Photonic crystals have a photonic band gap (PBG) in which light propagation and emission is prohibited. In particular, three-dimensional (3D) photonic crystals have a complete PBG in all directions, which might allow the complete control of light emission and propagation in devices. Here, we report the first demonstration of light propagation in a 3D photonic-crystal waveguide at optical communication wavelengths. A line defect is introduced into a 3D photonic crystal composed of nine stacked layers, having a complete PBG in the 1.55μm wavelength region. Light incident on the waveguide edge successfully propagates along the line-defect waveguide. The propagation characteristics agree with the calculated photonic band diagram of the structure. The calculated results indicate that lossless propagation becomes possible by increasing the number of layers in the device. These results are an important step toward the realization of multifunctional 3D photonic chips integrated within a small region.

We successfully developed a high-precision wafer alignment and bonding system for the fabrication of a variety of 3-D nanostructures. To control the wafer positions with high accuracy during the wafer-bonding process, we improved upon a design of the conventional mask-alignment stage. A stress sensor was incorporated to measure the load between the two wafers. In addition, the parallelism of the wafers was monitored by an optical interferometry system. To determine alignment errors in both the x and y directions simultaneously, we devised an alignment method consisting of crossed vernier scales. We demonstrated that the new alignment and bonding system allowed us to realize precise 3-D photonic crystals with the alignment inaccuracy of < 100 nm at most, and we show that the best experimental error achieved to date was < 25 nm. As this system has the benefit of more readily and intuitively determining the absolute positions of the two wafers, it can be applied to the fabrication of a wide variety of nanoscale multilayer devices.

We demonstrate GaN-based distributed-feedback surface-emitting lasers grown on sapphire substrates with a two-dimensional (2D) square-lattice photonic crystal (PhC) that forms a high-aspect-ratio void array (void diameter: 65 nm and depth: 220 nm). The 2D PhC layer acts as both distributed-feedback grating and p-type optical cladding of a separate confinement heterostructure. To form the 2D PhC in the nitride semiconductor layers, we developed an embedding process that uses mass-transport phenomena. Crystallographic facets appeared on the inner walls of the embedded voids. Room-temperature lasing action was observed at 406.0 nm for a PhC lattice constant of 162.5 nm. The threshold current density was 9.7 kA/cm2 for a 120-μm-square p-contact electrode.

We demonstrate the operation of two types of waveguides formed in three-dimensional (3D) photonic crystals (PCs). We first created a vertical waveguide by stacking acceptor-type defects, in which near-infrared light propagates in the stacking direction. Light is transmitted independent of polarization in this waveguide because electromagnetic waves couple to a degenerate mode derived from the structural symmetry of the defects. We then connected horizontal and vertical waveguides to form an L-shaped waveguide, which is able to guide near-infrared light from the horizontal to vertical direction in the 3D PC. We envisage the realization of more complex 3D optical interconnections by optimizing the waveguide structures and increasing the PC period in the vertical direction.

We designed three-dimensional (3D) photonic crystal (PC) waveguides by simultaneously introducing one acceptor-type and two donor-type line defects. The waveguides have an extremely large singlemode bandwidth, which covers more than 90% of the complete photonic band gap. The relatively large group velocity and the mode-field localization in the air core should prevent unintended nonlinear phenomena for ultra-short pulse propagation. These promising characteristics could only be achieved by using 3D PCs, which have the advantages of complete light confinement and no restrictions of the light cone.