Photoacoustic gas sensing points to real-time environmental monitoring

13 Jan 2025

University of Stuttgart speeds up quartz-enhanced photoacoustics to acquire data more rapidly.

A project at the University of Stuttgart has developed a method for more quickly detecting and identifying very low concentrations of gases.

The new approach, termed coherently controlled quartz-enhanced photoacoustic spectroscopy (QEPAS), could form the basis for detectors used in environmental monitoring, breath analysis and chemical process control.

Published in Optica, the initial results from the platform suggest it could be valuable for real-world gas sensing applications with extremely fast acquisition speed.

Photoacoustic techniques have made significant impact in biomedical studies, thanks to their ability to image hemoglobin molecules and indicate the layout of blood vessels.

They have also been employed in the detection of trace gas molecules, especially through the QEPAS variant of the principle in which photoacoustic waves are detected by a piezoelectric quartz tuning fork.

QEPAS can detect weak photoacoustic excitation and allows the use of extremely small sample volumes. But this method brings an intrinsic spectral resolution limit for fast wavelength sweeping, noted the Stuttgart project, a limit connected to the long ringing time of the quartz tuning fork.

"While the high quality factor of the tuning fork, which makes it ring for a long time, allows us to detect low concentrations through what scientists call resonant enhancement, it limits acquisition speed," said Simon Angstenberger from the Stuttgart Research Center of Photonic Engineering (SCoPE). "When we change wavelengths to obtain the fingerprint of the molecule, the fork is still moving. To measure the next feature, we must somehow stop the movement."

Putting the dampeners on

The project's solution involves a coherent control operation, in which the timing of laser pulses in shifted by half an oscillation cycle of the fork while the laser output stays at the same frequency.

This means a laser pulse arrives at the sample gas molecules located between the fork's prongs just as those prongs are moving inwards, said the project. The fork oscillation is then dampened, because as the gas gets hot and expands it will act against the movement of the prongs. After a few flashes of laser light and a few hundred microseconds, the fork stops vibrating and the next measurement can be performed.

"Adding coherent control to QEPAS enables ultra-fast identification of gases using their vibrational and rotational fingerprints," said Angstenberger. "Unlike traditional setups limited to specific gases or single absorption peaks, we can achieve real-time monitoring with a broad laser tuning range of 1.3 to 18 microns, making it capable of detecting virtually any trace gas."

The project built a proof of concept platform using a commercially available QEPAS gas cell to analyze a mixture with 100 parts per million of methane in the gas cell. Results showed that regular QEPAS scanning quickly blured the spectral fingerprint, but with the coherent control method it stayed clear and unchanged.

In its paper the team reported the acquisition of a complete methane spectrum spanning 3050 to 3450 nanometers in three seconds, a feat that would typically take around 30 minutes.

"This new technology could be used for climate monitoring by detecting greenhouse gases like methane, which is a potent contributor to climate change," said Angstenberger. "It also has potential applications in early cancer detection through breath analysis, and in chemical production plants for detecting toxic or flammable gas leaks and for process control."