Callister - An introduction - 8th edition

860 • Chapter 21 / Optical Properties
Fully silvered
(a)
(b)
(c)
Partially silvered
Figure 21.15 Schematic representations of the
stimulated emission and light amplification for
a ruby laser. (a) The chromium ions before
excitation. (b) Electrons in some chromium
ions are excited into higher energy states by
the xenon light flash. (c) Emission from
metastable electron states is initiated or
stimulated by photons that are spontaneously
emitted. (d) Upon reflection from the silvered
ends, the photons continue to stimulate
emissions as they traverse the rod length.
(e) The coherent and intense beam is finally
emitted through the partially silvered end.
(From R. M. Rose, L. A. Shepard, and J. Wulff,
The Structure and Properties of Materials, Vol.
4, Electronic Properties. Copyright © 1966 by
John Wiley & Sons, New York. Reprinted by
permission of John Wiley & Sons, Inc.)
(Just before
next reflection)
(At midcrystal)
(d)
(After
reflection)
(e)
Excited Cr atom
Cr atom in ground state
Semiconducting materials such as gallium arsenide may also be used as lasers
that are employed in compact disc players and in the modern telecommunications
industry. One requirement of these semiconducting materials is that the wavelength
associated with the band gap energy E g must correspond to visible light. That is,
from a modification of Equation 21.3, namely
l hc
(21.20)
E g
the value of l must lie between 0.4 and 0.7 m. A voltage applied to the material
excites electrons from the valence band, across the band gap, and into the conduction
band; correspondingly, holes are created in the valence band. This process is
demonstrated in Figure 21.16a, which shows the energy band scheme over some region
of the semiconducting material, along with several holes and excited electrons.
Subsequently, a few of these excited electrons and holes spontaneously recombine.
For each recombination event, a photon of light having a wavelength given by Equation
21.20 is emitted (Figure 21.16a). One such photon will stimulate the recombination
of other excited electron–hole pairs, Figure 21.16b–f, and the production of
additional photons that have the same wavelength and are all in phase with one
another and with the original photon; thus, a monochromatic and coherent beam
results. As with the ruby laser (Figure 21.15), one end of the semiconductor laser is
totally reflecting; at this end, the beam is reflected back into the material so that