ICMCTF2012 Session C1-1: Recent Advances in Optical Thin Films

Photonic crystals are nanostructures for light with periodic refractive index change. They look like periodic air-hole arrays in regular patterns. By manipulating the patterns and developing two- or even three-dimensional structures, various and flexible manipulations of photons become possible. Our research has demonstrated that photonic crystals indeed allow to manipulate photons almost on demand and could contribute to broad applications including communication, information, storage, processing, and even global energy issues.

For example, we have successfully demonstrated that photonic crystals can produce photonic nano-devices with the sizes less than 1/100,000 of conventional on-road devices while achieving excellent optical functions. These devices are very useful to increase the amount of information in optical communications. We have also shown that photonic crystals enable a nanocavity (a cage of light), which can confine light very strongly. The nanocavity can be used for slowing and even stopping light. In the present optical signal processing, light signals are at first converted to electronic signals to store the signals, and then re-converted to light signals. If we could directly store light as it is, the speed of the signal processing could be significantly increased. The nanocavity is also important for quantum information processing and communication, which are considered as the important candidates for the next generation communication and signal processing.

Moreover, we have demonstrated that photonic crystals can produce an unprecedented type of lasers, which cannot be achieved by the conventional technologies. We found that the photonic-crystal lasers can oscillate in a perfect single mode in a broad area and produce on-demand beam patterns with desired characteristics. These results will lead to the realization of various types of novel light sources; for example, a light source with extremely high output powers, a super-resolution light source which can be focused much smaller than the wavelengths, and a light source which can trap and manipulate nontransparent materials such as small pieces of metals. These light sources achieved by the photonic-crystal lasers should be very important for laser processing systems, next generation DVD systems, and versatile optical tweezers systems, etc.

Our works on photonic crystals will also contribute to address global energy issues. Photonic crystals can manage light emission and detection, which has great potentials to produce extremely high-efficient LEDs and solar cells. These are very important to save huge energies for lightings, and also to convert efficiently solar energy to electric one.

References:

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Pure titania (TiO₂) and carbon containing titania(C-TiO₂) thin films were prepared by reactive sputtering titanium metal target and graphite target in argon–oxygen plasma at 100 °C or below. The as-deposited thin films were amorphous and subsequently annealed at various temperatures of 300~800 °C in air and H₂ atmosphere. The effects of annealing on the thin films were systematically studied in terms of phase transformation, activation energy, crystallinity, oxygen vacancies and their optical and photocatalytic properties. The as-deposited TiO₂ transform to anatase at much lower temperature than C-TiO₂. At the same temperature, the H₂-annealed films achieve better crystallinity than the air-annealed. The activation energies of phase transformation for amorphous to anatase were obtained and the values are 126 and 47 KJ/mole for air- and H₂-annealed pure TiO₂, respectively. The result shows that C-TiO₂ need more energy for phase transformation than pure-TiO₂, and the H₂ atmosphere was able to lower the activation energy. Photocatalytic properties of these films were characterized by degradation of methylene blue under irradiation of visible light, and the C-TiO₂ film annealed at 800°C in H₂ possesses the best performance.

In this paper, we present our recent works on ZnO light-emitting devices including homojunction nanorod light-emitting diodes, optically pumped whispering-gallery-mode (WGM) lasing and electrically driven WGM heterojunction laser diodes.

Firstly, by applying plasma immersion ion implantation to modify the surface of ZnO nanorods grown from pure Zn and oxygen without catalyst, we found that the electronic defects only reside on the surface. This makes them easy to be doped to p-type. Then by ion implantation of As and P, we realized efficient pure UV emitting LEDs. For optically pumped WGM lasing, we clearly observed the evolution from spontaneous to stimulated emission from ZnO disks. The lasing process has pronounced excitonic signature, that is, inelastic exciton-exciton scattering. The observed lasing modes match well with the theoretical values derived from WGM lasing. The proof-of-concept sensing application based on ZnO WGM lasing is also demonstrated. Lastly, we fabricated the ZnO microrod/GaN heterojunction laser diode, the EL emission mechanism for this diode was discussed. At the current 12 mA, the WGM lasing with distinct multiple-mode structure was realized. This study indicates that the hexagonal ZnO microstructure is of potential in microlaser diodes.

Metallic oxides coatings, for example, TiO₂, are of increasing technological importance in a wide range of industrially relevant functional surfaces on components for the renewable energy sector, including the hydrogen economy. Reactive closed field unbalanced magnetron sputter ion plating (CFUBMSIP) provides an effective, industrially-compatible processing route for the deposition of metallic oxides, with controlled composition, structure, and mechanical, electronic and optical properties.

The current development of TiO₂-based CFUBMSIP coatings for applications is described, including the electrolytic production of hydrogen (water-splitting - from both direct UV and visible light photo-activity, and via conventional electrolysis), photo-voltaics (PV) with long-life and low cost, and photo-catalytic surfaces with anti-microbial properties.

The optical band gaps of the anatase and rutile TiO₂ phases result in excellent optical transmittance in the visible and near infra-red regions. Stoichiometric TiO₂ is also a good dielectric has a high refractive index. Ultraviolet (UV) light has an energy higher than the band gap (E_g) of TiO₂, resulting in electron-hole pair generation. If such excited electrons or holes can diffuse to the TiO₂ surface they can induce the formation of different kinds of radicals or ions (and, for example promote the splitting of water molecules, generating hydrogen and oxygen, or degrading or destroying biological matter, creating an antimicrobial surface), or the separated charge carriers can sustain an electrical current (as in a PV device). The intensity of visible light in the solar spectrum is around 9x that of the UV light responsible for the photo-activity of TiO₂. An efficient process of doping, e.g. the introduction of nitrogen, carbon or iron etc. into TiO₂ can narrow the band gap and thus shift the coatings optical response from UV to visible light.

Well crystallised anatase or rutile-phase TiO₂ structures will be critical for the PV cell application. Deliberately increasing the surface area of the thin film coatings can enhance efficiency in the PV application by increasing the absorption of optical radiation, and in electrolytic applications by increasing the density of active sites within a given area.

The production of well crystallised TiO₂ thin films by the reactive CFUBMSIP process is described. Process parameters, including Ar/O₂ ratio, heating, and elemental doping have been investigated systematically. The effects of various dopants, including N, Fe and Mn on the band gap of TiO₂ and the functional properties of the thin films are discussed, as is the potential for further optimization industrial scale-up.