Tunable photonic crystal lens with high sensitivity of refractive index.

We design a photonic crystal (PhC) lens whose focal length is highly tunable based on the frequency sensitive super-collimation (FSSC) phenomenon. Theoretically, an analytic expression of the focal length in PhCs is derived. The diffraction could be dramatically changed by modest change in refractive index of the dielectric rods in PhCs, because the sensitivity of the equi-frequency-contours around FSSC to refractive index is several orders larger than that in common bulk material. Numerically, we demonstrate that focal length can be nearly one order larger with only 0.2% refractive index change, from 28a (a is lattice constant) to 240a. With its micro-size, high sensitivity and feasibility by on-chip technology, such tunable lens has great potentials in modern optical systems.