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An optical unitary processor (OUP) is a programmable photonic circuit to achieve arbitrary unitary operation for various applications, including optical communication, deep learning, and quantum computing. Conventionally, OUPs are implemented by cascading 2 × 2 reconfigurable interferometers, but this scheme cannot easily be extended to multiple wavelength and polarization channels due to the strict requirement to employ 50:50 beam splitters. Here, we demonstrate that an OUP using multiport directional couplers (DCs) can realize independent unitary conversion of multiple wavelength and polarization channels simultaneously. This OUP is based on the multi-plane light conversion (MPLC) principle, which does not require a specific transformation at each layer, unlike the conventional scheme. Thanks to this unique robustness of the MPLC method and strong wavelength/polarization dependence of multiport DCs, we numerically show that independent unitary transformations can be applied to up to 16 channels (2 polarization × 4 wavelengths) using a single device.

We propose a novel ultra compact electro-absorption optical modulator based on a silicon rib waveguide and numerically demonstrate its performance. The proposed design employs two types of transparent conductive oxide (TCO) layers with different carrier densities to achieve both high modulation efficiency and low optical insertion loss. The thin TCO layer with high carrier density enables efficient modulation through the metal–oxide–semiconductor structure. On the other hand, the upper TCO layer with low carrier density allows low-resistance electrical contact for the top electrode without large optical loss. Using an indium tin oxide bilayer with optimized carrier densities, we numerically demonstrate a 4.3 dB extinction ratio and a 2.6 dB optical insertion loss with 1 µm device length. We estimate that the modulator operates under a low driving voltage of 1.3 V, exhibiting an ultra low energy consumption of 22.5 fJ/bit and a broad RC modulation bandwidth of over 40 GHz.

A compact, novel photonic crystal cavity aimed at applications with strict area limitations is presented. Optimization shows that the gentle confinement method previously used for line-defect cavities can be applied to more limited geometries. It also shows that it is paramount to consider the boundary region to minimize in-plane losses. The investigation show that a near optimum boundary thickness can be found by considering the boundary region as a Fabry-Pérot resonator. This optimization strategy is shown to be deterministic in terms of resonance wavelength. For an optimized air-clad, silicon cavity, finite-difference time-domain simulations give Q-values as high as 75,000 which is comparable to other photonic crystal cavities of similar size.

We propose a novel subwavelength metallic InP/InGaAs laser with a capsule-shaped cavity. By introducing curved metallic facets at the both ends, resonant modes can be strongly confined inside the center of the cavity, which not only improves the confinement factor and the Q factor, but also reduces the cavity volume. As an example case, we numerically demonstrate that the proposed structure can decrease the plasmonic loss and improve the Q factor from 197 to 297 and reduce the threshold current from 291 to 60 µA with the effective modal volume of 0.45 µm3. In addition, we demonstrate that there exists an optimal structure of the capsule-shaped cavity to minimize the current threshold.