Consider an interface $($at x$=0)$ between air and material characterized by a negative

permittivity $<0($in practice, silver possess the negative permittivity property in the visible

and near infrared spectra$).$ This interface can operate as an open waveguide that supports TM

modal fields that are guided along and are bounded within the interface. The magnetic field

component is described by H $=$ y$$U$($x$,$ z$)$