ACS Chemistry for Life

Quantum Cascade (QC) lasers are tiny pieces of semiconductors—generally micrometers to millimeters in size—that feature nanometer-deep troughlike structures known as quantum wells. The devices were invented and demonstrated at Bell Laboratories in 1994 by a team that included Federico Capasso, now a professor of applied physics at Harvard University.

“QC lasers operate like an electronic waterfall,” Capasso says. Each quantum well is associated with a characteristic electron energy level determined by the well’s depth. Electrons can tunnel between adjacent wells of successively lower energies in a cascading process reminiscent of water flowing down a flight of stairs. At each level, the electron emits a photon that has a select wavelength. By tailoring the depth of the wells, which are often made by growing layers of aluminum indium arsenide and gallium indium arsenide on indium phosphide, scientists can select the laser’s wavelength, Capasso explains.

With their ability to emit laser light across the mid-IR spectrum, the so-called fingerprint region for many types of molecules, QC lasers are natural tools for chemical analysis. Adam Erlich, vice president for marketing and business development at Block Engineering, points to the inherently high spectral radiance (or brightness) of QC laser systems as one of their key advantages relative to Fourier transform IR instruments. “That feature enables us to get high signal-to-noise ratios for small samples examined with our IR microscopy configuration and to quickly analyze samples from a distance of 6 inches to 2 feet” in a standoff, or remote, configuration, he says.

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Excerpted with permission, Chemical & Engineering News
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