24 Nov 2005
Introducing a specific defect structure into a photonic crystal waveguide bends light at acute angles.
By introducing a precise defect structure into a photonic crystal waveguide, scientists at NASA's Ames Research Centre, US, claim to have designed the first sub-wavelength mirror that can bend light at an angle sharper than 90°. Having simulated bending at 45°, Natalia Malkova and Cun-Zheng Ning believe their design can also support bending through a range of acute angles. (Applied Physics Letters 87 161113)
Photonic crystal waveguides come in two forms: continuous line defect waveguides and coupled-defect cavity waveguides. In the first variety, a missing line of rods in the photonic crystal defines the path of the light. In the second, defects are introduced at regular intervals. When the defects are in close proximity, they form a coupled-cavity which the light follows.
Malkova and Ning had been working with coupled-cavity waveguides trying to engineer mode-splitting when they came up with their idea.
Starting with a square lattice of dielectric rods, their design involves distorting the lattice at the corner of reflection to create a "control cell". Introducing a specific symmetry into the control cell redirects the light at an acute angle. The structure of the control cell is also repeated along the reflected path. (See diagram)
"First you have a defect and in our case this is a larger diameter rod than the ones surrounding it," Ning told Optics.org. "The trick is to distort the lattice by moving the smaller rods to set positions around the larger one."
Crucially, Ning adds the light can be reflected at a range of angles simply by changing the positions of the smaller rods. He expects this idea to work for angles as acute as 30° and wavelengths from the UV right through to the infrared.
After altering the lattice to give a permanent bending angle of 45°, Ning and Malkova began investigating ways of dynamically changing the bending direction. "We use electro-optical materials," said Ning. "By applying a field you change the rods' index of refraction and dynamically switch the light propagation direction."
The research is currently at the simulation stage and Ning is hoping to establish a collaboration to fabricate the photonic crystal waveguide.