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The bonding strength dependence on stress by the bowing of chips was investigated for the InP/Si chip-on-wafer plasma activated bonding. The high bonding strength of 20 MPa was achieved with tensile stress occurred by the strain-controlled interlayer between InP chips and Si substrates.

Investigation of thin film characteristics of InP/Si hybrid wafers by novel chip-on-wafer direct transfer bonding technology (CoW-DTB) was carried out. 36 InP chips were successfully bonded on Si wafers by CoW-DTB and photoluminescence properties were measured and compared by $1.3-\boldsymbol{\mu} \mathbf{m}$ AlGaInAs quantum wells samples with different bonding interface after removing InP substrate.

In III-V/SOI hybrid photonic devices, the mode size between III-V/SOI hybrid section and Si waveguide are very different. To obtain high coupling efficiency between them, we investigated taper length dependence of a double taper-type coupler which enables efficient mode conversion even with a short taper length.

As a light source for on-chip optical interconnections, we have been investigating membrane distributed-refiector (DR) lasers with low power consumption operation. This time we would like to present further improvements by adopting a buried-ridge-waveguide (BRW) structure so as to reduce its series resistance, hence energy consumption of the membrane DR laser. We investigated mode distributions of (a) flat type (conventional) buried heterostructure and (b) BRW structure, and calculated the distance between the p-side electrode and the active region, and the differential resistance of them. As the result of theoretical calculation, the differential resistance can be reduced to $180\ \Omega$ for the DFB section length of $30\ \mu\mathrm{m}$ , ridge height $\mathrm{d}=50\ \text{nm}$ and tilted mesa width $\text{Wd} , and this value is around 40% smaller than that of flat type BH structure. To realize such BRW structure, we controlled the growth period of InP by inserting GaInAs marker layers by OMVPE. As a result, the BRW structure with the ridge height $\mathrm{d}=50$ nm and the tilted mesa width $\text{Wd} =0.2\mu \mathrm{m}$ was successfully obtained with the growth time of 4.6 min.

An optical-vortex multiplexer/demultiplexer using vertically curved Si waveguides was developed. Multiplexing/demultiplexing with the lowest crosstalk of 23 dB was demonstrated for five multiple optical vortices.

Low-power consumption directly-modulated lasers are a key device for on-chip optical interconnection. We fabricated a GaInAsP/InP membrane DFB laser that exhibited a low-threshold current of 0.21 mA and single-mode operation with a sub-mode suppression ratio of 47 dB at a bias current of 2 mA. A high modulation efficiency of 11 GHz/mAsup1/2/supwas obtained. A 10 Gbit/s direct modulation using a non-return-to-zero 2sup31/sup-1 pseudo-random bit sequence signal was performed with a bias current of 1 mA, which is the lowest bias current ever reported for direct modulation of a DFB laser. A bit-error rate of 10sup-9/supwas successfully achieved.

In this work, electron beam lithography proximity effect correction (PEC) was experimentally studied for patterning of Si photonic waveguides with a relatively thick resist mask. Beam’s energy density distribution (EDD) was experimentally extracted by the line exposure method; however, exposure lines in this work were developed after cleavage with a high-contrast process to reduce developer-related effects. The measured line spread function was fitted to a 4-Gaussian function to model mid-range energy densities accurately. The extracted EDD showed less proximity effects compared to conventional Monte-Carlo simulation performed by a commercial software. PEC processes with both techniques were experimentally compared for a Si photonic waveguide pattern with different side-cladding trench widths. Microscopic images confirmed that the presented calibration method could achieve better development conditions near the required clearance dosage. Single-mode propagation loss for a 500 × 220 nm Si wire waveguide was reduced from 3.2 to 2.4 dB cm−1 using the presented process.

