Concentration of an optical field in an optical coaxial line

A study is reported of the characteristics of the principal wave in an optical coaxial metal waveguide. The real properties of metals at optical frequencies are taken into account. An approximate formula is derived for the dependences of the eigenvalues of the transverse wave vectors on the geometric parameters of a coaxial line and on its permittivity. The exact eigenvalues are determined for several values of the parameters of practical interest. It is shown that such a coaxial line can concentrate radiation with domains much smaller than the radiation wavelength.