Highly confined optical modes in nanoscale metal-dielectric multilayers

We show that a stack of metal-dielectric nanolayers, in addition to the long- and short-range plasmon polaritons, guides also an entire family of modes strongly confined within the multilayer---the bulk plasmon polariton modes. We propose a classification scheme that reflects specific properties of these modes. We report experimental verification of the bulk modes by measuring modal indices in a structure made of three pairs of silica $(\ensuremath{\sim}29\phantom{\rule{0.3em}{0ex}}\mathrm{nm})∕\text{gold}$ $(\ensuremath{\sim}25\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ layers.