Problem of the Week

Chapter 28: The Atom

Problem of the Week

Fingerprinting Pollution

<a onClick="window.open('/olcweb/cgi/pluginpop.cgi?it=gif::::/sites/dl/free/0078458137/193800/ch28_exhaust.gif','popWin', 'width=NaN,height=NaN,resizable,scrollbars');" href="#"><img valign="absmiddle" height="16" width="16" border="0" src="/olcweb/styles/shared/linkicons/image.gif"> (4.0K)</a> Shine a light at a molecule and it will absorb certain frequencies. Therefore, each chemical has an optical absorption fingerprint by which it can be identified. Chemical identification is important in such applications as monitoring pollution and detecting illegal drugs or explosives. In practice, however, scientists have had trouble making laser chemical identification work because they didn't have a laser that could emit varying frequencies of light. At least they didn't have the right type of laser until now.

Light
Amplification by
Stimulated
Emission of
Radiation

Quantum Cascade Laser
The quantum cascade laser, developed in 1994, is the world's first laser that can vary the wavelength of the light it emits over a relatively wide range, from 4 to 12 microns. The reason for the name, Quantum Cascade, is that QC lasers operate like an electronic waterfall. A QC laser consists of many layers of semiconducting material. When an electric current flows through the laser, electrons cascade down an energy staircase. At every step, the electrons make a quantum jump between energy levels and emit a photon in the process. The photons are reflected back and forth between mirrors, stimulating other quantum jumps and the emission of other photons. This amplification process enables a high output power.

What is the name of the region of the electromagnetic spectrum where the QC laser emits radiation?

What is the energy difference between some of the energy levels in the semiconducting material that makes up the QC laser?

For more information on developments of the quantum cascade laser, go to http://www.lucent.com/ideas2/ inventions/docs/qclaser.html,. Clicking will launch a new window.