Color center: also laser

Optically active centers may also occur as a result of stractural defects. These defects are usually called colour centers, and they produce optical bands in the colorless perfect crystal. We will also discuss the main features of color centers in this chapter (Section 6.5). From the practical viewpoint, color centers are used to develop solid state lasers. Moreover, the interpretation of their optical bands is also interesting from a fundamental point of view, as these centers can be formed unintentionally during crystal growth and so may give rise to unexpected optical bands. 

Chapter 6 is devoted to discussing the main optical properties of transition metal ions (3d" outer electronic configuration), trivalent rare earth ions (4f 5s 5p outer electronic configuration), and color centers, based on the concepts introduced in Chapter 5. These are the usual centers in solid state lasers and in various phosphors. In addition, these centers are very interesting from a didactic viewpoint. We introduce the Tanabe-Sugano and Dieke diagrams and their application to the interpretation of the main spectral features of transition metal ion and trivalent rare earth ion spectra, respectively. Color centers are also introduced in this chapter, special attention being devoted to the spectra of the simplest F centers in alkali halides. 

Fig. 2. Spectroscopy of electrons in fluids. Ensemble shows absorption maxima and coefficients in n-alcohols at picosecond (A ) and nanosecond (A) times, diols (B), amines (C), ethers (D), alkanes ( ), and color center (F). The absorption (F ) and stimulated (Fg) emission in KBr color center laser are also shown.

Although most experiments have so far been performed with dye lasers, the color-center lasers or the newly developed vibronic solid-state lasers such as the Tiisapphire laser, with broad spectral-gain profiles (Vol. 1, Sect. 5.7.3) are equally well suited for intracavity spectroscopy in the near infrared. An example is the spectroscopy of rovibronic transitions between higher electronic states of the H3 molecule with a color-center laser [24]. The combination of Fourier spectroscopy with ICLAS allows improved spectral resolution, while the sensitivity can also be enhanced [25, 34, 35]. 

Table I lists relevant data concerning the performance of the more common color center lasers. Since the operational lifetime is sometimes an issue with color center lasers, the approximate useful period of a single crystal is also listed. Unless otherwise noted, the powers are for cw operation.

This technique was first applied to the infrared region where many vibrational-rotational transitions of ions were measured with color-center lasers or diode lasers [6.105,6.109]. Meanwhile, electronic transitions have also been studied with dye lasers [6.110]. 

Solid state lasers include lasers based on paramagnetic ions, organic dye molecules, and color centers in crystalline or amorphous hosts. Semiconductor lasers are included in this section because they are a solid-state device, although the nature of the active center—recombination of electrons and holes—is different from the dopants or defect centers used in other lasers in this category. Conjugated polymer lasers, solid-state excimer lasers, and fiber raman, Brillouin, and soliton lasers are also covered in this section. 

To detect NH in the electronic ground state, the infrared absorption was applied in particular in solid matrices [21, 69] but also in the gas phase using a color center laser [70] and different frequency laser systems [71]. On the other hand, also the IR emission of NH(X v) was investigated experimentally and theoretically [72]. IR emission of the vibrational states of the X electronic state were also observed with Fourier transform emission spectroscopy [73] with low [74] and high resolution [75]. 

With the aid of the two-color Laser-Doppler-Anemometry (LDA), Bewersdorff was able to measure the axial and the radial turbulence intensities simultaneously and also the Reynolds shear stresses. The injection of polymer results in a damping of both intensities in the region of their maxima. In his Reynolds shear stress measurements he showed that the polymer injection results in a drastic damping, and the stress maximum is shifted towards the center of the pipe. In a homogeneous polymer solution the maximum of the Reynolds shear stress remains in the same position-as for water. Only in the region of the buffer zone are the shear stresses reduced. 

Some applications other than laser materials are the following luminescent materials for lighting, for display in cathode-ray tubes, and for X-ray radiography scintillator materials electroluminescent thin films glasses for solar concentrators colored materials for all types of applications (e.g., pigments). The greater part of these applications were reviewed in refs. 2 and 3. Optical centers can in many cases also be used as probes of the surroundings. 

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