Replacing part of a conventional optical circuit with a topological photonic system allows for various controls of optical vortices in the optical circuit. As an underlying technology for this, in this study, we have realized a topological converter that provides high coupling efficiency between a normal silicon wire waveguide and a topological edge waveguide. After expanding the waveguide width while maintaining single-mode transmission from the Si wire waveguide, the waveguides are gradually narrowed from both sides by using a structure in which nanoholes with C6 symmetry are arranged in a honeycomb lattice. On the basis of the analysis using the three-dimensional finite-difference time-domain method, we actually fabricated a device in which a Si wire waveguide and a topological edge waveguide were connected via the proposed topological converter and evaluated its transmission characteristics. The resulting coupling efficiency between the Si wire waveguide and the topological edge waveguide through the converter was –4.49 dB/taper, and the coupling efficiency was improved by 5.12 dB/taper compared to the case where the Si wire waveguide and the topological edge waveguide were connected directly.

A metamaterial is an artificial material designed to control the electric permittivity and magnetic permeability freely beyond naturally existing values. A promising application is a slow-light device realized using a combination of optical waveguides and metamaterials. This paper proposes a method to dynamically control the slow-light effect in a metamaterial-loaded Si waveguide. In this method, the slow-light effect (i.e., group index) is controlled by changing the phase of the control light incident on the device from a direction opposite to that of the signal light. The group index of the device could be continuously controlled from 63.6 to 4.2 at a wavelength of 1.55 µm.

Heterogeneous wafer bonding of InP/Si at room temperature is studied using surface-activated bonding (SAB) technology. To minimize the degradation of optical property while maintaining enough bonding strength, various bonding conditions including gas species of fast atom beam (FAB) were examined. The results show that a bonding strength of over 0.5 MPa can be obtained with less degradation of photoluminescence (PL) property of InP/Si hybrid wafer by combining Xe and Ar gases for FAB. Using this condition, hybrid wafer including quantum wells were fabricated, which showed sufficient PL property for the fabrication of hybrid photonic devices. The bonding conditions described in this study enabled realization of continuous wave (CW) operation of InP-based layers/SOI hybrid laser.

A transistor laser (TL) is a device that operates at a high-speed with multiple functions such as output control with low wavelength shift and signal mixing. By adopting a high heat dissipation structure with a high-speed compatible wide electrode pad and thick Au plating in TLs, improvement of temperature performance in 1.3 μm wavelength npn AlGaInAs/InP TL was demonstrated. As a result, continuous-wave operation of a 1.3 μm TL up to 90 °C was achieved. The thermal resistance was estimated to be 25 K W−1, based on the spectrum behavior, which is at least four times lower than the previously observed value.

Transistor Laser (TL) has multiple modulation methods by a combination of terminal connection because it has three electrical terminals unlike the laser diodes. In this report, for the voltage modulation of the TL, the light output response under a large signal voltage modulation was simulated. By considering the transient change of the current in the active layer, we succeeded in reproducing the light output having the peak in the opposite direction to the steady state observed with the actual device so far. Based on the simulation, the optimum collector-base capacitance in the actual device and the minimum required end-facet reflectance for securing a high extinction ratio were obtained.

Membrane distributed reflector (DR) laser exhibited a low threshold current (0.21 mA) and high modulation current efficiency factor of 12 GHz/mA1/2. A 20 Gbit/s direct modulation was obtained with the bias current of 1 mA, which corresponded to the energy cost of 93 fJ/bit.

In order to improve lasing characteristics of 1.3-μm wavelength transistor lasers, device structure was changed by focusing on optical confinement and heat dissipation. As the results, the highest continuous-wave operation temperature up to 90 °C was obtained for the first time.

We demonstrate high-speed direct-modulation of InAs/InGaAs quantum dot lasers grown on on-axis (001) silicon. The devices exhibited a 12.5 Gbit/s NRZ operation and a 25 Gbit/s PAM-4 operation confirmed from eye diagrams and BER measurements.

