Quantum Cascade Lasers

If the condensed history of infrared spectroscopy were written in biblical prose, the text might read: “In the beginning, dispersive instruments ruled. Then came the Fourier transform spectrometers, and the older instruments disappeared in a great purge.” The next verse in that ultrashort summary might not be recorded yet, but when molecular spectroscopy scribes put pen to parchment, they will certainly stress the field’s rapid advances due to the advent of quantum cascade (QC) lasers.

To give a sense of the progress made with these semiconductor-based light sources and their importance to chemical analysis, the field’s devotees stress the remarkable timescale over which they were developed and commercialized as well as the breadth of their applications.

“It has only been about 15 years since QC lasers were first demonstrated,” says chemistry Nobel Laureate Robert F. Curl Jr. of Rice University. “Yet in that short amount of time, these devices have become the most useful sources of tunable mid-infrared laser radiation.”

Some 20 companies, including newly launched start-ups and well-established manufacturers, now offer QC laser products ranging from the tiny semiconductor chips that generate the light to complete systems for chemical detection. And QC laser technology has quickly taken over a main role in chemical analysis in numerous arenas. Examples include gas analysis for industrial and commercial processes and chemical detection for medical, defense, and security applications. In addition, QC lasers figure prominently in advanced research systems designed by spectroscopists and other scientists.

The products and results keep coming. The past couple of months have brought an assortment of QC laser research papers in chemistry, physics, and applied physics journals. The same period has also seen new product launches. For example, Block Engineering, in Marlborough, Mass., just introduced a line of mid-IR spectrometers that includes LaserScan, a rugged handheld field instrument for remotely identifying unknown and potentially harmful substances.

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.

Simon Nicholson, sales director for Cascade Technologies in Stirling, Scotland, says high chemical selectivity and specificity, coupled with rapid detection, are the major advantages of Cascade’s QC-laser-based gas analysis instruments, relative to non-QC-laser systems. The company’s QC laser detectors are used in applications characterized by complex and sometimes rapidly changing gas mixtures. Examples include monitoring industrial emissions for SO_x , NO_x , CO, and other pollutants; analyzing automotive exhaust in real time; and checking aerosol cans for leaks on high-speed production lines.

At Fujifilm Imaging Colorants, in Grangemouth, Scotland, chemical engineering group leader Zac Meadows says his team found traditional reagent-based trap-and-analyze methods for quantifying emission by-products from diazotization processes to be “totally hopeless.” And FTIR analysis proved difficult and time-consuming because of the complex nature of the gas mixtures. “For fast-track development of new processes, we just could not trust our results from those other methods,” he says. “QC laser detection solved a big problem for us.”

Researchers in academic labs are also putting QC lasers through the paces. Many of them are using instruments from Daylight Solutions, in Poway, Calif. For example, Thomas G. Thundat, a professor of chemical and materials engineering at the University of Alberta, uses the technology for remote detection. “We have used QC lasers to obtain spectra of explosive residues on surfaces 50 meters away,” he says. His group reports a detection limit of about 100 ng/cm² (Anal. Chem. 2009, 81, 1952). “The beauty is that you can get this information within 0.8 seconds,” he adds.

And at Rice University, Frank K. Tittel and coworkers used a version of photoacoustic spectroscopy with a QC laser source to detect nitric oxide in concentrations as low as 15 ppb by volume with a five-second data acquisition time (Appl. Phys. B 2010, 100, 125). Capasso, Curl, and Tittel recently reviewed QC laser usage in chemical physics applications (Chem. Phys. Lett. 2010, 487, 1).

“The development of QC lasers is a clear breakthrough,” Thundat says. “There’s no doubt about it, this laser platform is definitely a game changer.”