Semiconductor nanocrystals can be fabricated using a number of technologies, differing in the environment in which nanocrystals appear, growth conditions, size range, and size distribution, as well as physical and chemical stability and reliability. Nanocrystals can be developed in inorganic glasses and crystals, in liquid solutions and polymers, or on a crystalline surface. In this chapter we provide a brief overview of these techniques and give a synopsis of nanocrystals developed by various techniques.

Nanocrystals in inorganic matrices

Glass matrices: diffusion-controlled growth

Fabrication of nanocrystals embedded in a glass matrix by means of diffusion-controlled growth is based on commercial technologies developed for fabrication of color cut-off filters and photochromic glasses. Color cut-off filters produced by Corning (United States), Schott (Germany), Rubin (Russia), and Hoya (Japan) are just glasses containing nanometer-size crystallites of mixed II-VI compounds (CdSxSe1−x). Empirical methods of diffusion-controlled growth of semiconductor nanocrystals in a glass matrix have been known for decades or even centuries (in the case of color stained glasses). Commercial photochromic glasses developed in recent years contain nanocrystals of I-VII compounds (e.g., CuCl, CuBr, AgBr). Typically, silicate or borosilicate matrices are used with the absorption onset near 4 eV (about 300 nm), thus providing optical transmission of the semiconductor inclusions to be studied over the whole visible range.

Growth of crystallites results from the phase transition in a supersaturated viscous solution.

In quasi-zero-dimensional structures under optical excitation there are, along with reversible processes that decay over the recombination time of electron-hole pairs, processes that result in a persistent change in the optical properties. These processes are controlled by the integral dose of the absorbed radiation rather than radiation intensity. Numerous examples of similar behavior can be found in photophysics and photochemistry of molecular structures. Similar to molecular structures, semiconductor nanocrystals embedded in a matrix or precipitated in a solution exhibit a variety of guest-host effects. Some of the phenomena related to the photo-induced modifications in absorption and/or emission features will be the subject of the present chapter. The main attention will be given to laser annealing, photodarkening, persistent spectral hole-burning, and photochemical reactions resulting in permanent spectral hole-burning. Finally, we consider the intercrystallite migration of carriers and its effect on luminescence kinetics.

Laser annealing, photodarkening, and photodegradation

Semiconductor-doped glasses exposed to prolonged illumination by laser light of a wavelength corresponding to resonant absorption by nanocrystals were found to exhibit systematically a number of photo-induced modifications. These include, first of all, a sharp decrease in the intrinsic edge luminescence versus impurity and defect related emission. Second, the lifetime of electron-hole pairs decreases by several orders of magnitude and reaches 10−11 s. Finally, additional structureless absorption with a coefficient on the order of 1 cm−1 appears in a wide spectral interval. The initial properties of the samples can sometimes be restored by heating to temperatures of 400–500°C.

We consider an ideal nanocrystal to be a bit of a crystal with a spherical or cubic shape, the so-called quantum dot. Such species do not exist in nature. Nevertheless, it has been very helpful for the physics of nanocrystals to use these simplified models to trace the basic effects arising from three-dimensional spatial confinement. An extension of the effective mass approximation towards spatially confined structures leads to a particle-in-a-box problem and provides a way to calculate the properties of nanocrystals that are not possible to analyze in other way because of the very large number of atoms involved. This approach fostered the systematic experiments that have determined the major advances in nanocrystal physics. At smaller sizes it converges with the results of the quantum-chemical approach, in which the given number of atoms in the nanocrystal is accounted for explicitly rather than the size.

In this chapter we consider systematically the properties of electron-hole pair states resulting from the effective-mass consideration. We see that an elementary excitation in the electron subsystem of a nanocrystal can be classified as exciton with an extension “exciton in a quantum dot.” Afterwards, a survey of quantum-chemical techniques along with the selected examples for semiconductor clusters will be given. Finally, the distinctive size ranges will be outlined to specify the steps of the evolution of properties and of the applicability of the different approaches and concepts to the mesoscopic structures confined in all three dimensions.

