The photoacoustic effect or optoacoustic effect is the formation of sound waves following light absorption in a material sample. In order to obtain this effect the light intensity must vary, either periodically (modulated light) or as a single flash (pulsed light).Rosencwaig, A. (1980) Photoacoustics and photoacoustic spectroscopy. Chemical Analysis: a Series of Monographs on Analytical Chemistry and Its Applications, Vol. 57. New York:John Wiley & Sons, . The photoacoustic effect is quantified by measuring the formed sound (pressure changes) with appropriate detectors, such as microphones or piezoelectric sensors. The time variation of the electric output (current or voltage) from these detectors is the photoacoustic signal. These measurements are useful to determine certain properties of the studied sample. For example, in photoacoustic spectroscopy, the photoacoustic signal is used to obtain the actual absorption of light in either opaque or transparent objects. It is useful for substances in extremely low concentrations, because very strong pulses of light from a laser can be used to increase sensitivity and very narrow wavelengths can be used for specificity. Furthermore, photoacoustic measurements serve as a valuable research tool in the study of the heat evolved in photochemical reactions (see: photochemistry), particularly in the study of photosynthesis.
Most generally, electromagnetic radiation of any kind can give rise to a photoacoustic effect. This includes the whole range of electromagnetic frequencies, from gamma radiation and X-rays to microwave and radio. Still, much of the reported research and applications, utilizing the photoacoustic effect, is concerned with the near ultraviolet/visible and infrared spectral regions.
History The discovery of the photoacoustic effect dates back to 1880, when Alexander Graham Bell was experimenting with long-distance sound transmission. Through his invention, called "photophone", he transmitted vocal signals by reflecting sun-light from a moving mirror to a selenium solar cell receiver. As a byproduct of this investigation, he observed that sound waves were produced directly from a solid sample when exposed to beam of sunlight that was rapidly interrupted with a rotating slotted wheel. He noticed that the resulting acoustic signal was dependent on the type of the material and correctly reasoned that the effect was caused by the absorbed light energy, which subsequently heats the sample. Later Bell showed that materials exposed to the non-visible (ultra-violet and infra-red) portions of the solar spectrum can also produce sounds and invented a device, which he called "spectrophone", to apply this effect for spectral identification of materials. Bell himself and later John Tyndall and Wilhelm Röntgen extended these experiments, demonstrating the same effect in liquids and gases. However, the results were too crude, dependent on ear detection, and this technique was soon abandoned. The application of the photoacoustic effect had to wait until the development of sensitive sensors and intense light sources. In 1938 Mark Leonidovitch Veingerov revived the interest in the photoacoustic effect, being able to use it in order to measure very small carbon dioxide concentration in nitrogen gas (as low as 0.2% in volume). Since then research and applications grew faster and wider, acquiring several fold more detection sensitivity.
While the heating effect of the absorbed radiation was considered to be the prime cause of the photoacoustic effect, it was shown in 1978 that gas evolution resulting from a photochemical reaction can also cause a photoacoustic effect. Independently, considering the apparent anomalous behaviour of the photoacoustic signal from a plant leaf, which could not be explained solely by the heating effect of the exciting light, led to the cognition that photosynthetic oxygen evolution is normally a major contributor to the photoacoustic signal in this case.
Physical mechanisms
Photothermal mechanism
Although much of the literature on the subject is concerned with just one mechanism, there are actually several different mechanisms that produce the photoacoustic effect. The primary universal mechanism is photothermal, based on the heating effect of the light and the consequent expansion of the light-absorbing material. In detail, the photothermal mechanism consists of the following stages:
conversion of the absorbed pulsed or modulated radiation into heat energy. temporal changes of the temperatures at the loci where radiation is absorbed – rising as radiation is absorbed and falling when radiation stops and the system cools. expansion and contraction following these temperature changes, which are "translated" to pressure changes. The pressure changes, which occur in the region where light was absorbed, propagate within the sample body and can be sensed by a sensor coupled directly to it. Commonly, for the case of a condensed phase sample (liquid, solid), pressure changes are rather measured in the surrounding gaseous phase (commonly air), formed there by the diffusion of the thermal pulsations.
