The Solid State Laser

The solid state laser is actually a specialized phosphor. It consists of a host and an activator, but the host must be single crystal. What we have to say about the solid state laser applies equally to all lasers, be they gaseous or solid state. 

You may now wonder what is meant by coherent light. Consider the Particle in a Box in Chapter 5, which we used as an introduction to the calculations of Quantum Mechanics. The only energy states which appear for the electron are those for which standing waves appear as a function of 1 , the length of the box . To generate coherent light, we set up exactly the same conditions in the form of a resonant cavity. All commercial lasers are based upon this principle, namely that the heart of the laser is this cavity. 

Return now to the generalized phosphor MXO4 A. Let us further suppose that the activator has a transition of Sq = Pi for excitation. We now know that the ground state electronic wavefunctions are symmetrical, whereas the Pi state will have a vectored orientation due to the nature of the p-electron orbital, as shown in the following diagram  

To this point, we have been concerned with phosphors consisting of many small particles, each of which lies at a different orientation to all of the others. This results in incoherent emission. But, if we now grow a single crystal containing the activator, we would find that, upon excitation of the ciystal phosphor, the emitted light is now polarized. The plane of polarization has a specific relation to the crystallographic axes of the 

For the iPi state, the polarization of the emitted photon is normal to the orientation of the lobes of the excited p-electron state. For a uni-axial cr3retal, the photons emerge along a lattice direction consistent with the direction of minimum electric field of the atoms of the structure. This tells us that the iPi state has relaxed to an orientation at the cation site 

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Most of the solid-state lasers employ as active material crystals or glasses doped with rare-earth or actinide ions, because these ions exhibit a large number of relatively sharp fluorescent lines, covering the whole visible and near-infrared spectrum 380) search for new laser materials and investigations of the characteristics of laser emission at different temperatures of the active material and with various pump sources have improved knowledge about the solid state spectra and radiationless transitions in laser media 38i). 

The first reported laser action in rare earth complexes was obtained by Lempicki and Samelson [656] for europium benzoylacetonate in alcoholic solution. The laser parameters for this complex have also been evaluated by Lempicki and coworkers [656, 660] who found a slightly better quantum efficiency (0.8) for europium benzoylacetonate than for ruby (0.7), the solid state laser. The laser action of europium benzoylacetonate has also been investigated by Schimitschek [661] and Bhatjmik et al. [662]. Some other complexes of Eu3+ viz. dibenzoylmethide [665,664], m-4,4,4-trifluoro-l(2-thienyl)-l,3-butanedione [665], thenoyl-trifluoroacetonate [666, 667] were also found to lase. 

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 laser-based methods make use of different types of laser sources. Many of them, such as the solid-state laser or the semiconductor laser, are under continuous development because of the discovery of new lasing materials or the optimization of frequency conversion techniques. The type of the analytical application and the nature of the required information determine the choice of a special laser source, particularly pulsed or continuous-wave (CW) operating mode. Pulsed laser sources with fixed wavelength are usually used for LA, whereas both tunable pulsed and CW laser sources are preferred for element-specific detection or plasma diagnostics. 

The information on an optical disc can be read-out with a light,pen. One possible design is shown in Fig. 8a. The light emitted by the solid-state laser is collimated and focused on the information layer, reflected there and on its way back split into two beams which fall on a detector array. This means that when the laser spot strikes a pit, interference will reduce the amount of reflected light. 

We wish to thank the S.R.C. for a grant to purchase the solid-state laser, and N.A.T.O. for a travel grant. 

The basic parameters of the solid-state laser operation are the following ... 

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