IR spectroscopy—it's not just for chemists

Infrared spectroscopy has long been a primary tool for chemical analysis. The tenet of spectroscopy is that the structure of molecules and ions can be determined from their spectra. In the infrared region, from 2 to 20 µm, most molecules and/or ions exhibit well-defined peaks, enabling the "fingerprint" characterization of chemical composition. Comparison of products from lot to lot in a manufacturing facility, "reverse engineering" of a competitor's formulations, and the study of interstellar gases are just a few ways that chemists use infrared spectroscopy. Now, IR spectroscopy is finding applications in physics, engineering, and optoelectronics research laboratories.

The maturation of Fourier-transform infrared (FT-IR) spectrometers over the past decade has contributed to this diversification. These interferometer-based instruments have inherent advantages over earlier dispersive spectrometers. High resolution capabilities result from the wavelength accuracy of laser referencing; high throughput and fast sampling come about because of the "multiplex" advantage (all frequencies are sampled at once), and the fast Fourier transform process presents the spectral results in seconds rather than minutes. Ten years ago, the world market for FT-IR systems numbered in the hundreds of units; estimates now indicate that more than $200 million of FT-IR instruments are sold each year.

Optoelectronic device design

Active development of infrared-emitting lasers and LEDs is underway in many laboratories. Such sources could be used to build specific analyzers for pollution detection, gas monitoring, process chemical control, and many more applications. Although optical parametric oscillators and difference frequency mixing can shift visible laser output to infrared wavelengths, these laboratory systems can be cumbersome. For compact instruments, compound semi conductor based IR-emitting optoelectronic devices are more desirable.

At last month's CLEO in Anaheim, CA, researchers from Lucent Technologies announced room-temperature mid-IR emission from semiconductor lasers (see p. 9). Other scientists at that organization aim to develop IR-emitting LEDs, and scientists at Sandia National Laboratories are also working on IR-emitting optoelectronic devices. These efforts have one thing in common—each group is using an FT-IR spectrometer (like the one on the cover) to characterize the device output (see p. 65), proving that IR spectroscopy is not just for chemists any more.