Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

GTBL042-19 GTBL042-Callister-v2 September 13, 2007 13:59

Revised Pages

778 • Chapter 19 / Optical Properties

Sunlight may be directly converted into electrical energy in solar cells, which

also employ semiconductors. The operation of these devices is, in a sense, the reverse

of that for the light-emitting diode. Ap-njunction is used in which photoexcited

electrons and holes are drawn away from the junction in opposite directions, and

become part of an external current.

Concept Check 19.7

Is the semiconductor zinc selenide (ZnSe), which has a band gap of 2.58 eV, photo-

conductive when exposed to visible light radiation? Why or why not?

[The answer may be found at http://www.wiley.com/college/callister (Student Companion Site).]

19.13 LASERS

All the radiative electron transitions heretofore discussed are spontaneous; that is,

an electron falls from a high energy state to a lower one without any external provo-

cation. These transition events occur independently of one another and at random

times, producing radiation that is incoherent; that is, the light waves are out of phase

with one another. With lasers, however, coherent light is generated by electron tran-

laser sitions initiated by an external stimulus; in fact,“laser”is just the acronym forlight

amplification bystimulatedemission ofradiation.

Although there are several different varieties of laser, the principles of operation

are explained using the solid-state ruby laser. Ruby is simply a single crystal of Al 2 O 3

(sapphire) to which has been added on the order of 0.05% Cr^3 +ions. As previously

explained (Section 19.9), these ions impart to ruby its characteristic red color; more

important, they provide electron states that are essential for the laser to function.

The ruby laser is in the form of a rod, the ends of which are flat, parallel, and highly

polished. Both ends are silvered such that one is totally reflecting and the other

partially transmitting.

The ruby is illuminated with light from a xenon flash lamp (Figure 19.13). Before

this exposure, virtually all the Cr^3 +ions are in their ground states; that is, electrons

fill the lowest energy levels, as represented schematically in Figure 19.14. However,

photons of wavelength 0.56μm from the xenon lamp excite electrons from the Cr^3 +

ions into higher energy states. These electrons can decay back into their ground state

Flash lamp

Coherent beam

Power source

Ruby

Figure 19.13 Schematic diagram

of the ruby laser and xenon flash

lamp. (From R. M. Rose, L. A.

Shepard, and J. Wulff,The

Structure and Properties of

Materials,Vol. IV,Electronic

Properties.Copyright©c1966 by

John Wiley & Sons, New York.

Reprinted by permission of John

Wiley & Sons, Inc.)