Kou Gondaira

Nanocavities at the surface of three-dimensional photonic crystals

We investigate nanocavities at the surface of three-dimensional (3D) photonic crystals, where the polarization-independent surface-mode gap can be utilized. We consider the formation of various nanocavities by introducing artificial defects utilizing the 3D structures around the surface and discuss the possibilities for increasing the Q-factors of the surface nanocavities with TE-like polarization based on the advanced designs of donor-type defects. We also introduce the design of acceptor-type defects and show that TM-like nanocavities are obtained. We then fabricate the designed nanocavities and examine their resonant characteristics; we successfully demonstrate TE-like nanocavities with Q-factors of ~40,000, which is four-times higher than previous surface cavities and as high as that of the cavities embedded inside 3D photonic crystals. TM-like nanocavities with Q-factors of ~22,000 are also demonstrated for the first time.

Role of Surface Mode on Light Out-Coupling Characteristics of waveguide in Three Dimensional Photonic Crystals

The radiation pattern and out-coupling efficiency of light from the end surface of interlayer (oblique) waveguides in stacked-stripe 3-D photonic crystals are numerically investigated. It is shown that the coupling to the surface mode of the 3-D photonic crystal decreases the out-coupling efficiency to free-space modes down to 60%, whereas the reflection at the surface is negligibly small. The efficiency can be improved to 90% by introducing a 2-D grating-like structure to create a surface-mode gap, where surface modes are prohibited.

Influence of structural fluctuations on Q factor of nanocavities at the surface of three-dimensional photonic crystals

We investigate the influence of structural fluctuations on the Q factor of nanocavities at the surface of 3D photonic crystals with a stacked-stripe structure. We analyze the Q factor of nanocavities by 3D finite-difference time-domain calculations in consideration of the misalignment of the position of the stacked layers and the fluctuation of the patterns in each layer. Misalignment of the order of ∼50  nm has almost no influence on the Q factor, but the pattern fluctuations of the order of nanometers in each layer, especially in the surface layer, seriously impact the decrease of the Q factor.

Control of radiation angle by introducing symmetric end structure to oblique waveguide in three-dimensional photonic crystal.

We investigate the radiation angle of an oblique waveguide in a stripe-stacked three-dimensional photonic crystal. We show that the output-light is radiated in a different direction from the oblique waveguide direction. Moreover, the radiation polar angle varies from 30° to 50° depending on the frequency. To inhibit the frequency dependence and obtain vertical radiation, we introduced a symmetric structure at the end of the waveguide. As a result of cancellation of the in-plane asymmetric wavenumber, the radiation polar angle is less than 6° from the surface-normal direction and does not depend on frequency.

Surface nanocavities in 3D photonic crystals

We design and demonstrate nanocavities with TE-like and TM-like polarizations at the surface of 3D photonic crystals using 3D design freedom in a 3D structure and the polarization-independent surface-mode gap.