Optik 124 (2013) 35–39 Contents lists available at SciVerse ScienceDirect Optik jou rnal homepage: www.elsevier.de/ijleo The effect of time-temperature gradient annealing on microstructure, optical properties and laser-induced damage threshold of TiO 2 thin films Amir Hassanpour a , Alireza Bananej b,∗ a Department of Physics, Khaje Nasireddine Toosi University of Technology, Tehran, Iran b Laser and Optics Research School, NSTRI, Tehran, Iran a r t i c l e i n f o Article history: Received 6 June 2011 Accepted 3 November 2011 Keywords: Thin film Laser-induced damage High-power lasers Annealing Porous ratio a b s t r a c t The effect of time-temperature gradient annealing on the TiO 2 single layers depositing in the same con- ditions by electron gun were discussed. Investigating the optical properties of the layers which were calculated by envelope method revealed that not only the extinction coefficient but also porous ratio of the layer may affect the laser resistance of the thin films. The experimental results are in a good agree- ment with theoretical analysis which is based on the well known Yoshida’s model. It can be seen the positive impact of higher porous ratio on the laser induced damage threshold values of the TiO 2 thin films. In addition, according to the XRD patterns of samples, it can be seen that our novel annealing procedure have a significant influence on the grain size of the TiO 2 material. Consequently, comparing the laser induced damage threshold showed that different annealing procedures may modify laser resistance of TiO 2 thin films by influencing their optical or structural properties. © 2011 Elsevier GmbH. All rights reserved. 1. Introduction As a consequence of advances in inertial confinement fusion (ICF) technology through high power lasers, there has been tremen- dous effort on improving output power of high power laser systems. While many successes achieved during these attempts, the speed of improvement were reduced because of some serious limitations. To overpass these limitations various series of efforts is still have been run by optic researchers around the world. Although production of optical thin films at first enhanced the desired optical properties of the components, their low laser induced damage threshold (LIDT) during intense laser radiation has made a big challenge to produce such films with some other spe- cial features. In fact, there is a coincidence that the power handling capability of optical materials, particularly optical coatings can be considered as the most important bottlenecks for scaling up the output power of high power laser systems. Very soon experimental results showed that LIDT parameter is affected not only by irradiated laser properties such as pulse duration [1], repetition rate [2] and spot size [3], but also by coat- ing characteristics which may vary during the deposition process. The deposition technique and parameters during nucleation and growth of the layer like deposition rate [4], temperature of the ∗ Corresponding author. E-mail addresses: arbananej@yahoo.com, abananeg@aeoi.org.ir (A. Bananej). substrate [5] and oxygen partial pressure [6] are highly important for the quality of the films and hence, magnitude of LIDT parameter. On the whole, due to large number of impressive parameters and complexity, study of LIDT is difficult and still is unsolved [7]. Post annealing of prepared samples has been considered as a practical method for improving the microstructure of the film materials for long years [8]. Therefore, we will investigate the effect of the time-temperature gradient annealing (TTGA) procedure on the microstructure of film materials and hence, LIDT as the desired parameter for high power optical coatings technology. Such a kind of annealing method in this approach is very useful to obtain favor- able optical properties and high LIDT especially in situation which has temperature rise limitation. In addition, we focus our study on the damage phenomena in long pulse regime where the ther- mal process and its related phenomena can be considered as the dominant factor [9]. According to the previous proposed models, LIDT in this regime is related to the structure of the film which can be analyzed based on porosity [10]. We also chose TiO 2 as the test material as one of the most suitable and practical materials for high power optical coatings technology due to its high refractive index, low absorption and high laser damage resistance in specific range of wavelengths. 2. Experimental All TiO 2 thin films were deposited by electron beam evapora- tion on BK7 substrates in the same terms. The films were deposited 0030-4026/$ – see front matter © 2011 Elsevier GmbH. All rights reserved. doi:10.1016/j.ijleo.2011.11.020