Truly a sparkling subject

Crystal-Field Engineering of Solid-State Laser Materials
March 9, 2001

Just over 40 years ago when the first, fleeting pulses of laser light were observed by Theodore Maiman, the active material used was ruby, and ever since then the development of solid-state lasers has been inextricably linked with gemstones. Whereas the more familiar lasers used in CD players and telecommunications are microscopic semiconductor chips, the field of high-power, short-pulse lasers has relied on crystals and glasses containing metallic dopants. The interior of the host material contains an intense electric field, the "crystal field", that shifts the energies of optical transitions of the metal ions and so determines the laser wavelength or colour. Natural gemstones provide only a restricted range of laser wavelengths, but by carefully selecting the host and the dopant, synthetic gems allow practically any wavelength to be produced: this is the origin of the expression "crystal-field engineering".

The authors have maybe too neatly divided their subject matter into experimental and theoretical chapters, making the overall presentation somewhat uneven. The early chapters are devoted to theoretical descriptions of the energy levels and optical spectra of ions in crystals. Rather surprisingly, there is a stand-alone chapter on group theory, which is insufficient for the absolute beginner and unnecessary for the expert; but it does serve the purpose of defining the notation used throughout the remainder of the book. The discussion of crystal-field theory is thorough: it includes colour centres as well as the more familiar transition-metal and rare-earth ions. Optical transitions are treated at the level of the Einstein coefficients, which is adequate for the relatively elementary treatment of lasing. The treatment of electron-lattice interactions is succinct and informative, and there are useful illustrations such as octahedrally coordinated Cr3+ (ruby laser) and Ti3+ (titanium sapphire laser).

The second half of the book deals with issues more directly related to laser performance, such as non-radiative transitions, donor-acceptor energy transfer and excited-state absorption. Up-conversion processes that form the basis of Er3+ fibre lasers and optical amplifiers give this chapter a topical flavour. Fast on the heels of this discussion comes a detailed treatment of covalency effects that takes the theoretical description of electronic states beyond the crystal-field approximation.

The concluding chapter contains the material promised in the title, namely crystal-field engineering. It provides a detailed and informative review of the properties and characteristics of the most successful transition-metal ion laser materials including Cr3+, Ti3+  and Co2+ in hosts such as garnet, alexandrite and colquiriite, and various tunable and fixed-wavelength rare-earth ion systems. Tunability is one of the major advantages of solid-state lasers: it can be achieved in special circumstances when a strong electron-lattice interaction gives rise to mixed electronic-vibrational (vibronic) states and consequently to spectral broadening of the emission. But the "engineering" aspects of the field are probably exaggerated. Unlike the case of semiconductor lasers, where the emission wavelength can be selected almost at will by band-structure engineering, the wavelength of solid-state lasers is determined by the choice of impurity ion and host material, and there is very little additional external control to be exercised.

This book is quite clearly a graduate-level textbook for materials scientists and condensed-matter physicists, but the wide-ranging review of laser materials will also make it an attractive reference book for specialists in the solid-state spectroscopy and laser fields.

John F. Ryan is professor of physics, University of Oxford.

Crystal-Field Engineering of Solid-State Laser Materials

Author - Brian Henderson and Ralph H. Bartram
ISBN - 0 521 59349 2
Publisher - Cambridge University Press
Price - £80.00
Pages - 398

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