Ryouichi Akimoto

Direct laser emission in the green–yellow spectral range remains a major challenge in semiconductor laser development due to material limitations. This study explores the formation dynamics and optical characteristics of CdSe quantum dots (QDs) induced by electron-beam irradiation as a strategy to bridge this spectral gap. Time-resolved diffraction analysis reveals that QD nucleation initiates rapidly within one minute of irradiation, with the formation rate strongly influenced by beam intensity. Spatially resolved photoluminescence (PL) and transmission electron microscopy measurements show distinct growth behaviors: enhanced Cd diffusion at the center of irradiated regions leads to smaller QDs and blueshifted emission, while Cd accumulation at the edges promotes larger QDs and redshifted emission. Under saturated irradiation conditions, the PL peak wavelength stabilizes within the 573–581 nm range, independent of initial CdSe thickness, indicating a self-regulating atomic rearrangement process. This convergence highlights the robustness of the electron-beam-induced QD formation mechanism, which enables wavelength-specific control over emission properties. The ability to engineer QDs with emission wavelengths spanning the green–yellow range demonstrates the potential of this approach for realizing compact, multi-wavelength optoelectronic devices. These findings establish electron-beam irradiation as a versatile tool for precise nanostructure engineering and spectral tuning in advanced photonic applications.

This study investigates the effects of electron beam irradiation on the lasing characteristics of optically pumped CdSe quantum dot (QD) lasers grown on GaAs substrates using molecular beam epitaxy. The experimental setup allows the creation of two types of CdSe QDs with distinct size differences, with and without electron beam irradiation during the epitaxy. Lasing wavelengths were observed at 534 nm in the non-irradiated region and between 548 and 557 nm in the irradiated regions, depending on the electron beam irradiation conditions, achieving multi-wavelength lasing in the green-yellow ranges on a single chip. Internal laser parameters were evaluated for both irradiated and non-irradiated regions across different cavity lengths. Photoluminescence (PL) spectra revealed significantly higher integrated PL intensity in the irradiated regions compared to the non-irradiated regions, attributed to reduced point defects. However, at the lasing threshold, the impact of these defects is negligible. Lasing spectra exhibited a notable blue shift in the irradiated regions with an increase in threshold intensity due to the state-filling effect, which also causes a decrease in internal quantum efficiency. The modal gain and transparent intensity were evaluated, showing lower gain values in the irradiated regions, consistent with the observed broadening of the PL spectrum. These findings highlight the dynamic nature of the state-filling effect and its impact on lasing performance, providing insights into the mechanisms affecting CdSe QD lasers under electron beam irradiation in green-yellow spectrum range.

Determining how to improve the non-uniformity of arrayed waveguide grating (AWG) is of great significance for dense wavelength division multiplexing (DWDM) systems. In this work, a silicon nanowire-assisted AWG structure is proposed, which can achieve high uniformity with a low insertion loss. The article compares the effect of nanowire number and shape on uniformity and insertion loss, finding that double nanowires provide the best performance. Double nanowires with a width of 230 nm and length of 3.5 μm can consist of a slot configuration between arrayed waveguides, both connecting to the star coupler and spacing 165 nm from the waveguides. Compared with conventional 8- and 16-channel AWGs with channel spacing of 200 GHz, the non-uniformity of the presented structure can be improved from 1.09 and 1.6 dB to 0.24 and 0.63 dB, respectively. The overall footprint of the device would remain identical, which is 276 × 299 or 258 × 303 μm² for the 8- or 16-channel AWG. The present high uniformity design is simple and easy to fabricate without any additional insertion loss, which is expected to be widely applied in the highly integrated DWDM systems.

The formation of quantum dots (QDs) induced by electron beam irradiation onto a highly strained CdSe layer on ZnSe (001) is investigated. In situ reflection high-energy electron diffraction is used for dual purposes: QD formation by electron beam irradiation and monitoring the progress of surface morphology. QD formation is confirmed by observation of nanoscale three-dimensional islands using transmission electron microscopy. It is also verified by imaging of an isolated single QD emission at room temperature by photoluminescence microscopy. Electron beam irradiation increases QD size both in height and lateral directions, resulting in the shift of emission from green to yellow spectral region.

We demonstrate a double-layer Si ₃ N ₄ multi-ring resonator system, where an S-bend waveguide is cross-coupled with a ring cavity loaded with two sub-ring resonators. The device performance is analyzed by using the transfer matrix method. The alignment-error-induced device performance variations are also discussed. The measured spectra coincide well with the simulated results. The device has high tuning flexibility and more design freedom with two sub-rings placed above the ring cavity. The spectrum profile can also be tuned by changing the cross-coupling coefficient. A heater placed above the resonator can shift the resonant wavelength effectively.

In this study, the operational durability of laser diodes (LDs) with CdSe quantum dot (QD) active region is examined through a detailed comparison with its BeZnCdSe quantum well (QW) counterpart. To focus on the impact on the difference caused by the nature of the active region, other layers such as the waveguide layer, cladding layer, and contact layer, which influence the mode profile of the optical confinement and electrical properties, are maintained under the same condition. Although the threshold current of the CdSe QD LD is higher than that of the BeZnCdSe QW LD by a factor of 1.6–1.7 under a fresh condition, the degradation of the former proceeds much more slowly than that of the latter. This is confirmed by a reliability test conducted under the current injection condition both below and above the lasing threshold. The result indicates that the stronger carrier localization in the CdSe QDs compared to that in the BeZnCdSe QWs prevents the interaction with remote defects in the active layer. Consequently, the CdSe QD LD is less sensitive to degradation due to recombination-enhanced defect reactions.

With a three-dimensional Si ₃ N ₄ integration, we demonstrate a dual-layer cross-coupled resonator system where an S-bend waveguide is coupled with a racetrack microring resonator loaded with a sub-microring. The device has a compact structure and provides more design freedom. The transfer matrix method is used to model the device. The resonance spectrum is significantly affected by the coupling coefficient between the waveguide and the resonator, generating versatile resonance features. A group of devices with a varying coupling length are implemented, exhibiting an evolution trend consistent with the simulation results. The resonance wavelength can be thermo-optically tuned by the micro-heater placed above the resonator. The three-dimensional Si ₃ N ₄ platform enables dense integration of high-performance devices, opening new avenues for future compact, high-density, and cost-effective photonic integrated circuits.

A vertically coupled suspended silicon nitride resonator sensor is experimentally demonstrated, which consists of a bottom-layer un-suspended access waveguide and a top-layer suspended microdisk cavity. The presented sensor can realize a high device sensitivity with a good mechanical stability. For a 40-μm-radius disk, more than 10 ⁴ quality factor at a wavelength of 1548.98 nm can be obtained with a moderate free-spectral range of 5.66 nm. The device sensitivity is measured by covering different organic liquids, and the sensitivity is about 554 nm/RIU. The fabrication of the proposed 3-D sensor is compatible with the conventional photolithography process, which would benefit its optical sensing applications.

A tunable three-dimensional silicon nitride racetrack resonator was experimentally demonstrated, with a microring resonator on a bottom layer and a feedback cross-coupled waveguide on a top layer. At a 1559.3-nm wavelength, an extinction ratio of more than 14 dB and free spectral range of 2.7 nm can be realized with a quality factor of about 1.88 x 10 ⁴ . The filter performance, such as the resonance wavelength and extinction ratio, can be thermo-optically tuned based on the electrical control of a heater above the feedback waveguide. The presented device has a potential to be applied as a tunable modulator/switch as well as a highly sensitive sensor.