introductory examples of spontaneous emission spectra influenced by ingredient of stimulated emission, basic concept of spontaneous and stimulated emission (Einstein coefficients) in an atomic system is reviewed. The notion of optical gain is introduced. Then an analogous treatment is repeated for semiconductors. Bernard–Duraffourg condition is derived and spectral shape of optical gain is dealt in detail with. The issue of stimulated emission in indirect bandgap semiconductors is mentioned, including note on a germanium laser. A number of exciton radiative processes capable to exhibit stimulated emission are exposed. Finally, common experimental techniques to measure optical gain (variable stripe length, scanning excitation spot, and pump&probe) are analyzed and compared. Modal gain and material gain are distinguished. Throughout the chapter, illustrative experimental and theoretical examples adopted from gain spectroscopy in various semiconductors clarify the text.

This chapter discusses the peculiar properties of lasers based on self-organized quantum dot arrays. A correlation between density of states, saturated gain and transparency current is discussed taking into account inhomogeneous broadening and higher-energy states. Ground-to-excited state lasing transition, which is observed in QD lasers, is explained. Empirical and analytical expressions are considered, which are capable of describing gain-current relation in self-organized QD lasers in the presence of wetting layer, matrix, and excited quantum dot states. A method to prevent gain saturation by multiple stacking of quantum dots is described in connection with the effect of the QD surface density on the threshold current. The effect of temperature on the gain and threshold characteristics is also discussed in realistic QD lasers.

Stimulated emission and lasing can be achieved easily in a number of semiconductor nanostructures. This chapter gives an overview of a series of physical mechanisms that were found experimentally to give rise to positive optical gain in quantum wells, quantum wires and nanocrystals. In quantum wells, these are radiative recombination of localized excitons, LO-phonon assisted exciton recombination and electron–hole plasma luminescence. In quantum wires the data are rather scarce; localized excitons and electron–hole plasma appears to be involved in lasing. Separately treated are the cases of nanocrystals dispersed randomly in a matrix and that of heterostructures with ordered quantum dots (grown by Stranski–Krastanow method). Exciton and biexciton mechanisms of optical gain in quantum dots are analyzed. The crucial competing role of Auger recombination is expressed via the filling factor. Prospects of random lasing in semiconductor nanostructures are outlined.

Silicon nanophotonics deals with unique luminescence properties of silicon nanocrystals sized approximately from one to several nanometers. These nanocrystals have the potential to become active medium in future light-emitting devices or even in a silicon laser. First, spontaneous photoluminescence in porous silicon and silicon nanocrystals is described and some preparative methods of these nanostructures are briefly mentioned. Then recent experimental data about search for optical gain in Si nanocrystals are critically considered. Peculiar luminescence behaviour of active planar waveguides made of Si nanocrystals is demonstrated. Selected ways of how to achieve electroluminescence in Si nanocrystals embedded in an insulating matrix are shown. Attractive combination of Si nanocrystals with Er3+ ions for lasing in the near infrared region is pointed out. Finally, possible biological applications of luminescent Si nanocrystals are briefly outlined.

Photonic structures, i.e. periodically ordered patterns with a period of the order of the light wavelength, affect strongly luminescence properties of embedded (nano)phosphors. Photonic crystals exhibit either a full band of forbidden photon energies, or a stop-band. This fact has impact on spontaneous luminescence spectral shape and decay rate. Concerning stimulated emission, slow group velocity of light may lead to significant enhancement of optical gain. Tiny disc microresonators tailor luminescence spectra into whispering gallery modes. Microcavities with distributed Bragg reflectors improve spectral purity and directionality of spontaneous luminescence. The concepts of a polariton laser and of single photon sources are briefly mentioned. Throughout the chapter, the text is accompanied with experimental examples.

Previous chapters discussed the crystal structure and bandstructure of III–V semiconductors. This chapter shifts to the book’s second major topic: electronic interactions with light. It introduces the main ideas about how light waves propagate in semiconductor crystals and induce absorption, spontaneous emission, and stimulated emission in bulk semiconductors. It also considers the differences between the electronic interactions with light in zinc-blende and wurtzite crystals and what happens as the energy gap of the semiconductor is reduced to zero or when the crystal is two-dimensional.