Photonic crystal lens for coupling of two planar waveguides

We report design, fabrication, and characterization of a new nanophotonic device comprising a two-dimensional photonic crystal (PhC) lens of size 3x4 µm fabricated in a silicon film of fused silica. The PhC lens is put at the output of a planar waveguide of width 4.5 µm to couple light into a planar waveguide of width 1 µm, each waveguide being of length 5 mm. A 1 µm 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 µm. The simulation has shown that the PhC lens has the maximal transmittance at 1.55 µm, with the waveguides coupling efficiency being 73%. The transmission spectrum measured has four local minima in the range 1.50-1.60 µm. The difference between the calculated and measured transmission spectrum is 29%. The focal spot size of the lens in air calculated at the FWHM is 0.32λ (where λ is the wavelength), which is less than the diffraction limit of 0.44 λ, defined by the sinc-function.