Light propagation in negative index materials (NIMs) is most unusual: light wave and energy travel in opposite directions. NIMs open the door to fundamentally new optical phenomena, and offer enormous potential for new device applications. Much of the current effort in this rapidly advancing field is aimed at realizing materials with a negative index at optical frequencies, where the wavelengths are five orders or magnitude smaller than in the microwave regime where negative refractive indices were first demonstrated.
NIMs do not exist in nature; they must be man made. Since characteristic lengths need to be on the nanometer scale, nanoparticles are ideal building blocks for NIMs. Interestingly, assemblies of nanoparticles form liquid crystal phases, where nanoparticles play the role of liquid crystal molecules.
In this talk, I will discuss some fundamental aspects of NIMs, identify key challenges to their realization and use in the visible regime, and indicate how unique aspects of nanoparticle liquid crystals might be used to meet these challenges.