Optical properties lasing

C2.1 GalnN quantum wells composition pulling effect C2.2 GalnN quantum wells microstructure C2.3 GalnN quantum wells optical properties C2.4 GalnN quantum wells effect of phase separation on lasing... 

It is now being widely discussed whether phase separation of GalnN into InN mid GaN due to their large differences in lattice constants plays a decisive role for the properties of GalnN/GaN quantum wells. Its influence on the optical properties and on lasing from such quantum wells is discussed in Datareview C2.5. 

It has been proposed recently that phase separation of GalnN into In-rich and Ga-rich phases has profound consequences on the optical properties and on the lasing properties of GalnN/GaN/AlGaN quantum well heterostructures [1,2]. The nanoscale compositional fluctuations resulting from phase separation are believed to lead not only to exciton localisation [2] but even to a quantum-dot-like behaviour [3], A more detailed discussion of the microscopic aspects of phase separation is given elsewhere in this volume. 

Low-excitation, low-temperature experiments like photoluminescence or photoluminescence excitation spectroscopy tend to indicate a considerable influence of localisation effects on the optical properties of GakiN/GaN quantum wells. Under high-excitation conditions typical for lasing, however, it is clearly seen that lasing from GalnN/GaN quantum well structures is due to a free-carrier plasma. 

An additional system prepared by colloidal chemistry which continues to attract considerable interest is that of QRs that exhibit electronic and optical properties that differ from those of QDs. For example, due to their cylindrical symmetry, QRs have a linearly polarized emission, as demonstrated by fluorescence measurements on single rods [41], leading to polarized lasing [14]. The powerful combination of optical and turmeling spectroscopy discussed here was also applied to investigations of the length- and diameter-dependence of the electronic level stmcture of CdSe QRs [40]. 

Fb centers involve an F-center beside two substitutional alkali impurities. Since the distribution of foreign alkali ions in an otherwise pure lattice is statistical, Fb centers are far fewer in number than Fa centers, and are obtained in substantial quantities only when the impurity dopant concentration is fairly high (approximately 1%). Like the Fa center there are two types of Fb center, also classified by their relaxation behavior. The Fb(I) center is formed when the two substitutional impurities lie along a common (100) axis. The Fb(I) center has optical properties similar to the F-center, which preclude the possibility of lasing action. The Fb(II) center is formed when the two alkali impurities are adjacent to one another along a (110) axis of the crystal (see Fig. 1). 

Cholesteric films that were prepared using surface interactions show substantially improved optical properties in comparison with samples obtained by the anisotropic deswelling method (Sect. 4.1.1). This is indicated by much lower threshold intensities in lasing experiments [103]. 

Because of the combined influence of the alignment and pitch of the helical modulation in different states, the material acquires a photonic band gap. The self assembled bio-derived photonic crystals thus have attractive optical properties for which CNC films are considered for development of newer materials for novel application. Especially the helical arrangement of the nanorods, converts these to materials with higher application potential in the field of optoelectronics. Such kind of films has attractive optical properties and possibly can be used in applications such as security papers and mirrorless lasing. Biological nanomaterials are also biodegradable. 

All physical properties of pyrylium salts (unsubstituted or substituted with alkyl and/or aryl groups) prove the aromaticity of these cations vibrational spectra [66], mass-spectral fragmentations [67], magnetic properties [68-70], and electronic absorption spectra [71]. It should be mentioned that there is a close similarity between the electronic absorption bands of pyrylium salts and those of benzene, easily recognized by the marked bathochromic effect of substituents in y-position of pyrylium salts on one of these bands. Two-photon absorption spectra of 2,4,6-triarylpyrylium cations [72] may be used in optical data storage, lasing, and photodynamic therapy. 

Before discussing the experimental evidence, it is worthwhile to consider lasing-related properties of quantum dots from a fundamental point of view. The theoretical description of the optical gain in bulk and quantum well semiconductors is discussed in Datareview C5.3. 

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