An energy cost of ridge-waveguide type membrane Distributed-Reflector laser was investigated for 10 Gbit/s and 20 Gbit/s operations. Thanks to its strong optical confinement and reduced resistance, the energy cost could be reduced to approximately 1/2 of that for conventional flat-top-waveguide type membrane DR laser.

We investigated high-power pulse semiconductor lasers based on epitaxially stacked double active layer through tunnel junction composed of MOCVD grown highly carbon-doped GaInAs on InP substrate. The peak pulse optical power was 50 W at a current of 65 A.

To realize next-generation photonic integrated circuits based on the III–V/Si hybrid integration platform using chip-on-wafer (CoW) direct bonding technologies, high-yield collective bonding of InP chips on Si substrates with a high bonding strength is required. This study demonstrates high-yield InP/Si CoW plasma-activated bonding using a chip holder with pockets, the depth of which is precisely controlled. Additionally, finite element simulations are used to determine that the stress-controlled interlayer consisting of InP-based epitaxial layers with tensile strain effectively suppresses stress at the InP/Si bonding interface, which affects the bonding strength. Thus, a high bonding strength of 20 MPa in 2 mm × 2 mm InP chips on the Si substrate was achieved by introducing a superlattice structure consisting of GaInAsP and InP (with tensile strain) as the stress-controlled interlayer.

Direct modulation lasers are attractive for short-to-mid range fiber communication system. Transistor lasers (TLs), which are hetero-bipolar transistor with an active layer in the base, have a potential to overcome modulation bandwidth limitation of conventional laser diodes (LDs). One of the reasons of the modulation bandwidth limitation of the conventional LDs is delay of carrier diffusion to quantum wells (QWs). With proper design of TLs with some current gain, this delay can be reduced and TLs can have wider modulation bandwidth under the emitter current modulation configuration. In the first part of the presentation, these theoretical analyses will be discussed. By theoretical analysis of TLs with rate equations taking account of carrier diffusion lifetime, 50 GHz modulation bandwidth can be expected with a current gain of ~1.7. Also, by applying voltage bias between the base and collector under the common-base configuration, which causes Frantz-Keldysh effect at the interface layer (GaInAsP in our case), internal loss can be controlled if the bandgap of layers at the interface is appropriately wider than the energy corresponding to the lasing wavelength. This is a kind of “built-in” electro absorption modulator. Therefore, voltage modulation, which corresponds to “loss” modulation, can be configured and has wider modulation bandwidth. The reason why loss modulation has wider modulation bandwidth is that the slope of decay curve of small signal modulation over a relaxation oscillation frequency is slow compared with that of conventional current modulation. In the second part, buried-hetero (BH) regrowth techniques of AlGaInAs QWs will be discussed. AlGaInAs TLs with lasing wavelength of 1.3 µm is very important for fiber communication system. By adopting proper annealing conditions in an organo-metallic vapor-phase-epitaxy (OMVPE) reactor, non-radiative recombination at the AlGaInAs/InP regrowth interface can be significantly reduced even after exposing in the air. Annealing gas, temperature, and the time are important parameters to find optimum annealing conditions. We found using PH3, instead of AsH3, as the annealing gas is effective to remove oxidation at the interface under 650°C, 45-min. annealing. Using this annealing condition, 1.3-µm AlGaInAs BH-LD was realized with a low threshold current density of 140A/cm2/well. Finally, the fabrication process and characteristics of 1.3-µm AlGaInAs TLs will be discussed. Several regrowth steps by MOVPE were required including above mentioned BH regrowth. CW operation up to the temperature of 40°C was demonstrated with both 2 and 3-terminal configurations. For 2-terminal configuration (this means the collector was floating), the threshold current of 33 mA was achieved with the cavity length of 1000 µm and stripe width of 1.7 µm. For 3-terminal configuration, the device shows optical characteristics (I-L) and transistor (collector current) characteristics, simultaneously, under common-emitter and common-base configuration. For the common-emitter configuration, threshold “base” current was reduced by applying voltage between the emitter and collector, On the other side, for the common-base configuration, we observed threshold “emitter” current increase by applying voltage between the base and collector due to Frantz-Keldysh effect. From transistor characteristics, we found Early effect also had an important role for TL characteristics.

