A shoebox-size chemical sensor to detect toxins, pollution
at a specific wavelength, which can be tuned by slightly changing the temperature of the laser. “It can achieve continuous coverage within a range of 8.7 to 9.4 micrometers,” says Harvard researcher Mikhail Belkin, who was involved in the work. The researchers plan to extend the range by adding more lasers to the array.
Quantum cascade lasers have important advantages over the thermal infrared sources in Fourier transform spectrometers. They are 6 to 10 orders of magnitude brighter, which makes them more sensitive. What’s more, they can be made much smaller. The thirty-two-laser array sits on a chip that is four millimeters long and three millimeters wide. Together with the control electronics, the setup is the size of a shoebox. By shrinking the electronics, the instrument could be made much smaller, perhaps even “compressed into a chip, say, two by two inches,” Belkin says. In a laboratory test, the researchers have demonstrated that their sensor detects three common liquids — acetone, isopropanol, and methanol — just as accurately and sensitively as a Fourier transform spectrometer.
The new sensor should be simpler than other laser technologies used for chemical sensing. Diode lasers, such as the ones employed in telecommunications, are used to measure atmospheric gases at sub-parts-per-billion concentrations. Dirk Richter, a research engineer at the University Corporation for Atmospheric Research, says that “diode lasers have demonstrated one of the highest performances for airborne sensing applications — at least one order of magnitude better than quantum cascade lasers.” These lasers, however, have to be cryogenically cooled, while quantum cascade lasers operate at room temperature. The sensor will also face competition from state-of-the-art quantum cascade laser sensors that a few companies are now selling. Aerodyne, a company in Billerica, Massachusetts, is marketing a quantum cascade laser sensor for monitoring car and aircraft emissions, greenhouse gases, and ozone depletion. The company’s instrument, which is about two feet long and wide, is transportable but still rather heavy and bulky, Tittel says. Poway, California-based Daylight Solutions sells a tunable quantum cascade laser. It uses an external cavity in which light is bounced back and forth between mirrors so that it gains sufficient energy to start lasing.
Note that the new sensor uses a diffraction grating instead of mirrors to reflect light. The grating structure is integrated with the laser array into the chip, keeping the instrument compact. Capasso expects that his group’s sensor will be much cheaper than existing ones when it is manufactured commercially. While it is hard to say exactly when the new portable sensor could be available in the market, Bruker Optics, a leading infrared spectrometer manufacturer, has shown interest in the technology, Capasso says.