The main physical picture, in this case, envisions the original temperature pulsations as origins of propagating temperature waves ("thermal waves"), which travel in the condensed phase, ultimately reaching the surrounding gaseous phase. The resulting temperature pulsations in the gaseous phase are the prime cause of the pressure changes there. The amplitude of the traveling thermal wave decreases strongly (exponentially) along its propagation direction, but if its propagation distance in the condensed phase is not too long, its amplitude near the gaseous phase is sufficient to create detectable pressure changes. This property of the thermal wave confers unique features to the detection of light absorption by the photoacoustic method. The temperature and pressure changes involved are minute, compared to everyday scale – typical order of magnitude for the temperature changes, using ordinary light intensities, is about micro- to millidegrees and for the resulting pressure changes is about nano- to microbars.
The photothermal mechanism manifests itself, besides the photoacoustic effect, also by other physical changes, notably emission of infra-red radiation and changes in the refraction index. Correspondingly, it may be detected by various other means, described by terms such as "photothermal radiometry", "thermal lens"Thermal Lens Spectroscopy. photonics.cusat.edu and "thermal beam deflection" (popularly also known as "mirage" effect, see Photothermal spectroscopy). These methods parallel the photoacoustic detection. However, each method has its special range of application.
Other
While the photothermal mechanism is universal, there could exist additional other mechanisms, superimposed on the photothermal mechanism, which may contribute significantly to the photoacoustic signal. These mechanisms are generally related to photophysical processes and photochemical reactions following light absorption: (1) change in the material balance of the sample or the gaseous phase around the sample; (2) change in the molecular organization, which results in molecular volume changes.Schulenberg, C.P.J. and Braslavsky, S.E. (1997) "Time-Resolved Photothermal Studies with Biological Supramoleculear Systems", pp. 57–81 in Progress in Photothermal and Photoacoustic Science and Technology Vol. III. A. Mandelis, and P. Hess (eds.). SPIE Optical Engineering Press Most prominent examples for these two kinds of mechanisms are in photosynthesis.Malkin, S. (1995) "The photoacoustic method – monitoring and analysis of phenomena which lead to pressure changes following light excitation", pp. 191–206 in Biophysical methods in photosynthesis. J. Amesz and A.J. Hoff (eds.) Advances in Photosynthesis. Vol. III. Kluwer
The first mechanism above is mostly conspicuous in a photosynthesizing plant leaf. There, the light induced oxygen evolution causes pressure changes in the air phase, resulting in a photoacoustic signal, which is comparable in magnitude to that caused by the photothermal mechanism. This mechanism was tentatively named "photobaric". The second mechanism shows up in photosynthetically active sub-cell complexes in suspension (e.g. photosynthetic reaction centers). There, the electric field which is formed in the reaction center, following the light induced electron transfer process, causes a micro electrostriction effect with a change in the molecular volume. This, in turn, induces a pressure wave which propagates in the macroscopic medium. Another case for this mechanism is Bacteriorhodopsin proton pump. Here the light induced change in the molecular volume is caused by conformational changes that occur in this protein following light absorption.
Detection of the photoacoustic effect
In applying the photoacoustic effect there exist various modes of measurement. Gaseous samples or condensed phase samples, where the pressure is measured in the surrounding gaseous phase, are usually probed with a microphone. The useful applicable time-scale in this case is in the millisecond to sub-second scale. Most often, In this case, the exciting light is continuously chopped or modulated at a certain frequency (mostly in the range between ca. 10–10000 Hz) and the modulated photoacoustic signal is analyzed with a lock-in amplifier for its amplitude and phase, or for the inphase and quadrature components. When the pressure is measured within the condensed phase of the probed specimen, one utilizes piezoelectric sensors inserted into or coupled to the specimen itself. In this case the time scale is between less than nanoseconds to many microseconds The photoacoustic signal, obtained from the various pressure sensors, depends on the physical properties of the system, the mechanism that creates the photoacoustic signal, the light-absorbing material, the dynamics of the excited state relaxation and the modulation frequency or the pulse profile of the radiation, as well as the sensor properties. This calls for appropriate procedures to (i) separate between the signals due to different mechanisms and (ii) to obtain the time dependence of the heat evolution (in the case of the photothermal mechanism) or the oxygen evolution (in the case of the photobaric mechanism in photosynthesis) or the time dependence of the volume changes, from the time dependence of the resulting photoacoustic signal.
Applications