We propose and demonstrate a method of making an infrared (∼60 THz) invisibility cloaking device by simply rolling a metamaterial film around an object that we want to hide.

We report the first demonstration of direct modulation of InAs/GaAs quantum dot (QD) lasers grown on on-axis (001) Si substrate. A low threading dislocation density GaAs buffer layer enables us to grow a high quality 5-layered QD active region on on-axis Si substrate. The active layer has p-modulation doped GaAs barrier layers with a hole concentration of 5 × 1017 cm−3to suppress gain saturation. Small-signal measurement on a 3 × 580 μm2 Fabry-Perot laser showed a 3dB bandwidth of 6.5 GHz at a bias current of 116 mA. A 12.5 Gbit/s non-return-to-zero signal modulation was achieved by directly probing the chip. Open eyes with an extinction ration of 3.3dB was observed at room temperature. The bit-error-rate (BER) curve showed no error-floor up to BER of 1 × 10−13. 12 km single-mode fiber transmission experiments using the QD laser on Si showed a low power penalty of 1 dB at 5 Gbit/s. These results demonstrate the potential for QD lasers epitaxially grown on Si to be used as a low-cost light source for optical communication systems.

We propose a novel slow-light Si-wire waveguide using metamaterials, which can be easily integrated with other Si photonics devices, and the group index of more than 40 was obtained in both theoretical and experimental investigations.

A novel self-holding switch is demonstrated by a magneto-optical waveguide switch. The switching state is flipped by a pulsed current and maintained without any power supply by virtue of the non-volatility of thin-film magnet.

A GalnAs/InP membrane p-i-n photodiode (PD) integrated with a back end DBR was theoretically investigated under a condition of the back reflection of −30dB. As the result, it was found that the 3dB bandwidth of 17 GHz can be obtained with the absorption section length of 12 μm. These are approximately 3 times faster and 1/3 times smaller than those of a similar PD without the back end DBR.

We demonstrate a monolithic integration of lateral-current-injection (LCI)-type membrane-based distributed-feedback (DFB) lasers and p-i-n-photodiodes (PDs) using a butt-jointed built-in (BJB) structure bonded on a Si substrate using benzocyclobutene. The BJB structure to be integrated to the membrane optical devices was prepared by organometallic vapor-phase epitaxy. A threshold current of 280 μA was obtained under a room-temperature continuous-wave condition by adopting a strongly index-coupled DFB laser with a surface grating structure and a λ/4-shift region. A low dark current of 0.8 nA was obtained with the p-i-n-PD at a bias voltage of –1 V, and its photocurrent property coincided with the light output property of the membrane DFB laser.

The direct modulation characteristics of a membrane distributed feedback (DFB) laser on a silicon substrate were investigated. Enhancement of the optical confinement factor in the membrane structure facilitates the fabrication of a strongly index-coupled (κ(I) = 1500 cm(-1)) DFB laser with the cavity length of 80 µm and a threshold current of 270 µA. Small-signal modulation measurements yielded a -3dB bandwidth of 9.5 GHz at 1.03-mA bias current, with modulation efficiency of 9.9 GHz/mA(1/2), which is, to the best of our knowledge, the highest value among those reported for DFB lasers.

Toward the realization of ultralow-power-consumption on-chip optical interconnection, two types of membrane-based GaInAs/InP p-i-n photodiodes were fabricated on Si host substrates by using benzocyclobutene bonding. A responsivity of 0.95 A/W was estimated with a conventional waveguide-type photodiode with an ∼30-μm-long absorption region. The fitting curves based on the experimental data indicated that an absorption efficiency above 90% could be achieved with a length of 10 μm. In addition, increased absorption per length of a photonic crystal waveguide-type photodiode was obtained because of the enhanced lateral optical confinement or the slow-light effect, enabling a further reduction in the device length.

