Page 268 - Tunable Lasers Handbook
P. 268

228     Norman P.  Barnes

                   In this notation, S  is the spin quantum number and A is a letter associated with
                   the symmetry of  the site of the active atom. These letters come from group the-
                   ory and are associated with character tables describing the group. For example,
                   if  the active atom occupied a site in the center of  a cube and the nearest neigh-
                   bors occupy sites in the center of  the faces of  the cube. the site is said to have
                   octahedral symmetry. Octahedral symmetry describes the situation because the
                   nearest neighbors form an octahedron. Octahedral designations include AI, A,. -
                   E. TI, and T,.
                       Absorption  and  emission  of  the  transition  metals  in  a laser  material  are
                   characterized primarily  by  band  structures. That is.  the  absorption  and  emis-
                   sion  spectra  are  characterized  by  broad  spectral  features.  It  is  the  broad
                   absorption features that allow transition  metal atoms to be efficient absorbers
                   of  broadband flashlamp radiation. Superimposed on the broad  absorption and
                   emission bands may be some relatively narrow line features. For example, the
                   Cr:A1,03 laser (the ruby laser) utilizes  the broad  absorption features for effi-
                   cient absorption of flashlamp radiation  and the narrow emission features asso-
                   ciated with the transition from the 2E  upper laser level to the JA2 ground level
                   for the lasing process.
                       Interactions of  the transition metal active atom with the laser material create
                    the broad absorption and emission features. Absorption or emission of  a photon
                   by  a  transition  metal  atom  can  change  the  crystal  lattice  and  the  number  of
                   phonons in the lattice. Reasons for the change in the lattice can be explained by
                    considering the size and orientation of  the orbits of  the electrons in the excited
                    state and ground state of  the transition metal. In the ground state, the 3d  elec-
                   trons tend to be closer to the active atom, whereas in the excited state the elec-
                    trons tend to be farther away. A resulting difference in size causes shifts in the
                   positions of  the nearest neighbors. Thus, associated with an electronic transition,
                    is a shift in the position of the nearest neighbors. That is. the absorption or emis-
                    sion of  a photon can cause a shift in the crystal lattice as shown in Fig. 3. Transi-
                    tions  can  also  occur  with  the  emission  or  absorption  of  both  a  photon  and
                   phonon.  Energy  differences between the  initial  and final energy levels can be
                    shared between the photon and the phonon. Because the phonon spectrum can
                    be wide, a wide variation of the phorion energies is possible. A wide variation of
                    photon energies is thus possible since only the  sum of  the photon and phonon
                    energies  must  add  up  to  the  energy  difference  between  the  energy  levels
                    involved. Transitions  that  involve  the  simultaneous  absorption  or  emission  of
                    both photons and phonons are referred to as vibi-onic ti-msitions.
                       Since the laser material is strongly involved in the absorption and emission
                    processes. the nature of the transition metal lasers depends strongly on the laser
                    material. Putting the same active atom in another laser material can. for example,
                    change the symmetry of the site of  the active atom. With a change in symmetry,
                    the entire nature of  the energy levels and thus the absorption and emission can
                    change. Even if  two laser materials with the same site symmetry are compared,
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