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Dr. C. Vijayan, Professor, |
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Dept. of Physics, IIT Madras |
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Links to other introductory articles: nanostructures , photonics, photonic materials An Overview of Photoacoustic Spectroscopy Salient features Photoacoustic spectroscopy (PAS) is a novel spectroscopic technique with several distinct features that make it a complementary technique to the conventional forms of spectroscopy. This is basically a branch of photothermal spectroscopy in which the heat generated due to the absorption of light energy in a sample is measured directly. In contrast, it may be noted that the optical absorption properties of a sample are studied by actually measuring the light transmitted by it in the conventional optical absorption spectroscopy. In PAS, the sample is periodically heated by the incidence of a chopped light beam and the pressure wave thus generated in the coupling gas enclosing the sample is detected by acoustic techniques. Such a scheme of experimentation provides several interesting special features, which make PAS a very attractive tool for the Chemist, the Physicist, the Materials Scientist and even the industrialist [1,2]. These features include a) the
basic simplicity of the experimental set up, In fact this technique has been successfully used in a wide variety of applications ranging from semiconductor characterization to eye-lens degradation studies. Let us see the basic principles of this form of spectroscopy that makes all this possible. Basic concepts The processes that occur in a solid on being illuminated by light depend very much on the nature of the sample and of course, the wavelength of the light incident. . Absorption, transmission, reflection, scattering and fluorescence are the possible processes that can occur. Absorption could be followed by partial radiative reemission. Nonradiative decay channels also are possible, which eventually result in an increase in the temperature of the sample. This heating up would depend on the (angular) chopping frequency w, the optical absorption coefficient b and the thermal diffusion coefficient a of the sample. Two characteristic lengths may be defined as Lop = (1/b) and Lth = (1/v{2a/w}) which typically are measures of the lengths over which the optical and thermal effects can have their influence. A theoretical analysis of PA response based on the thermal and optical properties of the sample has been given by Rosencwaig and Gresho [2]. In the case of optically transparent samples, Lop is greater than the sample thickness d and hence light is absorbed by the entire length of the sample. Such samples are considered to be thermally `thin' if Lth is greater than both Lop and d. Then the PA signal intensity is directly proportional to (db/w) where w is the (angular) chopping frequency of incident light. For thermally thick samples, only the light absorbed in the first layers of the sample contribute to the signal and the signal is proportional to at the product (b Lop)w. Thus we see that by varying the chopping frequency we can actually probe different layers of the sample, thus offering the possibility for depth-profiling. By increasing the frequency we are effectively increasing the thermal diffusion length Lth and hence observing PA signal originating from a deeper layer within the sample. PA depth profiling opens up exciting possibilities such as studying layered and amorphous materials and determination of thickness of thin films. In the case of optically opaque (Lop<d) samples, the PA signal strength varies as w-1 . Even for opaque samples, b can be evaluated by analyzing the PA signal whereas conventional optical spectroscopy is not possible. Experimental set-up The heart of the experimental set of a PA spectrometer is the PA cell. It encloses the sample in a leak-free gas atmosphere at a desired pressure. Mostly, air at atmospheric pressure is used due to the simplicity of experimentation. However, better signals can be obtained if the coupling has a pressure slightly above the atmospheric pressure. The sample holder is designed such that light can be incident on the sample through a window. The cell volume is kept a minimum for better signal strength. The cell is to be isolated from possible acoustic disturbances. The incident light from a white light source such as a xenon lamp of a few hundred watts power is chopped at a desired frequency using a mechanical chopper. The absorption of chopped light by the sample causes periodic heating, which alters the size of the sample periodically, leading to alternate condensation and rarefaction in the coupling gas. This pressure wave, normally in the acoustic frequency region, can be easily detected by a powerful microphone of sensitivity of more than 10 mV/Pa. A piezoelectric detector can be glued directly onto the sample the case of solid samples or to the wall of the container in the case of liquid samples, for the purpose of better acoustic impedance matching. The actual magnitudes