As a new field of acoustic research and development Photoacoustics (PA) is established at the Fraunhofer IBP. One part of the background represents the internationally recognized research and development of the Institute of Physical Chemistry (Prof. Peter Hess) of the University of Heidelberg. Therefore, the well known expert of gas phase photoacoustics, Prof. András Miklós, is integrated in these new activities at the Fraunhofer IBP. The applied research in photoacoustics will be directed mostly to the development of photoacoustic gas sensors and monitors for diverse applications in the industry, environment protection, medical diagnostics, biological problems, safety, quality control, etc. Additionally, fundamental research for improving the performance of the photoacoustic detection method and for developing novel PA sensors will also be carried out in the Fraunhofer IBP laboratory.
Principles of photoacoustics
The photoacoustic effect is based on the sensitive detection of acoustic waves generated by the absorption of pulsed or modulated monochromatic light via the transient localized heating and expansion in a gas, liquid, or solid. This effect is due to the transformation of at least part of the absorbed light energy into kinetic energy of the gas molecules by energy exchange processes . For gas sensing applications the wavelength of the monochromatic light has to be tuned to a strong absorption line of the molecule to be detected (target molecule). The sound pressure is proportional to the light power and the absorption coefficient of the gas at the wavelength of the light. Since the absorption coefficient is proportional to the concentration of the target molecule, the sound pressure will be proportional to the concentration of the target molecule in the gas.
Light sources and photoacoustic setup
Due to their narrow emission linewidth lasers are the best light sources for photoacoustic applications. Therefore, the first laser photoacoustic measurements have been performed mostly by powerful CO2 lasers, and very high sensitivities have been demonstrated for different trace gases in the air. For many practical applications, such as trace gas detection, compact tunable laser sources are needed. Single-mode DFB (distributed feedback) near-infrared (NIR) diode lasers, with their high spectral resolution, are used for this purpose despite their low power in the milliwatt range and a substantially lower absorption coefficient for overtone and combination bands in the NIR region. The application of such a laser source has been demonstrated, e.g. with the detection of ammonia . The current development of quantum-cascade (QC) semiconductor lasers, which radiate in the fundamental IR region, is providing a new impetus for future applications of photoacoustics with versatile laser sources in optical diagnostics and spectroscopy.
A relatively broad continuous tuning range, up to about 4.5 µm in the IR, is provided by optical parametric oscillators (OPOs), using periodically-poled lithium-niobate (PPLN) crystals. Such advanced tunable laser systems, consisting of a high repetition rate Nd:YAG pump laser and a PPLN-OPO in our case (see Fig. 1), have a sufficiently large tuning range to allow the analysis of gas mixtures and a spectral resolution of about 0.1 cm-1, sufficient to resolve the pressure-broadened spectral lines under normal conditions. Ultimate sensitivity in the sub-ppbV range can be achieved with OPOs, based on the efficient excitation of fundamental vibrational modes. This allows, for example, the selective detection of local changes in the concentration of methane in cities . Currently methane has a mean concentration of about 1.7 ppmV in the atmosphere. The OPO-based photoacoustic system is not suitable for practical applications because its complexity, size and price. However, it is an excellent tool for laboratory research and measurements.
The absorption of modulated light generates an acoustic signal (PA signal) in a detector cell, used for photoacoustic measurements. The PA signal can be amplified by tuning the modulation frequency to one of the acoustic resonances of the sample cell. In this resonant case the cell works as an acoustic amplifier and the photoacoustic signal is amplified by the Q factor of the resonance, which usually is in the range of 10 - 300.
