Photonic crystal lens for coupling two waveguides

We report the design, fabrication, and characterization of a new nanophotonic device comprising a two-dimensional photonic crystal (PhC) lens of size 3x4 microm fabricated in silicon-on-insulator. The PhC lens is put at the output of a planar waveguide of width 4.5 microm to couple light into a planar waveguide of width 1 microm, with two waveguides being of length 5 mm. A 1 microm off-axis displacement of the smaller waveguide leads to an 8-fold reduction of output light intensity, which means that the focal spot size at output of the PhC lens in silicon is less than 1 microm. The simulation has shown that the PhC lens has maximal transmittance at 1.55 microm, with the coupling efficiency being 73%. The focal spot size of the lens in air calculated at the FWHM is 0.32lambda (where lambda is the wavelength).