of the temperature-rise and size-change of the sample are very small and hence the PA signal from the acoustic detector has to be amplified. Lock-in amplification is found to be very advantageous. For PA spectroscopy the wavelength of the incident light is varied continuously using a scanning monochromator and the signal is monitored as a function of the wavelength. This spectrum has to be corrected for the spectral distribution of the light source and the dispersion of the monochromator. Carbon black, a perfect absorber in the entire visible region, is used as a standard reference for the purpose of spectral correction. Data acquisition and processing become easier and more accurate on computer interfacing. Scope and potential PA response is widely used is a variety of scientific and technical applications. Spectroscopy of otherwise difficult samples (such as powders, opaque materials etc.) is an area of immediate interest to Chemists and Materials Scientists. This also provides a direct measurement for nonradiative lifetimes. PAS has been used successfully as a spectroscopic tool for investigating and characterizing colored inorganic as well as organic materials including transition metal complexes, semiconducting materials, liquid crystals and metals. A correlative study of PAS and optical spectroscopy can be effectively used for obtaining a more complete picture of the electronic processes in materials. An area of special interest to the Chemist is the identification of species and their valence states in several situations such as catalysis and other chemical reactions. The effect of fluorescence quenchers can be studied by monitoring the PA and fluorescence signals simultaneously. Other related applications include photosynthesis and other photochemical reactions, quantum efficiency studies on organic dyes etc [3]. The possibility of studying coloring agents such as cyctochrome and hemoglobin samples has attracted the attention of biologists. Some biomedical applications such as blood examination also have been demonstrated. Usually blood specimens have to be rendered transparent by smearing a thin layer on a glass plate after some processing in order to record the optical absorption spectrum. However, PA spectrum of whole blood as such can be recorded to get the same information as that obtainable with optical absorption [2]. Similarly, PA spectrum of a whole leaf has been shown to provide information about coloring agents of the leaf. Another example is the study apple peal by depth profiling PA spectroscopy. Other medical applications of PAS include identification of bacterial states, study of animal and human tissues including teeth, bone, skin, muscle etc., analysis of drug in tissues, investigation of the photo-oxidative decay in human eye lenses etc. PA imaging and microscopy have been demonstrated to be efficient tools in obtaining visual information on a microscopic scale [4]. This is done by focusing light onto a spot on the sample and scanning it over the sample while recording the PA signal. Any local change in the thermal or optical property shows up as a change in the PA signal at that point of scanning. Depth-profiling and subsurface imaging can be done by varying the chopping frequency. This has potential as both a general analytic tool as well as a process control instrument in semicodnuctor industry. A recent study on semiconductor nanostructures Semiconductor nanostructures are of current topical interest due to the fact that their properties undergo considerable modification from those of the bulk due to quantum confinement effects. Nanoclusters of these materials embedded in dielectric hosts act are known to behave as quantum dots, described rightly as practical examples of `particle-in-a-box'. Such structures can be prepared by several chemical and physical methods such as ion-exchange reactions, molecular beam epitaxy, ion-implantation, sol-gel techniques etc . Quantum confinement leads to alterations in the band structure of the bulk. It also systematically shifts the band edge towards higher energy as the quantum dots get smaller and smaller. This is known as the signature `blue shift' in optical absorption and the process as `bandgap engineering'. The physical processes in such structures are governed by their excitonic energy levels, which are difficult to be probed by optical spectroscopy due their proximity to band-edge absorption, as the optical densities tend to be very large and transmission poor. However, we may seek a technique that directly measures the absorbed energy to investigate these systems. PA spectroscopy has been used successfully to obtain valuable spectroscopic information in such cases. In summary, it is quite established that the study of PA response has the potential to develop into a common useful analytic and research technique in a variety of scientific, industrial and medical applications both as a spectroscopic tool as well as a nonspectroscopic probe for thermal and elastic properties. Links to other introductory articles: nanostructures , photonics, photonic materials |