Photoacoustic spectroscopy approach achieves real-time detection of low gas concentrations

Researchers have developed a new method for quickly detecting and identifying very low concentrations of gases. The new approach, called coherently controlled quartz-enhanced photoacoustic spectroscopy, could form the basis for highly sensitive real-time sensors for applications such as environmental monitoring, breath analysis and chemical process control.

“Most gases are present in small amounts, so detecting gases at low concentrations is important in a wide variety of industries and applications,” said research team leader Simon Angstenberger from the University of Stuttgart in Germany. “Unlike other trace gas detection methods that rely on photoacoustics, ours is not limited to specific gases and does not require prior knowledge of the gas that might be present.”

In Optica, the researchers report the acquisition of a complete methane spectrum spanning 3,050 to 3,450 nanometers in just 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.”

Adding coherent control

Spectroscopy identifies chemicals, including gases, by analyzing their unique light absorption characteristics, akin to a “fingerprint” for each gas. To detect low gas concentrations quickly, however, requires not only a laser that can be tuned rapidly but also an extremely sensitive detection mechanism and precise electronic control of the laser timing.

In the new work, the researchers used a laser with an extremely fast tunable wavelength that was recently developed by collaborators at Stuttgart Instruments GmbH, a spin-off from the university. They also leveraged quartz-enhanced photoacoustic spectroscopy (QEPAS) as the sensitive detection mechanism.

This spectroscopy approach uses a quartz tuning fork to detect gas absorption by electronically measuring its vibrations at a resonant frequency of 12,420 Hz, induced by a laser modulated at the same frequency. The laser heats the gas between the fork’s prongs in rapid pulses, causing them to move and generating a detectable piezoelectric voltage.

“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,” explained Angstenberger. “This is because 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.”

To overcome this problem, the researchers developed a trick called coherent control. This involved shifting the timing of the pulses by exactly half an oscillation cycle of the fork while the laser output power remained at the same frequency.

This causes the laser pulse to arrive at the gas between the fork when its prongs move inwards. This trick dampens the fork oscillation because as the gas gets hot and expands it will act against the movement of the prongs. After a few flashes of laser light—over a few hundred microseconds—the fork stops vibrating and the next measurement can be performed.

Fast gas identification

“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 µm, making it capable of detecting virtually any trace gas.”

The researchers tested the new method using the laser developed by Stuttgart Instruments and a commercially available QEPAS gas cell to analyze a pre-calibrated methane mixture with 100 parts per million of methane in the gas cell. They showed that with regular QEPAS, scanning too quickly blurs the spectral fingerprint, but with the coherent control method, it stays clear and unchanged.

As a next step, the researchers plan to explore the limitations of the new technology to determine its maximum speed and lowest sensing concentration. They also want to use it to sense multiple gases, ideally at the same time.