HfS₂is the novel transition metal dichalcogenide, which has not been experimentally investigated as the material for electron devices. As per the theoretical calculations, HfS₂has the potential for well-balanced mobility (1,800 cm²/V·s) and bandgap (1.2 eV) and hence it can be a good candidate for realizing low-power devices. In this paper, the fundamental properties of few-layer HfS₂flakes were experimentally evaluated. Micromechanical exfoliation using scotch tape extracted atomically thin HfS₂flakes with varying colour contrasts associated with the number of layers and resonant Raman peaks. We demonstrated the I-V characteristics of the back-gated few-layer (3.8 nm) HfS₂transistor with the robust current saturation. The on/off ratio was more than 10⁴and the maximum drain current of 0.2 μA/μm was observed. Moreover, using the electric double-layer gate structure with LiClO₄:PEO electrolyte, the drain current of the HfS₂transistor significantly increased to 0.75 mA/μm and the mobility was estimated to be 45 cm²/V·s at least. This improved current seemed to indicate superior intrinsic properties of HfS₂. These results provides the basic information for the experimental researches of electron devices based on HfS₂.

We fabricated TM mode $\mathrm{In}\mathrm{Ga}\mathrm{Al}\mathrm{As}∕\mathrm{In}\mathrm{P}$ active waveguide optical isolators based on the magnetically induced nonreciprocal loss. We used epitaxially grown MnAs thin films as ferromagnetic electrodes of the semiconductor active waveguide optical isolators. We demonstrated TM mode nonreciprocal propagation $(8.8\mathrm{dB}∕\mathrm{mm})$ at $1540\mathrm{nm}$ with an excellent ferromagnetic electrode contact, which has greater semiconductor active waveguide optical isolator performance than that of our previously reported devices with $\mathrm{Ni}∕\mathrm{Fe}$ polycrystalline electrodes.

We developed a 1.5-μm band TM-mode waveguide optical isolator that makes use of the nonreciprocal-loss phenomenon. The device was designed to operate in a single mode and consists of an InGaAlAs∕InP ridge-waveguide optical amplifier covered with a ferromagnetic MnAs layer. The combination of the optical waveguide and the magnetized ferromagnetic metal layer produces a magneto-optic effect called the nonreciprocal-loss phenomenon—a phenomenon in which the propagation loss of light is larger in backward propagation than it is in forward propagation. We propose the guiding design principle for the structure of the device and determine the optimized structure with the aid of electromagnetic simulation using the finite-difference method. On the basis of the results, we fabricated a prototype device and evaluated its operation. The device showed an isolation ratio of $7.2\text{dB}/\text{mm}$ at a wavelength from 1.53 to $1.55\mu \text{m}$. Our waveguide isolator can be monolithically integrated with other waveguide-based optical devices on an InP substrate.

The in-plane and interlayer waveguide-type couplers between crystalline Si and amorphous-Si:H wire waveguides, for 2D/3D hybrid-material integration are presented in this paper. The in-plane-type coupler achieves stable coupling between two waveguides by using tapers located at the tips of the waveguides. The interlayer-type coupler can connect two waveguides, despite an interlayer distance of 1 μm, with a simple process flow, by introducing a trident structure. An experiment was conducted in which the in-plane and interlayer-type couplers realized low coupling losses (coupling efficiencies) of 0.16 dB (96%) and 0.49 dB (89%) per coupler, respectively.

We show the feasibility of controlling the magnetic permeability of optical semiconductor devices on InP-based photonic integration platforms. We have achieved the permeability control of GaInAsP/InP semiconductor waveguides by combining the waveguide with a metamaterial consisting of gate-controlled split ring resonators. The split-ring resonators interact magnetically with light travelling in the waveguide and move the effective relative permeability of the waveguide away from 1 at optical frequencies. The variation in permeability can be controlled with the gate voltage. Using this variable-permeability waveguide, we have built an optical modulator consisting of a GaInAsP/InP Mach–Zehnder interferometer for use at an optical communication wavelength of 1.55 μm. The device changes the permeability of its waveguide arm with controlling gate voltage, thereby varying the refractive index of the arm to modulate the intensity of light. For the study of variable-permeability waveguide devices, we also propose a method of extracting separately the permittivity and permeability values of devices from the experimental data of light transmission. Adjusting the permeability of optical semiconductors to the needs of device designers will open the promising field of ‘permeability engineering’. Permeability engineering will facilitate the manipulation of light and the management of photons, thereby contributing to the development of novel devices with sophisticated functions for photonic integration.

We fabricated GaInAsP/InP waveguide-integrated lateral-current-injection (LCI) membrane distributed feedback (DFB) lasers on a Si substrate by using benzocyclobutene (BCB) adhesive bonding for on-chip optical interconnection. The integration ofa butt-jointed built-in (BJB) GaInAsP passive waveguide was performed by organometallic vapor-phase epitaxy (OMVPE).By introducing a strongly index-coupled DFB structure with a 50-µm-long cavity, a threshold current of 230 µA was achieved for a stripe width of 0.8 µm under room-temperature continuous-wave (RT-CW) conditions. The maximum output power of 32 µW was obtained. The lasing wavelength and submode suppression ratio (SMSR) were 1534 nm and 28 dB, respectively, at a bias current of 1.2 mA.