Kenta Miura

Photonic crystals for the visible range fabricated by autocloning technique and their application

We fabricate photonic crystals for the visible range by the ‘autocloning’ technique, in which multilayers are stacked by an appropriate combination of sputter deposition and sputter etching. TiO2/SiO2 and Ta2O5/SiO2 are chosen as materials since they are transparent in the range and give a high contrast of refractive indices. The fabrication technique has flexibility regarding materials and size and is very reliable and reproducible even if the pitch is less than 0.2 μm. We also study the application of photonic crystals to birefringent elements such as waveplates and polarization selective gratings and experimentally verify that they are useful for optical pick-up systems.

Self-healing effects in the fabrication process of photonic crystals

We demonstrate a self-healing effect of unwanted defects in the autocloning process: autocloning is a previously proposed technique to fabricate multidimensional periodic nanostructures by stacking up corrugated multilayers. With the self-healing effect, aperiodic perturbations in the initial periodic shape of the structure immediately disappear and the surface shape becomes periodic in a few cycles of stacking. If the perturbations exist in photonic crystal, they cause light-scattering loss and also make the boundaries between photonic bands and band gaps unclear. In other words, they make the attenuation of light at the frequency in photonic band gaps weak. Consequently, to attain uniformity of the shape automatically with this effect is very important in the fabrication process for practical photonic crystal devices. In this letter, we verify this phenomenon experimentally, and discuss the mechanism by comparison with process simulation.

Intense photoluminescence from erbium-doped tantalum oxide thin films deposited by sputtering

Erbium-doped tantalum oxide films were prepared by radio-frequency magnetron sputtering. Visible light emission was observed from the films after annealing. We obtained PL peaks at 550 and 670nm. The effects of erbium concentration, annealing temperature, and annealing time on the light-emitting properties of the films are discussed. The strongest intensities of the 550 and 670nm peaks were observed from the samples with 0.96 and 0.63mol% erbium concentrations after annealing at 900°C for 20min, respectively.

Visible-Light Emission Properties of Erbium-Doped Tantalum-Oxide Films Produced by Co-Sputtering

The visible-light emission properties of erbium (Er)-doped tantalum-oxide films prepared by co-sputtering were evaluated. Photoluminescence (PL) peaks at wavelengths of 550 and 670 nm could be observed from these films by ultraviolet-laser excitation after annealing. The dependence of Er3+ concentration, annealing temperature, and annealing time on the PL peak intensities were investigated. Such light-emitting sputtered films can be useful as high-index materials of photonic crystals that can be applied to novel active devices.

Structural Analysis of Rf Sputtered Er Doped Ta2O5 Films

Thin films of Er-doped Ta2O5 have been synthesized by RF sputtering. The influence of annealing temperature, number of Er tablets and annealing time on the structural properties of grown films, has been studied. The samples annealed bellow 800°C show amorphous nature. However, the sample annealed at 800°C and above shows crystalline nature of the film with β–Ta2O5 (orthorhombic) and δ–Ta2O5 (hexagonal) phase. The crystalline structure of the film is disturbed with the increase in Er concentration.

Observation of Blue-Light Emission from Tantalum Oxide Films Deposited by Radio-Frequency Magnetron Sputtering

We obtained blue photoluminescence from tantalum oxide films deposited by radio-frequency magnetron sputtering after annealing. The maximum peak intensity of the photoluminescence was observed from a sample annealed at 600°C for 20min, and the peak wavelength was approximately 430nm. Tantalum oxide films that emit blue light may be useful materials for novel active optical devices utilizing Ta2O5/SiO2 multilayered photonic crystals.

In-plane light propagation in Ta/sub 2/O/sub 5//SiO/sub 2/ autocloned photonic crystals

We investigate the characteristics of in-plane light propagation in 2-D photonic crystals fabricated by the autocloning method, by which multiple layers are stacked while preserving periodic surface corrugation. A numerical analysis shows that Ta/sub 2/O/sub 5//SiO/sub 2/ autocloned photonic crystals are applicable to waveguides and lenses since a sufficient amount of change of the effective refractive index is obtained by adjusting the aspect ratio of the unit structure. It is also shown that the crystals have a wide bandgap applicable to Bragg reflectors and resonators. Experimental results reveal that in-plane propagation losses for the two directions are less than 1.4 dB/mm at /spl lambda/=1.55 /spl mu/m, showing the crystals are useful for low loss in-plane devices.

Fabrication of 3D photonic crystals by autocloning and its applications

3D photonic crystals can produce a number of fascinating optical devices utilizing the band gap, the dispersive passband, and unisotropic band structure. We previously proposed a process for fabricating a 3D periodic nanostructure which can behave as photonic crystals. The key technique in the process is the deposition of a multilayer by bias sputtering replicating periodically a corrugated layer pattern, which is named the autocloning effect. This paper reviews our recent works on 3D photonic crystals: the mechanism of the autocloning effect, propagation analysis in the 3D photonic crystal by an FDTD method, and applications such as plane-normal waveguides or wavelength-selective filters.

Modified Finite-Difference Time-Domain Method for Triangular Lattice Photonic Crystals

In this paper, a modified and easy finite-difference time-domain (FDTD) method based on a regular cartesian Yees lattice is developed for calculating the dispersion diagram of triangular lattice photonic crystals (PCs). Our method uses the standard central-difference equation, which is very easy to implement in any computing environment. The Bloch periodic boundary conditions are applied on the sides of the unit cell by translating the periodic boundary conditions to match with the directions of periodicity in the triangular lattice. Complete and accurate bandgap information is obtained by using this FDTD approach. Convergence, accuracy, and stability analysis were carried out, which ensures the reliability of this method. Numerical results for 2-D TE/TM modes in triangular lattice PC are in good agreement with results from 2-D plane wave expansion method. To ease the practical application of this method, clear explanations on the computer implementation are also provided.

Ge/SiO2 photonic crystal multichannel wavelength filters for short wave infrared wavelengths

Ge/SiO2 multilayer type two-dimensional photonic crystals (PhCs) for short wave infrared (λ~2000 nm) application are demonstrated. The PhCs are fabricated by the autocloning method, which stacks multilayer on a glass substrate with initial line and space pattern by bias-sputtering. We verified their multi-channel wavelength filtering operation. Long wave pass or short wave pass filtering function is observed. Typical PhC dimensions are: Ge and SiO2 thickness is 220 nm, total number of layers is 16, horizontal lattice constant is approximately 500 nm. Transmission losses are the order of 30%, which mainly originates from the absorption of Ge layer. Extinction coefficient (κ) of the sputtered Ge was estimated to be less than 0.05 for the wavelength range between 1600 and 2400 nm.