The highest photoacoustic detection sensitivity has been achieved with an optimized differential acoustic cell where two identical cylindrical resonator tubes, both equipped with a microphone, are used, as shown in Fig. 1. Since the photoacoustic signal is generated in only one tube, by resonantly exciting a longitudinal acoustic mode, the difference signal eliminates most of the background noise and distortions. This cell is compact, with an absorption length in the centimetre range. By using a laser-excited acoustic wave as the carrier wave and by modulating the radiation wavelength of a multi-kilohertz-repetition-rate optical parametric oscillator setup at a lower frequency than the acoustic frequency, a wavelength-amplitude double-modulation technique has been demonstrated and realized an additional enhancement factor of 35 in the detection sensitivity of methane with this technique .
A combined optical multipass-differential PA cell has been also developed and demonstrated  to obtain a ten-fold increase of the sensitivity. This PA cell design could be used as a basis for the development of a multipass PA detector for practical applications.
An acoustically fully open and optically multipass PA detector has been patented by the Fraunhofer IBP. This design could be used for developing PA monitors for open air use.
Trace gas detection of nitrogen compounds
Depending on the laser source, photoacoustics can be used for high resolution spectroscopy and, based on this capability, selective and sensitive detection of molecules in trace amounts. For trace gas monitoring, a large dynamic range and linear concentration dependence of the signal are essential, reaching five to six orders of magnitude in photoacoustics. With versatile low power NIR diode lasers, usually concentrations in the ppmV range can be detected. An example is the detailed investigation of adsorption and desorption processes of polar molecules such as ammonia at the walls, which limit the detection sensitivity in this class of molecules . Sensitive detection of NH3 may be used to study the emissions in agricultural environments, from car exhaust, and real-time monitoring of the breath of patients (e.g. liver disease). Of increasing importance is the investigation of the "nitrogen cycle", because the amount of reactive so-called fixed nitrogen in harmful species is increased rapidly by human activities. Molecule-specific detection and real-time monitoring of trace amounts of all nitrogen oxides (not only NOx) is important for monitoring changes in the atmosphere. For example, nitrous oxide (laughing gas) has been found to be one of the most effective greenhouse gases due to its strong absorption in transmission regions of the atmosphere. Due to its chemical stability and long lifetime it is also involved in stratospheric ozone depletion reactions. According to recent studies, the ambient concentration of N2O is ~310 ppbV. The OPO system has been used for sensitive monitoring of N2O . The large spectral range and the ~100 mW IR power of this light source allow discrimination with respect to other atmospheric species, such as H2O and CO2, as can be seen in Fig. 2.
Trace gas detection of ozone
A 9.5 µm pulsed quantum cascade (QC) laser and the differential photoacoustic detector were used to measure trace concentrations of ~100 ppbv ozone at ambient pressure with high selectivity. The QC laser was tuned by temperature variation between -41°C and 30.6°C and the corresponding wavelengths were calibrated by the well known photoacoustic spectrum of CO2. Good agreement was found between the measured spectrum and the simulated HITRAN spectrum of ozone. The photoacoustic signal showed a linear dependence on the ozone concentration in the investigated 100-4300 ppbv range (see Fig. 3). In comparison with recently published results, in which a similar QC laser in combination with an optical absorption technique was applied, an improvement in the ozone detection sensitivity of about a factor of 200 was achieved in experiments .
Current and future activities
The state-of-the-art tunable laser techniques and optimized photoacoustic setups are being applied to current problems of trace gas analysis in the ambient atmosphere, e.g. to investigate harmful compounds of the "nitrogen cycle". Detection of N2O molecules and of polar NO2 molecules with QC lasers was studied in 2005. Formaldehyde detection in gas mixtures and in ambient air is being studied at present.
To apply the photoacoustic technique to problems encountered in countries with specific pollution problems cooperations have been established with research groups in Brazil and Taiwan. In this respect, special versatile and easy to operate laser-based photoacoustic detection systems are being developed, which can be used in practical applications.
Photoacoustic methods and sensors with cheap IR emitters as light sources for detecting concentrations in the ppmv range are under development in the Fraunhofer IBP in cooperation with the Steinbeis Innovation Center Acoustics and Optics.