Researchers at the Delft University of Technology (Delft, the Netherlands) and the Eindhoven University of Technology (Eindhoven, the Netherlands) have used diffractionless collimated propagation in a planar photonic crystal to create a Mach-Zehnder interferometer (MZI) only 20 × 20 µm in size. For photonic crystals of a certain design, a beam of light can remain self-collimated, although only if it is propagating in certain directions; the mirrors and beamsplitter of the MZI must thus direct the beams only in these directions.
A prototype for operation at a self-collimation wavelength of 1510 nm was fabricated in a layer of silicon on a substrate of silica. The photonic crystal had a rectangular lattice of holes with a 105 nm radius and a 340 nm lattice constant; line defects serving as beamsplitters had holes with a 155 nm radius. The crystal showed a unidirectional output at 1510 nm; when the wavelength was detuned to 1620 nm, the self-collimation in both MZI arms was lost and the output dropped. The planar micro-optical device has potential as a compact modulator, as a micromechanical sensor, or in a lab-on-a-chip device. Contact Hoang Nguyen at [email protected].