  We propose a scheme to realize nonlinear coherent perfect absorption (CPA) at
the nanoscale using epsilon-near-zero (ENZ) plasmonic waveguides. The general
conditions to achieve CPA in a linear ENZ plasmonic waveguide are analyzed and
presented. The proposed ENZ waveguides support an effective ENZ response at
their cut-off frequency, where the CPA effect occurs under the illumination of
two counter-propagating plane waves with equal amplitudes and appropriate phase
distributions. In addition, the strong and uniform field enhancement inside the
nanochannels of the waveguides at the ENZ resonance can efficiently boost Kerr
nonlinearities, resulting in a new all-optical switching intensity-dependent
CPA phenomenon which can be tunable with ultrafast speed. The proposed
free-standing ENZ structures combine third-order nonlinear functionality with
standing wave CPA interference effects in a nanoscale plasmonic configuration,
thus, leading to a novel degree of tunable light-matter interactions achieved
in subwavelength regions. Our findings provide a new platform to efficiently
excite nonlinear phenomena at the nanoscale and design tunable coherent perfect
absorbers.
