Hybrid distributed Bragg reflector laser on Si with a transfer printed InAs/GaAs quantum dot amplifier.

We demonstrate a hybrid integrated laser by transfer printing an InAs/GaAs quantum dot (QD) amplifier on a Si waveguide with distributed Bragg reflectors (DBRs). The QD waveguide amplifier of 1.6 mm long was patterned in the form of an airbridge with the help of a spin-on-glass sacrificial layer and precisely integrated on the silicon-on-insulator (SOI) waveguide by pick-and-place assembly using an elastomer stamp. Laser oscillation was observed around the wavelength of 1250 nm with a threshold current of 47 mA at room temperature and stable operation up to 80°C. Transfer printing of the long QD amplifiers will enable the development of various hybrid integrated laser devices that leverage superior properties of QDs as laser gain medium.

Carrier conversion from terahertz wave to dual-wavelength near-infrared light for photonic terahertz detection in wireless communication.

THz waves are promising wireless carriers for next-generation wireless communications, where a seamless connection from wireless to optical communication is required. In this study, we demonstrate carrier conversion from THz waves to dual-wavelength NIR light injection-locking to an optical frequency comb using asynchronous nonpolarimetric electro-optic downconversion with an electro-optic polymer modulator. THz wave in the W band was detected as a stable photonic RF beat signal of 1 GHz with a signal-to-noise ratio of 20 dB via the proposed THz-to-NIR carrier conversion. In addition, the results imply the potential of the photonic detection of THz waves for wireless-to-optical seamless communication.

High-performance Ge/Si electro-absorption optical modulator up to 85°C and its highly efficient photodetector operation.

We studied a high-speed Ge/Si electro-absorption optical modulator (EAM) evanescently coupled with a Si waveguide of a lateral p-n junction for a high-bandwidth optical interconnect over a wide range of temperatures from 25 °C to 85 °C. We demonstrated 56 Gbps high-speed operation at temperatures up to 85 °C. From the photoluminescence spectra, we confirmed that the bandgap energy dependence on temperature is relatively small, which is consistent with the shift in the operation wavelengths with increasing temperature for a Ge/Si EAM. We also demonstrated that the same device operates as a high-speed and high-efficiency Ge photodetector with the Franz-Keldysh (F-K) and avalanche-multiplication effects. These results demonstrate that the Ge/Si stacked structure is promising for both high-performance optical modulators and photodetectors integrated on Si platforms.

Graphene perfect absorber based on degenerate critical coupling of toroidal mode.

Graphene is a two-dimensional material with great potential for photodetection and light modulation applications owing to its high charge mobility. However, the light absorption of graphene in the near-infrared range is only 2.3%, limiting the sensitivity of graphene-based devices. In this study, we propose a graphene perfect absorber based on degenerate critical coupling comprising monolayer graphene and a hollow silicon Mie resonator array. In particular, monolayer graphene achieves perfect absorption by controlling the periods and holes of the Mie resonators. The proposed graphene perfect absorber can significantly improve the sensitivity of graphene-based devices.

Modulation bandwidth improvement of III-V/Si hybrid MOS optical modulator by reducing parasitic capacitance.

In this work, we numerically and experimentally examined the impact of parasitic capacitance on the modulation bandwidth of a III-V/Si hybrid metal-oxide-semiconductor (MOS) optical modulator. The numerical analysis revealed that the parasitic capacitance between the III-V membrane and the Si slab should be considered to achieve high-speed modulation, particularly in the case of a thick gate oxide. We also fabricated a high-speed InGaAsP/Si hybrid MOS optical modulator with a low capacitance using a SiO2-embedded Si waveguide. The fabricated device exhibited a modulation efficiency of 0.245 Vcm and a 3 dB bandwidth of up to 10 GHz. Clear eye patterns with 25 Gbps non-return-to-zero (NRZ) modulation and 40 Gbps 4-level pulse amplitude modulation (PAM-4) were obtained without pre-emphasis.

High-efficiency and high-speed narrow-width MOS capacitor-type Si optical modulator with TM mode excitation.

We developed a high-speed and high-efficiency narrow-width metal-oxide-semiconductor (MOS) capacitor-type Si optical modulator (Si-MOD) by applying TM optical mode excitation. We designed and fabricated an optical-mode-converter structure from TE to TM mode. Even in the case of a 200-nm width, the Si MOS-MOD showed high-modulation efficiency in TM mode (about 0.18 Vcm), and the electrical capacitance decreased as the MOS junction width decreased. We also demonstrated high-speed operation at 32 Gbps and 40 Gbps for the 30-µm-long Si MOS-MOD in TM mode.

Taperless Si hybrid optical phase shifter based on a metal-oxide-semiconductor capacitor using an ultrathin InP membrane.

