IPF 2010: Quantum cascade lasers enter the marketplace

IPF 2010 Federico Capasso stood unassumingly against the backdrop of a LaserFest sign at the 2010 Industrial Physics Forum in Rochester, New York. The Harvard University physicist’s eyes were fixed on his PowerPoint slides. He seemed unaware that the 150-seat presentation room was filled to nearly twice its capacity.

Evidently, Capasso and his topic, the physics and technology of quantum cascade lasers (QCLs), were big draws at this year’s IPF, held in conjunction with the 94th Frontiers in Optics Conference. Despite his academic affiliation, Capasso is eminently qualified to present at the IPF. Not only did he and his colleagues develop QCLs while working at Bell Labs in the 1990s, but QCLs, which are based on semiconducting nanomaterials and emit in the IR range, have already found several industrial applications (see the 2002 Physics Today article by Capasso, Claire Gmachl, Deborah Sivco, and Alfred Cho).

Unlike traditional LEDs, which emit photons when electrons and holes recombine, QCLs emit photons when electrons tunnel through nanometer-thin layers of a semiconductor. In essence, the electrons cascade down an energy staircase of quantum wells. Because the electrons emit a photon at each step in the staircase, QCLs attain a high level of quantum efficiency. Moreover, the laser can be made multispectral by varying the composition, and consequently the bandgap, of the semiconductor materials.

From promise to products

When QCLs debuted in 1994, their promise was clear, thanks to two rare properties. QCLs can emit at more than one wavelength and in the region of the IR spectrum where molecules’ characteristic and identifying features reside. A host of commercial applications came to mind in such fields as atmospheric chemistry and explosives detection.

However, it was also clear that obstacles lay in the path toward making QCLs practical. In her 1994 Physics Today news story , my colleague Barbara Goss Levi noted that the prototypes required high electric current, and therefore compensatory cooling, to reach useful intensities.

Within two years, Capasso and dozens of other academic and industrial researchers had succeeded in increasing output power while decreasing input current, mostly by modifying the staircase structure and fine-tuning the material composition. Now, 16 years after their discovery, QCLs exist as portable, commercial, and continuous-wave, room-temperature devices that lase efficiently from the 3- to 25-μm molecular-fingerprint region up to 300 μm with sensitivities in the parts per billion.

In his talk Capasso listed 17 companies pursuing or already selling QCL-based products. He highlighted Pranalytica , which came out first with a commercial continuous-wave, room-temperature QCL. Among the California-based company’s products is a mid-IR flashlight useful for soldiers, with battery life up to 24 hours, shown here, and a laser that essentially blinds the IR sensor of heat-seeking missiles.

Another company, Daylight Solutions , in collaboration with Rice University Nobel chemist Robert Curl, has developed a real-time breath analyzer that detects ammonia (a marker for kidney disease).

Earth scientists have also employed the QCL for spectroscopic analysis of atmospheric chemicals. Using aircraft-borne QCL sensors, NASA scientists, in collaboration with Capasso’s Harvard lab, have measured trace concentrations of atmospheric methane and nitrous oxide. And a group based at Princeton University measured ozone levels during the 2008 Olympics in Beijing.

Massachusetts-based Aerodyne Research sees profitable opportunities in a world where carbon dioxide and other industrial byproducts are increasingly regulated. In a presentation that followed Capasso’s, Aerodyne president Charles Kolb told the IPF that he is investing in QCLs as a sensitive and flexible tool for measuring industrial emissions. Aerodyne, in collaboration with Capasso’s group, conducted measurements of greenhouse gases in the stratosphere and troposphere, detecting concentrations as low as 30 parts per billion by volume.

Aerodyne is also applying QCLs at ground level. The company has developed QCL systems that measure the emissions from airplanes taxiing and taking off and from cars and trucks on the highway. Aerodyne’s roadside monitor sucks in the exhaust of passing cars and analyzes it within seconds. Not stopping there, the company also has what Kolb calls “a mobile lab on a FedEx truck,” shown here, that can tail vehicles and monitor their emissions as a function of time and location.

Using its QCL systems, Aerodyne uncovered a rare piece of environmental good news from, of all places, the center of the US oil industry: Houston, Texas. Aerodyne compared its 2009 data on gasoline- and diesel-vehicle emissions in the city with similar data collected in 2000. It found that the concentrations of carbon monoxide and nitrous oxides dropped by 40% and 50%, respectively. Now we don’t have to hold our breaths while we wait for electrical vehicles to take over the world.

Jermey N. A. Matthews

All the talks at IPF 2010 were recorded and are now available on video .