Sub-wavelength resolution in a photonic crystal superlens
Materials that can bend light in the opposite direction to normal (’left-handed’ materials) reverse the way in which refraction usually works-this negative refractive index is due to simultaneously negative permeability and permittivity. Here we demonstrate negative refraction of electromagnetic waves in a two-dimensional dielectric photonic crystal that has a periodically modulated positive permeability and a permeability of unity. This experimental verification of negative refraction is a step towards the realization of a ’superlens’ that will be able to focus features smaller than the wavelength of light. Our structure consists of a square array of alumina rods in air. To obtain the frequency range within which the negative refraction and the other peculiar properties incorporated with it, we have calculated the the equal frequency contours of our photonic crystal with the photonic plane wave expansion method. We found out that 13.7 GHz is the optimum frequency for negative refraction. We took transmission measurements to confirm our structure’s predicted negative refraction: we used the the interfaces of the photonic crystal in the Γ − M direction. Our experiments were simulated with the finite-difference time-domain method (FDTD). The negative index of refraction was determined to be −1.94, which is close to the theoretical value of −2.06 calculated by the FDTD method. Since we know that we have a negative refractive material , we used our crystal to test the superlensing effect that was predicted for negative refractive materials. We have demonstrated that the image of two coherent point sources separated by a distance of λ/3 can be resolved. We have extended our approach to the case with two incoherent point sources, and we have achieved subwavelength resolution for this configuration as well. To our knowledge, this is the first demonstration of subwavelength resolution of electromagnetic waves in a negative index material.