In this chapter we consider the optical processes in nanocrystals that can be interpreted in terms of creation and annihilation of a single electron-hole pair within a crystallite. Size-dependent absorption and emission spectra and their fine structures, as well as size-dependent radiative lifetimes, will be discussed for nanocrystals of II-VI, I-VII compounds and, where possible, for nanocrystals of III-V compounds and of group IV elements. Nontrivial aspects of excitonphonon interactions that manifest themselves in homogeneous linewidths and/or intraband relaxation processes will be outlined. Challenging experiments providing the optical information relevant to a single nanocrystal will be discussed as well. Most of these results have become possible because of a number of the spectrally and spatially selective techniques described in Chapter 4. An influence of a microcavity on spontaneous emission of nanocrystals, the competitive recombination mechanisms, and the electric field induced effects will be analyzed as well.

Size-dependent absorption spectra. Inhomogeneous broadening and homogeneous linewidths

Experimental evidence for quantum-size effects in real nanocrystals

In the early 1980s A. I. Ekimov and A. A. Onushchenko (Ekimov and Onushchenko 1982; Ekimov and Onushchenko 1984) and L. Brus with coworkers (Brus 1983; Rossetti, Nakahara, and Brus 1983) published pioneering articles in which size-dependent absorption spectra of semiconductor nanocrystals resulting from quantum confinement were demonstrated for the first time. During the same period S. V Gaponenko et al. reported on inhomogeneous broadening of the optical absorption spectra of glasses doped with semiconductor nanocrystals (Gaponenko, Zimin, and Nikeenko 1984).

The advances in physics and the technology of semiconductor nanocrystals that were summarized in Chapters 2–7 provide comprehensive knowledge on the optical and electronic properties of nanocrystals and make it possible to create novel mesoscopic materials with desirable parameters by means of stoichiometry and size control. In these chapters the intrinsic properties of nanocrystals were discussed, implying the absence of any cooperative effect on the properties of a given nanocrystal ensemble. In recent years significant progress has been made in moving from randomized nanocrystals towards spatially organized structures like nanocrystal superlattices, quantum dot solids, and photonic crystals. The principal results obtained in the field will be reviewed in this chapter.

Superlattices of nanocrystals: quantum dot solids

There are several ways to develop a nanocrystal superlattice, that is, a structure consisting of identical nanocrystals with regular spatial arrangement. The first is to use zeolites, which form a skeleton with regular displacement of extremely small cages, the size of a cage being typically about 1 nm. A number of clusters such as CdnSm and ZnnSm can be embedded in these cages, the cluster size distribution and geometry being controlled by the topology of the three-dimensional host surface (Wang et al. 1989; Stucky and MacDougall 1990; Bogomolov and Pavlova 1995 and references therein). Using various zeolites as frameworks for semiconductor clusters makes possible the study of regular three-dimensional cluster lattices with variable intercluster spacing.

Because of inevitable size distribution, shape variations, different concentration of impurities and defects, and fluctuations of local environment and charge distribution, every ensemble of nanocrystals dispersed in some solid or liquid medium possesses inhomogeneously broadened absorption and emission spectra. Therefore, a number of properties inherent in molecular and atomic inhomogeneously broadened spectra can be a priori foreseen for nanocrystals. These include spectral hole-burning, fluorescence line narrowing under selective excitation, and decay time distribution. At the same time, spectrally selective techniques developed for inhomogeneously broadened molecular and atomic structures have been successfully applied to nanocrystals providing evaluation of individual parameters smeared as a result of inhomogeneous broadening. This chapter gives a brief overview of specific phenomena inherent in all spectrally inhomogeneous media and a survey of the relevant experimental techniques including nonlinear pump-and-probe spectroscopy, fluorescence excitation spectroscopy, and single molecule spectroscopy.

