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,