Negative refraction and imaging using plasmonic slabs and layers

Double-negative medium, with permittivity and permeability simultaneously negative, exhibits a negative value of the refractive index and the resulting negative refraction has been demonstrated. One interesting property of a double-negative medium is that a thin flat slab can serve as a superlens, but realizing double-negative media in optical frequencies is rather difficult that there is a strong urge to find substitutes that are easier to realize and more robust to operate. Double negativity is actually not a necessary condition for negative refraction. We derive the condition for which a thin slab with given permittivity and permeability can focus a point source to a far-field image, and we find that the "good lens" condition manifests as a quantization rule for the transmission phase. The perfect lens with ε=μ= -1 is a special case of the general rule. The results also imply that a metallic plasmonic thin film, being a "single negative" medium, can focus the TM wave into a far-field image. We also find configurations in which two metallic films can give a bright image via resonance tunnelling. In addition, we show that a simple plasmonic waveguide array can give all-angle wide frequency bandwidth negative refraction from the visible to infrared frequencies. The phenomena can be understood from using theoretical analysis from the simple effective medium approach to including surface plasmon effect. The analytical results are confirmed by finite-difference time-domain numerical simulations. Our result provides an alternative way to construct robust all-angle negative refractive materials operating in a wide range of frequency from the near-infrared to the visible range.