Selection of Laser Material

Laser performances are determined by the properties of laser materials. Therefore, the selection of laser materials is an important step in building a high performance solid-state laser. The absorption and emission properties of the doping ions should 

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Due to the special characteristics of the laser emission process and the parasitic non-radiative de-excitation, it is necessary to carefully select the laser materials, including both the active ions and host materials. In addition, the characteristics of dopants and the states of doping have also played a crucial role in determining the performances of laser materials and thus the solid-state lasers. The efficiency and effectiveness of doping is mainly determined by the degree of matching in ionic radii between the dopant ions and substituted cations. The Shannon ionic radii of the ions in condensed state with anionic coordination number of 6 and 8 are rs = 0.103-0.115 nm and rg = 0.113-0.128 nm, respectively. In both cases, the radius decreases with increasing atomic number [79]. These ions can substitute for host cations with similar ionic radius, such as Ca ", La ", Gd ", Y ", Lu ", ... 

The tuning range of vibronic solid-state lasers can be widely varied by a proper choice of the implanted ions and by selecting different hosts. This is illustrated in Fig. 5.80a, which shows the spectral ranges of laser-excited fluorescence of the same Cr + ion in different host materials [424] while Fig. 5.80b shows the tuning ranges of laser materials where different metal ions are doped in a MgF2 crystal. 

When considering the optical resonances of the sample to aid in the selection of laser wavelength, the self-absorption of the sample also needs to be considered. When exciting organic materials or aqueous solutions in the NIR, this can mean absorption due to overtones and combinations of vibrational fundamentals. The self-absorption will affect relative intensities, which can severely affect the ability for quantitation. 

Principles and Characteristics Laser ablation is conceptually very simple, but mechanistically complicated. The process involves coupling of the photon energy of a laser pulse (typically about 20-30 ns wide, with an energy of 1-10 Jcm ) into the surface of a solid, resulting in evaporation and ejection of various species from the surface (the so-called plume ) within 10 to 10 s. The first experiments were carried out in 1962 [32]. When focused to a small area, a laser beam provides enormous power densities and electromagnetic fields. The plume , presumably a plasma, is accompanied by shock waves and electrical breakdown. The ejected material may eventually be deposited as a thin film. It is possible, by suitable selection of laser power and focus, to ablate a range of plastic materials in a controlled manner. For some matrices the polymer melts and diffuses away from the centre of the ablation site, leading to the forma-... 

In the discussion in Section 9.1.6 of harmonic generation of laser radiation we have seen how the high photon density produced by focusing a laser beam into certain crystalline materials may result in doubling, tripling, etc., of the laser frequency. Similarly, if a laser beam of wavenumber Vl is focused into a cell containing a material which is known to absorb at a wavenumber 2vl in an ordinary one-photon process the laser radiation may be absorbed in a two-photon process provided it is allowed by the relevant selection rules. 

To erase information by the transition amorphous — crystalline, the amorphous phase of the selected area must be crystallized by annealing. This is effected by illumination with a low power laser beam (6—15 mW, compared to 15—50 mW for writing/melting), thus crystallizing the area. This crystallization temperature is above the glass-transition point, but below the melting point of the material concerned (Eig. 15, Erase). 

Resonant Excitation Excitation by a laser, which is resonant with an electronic transition of the material under investigation, can increase the Raman cross-section by approximately 10. The transitions and thus the resonance wavelengths are specific for the substances. Resonance excitation thus leads to selectivity that can be useful for suppressing bulk bands, but can also complicate the detection of mixtures of substance with different absorption spectra. 

Also, direct determination of additives by means of laser desorption in solid polymeric materials rather than in polymer extracts has been reported [266], Takayama et al. [267] have described the direct detection of additives on the surface of LLDPE/(Chimassorb 944 LD and Irgafos P-EPQ) after matrix (THAP)-coating. As shown in Scheme 7.13, direct inlet mass spectrometry is also applicable to transfer TLC-MS and TLC-MS/MS analyses without the need for prior analysis. For direct sample introduction a small amount of the selected... 

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