Photoacoustic effect is the generation of acoustic waves as a result of light absorption in a material. As an example, consider a laser beam that is passed through a gas sample, which is enclosed in a cell of a constant volume. The laser energy absorbed by the sample molecules leads to local heating of the gas, which causes a pressure increase. If the optical excitation of molecules is done periodically, by modulating the laser power or frequency, also the pressure change is periodic. This acoustic wave at the modulation frequency can be detected with a microphone. The microphone signal is directly proportional to the absorbed power, which makes it possible to determine the concentration of absorbing molecules in the sample. In addition to analysis of gas phase samples, photoacoustic spectroscopy (PAS) is commonly applied to measurements of condensed phase samples.
- World-record trace-gas detection sensitivity using cantilever-enhanced PAS: < 1 ppt volume mixing ratio of HF in air, corresponding to a detection limit of < 1000 molecules/mm3. Scientific reports 8, 1848 (2018)
- World-record normalized noise equivalent absorption (NNEA) of photoacoustic spectroscopy: 1.75 × 10−12 W cm−1 using cavity-enhanced cantilever-enhanced photoacoustic detection. Analyst 144, 2291 (2019)
- Photoacoustic optical frequency comb spectroscopy: New method and the first reported infrared spectrum of radiocarbon methane 14CH4. Opt. Lett. 44, 1142 (2019)
- Next-generation optical power detector; from nW to watt level and from UV to far-infrared with a single device: arXiv:2104.06930 (2021)
In our photoacoustic experiments we use a micromachined silicon-cantilever microphone developed by Gasera Ltd. This technology offers an excellent photoacoustic detection sensitivity and a large linear dynamic range. An important general advantage of PAS is that it works at any wavelength, which makes it possible to measure strong vibrational transitions of molecules if only a suitable light source is available. For this reason, development of coherent mid-infrared sources is an integral part of our research on photoacoustic trace-gas analysis. In particular, we have developed several high-power CW-OPOs for high-resolution spectroscopy in the fundamental C-H stretching region at c.a. 3000 cm-1.
The following two paragraphs summarize our work on photoacoustics with the cantilever microphone technology: (1) Cantilever-enhanced photoacoustic spectroscopy and (2) Development of an optical power detector with spectrally broad and flat responsitivity.
Cantilever-enhanced photoacoustic spectroscopy (CEPAS) is one of the most sensitive methods for selective and quantitative trace gas detection. Typical applications of trace gas analysis include air-quality monitoring, detection of impurity gases in industrial processes, and analysis of biomarker gases in exhaled breath. All these applications require the capability of quantifying target gases at parts-per-billion (ppb) or even parts-per-trillion (ppt) volume mixing ratios. Due to typically complex gas matrices (air, exhaled breath, etc.) with strong spectral interferers, such as water and carbon dioxide, excellent spectroscopic resolution is needed to measure the concentrations of target molecules precisely and selectively. We reach the combination of high detection sensitivity and selectivity by applying the CEPAS method with state-of-the-art mid-infrared light sources developed by our team.
Since the photoacoustic signal is directly proportional to the absorbed optical power, a high-power laser is used to maximize the trace-gas detection sensitivity. As an example, we have reached a world-record (650 ppq) detection limit for hydrogen fluoride (HF) using a high-power CW-OPO that was optimized for 2.5 µm wavelength region, which accommodates strong spectral features of HF while minimizing spectral inferference due to water . The combination of a widely tunable CW-OPO and CEPAS has also enabled the first high-resolution studies of radiocarbon methane, 14CH4 . The precise measurements and analysis of the ro-vibrational structure of 14CH4 is a necessary prerequisite for developing laser-spetroscopic radiocarbon methane detectors, which find applications in radioactive emissions monitoring, in measurements of the biofractions of gas mixtures, as well as in apportioning of methane emission sources. Another radioactive gas-phase species of high importance in similar applications is radiocarbon dioxide, 14CO2. Together with our colleagues at VTT, we have recently shown the suitability of CEPAS for parts-per-trillion level detection 14CO2 .
