Normally, if photons can travel in a certain direction through a medium, they can also travel in the reverse direction—a phenomenon called time-reversal symmetry. But a microwave magneto-optical photonic-crystal waveguide fabricated by researchers at Massachusetts Institute of Technology (Cambridge, MA) and Yale University (New Haven, CT) breaks this symmetry. The effect is an electromagnetic analog of the so-called quantum Hall effect seen in condensed-matter physics. The photonic crystal contains an array of ferrite rods in air, with a 40 mm lattice constant; the array is bounded on one side by a nonmagnetic metallic cladding and subjected to a large external magnetic field.
The result is an electromagnetic “chiral edge state” that allows photons to travel only in certain directions. Because of this, photons that are propagating cannot be backscattered, even from normally scattering obstructions. To confirm this, the researchers placed scatterers with sizes up to 1.65 times the lattice constant in the array, but the objects caused virtually no reflections or backscattering. If the microwave photonic crystal could be reduced in size so that it worked at optical wavelengths, the result would be a class of previously impossible optical devices. Contact Zheng Wang at [email protected].