Thermal effects induced by laser irradiation of solids
A part of incident energy is absorbed within the irradiated sample when a solid is exposed to the influence of laser radiation, to more general electromagnetic radiation within the wide range of wavelengths (from microwaves, to infrared radiation to X-rays), or to the energy of particle beams (electronic, protonic, or ionic). The absorption process signifies a highly selective excitation of the electronic state of atoms or molecules, followed by thermal and non-thermal de-excitation processes. Non-radiation de-excitation-relaxation processes induce direct sample heating. In addition, a great number of non-thermal processes (e.g., photoluminescence, photochemistry, photovoltage) may also induce heat generation as a secondary process. This method of producing heat is called the photothermal effect. The photothermal effect and subsequent propagation of thermal waves on the surface and in the volume of the solid absorbing the exciting beam may produce the following: variations in the tempe...rature on the surfaces of the sample; deformation and displacement of surfaces; secondary infrared radiation (photothermal radiation); the formation of the gradient of the refractivity index; changes in coefficients of reflection and absorbtion; the generation of sound (photoacoustic generation), etc. These phenomena may be used in the investigation and measurement of various material properties since the profile and magnitude of the generated signal depend upon the nature of material absorbing radiation. A series of non-destructive spectroscopic, microscopic and defectoscopic detecting techniques, called photothermal methods, is developed on the basis of the above-mentioned phenomena. This paper outlines the interaction between the intensity modulated laser beam and solids, and presents a mathematical model of generated thermal sources. Generalized models for a photothermal response of optically excited materials have been obtained, including thermal memory influence on the propagation of thermal perturbation. Focus is on optically opaque media. The derived models are compared to existing models neglecting the thermal memory effect. In this way it has been possible to determine the range of value of existing models and to indicate the additional employment of photothermal methods in determining through experimentation the thermal memory properties of solids. These properties have not as yet been experimentally determined in any medium, nor has the methodology for the experimental measurement of thermal memory parameters been suggested in the literature. Their recognition is highly significant not only for further fundamental research, but for many modem applications as well.
Source:AIP Conference Proceedings, 2004, 740, 221-232
- 22nd Summer School and International Symposium on the Physics of Ionized Gases, Aug 23-27, 2004, Tara Natl Pk, Yugoslavia