Population-induced optical nonlinearity and spectral hole-burning

Every real system consisting of particles with discrete energy levels exhibits absorption saturation under intense optical excitation. The only example of a nonsaturable system is an ensemble of ideal harmonic oscillators that possess an infinite number of equally spaced energy levels, optical transitions allowed only for a couple of neighboring levels, and the probability of optical transitions being proportional to the level number (Stepanov and Gribkovskii 1963).

Electronic states and probabilities of optical transitions in molecules and crystals are determined by the properties of atoms and their spatial arrangement. An electron in an atom possesses a discrete set of states, resulting in a corresponding set of narrow absorption and emission lines. Elementary excitations in an electron subsystem of a crystal, that is, electrons and holes, possess many properties of a gas of free particles. In semiconductors, broad bands of the allowed electron and hole states separated by a forbidden gap give rise to characteristic absorption and emission features completely dissimilar to atomic spectra. It is therefore reasonable to pose a question: What happens on the way from atom to crystal? The answer to this question can be found in the studies of small particles with the number of atoms ranging from a few atoms to several hundreds of thousands atoms. The evolution of the properties of matter from atom to crystal can be described in terms of the two steps: from atom to cluster and from cluster to crystal.

The main distinctive feature of clusters is the discrete set of the number of atoms organized in a cluster. These so-called magic numbers determine unambiguously the spatial configuration, electronic spectra, and optical properties of clusters. Sometimes a transition from a given magic number to the neighboring one results in a drastic change in energy levels and optical transition probabilities.

The dramatic reduction in transmission loss of optical fibers coupled with equally important developments in the area of light sources and detectors have brought about a phenomenal growth of the fiber optic industry during the past two decades. Indeed, the birth of optical fiber communications coincided with the fabrication of low-loss optical fibers and operation of room temperature semiconductor lasers in 1970. Since then, the scientific and technological progress in the field has been so phenomenal that optical fiber communication systems find themselves already in the fifth generation within a span of about 25 years. Broadband optical fiber amplifiers coupled with wavelength division multiplexing techniques and soliton communication systems are some of the very important developments that have taken place in the past few years, which are already revolutionizing the field of fiber optics. Although the major application of optical fibers has been in the area of telecommunications, many new related areas such as fiber optic sensors, fiber optic devices and components, and integrated optics have witnessed considerable growth. In addition, optical fibers allow us to perform many interesting and simple experiments permitting us to understand basic physical principles.

With the all-pervading applications of optical fibers, many educational institutions have started courses on fiber optics. At our Institute, we have a threesemester M.Tech. program on Optoelectronics and Optical Communications (jointly run by the Physics and Electrical Engineering Departments) in which we have an extensive coverage of the theory of optical fibers and optical fiber communications and also many experiments and projects associated with it.

Introduction

The most important and widely exploited application of optical fiber is its use as the transmission medium in an optical communication link. The basic optical fiber communication system consists of a transmitter, an optical fiber, and a receiver. The transmitter has a light source, such as a laser diode, which is modulated by a suitable drive circuit in accordance with the signal to be transmitted. Similarly, the receiver consists of a photodetector, which generates electrical signals in accordance with the incident optical energy. The photodetector is followed by an electronic amplifier and a signal recovery unit.

Among the variety of optical sources, optical fiber communication systems almost always use semiconductor-based light sources such as light-emitting diodes (LEDs) and laser diodes because of the several advantages such sources have over the others. These advantages include compact size, high efficiency, required wavelength of emission, and, above all, the possibility of direct modulation at high speeds.

In this chapter, we discuss the mechanism of light generation, basic device configurations, and relevant output characteristics of the light source. In Section 11.2 we discuss the basic requirements that the source should meet to be suitable for use in an optical fiber communication system. In Section 11.3 we briefly present an elementary account of the principle of operation of a laser. In Section 11.4 we discuss basic semiconductor physics relevant to the operation of a semiconductor laser followed by the device structure and characteristics in Section 11.5. Finally, in Section 11.6 we briefly discuss the characteristics of LEDs that are relevant to a fiber optic communication link.