A new method for real-time detection and identification of very low concentrations of gases adds coherent control to quartz-enhanced photoacoustic spectroscopy, which uses a quartz tuning fork to detect gas absorption. Credit: Florian Sterl, Sterltech Optics GmbH, edited
A groundbreaking method known as coherently controlled quartz-enhanced photoacoustic spectroscopy has been developed to detect and identify gases at very low concentrations rapidly.
This new technique, with promising applications in environmental monitoring, early cancer detection, and chemical process safety, allows for comprehensive gas analysis in mere seconds, a process that traditionally took much longer.
Enhanced Sensitivity in Trace Gas Detection
Researchers have unveiled a groundbreaking method for detecting and identifying trace amounts of gases with exceptional speed and precision. Known as coherently controlled quartz-enhanced photoacoustic spectroscopy, this innovative approach could pave the way for highly sensitive, real-time sensors used in areas like environmental monitoring, medical 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,” explained Simon Angstenberger, the research team leader 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.”
Published today (January 9) in Optica, the journal of the Optica Publishing Group, the team demonstrated their method by capturing a complete methane spectrum (3050 to 3450 nanometers) in just three seconds—an accomplishment that typically takes 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.”
Technological Leap in Spectroscopy
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.
Revolutionizing Gas Identification Speed
“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.
Reference: “Coherent Control in Quartz-Enhanced Photoacoustics: Fingerprinting a Trace Gas at ppm-Level within Seconds” by S. Angstenberger, M. Floess, L. Schmid, P. Ruchka, T. Steinle and H. Giessen, 9 January 2025, Optica.
DOI: 10.1364/OPTICA.544448
