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A magneto-optical switch responding to signal with 200 ps rise time was demonstrated. The switch uses current-induced magnetic field to modulate the magneto-optical effect. Impedance-matching electrodes were designed to apply high-frequency current and accommodate the high-speed switching. A static magnetic field generated by a permanent magnet was applied orthogonal to the current-induced ones and acts as a torque and helps the magnetic moment reverse its direction which assist the high-speed magnetization reversal.

Hybrid GaInAsP/SOI ridge waveguide Fabry–Pérot lasers with reduced thermal resistance were fabricated and measured. The lasers were formed by room-temperature surface-activated bonding of InP and silicon-on-insulator wafers. Thin SiO2 film was introduced as ridge-sidewall insulation to increase heat flow to the p-electrode side-metal. By incorporating a thermal shunt structure and Au electroplating, a single-facet output power of 20 mW and lasing operation up to 110 °C were achieved for a cavity length of 2.0 mm under continuous-wave conditions. The proposed structure showed low thermal resistance of 14.3 K W−1 and a threshold current density of approximately 0.7 kA cm−2.

A mode-selective topological waveguide has been proposed. By performing appropriate band tuning in two photonic structures that comprise topological waveguides, we can realize topological waveguides which allow optical vortex modes with a specific charge numbers.

In this study, we propose a defect structure that enhances the vertical coupling efficiency of circularly polarized light incident on topological waveguides consisting of triangle nanoholes with C6v symmetry arranged in honeycomb lattice. The defect structure was formed by removing triangle nanoholes from a certain hexagonal unit cell around the topological waveguide. As a result of comparing the coupling efficiency with and without the defect structure through three-dimensional finite-difference time-domain analysis, significant improvement in the vertical coupling efficiency was observed over the entire telecom C band (4460%@1530 nm). In addition, it was also found that the wavelength showing maximum coupling efficiency can be controlled over the entire C band by changing the arrangement of the dielectric around the defect structure.

Bonding position accuracy of InP/Si direct transfer bonding for heterogeneous integration were investigated. It was revealed that large difference of coordinate misalignment between X and Y directions exists and this may be strongly related to the tension of adhesive sheet.

We present a wavelength tunable vertical-cavity surface-emitting laser (VCSEL) constructed by die-bonding a half-cavity InGaAs laser diode (LD) chip onto a silicon-on-insulator chip with a microelectromechanical system electrostatic diaphragm mirror that functions as a Fabry–Perot interferometer. As a result of the short cavity length, the integrated tunable LD has single-mode lasing characteristics with an extremely large coherence length of 150 m or more. The developed wavelength tunable LD is used to perform swept-source optical coherence tomography with a large scan depth, which is applicable to ophthalmic observation for the diagnosis of pathologic nearsightedness based on the measurement of the axial length of an eye.

We demonstrate a membrane photonic integrated circuit (MPIC) that includes a membrane distributed feedback (DFB) laser and a p-i-n photodiode with a buried-ridge-waveguide (BRW) on a Si substrate, using a-Si nanofilm-assisted room-temperature surface activated bonding (SAB) for on-chip optical interconnection. The BRW structure enhanced the lateral optical confinement compared with that of the conventional flat structure. The directly bonded membrane DFB laser using SAB had a lower thermal resistance and higher output power than the previous structure using a benzocyclobutene (BCB) bonding layer. The DFB laser had a low threshold current of 0.27 mA at 25 °C. The maximum detected photocurrent and slope efficiency were 0.95 mA and 0.203 mA/mA, respectively, at 25 °C. The MPIC was successfully operated at temperatures up to 120 °C. The 3-dB bandwidths of 16.8 GHz and 10.1 GHz were achieved at 25 °C and 80 °C, respectively, and 25 Gbps and 15 Gbps non-return-to-zero (NRZ) 215-1 pseudo-random bit sequence signals were recorded at 25 °C and 80 °C, respectively.

