Papers

ICMCTF2010 Session C1: Recent Advances in Optical Thin Films

Wednesday, April 28, 2010 1:30 PM in Room Royal Palm 1-3

Wednesday Afternoon

Start	Invited?	Item
1:30 PM		C1-1 Colloidal Monolayer and Patterned Films Made of Titania Sub-Micron Particles Prepared by Sol-Gel Process Sabine Portal, Carles Corbella, Roger Amade, Oriol Arteaga, MªÀngels Vallvé, Jordi Ignés-Mullol, Enric Bertran (University of Barcelona, Spain) In this work we present a new process to produce periodic patterned films and compact monolayers of titania particles. Spherical particles of titania were prepared by sol-gel method from hydrolysis of titanium ethoxide precursor in ethanol. The particle size was centered at around 500 nm with a dispersion of 10%. Langmuir-Blodgett self-assembled monolayers of the synthesized titania particles were deposited on 5 cm² –substrates of silicon and glass. Depending on the size distribution, the particle monolayers were characterized by either hexagonal or disordered compact structures. Templates on silicon wafers were realized by Electron Beam Lithography and the grooves were filled with titania particles by spin-coating. Morphological properties (particle roughness, shape) were characterized by Atomic Force Microscopy and Scanning Electron Microscopy. Transmittance spectra and ellipsometry measurements provided the optical properties of the colloidal films, which were connected to the degree of their structural order and compactness. The effects of film characteristics on wettability were evaluated by contact angle measurements. Titania colloidal films can find application in photonics due to titania high refractive index but also as smart surfaces with controllable photo-switched wettability and photocatalytic properties. Titania templates are promissing for applications in bioengineering.
1:50 PM		C1-2 Centrifuge Technology: A Breakthrough in Quantitative Adhesion Testing of Optical Coatings Uwe Beck, Georg Reiners (BAM, Germany); Dietmar Lerche, Uwe Rietz (LUM GmbH, Germany); Hansjörg Niederwald (Carl Zeiss Jena GmbH, Germany) Adhesion testing of coatings is of fundamental interest for quality assurance and still a challenge regarding reliable quantitative results. Because of the huge variety of coating/substrate systems in terms of materials and large thickness range, adhesion tests display the same variety as coating/substrate systems. Almost all tests are qualitative rather than quantitative. Except for the pull-off test (DIN EN 26922), testing is not directly correlated to the adhesive strength in terms of force per area. For optical coatings and films, the standardized tests according to ISO 9211 apply, i.e. abrasion tests (cheese cloth or eraser test) and adhesion tests (tape or cross-hatch test) with different degrees of severity. In fact, instead of the adhesive strength, all these tests provide only qualitative information on the abrasion or adhesion resistance of a particular coating/substrate system. The adhesive strength as physical quantity is not measured and tests are hence not comparable. In this respect, the patented centrifuge test is a breakthrough in adhesion testing, in particular for optical coatings. It has manifold advantages. First, it can be easily run as multiple-sample test instead of all the other single-sample tests. Second, it is much easier and faster than any other test. Third, adhesive strength is measured in absolute numbers (N/mm²). Fourth, compared to the pull-off test, it is advantageous that sample clamping is one-sided instead of two-sided. The centrifuge technology additionally enables tests under defined climates or hazardous environments. Moreover, versatile test conditions (alternating loads at various load rates) important to fatigue testing can be easily realised by the variation of the number of revolutions. The desk-top fracture analyser LUMiFrac made by LUM may analyse simultaneously up to eight different samples. For optical layer stacks provided by Carl Zeiss, conventional tests are discussed and compared in detail with the centrifuge test. It could be shown that the centrifuge test has remarkable advantages with regard to accuracy, precision and reproducibility. Hence, comparability is guaranteed. In addition, the centrifuge test was able to discriminate the adhesive strength for coating/substrate systems until failure that did not fail in any of the standardized tests. Finally, further activities on the standardization of the centrifuge test are briefly discussed. The work was partially supported by a grant (IWO 070171) of the Federal Ministry of Economics.
