ICMCTF2014 Session C1: Recent Advances in Optical Thin Films and Nanomaterials

Optical interference coatings find wide application in science and industry. It is hard to think of an optical instrument or system that does not benefit from their use, and many systems would not be possible without them.

The challenges of the optics and security industries are constantly driving better performance and the development of improved and new optical coating technologies. A few of the application areas and how they drive the technology include: Space. Applications for optical coatings in space include antireflective protective covers for solar cells, solar thermal control coatings, and filters for instrumentation. An enabling type of filter used in space-based sensors is the linear variable filter, or LVF. The spectral performance of this type of filter varies across one dimension, making it ideal for use with arrayed detectors such as CCDs. LVFs are also used in many ground-based devices such as miniature spectrometers. Document Security. The availability of color photocopying machines in the 1970s initiated a need for new anti-counterfeiting technologies for banknotes and secure documents. Beginning in the 1980s security devices based on optical interference have been employed since they shift in color as a document is tilted, an effect that cannot be readily duplicated by a photocopier. Challenges today include the creation of effects that are more eye-catching and even more difficult for counterfeiters to replicate. Telecommunications. The 1990s saw a major build-out of the fiber-optic telecommunications infrastructure. To bring the cost of the deployment down, signal streams were multiplexed by sending multiple laser wavelengths through each fiber. Optical filters were adopted for combining and separating the wavelengths, for correcting the wavelength-dependent amplification of optical amplifiers, and other uses. These requirements drove major improvements in optical coating technology during the mid-1990s through the early 2000s, and optical filters are still widely used in telecommunications today. Consumer Electronics. The processing of multiple devices at the wafer level in the field of electronics has a long history dating back to the 1960s. Until recently, however, sensors that employ filters and electronic detectors were constructed using costly unit-by-unit assembly methods. In the past few years processes have been developed which enable optical filters to be deposited directly onto semiconductor wafers, enabling the integration of detectors, filters, and circuitry at the wafer level. This advance has been driven by the need for inexpensive sensors for devices such as smart phones and tablet computers.

Surfaces of optical devices such as touch screens, correction glasses, low emissivity or smart windows are exposed to everyday mechanical and tribological interactions. This clearly calls for the development of multi-functional films possessing controlled optical characteristics coupled with superior tribo-mechanical properties, and it also stimulates the development of new tools and methods for their assessment.

In this work we present three in situ real-time techniques that provide instantaneous information about the mechanical response of optical coatings exposed to different types of solicitations: a) Analysis of the dynamics of solid particle erosion of coatings deposited on a Quartz Crystal Microbalance; b) Evaluation of the mechanical stress to assess the contribution of each individual layer on the total stress, as well as the effect of environmental conditions; and c) In situ scratch and wear testing to study defect initiation, progression and propagation.

Using the above techniques, we show specific examples of the mechanical properties of optical films on glass and plastic substrates. We discuss how the combination of these techniques with other complementary characterization methods can provide a better understanding of the mechanisms governing the tribo-mechanical performance of optical coating systems.

Vanadium dioxide (VO₂) is a well known thermochromic material of interest for applications in smart windows. However, the high deposition temperature, typically over 400°C, required to obtain the appropriate VO₂ crystalline phase represents one of the major challenges for its implementation on a large scale. In our previous work, we showed that high quality VO₂ films could be obtained by the HiPIMS deposition technique at a lower substrate temperature (300 ⁰C) compared to DC or RF magnetron sputtering. In the present study, the HiPIMS process is further optimized through detailed pulse parameters management including pulse frequency and the instantaneous power. We correlate these parameters with the microstructure, crystallinity and composition of the VO₂ films, as well as with their thermochromic properties, namely the transition temperature, and the IR switching efficiency.

We have been investigating the characteristics of heavily Ga-doped ZnO (GZO) films with carrier concentrations of 1.1 to 1.2×10²¹ cm^-3, which can be promising low-loss alternatives to metals in near-infrared (NIR) wavelength ranges for use in plasmonics. Plasmonics offers the potential to confine and guide light below the diffraction limit and promises a new generation of highly miniaturized photonic devices. Current plasmonic devices based on metals or metal alloys at telecommunication and optical frequencies face significant challenges due to losses encountered in the materials. We have deposited GZO films with various thicknesses in the range from 100 to 350 nm by ion-plating with dc arc discharge. Temperature of glass substrates was 200ºC. Hall effect measurement results of 105-nm-thick and 344-nm-thick GZO films show electrical resistivity of 2.5×10^-4 Ωcm, carrier concentration of 1.08×10²¹ cm^-3 and Hall mobility of 23 cm²/Vs and electrical resistivity of 1.8×10^-4 Ωcm, carrier concentration of 1.23×10²¹ cm^-3 and Hall mobility of 29 cm²/Vs, respectivley. Hall mobility monotonically increases with increasing thicknesses up to 344 nm, whereas carrier concentration changes a little. Analysis of spectroscopic ellipsometry measurement shows that 105-nm-thick and 344-nm-thick GZO films exhibit negative real permittivity that is an essential property of any plasmonic material at wavelenghth of more than 1256 nm and 1198 nm, respectively. Note that, for all the GZO films, we find that values of the imaginary part of the dielectric function, which describes the losses encountered in polarizing the films, were of less than 0.6 in the range of wavelength smaller than 1500 nm in the NIR wavelength region. Taking into account that just the losses are relevant in defining the quality factor of transformation optics (TO) devices and superlens, the GZO films can be a promising low-loss material for the plasmonics in the NIR wavelength region, and can work well at the telecommunication wavelength.

The outdoor glass insulators installed in the electrical power substation undergoes frequent surface flashover phenomenon due to atmospheric contamination. Synthesis of hydrophobic dielectric coating over the insulator surface is one such technique to overcome this problem. In this paper, we report the deposition of DC sputtered HfO_xN_y films over glass insulators. The coatings were developed in two different sputtering gas atmosphere namely Ar and He by varying oxygen partial pressure from 2.5% to 20% keeping nitrogen flow constant. The structural, hydrophobic, optical and electrical properties of the deposited films were characterized using XRD, AFM, EDS, contact angle goniometer, UV-vis-NIR spectrophotometer, impedance analyzer and four probe method. The average crystallite size and the deposition rate were higher for the films deposited in N₂+Ar atmosphere in comparison to the film deposited in N₂+He environment. The nano roughness of the samples estimated from the AFM micro graphs was correlated with the crystallite size and nitrogen content of the films. All the deposited samples were hydrophobic with highest contact angle of 94.3° for the film deposited at 2.5% oxygen partial pressure in Ar environment. The surface energy of the HfO_xN_y films was estimated from water contact angle using Wu and Owens methods. The deposited samples shows the optical transmission > 85%. The refractive index and the bandgap gap calculated from the optical records were significantly affected by the atomic mass of the sputtering gas. The electrical resistivity of the films was found out to be packing density dependent and shows a decreasing trend with the rise of O₂ partial pressure. The dielectric constant of the films was high in comparison to HfO₂ film and was thickness dependent.