We present a novel optical phase modulation scheme on a Si photonic platform that uses a III-V/Si hybrid metal-oxide-semiconductor (MOS) capacitor formed by bonding an n-type InGaAsP membrane on a p-type Si waveguide. We numerically revealed that the phase modulation efficiency was improved by a factor of 7 × 8 owing to electron accumulation at the InGaAsP MOS interface when the n-type Si layer in a Si MOS optical phase shifter was replaced by an n-type InGaAsP layer. To realize the III-V/Si hybrid MOS capacitor, we developed an Al $_2$ O $_3$ bonding interface deposited by atomic layer deposition that enabled a low interface density of < 10 $^12$ cm $^{-2}$ eV $^{-1}$ at an InGaAsP MOS interface, which was essential for electron accumulation. We demonstrated a modulation efficiency of 0.047 Vcm at a 1.55 μm wavelength owing to the electron-induced change in the refractive index of InGaAsP. Since no holes were induced in the III-V layer of the III-V/Si hybrid MOS capacitor, we avoided large hole-induced absorption in InGaAsP. As a result, when we had a - phase shift, we obtained optical absorption of 0.23 dB, approximately 10 times smaller than that of a Si MOS optical phase shifter. We found by numerical analysis that the efficient low-loss III-V/Si hybrid MOS optical phase shifter improved markedly the optical modulation amplitude, indicating its suitability for high-speed modulation beyond 100 Gbps. We also demonstrated a Mach-Zehnder interferometer optical switch using the proposed optical phase shifter with a switching time of less than 20 ns. We achieved an extremely low switching power of approximately 1 nW, enabling a large-scale optical switch and universal photonic integrated circuits. We also discuss the feasibility of a photonic neural network for deep learning.

The realization of a silicon optical phase shifter marked a cornerstone for the development of silicon photonics, and it is expected that optical interconnects based on the technology relax the explosive datacom growth in data centers. High-performance silicon optical modulators and switches, integrated into a chip, play a very important role in optical transceivers, encoding electrical signals onto the light at high speed and routing the optical signals, respectively. The development of the devices is continuously required to meet the ever-increasing data traffic at higher performance and lower cost. Therefore, heterogeneous integration is one of the highly promising approaches, expected to enable high modulation efficiency, low loss, low power consumption, small device footprint, etc. Therefore, we review heterogeneously integrated optical modulators and switches for the next-generation silicon photonic platform.

Current infrared thermal image sensors are mainly based on planar firm substrates, but the rigid form factor appears to restrain the versatility of their applications. For wearable health monitoring and implanted biomedical sensing, transfer of active device layers onto a flexible substrate is required while controlling the high-quality crystalline interface. Here, we demonstrate high-detectivity flexible InAs thin-film mid-infrared photodetector arrays through high-yield wafer bonding and a heteroepitaxial lift-off process. An abruptly graded In_xAl_1–xAs (0.5 < x < 1) buffer was found to drastically improve the lift-off interface morphology and reduce the threading dislocation density twice, compared to the conventional linear grading method. Also, our flexible InAs photodetectors showed excellent optical performance with high mechanical robustness, a peak room-temperature specific detectivity of 1.21 × 10⁹ cm-Hz^1/2/W at 3.4 μm, and excellent device reliability. This flexible InAs photodetector enabled by the heteroepitaxial lift-off method shows promise for next-generation thermal image sensors.