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We report the design, fabrication and characterization of a photonic crystal directional coupler with a size of ∼20×20 μm2fabricated in silicon-on-insulator material. Using a scanning near-field optical microscope we demonstrate a high coupling efficiency for TM polarized light at telecom wavelengths. By comparing the near-field optical images recorded in and after the directional coupler area, the features of light distribution are analyzed. Finally, the scanning near-field optical microscope observations are found to be in agreement with the transmission measurements conducted with the same sample.

Nano-photonic technologies of GaAs-based two-dimensional photonic crystal (2DPC) slab waveguides (WGs) and InAs-based quantum dots (QDs) are reviewed for a symmetrical Mach?Zehnder (SMZ) type, ultra-small and ultra-fast all-optical switch (PC-SMZ) and logic device. As the first phase, ultra-fast (~ps) and ultra-low energy (~100?fJ) switching has been demonstrated using a chip 600??m?300??m in size. The second phase is to create a PC-SMZ-based ultra-fast photonic logic switch with a latch function for a future ultra-fast photonic digital processor. One of the priority subjects is to establish a new design method, i.e., topology optimization (TO) method of 2DPC-WGs with wide/flat bandwidth, high transmittance and low reflectivity. Another one is to develop selective-area-grown, high-density and highly uniform InAs QDs with large optical nonlinearity (ONL) by using a metal-mask (MM) molecular beam epitaxy (MBE) growth method. Recent results regarding these two subjects encourage us to reach the final goal.