On-chip electro-optic modulator based on left-handed photonic crystals

Official URL: https://dx.doi.org/10.1117/12.3000184

To achieve more compact integrated photonic devices, reducing size of elements is crucial. A factor that limits size-reduction is electro-optic components that require large optical interaction length. In this work, we designed and fabricated an optical modulator where a photonic crystal structure is used to create large phase difference in short distance. Our design is a 2x2 Mach-Zehnder interferometer on the platform of silicon-on-insulator. A left-handed photonic crystal structure that is designed to operate at 1.55 um is placed on one arm of the interferometer to add phase to light. The phase difference between two arms yields amplitude modulation at the output of the interferometer. The photonic crystal is hexagonal air hole lattice and used to switch between negative and positive effective refractive indices. This change is triggered by applying voltage which decreases the refractive index of silicon from 3.480 to 3.477 due to plasma dispersion effect, and causes photonic band-to-band transition. By this way, effective refractive index of the structure jumps from negative to positive values. To be able to realize this, photonic crystal region is sandwiched between n-doped and p-doped materials, which creates a p-i-n diode. By taking the advantage of band-to-band transition at left-handed photonic crystal, we experimentally demonstrated that interaction length is reduced from 255 um to 4.4 um. This reduction leads to low optical insertion loss as well as more compact devices.