Fabrication by means of X-ray lithography of two-dimensional GaAs/AlGaAs photonic crystals with an unconventional unit cell

Two-dimensional photonic crystals have been fabricated by x-ray lithography and reactive ion etching on an air/GaAs/AlGaAs asymmetric waveguide. The shape of the lattice unit cell has been varied by exploiting x-ray diffraction effects and nonlinear response of resists during the development process. Rings with or without a central pillar have been fabricated with a resolution down to 50 nm. Lithographic details are described to show the accuracy of this fabrication technique. Optical characterization has been performed showing the presence of a well defined photonic band structure as well as band anti-crossing. The results are discussed and compared with theoretical calculations of the photonic band dispersion. Structures with high dielectric fraction are shown to yield photonic modes with small line widths and low propagation losses. The reliability and the precise control of the fabricated sample structures make this lithographic method useful for a full investigation of optical properties on the dependence of the lattice unit cell shape.