Another approach that we have demonstrated with CEPAS is to enhance the laser power in an optical power build-up cavity, so as to reach ppt-level trace gas detection limits (noise-equivalent concentration) even with a simple low-power near-infrared semiconductor laser .
Fig. 1. Left: The basic principle of cavity-enhanced CEPAS spectroscopy. Right: Allan deviation determined from a measurement of acetylene at 6529 cm-1.
Unlike other highly sensitive photoacoustic spectroscopy techniques, the cantilever-enhanced photoacoustic method does not require the use of acoustic resonance for signal enhancement. As a result, the CEPAS method is also applicable to Fourier Transform Infrared Spectroscopy. We have utilized this favorable property to develop photoacoustic frequency comb spectroscopy, which can be used to record broad molecular spectra with high detection sensitivity and selectivity [5-6]. For more details about this new frequency comb spectroscopy method, see the Frequency-comb spectroscopy page.
Electromagnetic power detector based on photoacoustic effect
Power detector is a central component in nearly all measurement and imaging technologies that use electromagnetic radiation for either scientific or industrial purposes. Currently a large selection of different detectors is needed to cover all the required wavelengths, power levels and sensitivities, especially in research laboratories. We are developing a general-purpose room-temperature photoacoustic power detector that can be used for traceable detection of electromagnetic radiation over a wide range of frequencies (UV to far-IR/THz) and power (nW to W).
With our first detector prototype we have demonstrated an exceptionally large linear dynamic range of eight orders of magnitude, covering power levels from approximately 10 nW to 1 W . The prototype detector was characterized for a wide range of wavelengths, from UV (325 nm) to long mid-infrared (25 µm). We are currently working on the second-generation protype designed for an extended wavelength region, in order to reach the far-IR/THz region, where traceable power measurements are challenging to perform.
Our power detector is a sort of thermal detector, where a black material strongly absorbs optical radiation. The absorber is placed inside a gas cell enclosed by a window that transmits the incident optical waves. The absorbed optical power heats up the absorber surface, leading to a pressure increase (acoustic wave) in the surrounding gas. The acoustic signal, which is directly proportional to the incident optical power, is measured with a silicon-cantilever microphone, similar to that described above. Owing to wavelength-independent nature of the photoacoustic effect, the power detector can be designed for any wavelength, and its properties (sensitivity, spectral responsivity, etc.) can be tailored by the choice of absorber material. We have studied several different carbon-based absorbers, see Fig. 2 for a couple of examples .
Fig. 2. (a) and (b): Scanning electron microscope images of candle-soot and black-paint absorber materials, respectively. Figures (c) and (d) show the respective spatial responsivity maps of photoacoustic power detectors based on these two absorbers. The diameter of the absorbing area is 10 mm.
 T. Tomberg, M. Vainio, T. Hieta, L. Halonen, "Sub-parts-per-trillion level sensitivity in trace gas detection by cantilever-enhanced photo-acoustic spectroscopy," Scientific reports 8, 1848 (2018)
 S. Larnimaa et al., "High-resolution analysis of the ν3 band of radiocarbon methane 14CH4," Chem. Phys. Lett. 750, 137488 (2020)
 M. Fatima e al., "Radiocarbon dioxide detection using cantilever-enhanced photoacoustic spectroscopy," Opt. Lett. 46, 2083 (2021)
 T. Tomberg, T. Hieta, M. Vainio, L. Halonen, "Cavity-enhanced cantilever-enhanced photo-acoustic spectroscopy," Analyst 144, 2291 (2019)
 I. Sadiek, T. Mikkonen, M. Vainio, J. Toivonen, A Foltynowicz, "Optical frequency comb photoacoustic spectroscopy," Physical Chemistry Chemical Physics 20, 27849 (2018)
 J. Karhu et al., "Broadband photoacoustic spectroscopy of 14CH4 with a high-power mid-infrared optical frequency comb," Opt. Lett. 44, 1142 (2019)
 J. Rossi et al., "Optical power detector with broad spectral coverage, high detectivity and large dynamic range," arXiv:104.06930 (2021)
 J. Rossi et al., "Photoacoustic characteristics of carbon-based infrared absorbers," arXiv:2012.01568 (2020)