Integrated silicon nitride waveguides of 100 nm height can achieve ultralow propagation losses below 0.1 dB/cm at the 1550 nm wavelength band but lack the scattering strength to form efficient grating couplers. An enhanced grating coupler design based on an amorphous silicon layer on top of silicon nitride is proposed and demonstrated to improve the directionality of the coupler. The fabrication process is optimized for a self-alignment process between the amorphous silicon and silicon nitride layers without increasing waveguide losses. Experimental coupling losses of 5 dB and a 3 dB bandwidth of 75 nm are achieved with both regular and focusing designs.

An ultra-compact (6 μm length) electro-absorber modulator with transverse magnetic (TM) to transverse-electric (TE) conversion is proposed. The device performance is controlled by means of the semiconductor-to-metal transition of the vanadium dioxide. For the insulating state, the device performs as a TM–TE converter with insertion losses of 0.3 dB and extinction ratio of 36 dB at a wavelength of 1.55 μm. Changing to the metallic state, the TE generated component is attenuated due to the increase of losses in the VO2 and the mode mismatch. This electro-absorber modulator shows a broadband operation with an extinction ratio higher than 10 dB and insertion losses below 0.5 dB for a range of 60 nm covering the whole C-band.

The optimization of grating couplers is usually realized by multiple simulations using specific computational software for this task. Many grating parameters must be analyzed and designed to get the maximum coupling efficiency and the transmission spectrum centred at the wavelength of operation. However, these simulations may take a long time and consume high computational resources depending on the simulation resolution. This work is focused on finding a method to optimize the grating parameters with the lowest number of simulations. In this way, closed-form expressions are presented to get the optimal values for the period and fill-factor, which are the main parameters in the grating design. The usefulness of the proposed approach is shown for the design of silicon grating couplers operating at 1.31 µm and 1.55 µm and both TE and TM polarizations.

Transparent conducting oxides (TCOs) have emerged as both particularly appealing epsilon-near-zero (ENZ) materials and remarkable candidates for the design and fabrication of active silicon nanophotonic devices. However, the leverage of TCO’s ultrafast nonlinearities requires precise control of the intricate physical mechanisms that take place upon excitation. Here we investigate such behavior for ultrafast all-optical phase switching in hybrid TCO-silicon waveguides through numerical simulation. The model is driven from the framework of intraband-transition-induced optical nonlinearity. Transient evolution is studied with a phenomenological two-temperature model. Our results reveal the best compromise between energy consumption, insertion losses and phase change per unit length for enabling ultrafast switching times below 100 fs and compact active lengths in the order of several micrometers.

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