A Compact Chemical Sensor

A portable array of lasers could be used to monitor pollution and detect toxins.

Researchers at Harvard University have made a shoebox-size sensor that could be used to detect extremely low concentrations of a broad variety of chemicals. The heart of the sensor is an array of tiny but powerful lasers that can be tuned over a wide range of wavelengths.

Dime-size laser: A laser array forms the core of a shoebox-size chemical sensor. The array is fabricated on a chip that is about one quarter the size of the dime it sits on in this image.

Sensors for detecting small amounts of gases have numerous applications. They are used to monitor toxic pollutants in the air and water, to test for chemicals in blood or on breath, and to detect impurities in natural gas or commercially synthesized chemicals. The commonly used tool for these applications, a Fourier transform infrared spectrometer, can detect a range of chemicals, but it is not very sensitive, detecting levels in the low parts-per-million range. The instrument is also heavy and bulky–about the size of a large suitcase–restricting the device to laboratory-type settings.

Harvard physics professor Federico Capasso is working on a compact, cheap sensor that can be carried discretely into airports, buildings, industrial manufacturing sites, and hospitals, and detect parts-per-billion volumes. The new device, which Capasso and his colleagues will showcase in May at the 2008 Conference on Lasers and Electro-Optics, is a step toward such an instrument. It is based on a special type of laser known as a quantum cascade laser, which Capasso and others developed at Bell Labs in 1994.

Quantum cascade lasers can be designed to emit light at any wavelength between 3 and 15 micrometers. This region of the spectrum, the mid-infrared, is important for chemical sensing. Most chemical bonds inside molecules respond to these frequencies by vibrating. Researchers can identify the chemical composition of materials based on the way the molecules vibrate.

Right now, quantum cascade lasers are used mostly in laboratories to detect gases at levels as low as one part per billion. The lasers emit light in a narrow range of mid-infrared wavelengths and can target at the most one to three molecules, says Frank Tittel, an electrical- and computer-engineering professor at Rice University, who has developed several laser spectrometers used by NASA and the EPA. The challenge is to make a portable and affordable laser source that emits a broad range of wavelengths.

The new sensor contains an array of 32 lasers, which the researchers fabricated using standard semiconductor processing methods. A microcontroller chip controls the lasers. Each laser emits light 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 key 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 32-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.” But these lasers 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, MA, 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. Daylight Solutions, based in Poway, CA, 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.

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.

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