Nanofabricated three dimensional photonic crystals operating at optical wavelengths

We describe the nanofabrication of the first three-dimensional (3-d) photonic crystals with a forbidden photonic bandgap lying in the near infrared region of the electromagnetic spectrum, 1.1 μm < λ < 1.5 μm, just beyond the electronic band-edge of GaAs. We fabricated these structures by chemically assisted ion beam etching through a triangular hole array mask, defined by electron beam lithography on GaAs. The 3-d forbidden photonic bandgap was spectrally tuned by 2-d lithographic control of the 3-d spatial periodicity. Optical transmission spectra were generally in good agreement with microwave frequency transmission on centimeter scale models. Nevertheless, we find that the mid-gap optical reflectivity is surprisingly sensitive to structural errors in the photonic crystal, degrading the optical rejection from an expected 95% to an observed 80%. We suggest that mid-gap attenuation in the most vulnerable k-space directions, rather than overall thickness, is the relevant Figure-of-Merit for nano-scale photonic crystals.