We examined the applicability of neural networks (NNs) and the covariance matrix adaptation evolution strategy (CMA-ES) for designing highefficiency grating couplers on a Si-on-insulator (SOI) photonics platform operating at a wavelength of 1550 nm. We trained a NN to predict the wavelength dependences of coupling efficiency instead of using the finite-difference time-domain method, which enabled us to calculate analytically the gradient of coupling efficiency with respect to the design parameters. As a result, we found that a gradient-based method can be applied to find a more optimal design from an apodized grating coupler. In contrast, the CMA-ES, which is one of the most efficient evolutionary strategies, enabled us to explore a large design-parameter space efficiently without any constraint in an initial design of a grating coupler. As a result, we successfully obtained the optimal design from a uniform grating coupler. The optimized grating coupler exhibited a coupling efficiency of 74.7% when the thickness of the Si layer is 220 nm. The CMA-ES also enabled us to optimize the thickness of the Si gratings simultaneously. Through the optimization, we achieved a coupling efficiency of 92.1% when the Si thickness is 293 nm.

Integrated mid-infrared (MIR) photonics has been widely investigated for the past decade, where germanium (Ge) is a promising optical material in this regime. In this work, we studied the origin of optical losses in Ge waveguides on a Ge-on-insulator (GeOI) wafer fabricated using Smart-cut. We observed that the high optical loss was mainly due to the holes in Ge films, which were generated by crystal defects formed by hydrogen ion implantation for Smart-cut. Furthermore, we found that the carrier concentration profile after the splitting process in remaining Ge films depends on the hydrogen ion implantation energy and initial background doping concentration of Ge wafers. A higher proton implantation energy can lead to deeper penetration of hydrogen ions into Ge films with less damage remaining near the implantation surface, resulting in the successful fabrication of an n-type GeOI wafer with a low carrier density. As a result, we experimentally demonstrated a low-loss Ge waveguide on an n-type GeOI wafer with a propagation loss as low as 2.3 ± 0.2 dB/cm. This work suggests an approach to tailor the carrier type in a Ge film formed using Smart-cut for large-scale MIR Ge photonic integrated circuits.

A phototransistor is a promising candidate as an optical power monitor in Si photonic circuits since the internal gain of photocurrent enables high responsivity. However, state-of-the-art waveguide-coupled phototransistors suffer from a responsivity of lower than 103 A/W, which is insufficient for detecting very low power light. Here, we present a waveguide-coupled phototransistor operating at a 1.3 μm wavelength, which consists of an InGaAs ultrathin channel on a Si waveguide working as a gate electrode to increase the responsivity. The Si waveguide gate underneath the InGaAs ultrathin channel enables the effective control of transistor current without optical absorption by the gate metal. As a result, our phototransistor achieved the highest responsivity of approximately 106 A/W among the waveguide-coupled phototransistors, allowing us to detect light of 621 fW propagating in the Si waveguide. The high responsivity and the reasonable response time of approximately 100 μs make our phototransistor promising as an effective optical power monitor in Si photonic circuits.

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