Infrared refractive index is an indispensable parameter for various fields including infrared photonics. To date, critical-angle refractometers, V-block refractometers, and spectroscopic ellipsometry have been commonly used to measure the refractive index. Although every method has an accuracy of four decimal places for the refractive index, a measurable wavelength region is limited up to about 2 µm. In this study, we demonstrated a metamaterial infrared refractometer for determining broadband complex refractive index. Using the device, a broadband (40–120 THz; wavelength 2.5–7.5 µm) and high-precision(< 5 ×10−3) complex refractive index of polymethyl methacrylate was measured for the first time.

In this study, we developed a photonic band microscope based on hyperspectral Fourier image spectroscopy. The developed device constructs an infrared photonic band structure from Fourier images for various wavelength obtained by hyperspectral imaging, which make it possible to speedily measure the dispersion characteristics of photonic nanostructures. By applying the developed device to typical photonic crystals and topological photonic crystals, we succeeded in obtaining band structures in good agreement with the theoretical prediction calculated by the finite element method. This device facilitates the evaluation of physical properties in various photonic nanostructures, and is expected to further promote related fields.

We have fabricated III–V gain region/Si waveguide hybrid lasers with an InP-based two-storied ridge structure using direct bonding technology. Continuous-wave operation at a temperature of 20 °C with a threshold current as low as 30 mA was obtained for a hybrid laser consisting of a Fabry–Perot cavity with facets formed on the Si waveguide, and it was achieved thanks to a high optical coupling efficiency of over 80% at the interface between the III–V gain region and the Si waveguide using a two-storied ridge structure and a taper waveguide. The wavelength tunable laser which is comprised of a Si cavity with a loop mirror and double ring filters and III–V gain regions with laser and semiconductor optical amplifier (SOA) sections was demonstrated by utilizing this hybrid structure. It exhibited a wide wavelength tuning range of 48.6 nm and an optical amplification of 10 dB by SOA.

We propose a method for selectively propagating optical vortex modes with specific charge numbers in a photonic integrated circuit (PIC) by using a topological photonic system. Specifically, by performing appropriate band tuning in two photonic structures that comprise a topological waveguide, one specific electromagnetic mode at the Γ point of a band diagram can be excited. Based on theoretical analysis, we successfully propagated optical vortex modes with specific charge numbers over a wide range in the C band in the proposed topological waveguide. The proposed method could be useful in controlling optical vortex signals at the chip level in future orbital angular momentum multiplexing technologies.

Replacing conventional electrical global wiring to optical wiring is a promising solution to overcome interconnect bottleneck of Si-LSIs.

Novel magneto-optical isolator and self-holding optical switch with an a-Si:H microring resonator are demonstrated. The devices are driven by the remanence of integrated thin-film magnet and, therefore, maintain their state without any power supply.

Recently a novel photonic integration technology, based on a thin InP-based membrane, is emerging. This technology offers monolithic integration of active and passive functions in a sub-micron thick membrane. The enhanced optical confinement in the membrane results in ultracompact active and passive devices. The membrane also enables approaches to converge with electronics. It has shown high potential in breaking the speed, energy and density bottlenecks in conventional photonic integration technologies. This paper explains the concept of the InP membrane, discusses the versatility of various technology approaches and reviews the recent advancement in this field.

We propose an Si-based optical vortex beam splitter consisting of topological edge state waveguides with C6 symmetry. The device can continuously change the branching ratio of the propagate light in the range from 16% to 84%.

We propose and demonstrate a near-infrared invisibility cloaking by using a double-layered metamaterial film. 1020-nm-band light is guided so as not to hit a tungsten wire around which the film is wound.

We demonstrate a novel self-holding function of a magneto-optical waveguide switch. The switching state is flipped by a pulsed current and maintained without any power supply by virtue of the nonvolatility of the thin-film magnet. Extinction ratios up to 15.4 dB were demonstrated. The switch state was controlled by a 1- $\mu \text{s}$ pulsed electrical current.

