A novel design of large bandwidth, fabrication tolerant, CMOS-compatible
compact tapers (15 um) have been proposed and experimentally demonstrated in
silicon-on-insulator. The proposed taper along with linear grating couplers for
spot-size conversion exhibits no degradation in the coupling efficiency
compared to a standard focusing grating in 1550 nm band. A single taper design
has a broadband operation over 600 nm that can be used in O, C and L-band. The
proposed compact taper is highly tolerant to fabrication variations; 80 nm
change in the taper width and 200 nm in end waveguide width varies the taper
transmission by <0.4 dB. The footprint of the device i.e. taper along with the
linear gratings is ~ 250 {\mu}m2; this is 20X smaller than the adiabatic taper
and 2X smaller than the focusing grating coupler
We demonstrate an ultra-compact waveguide taper on a silicon nitride platform. The proposed taper provides a coupling efficiency of 95% at a length of 19.5 μm in comparison to the standard linear taper of length 50 μm, which connects a 10 μm wide waveguide to a 1 μm wide photonic wire. The taper has a spectral response >75% spanning over 800 nm and resilience to fabrication variations; ±200 nm change in taper and end waveguide width varies transmission by <5%. We experimentally demonstrate taper insertion loss of <0.1 dB/transition for a taper as short as 19.5 μm, and reduce the footprint of the photonic device by 50.8% compared to the standard adiabatic taper. To the best of our knowledge, the proposed taper is the shortest waveguide taper ever reported in silicon nitride.
We compute the dielectric properties of freestanding and metal-supported borophene from first-principles time-dependent density functional theory. We find that both the low-and high-energy plasmons of borophene are fully quenched by the presence of a metallic substrate at borophene-metal distances smaller than 9Å. Based on these findings, we derive an electrodynamic model of the interacting, momentum-dependent polarizability for a two-dimensional metal on a model metallic substrate, which quantitatively captures the evolution of the dielectric properties of borophene as a function of metal-borophene distance. Applying this model to a series of metallic substrates, we show that maximizing the plasmon energy detuning between borophene and substrate is the key material descriptor for plasmonic performance.
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