Compact Optical Antenna Coupler for Silicon Photonics Characterized by Third-Harmonic Generation

Themistoklis Sidiropoulos Physcis, Imperial College London, London, UK Michael Nielsen Physcis, Imperial College London, London, UK Tyler Roschuk Physcis, Imperial College London, London, UK Anatoly Zayats Physics, King's College London, London, UK Stefan Maier Physcis, Imperial College London, London, UK Rupert Oulton Physcis, Imperial College London, London, UK

With the rise of integrated silicon photonic circuits, the problem of efficiently coupling light into photonic waveguides at telecommunication wavelengths has become a relevant one. While grating couplers typically have efficiencies in excess of 20%, this comes with a trade-off between complicated optimized designs and bulky device sizes. Moreover, grating couplers suffer from high angular sensitivity, high dispersion and short bandwidths. For these reasons, the straight forward method of direct end-fire excitation is the most common method of coupling despite efficiencies as low as 1%.

In this work, we present a compact plasmonic-photonic coupler consisting of gold nanoparticles positioned above a silicon photonic waveguide that shows efficient broadband coupling of free-space radiation into the photonic waveguide via localised surface plasmon (LSP) resonances.

Numerical simulations suggest for 250nm wide nanoparticles excited at a wavelength of 1.5µm, coupling efficiencies of 13% for a single particle and 20% an identical particle pair into both waveguide directions. Directional coupling can be obtained by adjusting the relative phase between LSPs through the variation of one particle’s width. For the directional couplers a maximum coupling efficiency of 27% into one direction is found for a relative phase of π/4 caused by multiple scattering effects. Yet, a simple dipole model highlights a subtle interplay between multiple scattering and directionality; namely, strong multiple scattering between the nanoparticles limits the coupling efficiency and optimum directionality minimizes the coupling efficiency. The numerical simulations also show a 500nm coupling bandwidth in the telecommunications band and low group delay dispersion, enabling the faithful coupling of pulses as short as 50fs.

Experimental verification of the simulations was conducted through the addition of a single particle out-coupler to extract the light out of the waveguide again, yielding a 19% experimental in-coupling efficiency. Directionality of the in-coupling was verified by varying the particle separation and relative phase difference. The strong coupling effects were investigated through the nonlinear effects of second harmonic generation (SHG) and third harmonic generation (THG) as seen at the in- and out-couplers. The broadband spectral response of the plasmonic-photonic couplers was examined using a supercontinuum generated white light source.

t.sidiropoulos10@imperial.ac.uk









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