The energy consumption of light sources must be reduced to realize on-chip optical interconnects. This study proposes a buried-ridge-waveguide (BRW) structure to enhance the lateral optical confinement. An increase in the lateral optical confinement induces the reduction in the threshold current, and it also provides a differential resistance due to a reduction in the distance between the electrode and active region. The stripe width and ridge height are designed considering the internal loss and differential resistance. A comparison between the BRW and conventional flat structures demonstrates a 20% and 35% reduction in the threshold current and differential resistance, respectively.

To advance on-chip optical interconnections, membrane distributed-reflector (DR) lasers with low threshold current and high-efficiency operation at one side output were realized. First, a membrane distributed Bragg reflector (DBR) laser with 80- μ m-long active section and 50- μ m-long DBR section was fabricated to clarify the DBR reflectivity. An external differential quantum efficiency of 35% for the output from the front facet was obtained, and the DBR reflectivity was estimated to be 75%. Next, a membrane DR laser with 61- μ m-long distributed feedback section and 50- μ m-long DBR section was fabricated. A threshold current of 0.48 mA, external differential quantum efficiency from the front side waveguide of 26%, and light output ratio from the front to the rear sides of 13 were obtained. The lasing spectrum showed a single-mode operation with a side-mode suppression-ratio (SMSR) of 40 dB. Finally, small-signal direct modulation was carried out and a modulation current efficiency factor of 7.9 GHz/mA1/2 and 7 GHz/mA1/2 were, respectively, obtained for the 30- μ m-long and 61- μ m-long devices.

On-chip optical interconnection is a promising technology for wiring future large-scale integrated circuits, as a means to mitigate the considerable power dissipation of traditional wiring layers. Here, we fabricate an integrated optical link using a membrane distributed-feedback (DFB) laser and a p-i-n photodiode (PD) in a butt-jointed built-in coupling geometry. The optical link is formed on a Si substrate by benzocyclobutene bonding. The integrated DFB laser shows a low-threshold current of 0.48 mA. Light transmission between the DFB laser and the p-i-n PD is confirmed with static measurements of the optical link. The optical link has a 3-dB bandwidth of 11.3 GHz at a 2.73 mA DFB laser bias current and a –3 V p-i-n PD bias voltage. A data transmission experiment of the optical link is performed, using a nonreturn to zero, pseudorandom-bit-sequence with a word length of 231-1 signals. With a DFB laser bias current of 2.5 mA, 10 Gbit/s data transmission with a bit-error-rate of 6 × 10–7 is successfully achieved.

Record high power conversion efficiency operation of a GaInAsP/InP membrane distributed-reflector laser on silicon substrate was realized. In order to enhance the efficiency, both an improvement of external differential quantum efficiency and a reduction of differential resistance were achieved. As a result, an external differential quantum efficiency from the front facet of 36%, and a maximum power conversion efficiency of 14.6% were obtained for a device with a 40- $\mu \text{m}$ -long active region.

We demonstrate a silicon Mach-Zehnder modulator (MZM) based on hydrogenated amorphous silicon (a-Si:H) strip-loaded waveguides on a silicon on insulator (SOI) platform, which can be fabricated by using a complementary metal-oxide semiconductor (CMOS) compatible process without half etching of the SOI layer. Constructing a vertical p-n junction in a flat etchless SOI layer provides superior controllability and uniformity of carrier profiles. Moreover, the waveguide structure based on a thin a-Si:H strip line can be fabricated easily and precisely. Thanks to a large overlap between the depletion region and optical field in the SOI layer with a vertical p-n junction, the MZM provides 0.80- to 1.86-Vcm modulation efficiency and a 12.1- to 16.9-dBV loss-efficiency product, besides guaranteeing a 3-dB bandwidth of about 17 GHz and 28-Gbps high-speed operation. The αVπL is considerably lower than that of conventional high-speed modulators.