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.
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 about 3000 cm-1.
The following paragraphs summarize our work on photoacoustics with the cantilever microphone technology, applied to (1) Highly sensitive gas spectroscopy, (2) Precise measurements of light-absorbing aerosols, such as black and brown carbon, and (3) Universal optical power detector with spectrally broad and flat responsivity.
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 parts per quadrillion) detection limit for hydrogen fluoride (HF) using a high-power CW-OPO that was optimized for the 2.5 µm wavelength region, which accommodates strong spectral features of HF while minimizing spectral interference due to water [1]. The combination of a widely tunable CW-OPO and CEPAS has also enabled the first high-resolution studies of radiocarbon methane, 14CH4 [2]. The precise measurements and analysis of the ro-vibrational structure of 14CH4 is a prerequisite for developing laser-spectroscopic radiocarbon methane detectors, with 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 of 14CO2 [3].
Another approach that we have demonstrated with CEPAS is to enhance the laser power in an optical power build-up cavity. This approach leads to world-record normalized noise equivalent absorption (NNEA) of photoacoustic spectroscopy (1.75 × 10−12 W cm−1) and parts-per-trillion level trace gas detection limit even with a simple low-power near-infrared semiconductor laser [4].
Fig. 1. Schematic of a CEPAS experiment for trace gas measurements. Laser intensity or wavelength is modulated, which leads to an acoustic signal upon light absorption by the sample molecules. The microphone signal detection (demodulation) is done using either lock-in detection (typically at second harmonic of the modulation frequency) or fast Fourier transform (FFT).
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.
Another interesting feature of CEPAS is its small sample volume. The small volume is highly useful for measurements of samples with limited amount, such as the above-mentioned radioactive samples or column effluents of gas chromatography. For more details on analysis of complex gas mixtures by a combination of gas chromatography and broadband CEPAS, see [7].
Black carbon (soot nanoparticles) is an air pollutant that leads to millions of premature deaths annually. It is also the second-most important cause of global warming after carbon dioxide. Black carbon is produced in incomplete combustion, for example due to traffic and residential wood burning. The emissions can travel thousands of kilometers in the atmosphere.
Fig. 2. Black carbon in the atmosphere causes premature mortality and global warming. (Artwork by Kajsa Roslund).
Precise measurements of the light absorption caused by black carbon at levels relevant for atmospheric research is challenging. Photoacoustic detection is well suited for the purpose, because it gives a signal directly proportional to the absorbed optical power, without being affected by light scattering. We have recently shown that CEPAS can provide significant improvement in the detection sensitivity of black carbon and other light-absorbing aerosols in comparison to conventional photoacoustic techniques [8]. The measurement instrumentation is simple and miniaturizable, paving the way for comprehensive black-carbon monitoring infrastructure with high temporal and spatial resolution. As a proof of concept, we are running the first field tests with our prototype instrument at the SMEAR III atmospheric monitoring station in Helsinki. In addition to black carbon, our research targets precise characterization of brown carbon, which has different optical properties, and thus different radiative forcing than black carbon. The work is a collaboration with the Aerosol Composition group of the Finnish Meteorological Institute (FMI).
The work is supported by Jane and Aatos Erkko Foundation (JEAS) and the Academy of Finland.
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 have developed 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 [9]. The prototype detector was characterized for a wide range of wavelengths, from UV (325 nm) to long mid-infrared (25 µm). The spectral coverage has recently been extended to the THz region (> 200 µm, or 1.4 THz), potentially enabling traceable THz power measurements (see Figure 3 below).
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 materials [10].
The work is supported by the Academy of Finland.
Fig. 3. (a) Principle of the photoacoustic optical power detector. A chopped light beam is directed onto a light absorber, which is placed inside a photoacoustic cell sealed by a window. The microphone signal is directly proportional to the absorbed power. (b) Spectral response of the optical power detector for two different absorber materials – soot and carbon nanotubes (CNT). The CNT absorber shows flat response from UV to THz.
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[2] S. Larnimaa et al., "High-resolution analysis of the ν3 band of radiocarbon methane 14CH4," Chem. Phys. Lett. 750, 137488 (2020)
[3] M. Fatima e al., "Radiocarbon dioxide detection using cantilever-enhanced photoacoustic spectroscopy," Opt. Lett. 46, 2083 (2021)
[4] T. Tomberg, T. Hieta, M. Vainio, L. Halonen, "Cavity-enhanced cantilever-enhanced photo-acoustic spectroscopy," Analyst 144, 2291 (2019)
[5] I. Sadiek, T. Mikkonen, M. Vainio, J. Toivonen, A Foltynowicz, "Optical frequency comb photoacoustic spectroscopy," Physical Chemistry Chemical Physics 20, 27849 (2018)
[6] J. Karhu et al., "Broadband photoacoustic spectroscopy of 14CH4 with a high-power mid-infrared optical frequency comb," Opt. Lett. 44, 1142 (2019)
[7] T. Tomberg et al., "Broadband Laser-Based Infrared Detector for Gas Chromatography," Anal. Chem. 2020, 92, 14582 (2020)
[8] J. Karhu et al., “Cantilever-enhanced photoacoustic measurement of light-absorbing aerosols,” Aerosol Science and Technology 56, 92 (2022)
[9] J. Rossi et al., "Optical power detector with broad spectral coverage, high detectivity and large dynamic range," Opt. Lett. 47, 1689 (2022)
[10] J. Rossi et al., "Photoacoustic characteristics of carbon-based infrared absorbers," Photoacoustics 23, 100265 (2021)