2:10 PM		C1-3 Mechanisms of Sub-Picosecond Pulse Laser-Induced Damage to Optical Thin Films Wolfgang Rudolph, Luke Emmert, Duy Nguyen (University of New Mexico); Carmen Menoni, E Krous, Dinesh Patel (Colorado State University) Despite the long history of laser damage studies, this topic has remained of fundamental as well as practical interest for its importance for laser micro- and nano-structuring and the developments of optical components for high power lasers, in particular for sub-ps pulse lasers. Fundamental processes leading to laser breakdown in dielectric thin films are reviewed. In the sub ps laser regime, the damage thresholds are very deterministic and can be related to fundamental film material and film deposition parameters. The effect of native material defects and transient laser-induced defects will be discussed. Scaling laws of the damage threshold with respect to pulse duration, material bandgap, and the number of excitation pulses will be derived and compared to experimental results. Finally, we will describe how the environment (ambient gas and pressure) affects the film’s resistance to laser-induced damage.
2:50 PM		C1-5 Synthesis, Structure, and Optical Properties of Au-TiO₂and Cu-TiO₂Nanocomposite Thin Films Alpana Ranade, Michael Graham, Yip-Wah Chung (Northwestern University) Sunlight incident on vehicles such as cars and planes gives rise to two problems. First, absorption of ultraviolet radiation contributes to degradation of fabrics and polymers in vehicles. Second, absorption of visible and infrared radiation results in vehicle heating that typically requires the use of an oversized air conditioning unit for passenger comfort. This in turn reduces the fuel economy. Therefore, it is desirable to develop materials for these vehicles that are transparent to the visible, absorbing to ultraviolet and reflecting to infrared. There are two general approaches to achieving efficient infrared reflection: optical interference and plasma resonance. This talk describes research studies based on the second approach. We start with a titanium dioxide matrix, which is transparent to visible light and absorbs ultraviolet at wavelengths < 400 nm. Incorporation of appropriate metal atoms into this matrix results in a film that acts as a plasma that reflects light in the infrared with wavelength greater than a critical value. The degree of reflectance enhancement is a function of the metal concentration and morphology. In this talk, we will present results of Au-TiO₂ and Cu-TiO₂ films. These films are made by reactive magnetron sputtering. Their properties are evaluated by transmission and scanning electron microscopy (metal morphology and particle size distribution), atomic force microscopy (surface roughness), x-ray diffraction (structure), and UV-VIS-IR spectroscopy (optical properties). Successful development of thin films with the above desirable attributes will be presented.
3:10 PM		C1-6 Persistent Conductivity in Post-Growth Doped ZnO Films Following Pulsed UV Laser Irradiation Gregory Exarhos (Pacific Northwest National Laboratory); Lisa Wang (Carleton College) Solution and rf sputter deposited doped ZnO films were subjected to multiple 4-nsec pulses of 355 nm or 266 nm light from a Nd:YAG laser at fluences between 5 and 150 mJ/cm². Film densification, change in refractive index, and an increase in conductivity were observed following room temperature irradiation in air, a carbon monoxide reducing environment, or under vacuum. At these fluences, films did not damage catastrophically and high visible transmissivity persisted. The increase in conductivity is attributed to formation of oxygen vacancies and subsequent promotion of free carriers into the conduction band. The effects were most pronounced in films irradiated under vacuum. All treated films became insulating again upon equilibration in air at room temperature after several days. Films were characterized by means of ellipsometry, UV-VIS-NIR transmission spectroscopy, visible and UV Raman spectroscopy and Hall measurements. Analysis of interference fringes in measured transmission spectra allowed evaluation of optical properties and confirmed results obtained from ellipsometry measurements. Raman measurements showed an increase in LO mode intensity with respect to TO mode intensity as the films became more conducting in accord with previous work on conducting ZnO films. Results of this study are not only important for the continued development of transparent conducting oxide films that find use in photovoltaic cells and solid state lighting modules, but also provide compelling evidence for the role of free carriers as initiators of the laser damage process in these wide bandgap metal oxide films.