Abstract
We adapted a fast Fourier transform-based Beam Propagation Method (FFT-BPM) to investigate waveguide discontinuities in plasmonic waveguides. The adaptation of the FFT-BPM to treat transverse magnetic (TM) fields requires the circumvention of two major difficulties: the mixed derivatives of the magnetic field and waveguide refractive index profile in the TM wave equation and the step-like index change at the transverse metal-dielectric boundary of the plasmonic guide and the transverse boundaries of the dielectric waveguide as well. An equivalent-index method is adopted to transform TM fields to transverse electric (TE) ones, thus enabling the benefit of the full power and simplicity of the FFT-BPM. Moreover, an appropriate smoothing function is used to approximate the step-like refractive index profile in the transverse direction. At the junction plane, we used an accurate combined spatial-spectral reflection operator to calculate the reflected field. To validate our proposed scheme, we investigated the modal propagation in a silicon waveguide terminated by air (like a laser facet in two cases: with and without a coating layer). Then we considered a subwavelength plasmonic waveguide (metal-insulator-metal MIM) butt-coupled with a dielectric waveguide, where the power transmission efficiency has been calculated and compared with other numerical methods. The comparison reveals good agreement.
Highlights
Active and passive plasmonic devices are becoming one of the most promising candidates that could overcome the size mismatch between micro scale photonic devices and nanoscale electronic devices [1,2]
Merging nanoelectronics and nano-optics enables the development of active subwavelength scale optics and an abundance of nano-optoelectronic devices and functionalities, such as tunable metamaterials, nanoscale optical processing, and strongly enhanced light-matter interactions for quantum devices and biosensing applications [3]
The waveguides terminated by air were studied using a variety of techniques [19,20,21,22,23,24,25] in
Summary
Active and passive plasmonic devices are becoming one of the most promising candidates that could overcome the size mismatch between micro scale photonic devices and nanoscale electronic devices [1,2]. It is worthy to note that subwavelength optical confinement modes, allows the efficient and beneficial use of compact photonic devices like nanoscale photodetectors; improving noise immunity, response speed and power dissipation in optical communication devices [8,9]. Three major problems should be considered carefully when dealing with step discontinuity and plasmonic devices: The transverse magnetic nature of modes, the abrupt index change in the transverse direction and in the propagation direction at the junction plane [15]. We claim that the method presented in this paper could be extended to other types of interesting waveguide couplers, like the evanescent coupling between MIM-dielectric and MIM-MIM waveguides Such coupling could be useful and efficient for nanoscale wavelength filters and ultra-fast optical switching as well as optical sensing application
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