Photodeflection signal formation in photothermal measurements: comparison of the complex ray theory, the ray theory, the wave theory, and experimental results

Abstract

A comparison is made of three methods for modeling the interaction of a laser probe beam with the temperature field of a thermal wave. The three methods include: (1) a new method based on complex ray theory, which allows us to take into account the disturbance of the amplitude and phase of the electric field of the probe beam, (2) the ray deflection averaging theory of Aamodt and Murphy, and (3) the wave theory (WT) of Glazov and Muratikov. To carry out this comparison, it is necessary to reformulate the description of the photodeflection signal in either the WT or the ray deflection averaging theory. It is shown that the differences between calculated signals using the different theories are most pronounced when the radius of the probe beam is comparable with the length of the thermal wave in the region of their interaction. Predictions of the theories are compared with experimental results. A few parameters of the experimental setup are determined through multiparameter fitting of the theoretical curves to the experimental data. A least-squares procedure was chosen as a fitting method. The conclusion is that the calculation of the photodeflection signal in the framework of the complex ray theory is a more accurate approach than the ray deflection averaging theory or the wave one.

Determination of Structural Parameters of Thin-Film Photocatalytic Materials by BDS

“…The change in the refractive index and its gradient deflects the probe beam and change its phase [9,14]. The deflection of the PB is expressed by the change in the coordinate of its trajectory as [9,14] …”

“…The deflection of the PB results in the change of its divergence and thus in the change of the PB electric field amplitude A [9,14]. In the detector plane (z D ), the amplitude is expressed by [4,9] A…”

“…The deflection and phase change of the PB due to the change in the refractive index and its gradient is detected by the use of a quadrant photodiode and arises from differences in illumination between its halves [9,14]:…”

Application of complex geometrical optics to determination of thermal, transport, and optical parameters of thin films by the photothermal beam deflection technique

“…As a result of PB interaction with the ThW, we deal with two effects-deflection of the rays and change of their phase [9,10].…”

“…This is either the deflection of the beam or its phase change at thermal gradients. Thus, neither of them predicts PDS signal behavior properly, and deviations from true values are pronounced, particularly for a high modulation frequency of the temperature field (TF) [9,10]. Because of this, an approach that takes into account beam deflection as well as the phase shift and is based on complex geometrical optics equations, was introduced [the complex ray theory (CRT)] [9,10] but has so far been applied only to describe PDS signals in bulk macroscopic materials where relatively low modulation frequencies are used.…”