Fabrication of mid-index contrast ring resonators with a well-defined gap is very challenging, while gapless rings suffer from substantial coupling losses. To avoid these issues, we propose a gapless design based on a step-like structuring of the bus/ring waveguides. As we show with numerical simulations, our design allows to spatially confine the modes to the steps formed on the bus/ring waveguides thus mitigating the coupling losses. Additionally, the step on the ring waveguide reduces the bending losses. Finally, we show how varying the heights and widths of the steps allows to tailor the transmittance and quality factor of the ring.
Performance of multimode interference couplers is constrained by the phase errors caused by the deviations of the modes propagation constants from the required quadratic dependence upon mode number. In this work, we show that by creating a rectangular grating on the top surface of the coupler with a spatial frequency matching the intensity distribution of the i-th order mode, it is possible to control the phase errors of all spatial modes up to the i-th order. The effect of the grating on the propagation constants of higher-order modes is studied using perturbation-based and strict vector calculus. The efficacy of the method was demonstrated using numerical examples of an MMI-based two-mode (TE0 and TE1) coupler in the 1.31 μm wavelength region and the singlemode (TE0) 1.31/1.55 μm wavelength splitter, both made of medium index contrast material (TiO2:SiO2).
We demonstrate how to tailor the size-dependent enhanced transmission and absorption of 1D subwavelength semiconductor-based metamaterial high contrast gratings. We focus on 3 spectral regimes generic for most semiconductors, where the refractive index (n) and extinction coefficient (k) of the semiconducting material satisfy the following conditions: n >> k, n ~ k and n < k. We show that the transmission into such structures can be enhanced by reducing the bar width, increasing the grating period, or tapering the grating sidewalls. Moreover, thanks to the slow-light phenomenon, the absorption of the grating can be enhanced as compared to bulk semiconductors.
Understanding the interactions of light with periodic arrays of Si fins is of the utmost importance for nanoelectronics, where laser light is used for the fabrication and metrology of fin field effect transistors (finFETs). However, due to their nanoscale dimensions and periodic arrangement, these structures exhibit complex photonic properties. In this work, we explain theoretically how the reflectance of semi-infinite periodic arrays of Si fins embedded in SiO2 varies with the fin pitch (i.e. spatial periodicity) and fin width. The results are corroborated with band structure calculations, showing that the spectra of both polarizations, parallel (TE) and perpendicular (TM) to the fin sidewalls, can be understood based on the excitation of one waveguide mode. First, we demonstrate that increasing the pitch decreases the reflectance from the arrays, for both polarizations. Moreover, TE spectra resemble that of bulk Si and are much higher as compared to TM, which are similar to the bulk SiO2 spectrum. The difference is attributed to the fact that TE mode is mostly confined inside the fin, whereas TM is spread in the SiO2. Subsequently, we show that the reflectance from the arrays increases as a function of the fin width. TE reflectances are again mostly sensitive towards Si dispersion and higher than TM counterparts. Interestingly, for TM illumination a transition from the SiO2- to Si-like spectra is observed for the fins of increasing width. The transition is caused by the change in the fraction of the electric field propagating inside the fin. The developed insight will facilitate design, fabrication and metrology of optoelectronic, photovoltaic and nanoelectronic devices.
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