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. Credit: Capasso Group, Harvard University

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.

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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.