Photothermal model anslysis and experiment of surface thermal-lensing

高功率激光技术的发展对光学薄膜的抗激光损伤能力提出了越来越高的要求。大量的薄膜激光损伤实验表明，薄膜中纳米尺度的吸收缺陷导致了激光诱导损伤，是薄膜激光损伤的源头。对这些缺陷进行探测，可以为进一步分析缺陷引入的原因，从而改进薄膜的制备和储存过程，提高薄膜激光损伤阈值奠定基础。表面热透镜技术作为一种无损的光热探测技术，具有较高的灵敏度和三维分辨能力（径向和纵向），在薄膜弱吸收和缺陷检测方面得到了广泛应用。 从温度场理论出发，分析了单层TiO2膜的厚度及热物参数对其表面温升的影响情况，认为薄膜表面温升的变化规律与其吸收的变化规律相同，在此基础上提出了一种由某一波长下的吸收值推得任意波长下吸收值的方法。 从实验上研究了单层HfO2膜在特定波长（1064nm）下的吸收值和其在1064nm、532nm及355nm波长下激光损伤阈值的关系；结果发现薄膜特定波长下的吸收值也可以定性的反映其它波长的吸收大小，薄膜的激光损伤阈值由膜层的吸收平均值与吸收均匀性共同决定。 详细推导了薄膜中含有吸收层的五层膜系统的温度场和形变场，并进行了数值分析，在此基础上，提出了一种利用表面热透镜技术的变频测量对膜层中的热缺陷进行深度探测的方法，利用此方法同时可以对膜层热扩散系数进行探测。实验上对该方法进行了验证，在已知膜层热扩散系数的前提下，探测到膜层缺陷深度的分辨率可达几十纳米；在已知膜层中缺陷深度的前提下，通过变频测量，得到了膜层样品的热扩散系数。 综合利用表面热透镜技术中探测到的衍射光的强度和相位信息，对膜层中的吸收缺陷和热阻缺陷进行了分析和探测。当薄膜中存在这两种缺陷时，探测到的光热信号强度都得到了增强，然而相位的变化趋势不同。这个现象在实验和理论上都得到了验证，从而提出了一种加强的表面热透镜技术，在探测膜层中缺陷吸收大小的同时也可对缺陷的种类进行判断。With the development of high-power laser technology, the laser induced damage threshold of optical coating needs to be higher and higher. A lot of experimental results indicate that absorbing nanometer particles are responsible for the initiation of the laser-induced damage. If these absorbing defects could be detected and their introduced reasons be analyzed, the coating fabrication and storage process will be improved. Then the defects will be reduced from the origin and the laser induced damage threshold will be enhanced. As a photothermal technique, surface thermal-lensing is widely used for its high sensitivity and three dimensional spatial resolving capability. Temperature distribution in single-layer TiO2 films was analyzed based on temperature field theory. Through our calculations, The surface temperature rise depends on film thickness and photothermal parameters of films. The optical absorption variation was obtained to be similar to that of surface temperature rise in films. A method is proposed to evaluate optical absorption at random wavelength through absorption at a certain wavelength. The absorption at 1064nm and the damage threshold at different wavelengths of single-layer HfO2 films were measured and analyzed. The results showed that the laser damage resistance of single-layer HfO2 film illuminated with different wavelengths (fundamental frequency 1064nm, second harmonic 532nm, third harmonic 355nm) can be deduced qualitatively by the measured absorption at 1064nm. The laser induced damage threshold was attributed to the average value and uniformity of coating absorption. The temperature field and deformation field of five layers system with a absorptance layer were deduced and a numerical analysis was made. A method which is surface thermal-lensing by varing modulation frequency was put forward to detect the depth of defects and the thermal diffusivity of films based on the numerical analysis. It was proved in the experiment. If the thermal diffusivity of films was known, the depth of defect layer was got and the resolving capability was tens of nanometers. If the longitudinal depth of defect layer was known, the thermal diffusivity of films was got. The surface thermal-lensing was used to measure the weak absorption of optical films. Different trends of surface thermal lensing phase signals were observed in films with two types of defects, i.e., absorptive defects and heat resistant defects. Theoretical analysis was made, and it was in good agreement with the experimental results. Therefore, an enhanced surface thermal-lensing technique was proposed to deduce both the magnitude of absorption and the type of defect in optical films, by considering signal intensity with its phase status