Study On Photothermal Deflection Technique With Step Optical Excitation

The photothermal deflection (PTD) technique, which is developed from the photothermal effect, is a kind of the nondestructive detecting techniques. Because of some advantages, such as high sensitivity, sample-non-pretreatment and real-time detection, the PTD technique has been widely applied to the area of physics and material sciences etc. Traditional PTD techniques generally use the periodic modulated or pulsed beam as the excitation sources, which mainly focuse on the amplitude and the phase of signals. The features of real-time signals are less studied by traditional PTD techniques. The experimental system, the experimental process and the data-processing of traditional PTD methods are comparatively complex. Compared with the traditional PTD method, the PTD technique with step optical excitation has many advantages such as simpler equipment, easier adjustment and faster measurement. After making a brief review of the development and application of the PTD technique, the main contents about PTD with step optical excitation in this thesis are the following:1. The PTD technique with step optical excitation is experimentally studied in detail. The experiment system is established. The system structure and the adjustment method are introduced in the thesis. The physical parameters of samples and the experiment conditions, which have effect on PTD signals, are studied. The relationship between PTD signals and the power of excitation beams is explored. The regularity between the real-time signals and the sample's thermal diffusivity, as well as the distance from probe beams to the sample surface, is acquired and discussed. Some suggestions about the distance from probe beam to the sample surface and the power of excitation beams are given in the thesis.2. The PTD phenomena with step optical excitation are theoretically studied in detail. A new one-dimensional theoretical model is proposed. The temperature distribution of the sample and the air over sample surfaces are deducted. According to temperature distribution, the mathematical expression of PTD signals is finally obtained. By numerical simulation, the temperature and the temperature gradient in the sample and the air over sample surfaces, which are varied with the time and the space, are analyzed. The influence of the distance between probe beams to the sample surface, as well as the thermal diffusivity of samples, on the PTD signals are theoretically analyzed and discussed. The theoretical model can fit the signal curves. The theoretical results can explain the experimental phenomena very well.3. Two new methods for measuring the thermal diffusivity of materials are proposed. Based on the signal characters of the PTD with step optical excitation, the characteristic time comparison method and the up-down signal fitting method are introduced for measuring the thermal diffusivity of materials, respectively. A simplified theoretical model is proposed in the up-down signal fitting method, which can fit the experimental signals very well. By practically measuring the thermal diffusivity of samples, it is shown that two methods can obtain the satisfied results quickly and easily. Finally, two methods are discussed and compared each other, and the way to improve the measurement accuracy is proposed.