We present remedies to two fundamental difficulties facing the applicability of the traditional
FFT-based beam propagation method (FFT-BPM) when investigating the propagation and
transmission of transverse magnetic (TM) optical beams in subwavelength step-index waveguiding structures. To the best of our knowledge, the FFT-BPM is introduced for the first time to
assess the plasmonic-dielectric waveguide interconnects. At the junction plane, we modified the
FFT-BPM algorithm by including a combined spatial-spectral reflection operator formalism to
calculate the reflected field. As a test, we calculated the optical power transmission efficiency
between plasmonic and dielectric waveguide interconnect. A comparison between our results,
and those obtained by full-modal matching using finite-difference frequency-domain (FDFD),
reveals good agreement. Such interconnecting structure is crucial in many applications as biosensors, optical near-field probes, and interfacing elements involving high-contrast refractive
index materials. We believe that rehabilitating the classical FFT-BPM to handle nanoscale waveguiding structures, which include metal-dielectric interfaces, will be of prime importance in
the development, analysis and assessment of nano-photonics devices.