We propose a III-V/Si hybrid metal-oxide-semiconductor (MOS) optical phase shifter using an ultrathin InP membrane, which allows us to eliminate the III-V taper required for mode conversion between Si and hybrid waveguides. We numerically revealed that thinning a III-V membrane can reduce the insertion loss of the phase shifter while maintaining high modulation efficiency because the optical phase shift is induced by carrier accumulation at the MOS interface. We experimentally demonstrated the proposed optical phase shifter with an ultrathin InP membrane and achieved the modulation efficiency of 0.54 Vcm and the insertion loss of 0.055 dB. Since the taperless structure makes the hybrid integration easier and more flexible, the hybrid MOS optical phase shifter with an ultrathin III-V membrane is promising for large-scale Si programmable photonic integrated circuits.

High-speed Ge/Si electro-absorption optical modulator in C-band operation wavelengths.

We studied a high-speed electro-absorption optical modulator (EAM) of a Ge layer evanescently coupled with a Si waveguide (Si WG) of a lateral pn junction for high-bandwidth optical interconnect. By decreasing the widths of selectively grown Ge layers below 1 µm, we demonstrated a high-speed modulation of 56 Gbps non-return-to-zero (NRZ) and 56 Gbaud pulse amplitude modulation 4 (PAM4) EAM operation in the C-band wavelengths, in contrast to the L-band wavelengths operations in previous studies on EAMs of pure Ge on Si. From the photoluminescence and Raman analyses, we confirmed an increase in the direct bandgap energy for such a submicron Ge/Si stack structure. The operation wavelength for the Ge/Si stack structure of a Ge/Si EAM was optimized by decreasing the device width below 1-µm and setting the post-growth anneal condition, which would contribute to relaxing the tensile-strain of a Ge layer on a Si WG and broadening the optical bandwidths for Franz-Keldysh (FK) effect with SiGe alloy formation.

First demonstration of SiGe-based carrier-injection Mach-Zehnder modulator with enhanced plasma dispersion effect.

We demonstrate a strained Si0.91Ge0.09-based carrier-injection Mach-Zehnder (MZ) optical modulator using the enhanced plasma dispersion effect in strained SiGe through mass modulation for the first time. The SiGe modulator has an injection current of 1.47 mA for a phase shift of π which is lower than that for a Si modulator. Also, it is expected that the injection current can be further reduced by increasing the strain and Ge fraction, enabling operation at an injection current of less than 1 mA. As an example of the dynamic characteristics, 10 Gbps modulation with clear eye opening was obtained by the pre-emphasis method.

Demonstration of record-low injection-current variable optical attenuator based on strained SiGe with optimized lateral pin junction.

We demonstrate a strained SiGe variable optical attenuator (VOA) with a lateral pin junction which exhibits record-low injection-current for 20-dB attenuation. We optimize the distance between the highly doped p + and n + regions in the lateral pin junction to effectively inject electrons and holes, taking into account the propagation loss. In conjunction with the enhanced free-carrier absorption in strained SiGe, the SiGe VOA with the optimized lateral pin junction exhibits 20-dB attenuation by 20-mA/mm injection current, which is 1.5 times lower current density than that of the Si VOA. The SiGe VOA also shows better RF response than the Si VOA due to the short carrier lifetime in SiGe, allowing us to achieve efficient and fast attenuation modulation simultaneously. Furthermore, 2-GHz switching and error-free transmission of 4 × 12.5 Gbps WDM signal have been also achieved.

Differential receivers with highly -uniform MSM Germanium photodetectors capped by SiGe layer.

Waveguide integrated MSM (metal-semiconductor-metal) Germanium (Ge) photodetectors (PDs) with a SiGe capping layer were exploited for silicon photonics integration. Under optimized epitaxial growth conditions, the capping layer passivated the Ge surface, resulting in sufficiently low dark current of the PDs. In addition, the PDs exhibited a narrower distribution of the dark current than PDs with a Si capping layer, probably due to the lower surface leakage current. Low-noise differential receivers with uniform MSM Ge PDs exhibiting 10 Gbps data transmission were realized.

Demonstration of 12.5-Gbps optical interconnects integrated with lasers, optical splitters, optical modulators and photodetectors on a single silicon substrate.

One of the most serious issues in information industries is the bandwidth bottleneck in inter-chip interconnects. We propose a photonics-electronics convergence system to solve this issue. We fabricated a high density optical interposer to demonstrate the feasibility of the system by using silicon photonics integrated with an arrayed laser diode, an optical splitter, silicon optical modulators, germanium photodetectors, and silicon optical waveguides on a single silicon substrate. Error-free data transmission at 12.5 Gbps and a transmission density of 6.6 Tbps/cm2 were achieved with the optical interposer. We believe this technology will solve the bandwidth bottleneck problem in the future.

First demonstration of high density optical interconnects integrated with lasers, optical modulators, and photodetectors on single silicon substrate.

Engineers are currently facing some technical issues in support of the exponential performance growths in information industries. One of the most serious issues is a bottleneck of inter-chip interconnects. We propose a new "Photonics-Electronics Convergence System" concept. High density optical interconnects integrated with a 13-channel arrayed laser diode, silicon optical modulators, germanium photodetectors, and silicon optical waveguides on single silicon substrate were demonstrated for the first time using this system. A 5-Gbps error free data transmission and a 3.5-Tbps/cm(2) transmission density were achieved. We believe that this technology will solve the bandwidth bottleneck problem among LSI chips in the future.