Flat-top Beam Excitation Of Surface Thermal Lensing Technique For Measuring Film Absorption Study

Absorption is of great importance to evaluate optical thin films. Especially in high power laser applications, absorption can cause thermal distortion to optical components, then change the transmission characteristics of the incident beam, and also lower the laser induced damage thresholds(LIDT). Precise measurement of the absorption is also very important for film designers to optimize the film-deposition parameters. However, traditional methods, including spectrophotometric, extinction coefficient method, are not sensitive enough to measure absorption of thin films in the 10^-4～10^-7 range which is often used in the modern optical system. Surface thermal lens (STL) technique was developed by Kuo and Saito respectiv- ely in the early 1990s. It is a sensitive photothermal methods and widely used in the thin film characterizations not only for its high precision but also for the ability to get the fu- ll distribution of the surface deformation. In former STL instruments, a laser beam with a Gaussian distributed intensity was commonly used as the excitation source. Nevertheless, the output of a high power laser is usually a superposition of multi-modes rather than a Gaussian distribution (TEM00). Therefore, it is very convenient to add an aperture to get a top-hat beam. Moreover, higher sensitivity can be achieved when a top- hat beam is used as the excitation source.In this paper, the STL technique with cw modulated top-hat beam excitation is investigated theoretically and experimentally.1. The theoretical model based on the Fresnel diffraction integral is developed to describe the STL signal excited with the cw modulated top-hat beam for the first time to the best of our knowledge.2. The three-dimensional profile of the temperature rise, the radial profile of the surface deformation and the diffraction signal of the probe beam obtained with both top-hat and Gaussian beam excitations are compared by numerical calculations. The dependence of the surface thermal lens amplitude on the experimental parameters is analyzed. Numerical results show that the top-hat beam STL instrument is more sensitive than the Gaussian beam STL instrument, with a maximum sensitivity enhancement factor of approximately 2.3. Corresponding STL experiment setup is established. Top-hat intensity distribution...