ICMCTF2013 Session C1-1: Recent Advances in Optical Thin Films

Plasmonics is the technique to control and utilize surface plasmon (SP) generated around nano-structured metals. In 2004, for the first time, we have reported that the plasmonics is very useful to increase the emission efficiencies of light emitting materials. Huge enhancement of the photoluminescence from InGaN/GaN-based QWs was obtained when nano-structured Ag layers were deposited 10 nm above the QWs[1]. The coupling between the exciton and the SP becomes remarkable when the emission energy is close to the SP frequency[2]. The SP-enhanced internal quantum efficiencies (IQEs) can reach almost 100% at the blue emission region[3]. One of the most important advantages of this technique is the ability to apply to not only InGaN-based materials but also various materials. We observed similar huge enhancement effcts for several organic films, CdSe-based nanoparticles and also silicon-based nanostructures, etc. The SP-exciton coupling would lead to high efficiency optoelectronic devices such as "plasmonic LEDs" and "Plasmonic solar cells".

Until now, several types of the plasmonic LEDs and solar cells have been proposed and reported, however, these are still far from practical utilizations. Further optimization of the metal nanostructure and tuning of the SP coupling process are required to develop both devices. Therefore we are designing more effective plasmonic nanostructures by using the 3-dimensional finite difference time domain (3D-FDTD) calculations and the nanofabrication processes with several bottom-up techniques. For example, we can control the resonance spectra of localized SP (LSP) mode by using the Ag nanoparticles with various diameters. By optimization of the Ag particle size, we achieved high efficient green emission from InGaN/GaN QWs, which has been very difficult to improve the emission efficiency. Moreover we succeeded to control the LSP resonance spectra with much wider wavelength region by employing 2 dimensional (2D) nanosheet structure of Ag nanoparticles with 5nm diameter[4]. These plasmonic nanostructures would bring the new type of high-efficiency LEDs and solar cells. The detail of the recent progress in plasmonics applied to several optoelectronic devices will be discussed at the conference.

1) K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, A. Scherer: Nat. Mater. 3, 601 (2004).

2) K. Okamoto, A. Scherer, and Y. Kawakami: Phys. Stat. Sol. C, 5, 2822 (2008) .

3) K. Okamoto, Y. Kawakami: IEEE J. Select. Top. Quantum Electron.15, 1190 (2009).

4) K. Okamoto, B. Lin, K. Imazu, A. Yoshida, K. Toma, M. Toma, and K. Tamada, Plasmonics (2012), in press.

The aim of this work is to develop hydrophobic coatings for outdoors insulators using sputtering technique. Hafnium oxide is characterized by high dielectric constant, large band gap (5.6eV), high refractive index (2.1) , and good mechanical ,thermal and chemical properties. Hence HfO2 is suitable as a protective coating for outdoor insulators used in the transmission line and transformers. Hafnium oxide coatings were deposited on glass substrates by DC magnetron sputtering technique at sputtering pressure of 10mtorr, 15mtorr and 20mtorr.The film was characterized by techniques like X ray diffraction(XRD),atomic force microscopy(AFM),water contact angle meter and UV-NIR spectrophotometer. The average crystallite size calculated from XRD peaks shows that it increases with increase in sputtering pressure. The roughness calculated from AFM images shows the similar trend. The hydrophobicity was investigated using water contact angle meter and found correlation with the roughness calculated from AFM. The effect of sputtering pressure was also investigated on optical band gap and refractive index calculated from transmission and absorption data.

In the last decade, the different compositions of Bismuth Titanate Oxide (BixTiyOz) has been researched it due to their physical properties such as: electric, ferroelectric and optoelectronic behavior that allow it be used as ceramic capacitors, transducers, sensors, memory devices, optoelectronic devices, piezoelectric technology, acousto-electronics and acousto-optics applications. For these reasons in this work, we present the results obtained in the growth of the Bi_xTi_yO_z thin films through rf magnetron sputtering The films were grown on common glass substrate and has been evaluated the microstructure and optical behavior as a function of the substrate temperature and power applied at target. The microstructure analysis was carried out by x-ray diffraction (XRD) and optical response was evaluated by means transmittance measurements. The XRD results have shows that the films growth from room temperature to 573 K and power from 100 W to 200W are amorphous. Moreover, the films growth at 623 K and power from 150 to 200 W shows preferential orientation along (622) plane of Bi₂Ti₂O₇ cubic face centered phase.

Using the transmittance values and through Swanepoel method we calculated the refractive index, thickness and absorption coefficient of the amorphous and crystalline films. The mean values in amorphous films found are: n=2.43 (λ=463nm); α= 1.3x10⁴ cm^-1; 285nm and in crystalline films n=3.31 (λ=424nm); 255.68nm and α= 2.2x10⁴ cm^-1. The energy gap was determined used the Urbach's formula (2.8 eV). The values of refractive index and energy gap are very near of the Bi₂Ti₂O_7.