Low propagation losses in single-line-defect photonic crystal waveguides on GaAs membranes

We have fabricated and characterized straight single-line-defect two-dimensional photonic-crystal waveguides on GaAs films with lengths of 1, 4, and 10 mm. By optimizing key processes for smooth top and bottom waveguide-surface, as well as fine patterning process of the air holes, extremely low propagation loss was achieved. The optimization includes wet etching process of a sacrificial AlGaAs clad layer and oxygen plasma process for complete resist removal. AFM measurement resulted that the surface roughness is less than 1 nm at the top surface of the line-defect waveguide. From the transmittance spectra for different-length samples, the propagation loss is estimated as small as 0.76±0.5 dB/mm. Besides the loss for the straight waveguide, the loss per bend for the 60° bend waveguide was estimated as 0.3 dB/bent with the bandwidth of broader than 40 nm. The present results are promising for key passive elements such as photonic-crystal symmetric Mach-Zehnder switches needed in future optical communication applications.