Equal-phase-velocity propagation for the TE and TM modes of planar metal-clad optical waveguides

Abstract

An asymmetrical four-layered metal-clad dielectric optical waveguide can support the TEN and TMN modes forN⩾0 with the same phase-velocity if the core thickness,t3, and the buffer layer thickness,t2, are suitably chosen. Only metals that have a negative real part ∈′1 of their complex permittivity\(\hat \in _{\text{1}} = \in '_1 - j \in ''_1 \)at optical frequencies are considered as the cladding material. It is shown that the application of a lossless approximation which set ∈″1=0 enables the development of a real expression which may be solved directly for the values oft2 andt3 that are required to achieve the equal-phase-velocity characteristic. Parametric plots for the guide dimensions are presented for representative waveguides and it is shown that an appropriate choice oft2 andt3 will reduce the absorption losses of the TE and TM modes to negligible levels. This property of the waveguide will find application in thin film optical devices such as polarization rotation modulators that require the simultaneous propagation of TE and TM polarized waves.