Photoacoustic sensors efficiently detect trace gas
Increasing awareness and new regulations for safety and emission control have created a strong demand for trace gas sensors that are compact and portable, as well as low cost and reliable. Quantum-cascade lasers (QCLs) have given new impetus to the development of optical gas sensors thanks to their tunability in the spectral region from 3-20μm (fingerprint region). QCLs show excellent properties in their narrow line width, average power (up to a few watts), and room-temperature operation. In combination with these laser sources, photoacoustic spectroscopy (PAS) offers the advantage of high sensitivity, selectivity, compact setup, fast time-response, and simple optical alignment.
PAS is based on the photoacoustic effect,1 the conversion of light to sound in absorbing materials. Viengerov2 used PAS for the first spectroscopic gas analysis in 1938. The photoacoustic signal is traditionally detected using a resonant acoustic cell equipped with a sensitive microphone.3 Recently, alternative transducers such as a quartz tuning fork (TF),4 optimized capacitive microelectromechanical systems microphones5, or a silicon cantilever6 have been demonstrated. We designed and fabricated photoacoustic gas sensors based on different setup configurations for detection of nitric oxide (NO) and formaldehyde (CH2O). The devices exhibit state-of-the-art sensing performance.
The realized photoacoustic sensors use commercial QCLs as light sources. We have investigated different sensor schemes. One is based on resonant photoacoustic H- and T-cell sensors and electret microphones. Under resonant conditions, the cells work as an acoustic amplifier. The absorbed laser energy accumulates in the resonator's acoustic mode. The amplitude of the acoustic wave is scaled by a quality factor (Q ) of 20–200.7–12 An alternate scheme is based on quartz-enhanced photoacoustic spectroscopy (QEPAS). In this technique, the modulated laser light is focused between two prongs of a quartz TF, where the absorbing gas generates acoustic pressure waves that excite a resonant vibration and then are converted into an electrical signal by the piezoelectric effect.13
The resonant photoacoustic cell characterized by H-cell geometry consists of a cylindrical stainless steel resonator with two λ/4 long buffer volumes connected to its ends. This configuration helps to reduce the background signal, via destructive interference, caused by the heating of the two zinc selenide (ZnSe) windows.
We designed the resonator to be excited in its first longitudinal mode (1380Hz). It was equipped with four electret microphones placed on the antinode of the acoustic mode (see Figure 1a). We used the sensors to detect NO and CH2O down to 150 parts per billion (ppb).
The resonant T-cell consists of two intersecting volumes: an optical-absorption volume and an acoustical-resonance cylinder. The internal gold-coated walls of the optical cavity have been shaped to produce multiple light reflection and subsequent focusing in the cavity center. An electret microphone is mounted at the end of the resonance cylinder in a design that does not require a collimated laser beam (see Figure 1b). Using this configuration, we saw a five times better detection limit (30ppb) for CH2O.
Another group developed a QEPAS NO sensor using a continuous-wave, thermoelectrically cooled, external-cavity QCL light source13 (see Figure 2). To enhance the QEPAS signal, We coupled the TF with an acoustic-organ-pipe type of microresonator (MR). The QEPAS spectrophone, consisting of the TF and the MR, was put inside a vacuum-tight cell with ZnSe windows. A NO concentration of 15 ppb resulted in a noise-equivalent signal.
In summary, the development of compact gas sensors is important for environmental applications. We developed two different approaches using QCLs for sensing trace NO and CH2O. The former was based on standard PAS and resonant cells. The result was a better detection limit using an innovative T-cell. The second approach was based on QEPAS, with the sensor's detection sensitivity at 15ppb. Future improvements will involve implementing a low-loss, mid-infrared, single-mode fiber system to couple the QCL source with the spectrophone and convert our sensor into a portable device.
The authors acknowledge financial support from both the Puglia region's “Intervento Cod. DM01, Progetti di ricerca industriale connessi con la strategia realizzativa elaborata dal Distretto Tecnologico della Meccatronica” (Speech Cod DM01, industrial research projects related to strategy roll-out by district technology of mechatronics).
Cinzia Di Franco received her Laurea degree in chemistry from the University of Bari. In 2000, she joined the research and development division of STMicroelectronics, Catania. Since April 2003 she has been a researcher at CNR-IFN. She authored or co-authored 28 refereed papers and holds one patent.
Angela Elia received her PhD in physics from the University of Bari. She has been a postdoctoral fellow in the physics department there since 2006. She focuses on the development of laser-based gas sensors. She is co-author of 27 publications and of two invited chapters.
Vincenzo Spagnolo has been an assistant professor of physics at the Polytechnic University since January 2004. He focuses on thermal modeling of quantum-cascade lasers and gas sensing. He holds two patents and is co-author of more than 70 referred articles.
Pietro Mario Lugará focuses on stimulated emission, optical gain, and Raman spectroscopy in bulk semiconductors and multiple-quantum well structures of optoelectronic devices for real-time monitoring of industrial welding processes, non-invasive optical oximetry, and photoacoustic sensing of air pollution.
Gaetano Scamarcio is a full professor of physics. He did his postdoctoral work at Max Planck Institute for Solid State Research in Stuttgart, and worked at Bell Laboratories and Lucent Technologies in Murray Hill, NJ. He authored more than 130 papers. His research interests include quantum-cascade lasers and optoelectronic sensors for mechatronics.