Researches and developments of Si-photonics are hot topics in photonics field. This is because the technology has potentials to realize low-cost and compact products for various applications such as optical interconnection, sensing, LiDAR (Light Detection and Ranging)and so on. For passive components, most of components such as waveguides, splitters, combiners, filters, are being established. On the other hand, active components, especially semiconductor lasers, are still under developments since Si has indirect bandgap property and cannot achieve efficient emission. To overcome this problem, hybrid lasers, composed of III-V layer on top of Si or SOI substrates, were proposed and demonstrated. In this talk, I will review the progress of two types of hybrid lasers in our group, one is membrane lasers and the other is direct bonded hybrid lasers.

We demonstrate a vertical p-n junction silicon Mach-Zehnder modulator constructed with hydrogenated amorphous silicon strip-loaded waveguides on a flat SOI platform. A 3-mm-long phase shifter shows 0.80- to 1.86-Vcm modulation efficiency, 7.3- to 16.9-dBV loss-efficiency product, 3-dB bandwidth of 17 GHz, and 25-Gb/s operation.

High efficiency operation of membrane distributed-reflector laser was demonstrated by both an improvement in differential quantum efficiency and a reduction of differential resistance. A threshold current of 0.44 mA, an external differential quantum efficiency for the front-side output of 36%, and a maximum power conversion efficiency of 14.6% were obtained for a device with a DFB section length of 40 μm.

The double taper-type mode convertor for direct bonded GaInAsP/SOI hybrid photonic devices was proposed and the characteristics of the fabricated structure using plasma activated bonding were measured. The calculated and measured results showed the coupling efficiencies of 90% and 75%, respectively.

Bonding conditions and characteristics of N2-plasma activated bonding for GaInAsP/SOI hybrid lasers are explained. SEM and TEM images reveal high quality bonding interface and less damage quantum wells, which are enough to realize hybrid lasers.

Low temperature wafer scale direct bonding technology using plasma activated bonding (PAB) for heterogeneous photonic device integration is reviewed. Nitrogen plasma irradiation in a high vacuum chamber allows tight strength bonding between InP-based and SOI wafers with the bonding temperature of 150°C as well as low damage to GaInAsP quantum wells (QWs). In contrast Argon irradiation cause poor bonding strength for InP. Using this technology, lasing operations of 1.55-μm GalnAsP/SOI hybrid lasers were demonstrated.

A waveguide optical isolator operating in TE mode was demonstrated with an isolation of 17.9 dB. Amorphous silicon core along lateral walls of magneto-optical garnet was fabricated to induce nonreciprocal phase shift in TE mode.

We propose organic membrane photonic integrated circuits (OMPICs), which can collectively integrate various optical elements into a thin film and can realize wearable devices and high-speed optical signal processing by making more flexible photonic integrated circuits. In OMPICs, vertical grating couplers are effective as input and output ports. In this study, we propose three types of structure: an SU-8/Cytop grating coupler, a metal grating coupler, and a modified metal grating coupler with a metal mirror and an SU-8/Cytop distributed Bragg reflector. We analyze the optical coupling efficiency in each case.

In this work, GaInAs/InP membrane p-i-n photodiodes integrated with a back-end distributed Bragg reflector were numerically investigated for a backreflection of −30 dB. The results showed that a 3-dB bandwidth of 17 GHz and reverse bias of 1 V can be obtained for an absorption section length of 12 μm. The 3-dB bandwidth is approximately twice faster and the absorption length is 1/3 times smaller compared to those of a similar photodiode without the back-end DBR.

The papers in tihs special section were presented at the IEEE International Semiconductor Laser Conference, which was last held in Palma de Mallorca, Spain, in September 2014 and covered a variety of recent research advances in semiconductor lasers.

Toward the light source for on-chip interconnection, a current-injection-type membrane distributed feedback laser with a surface gating structure is demonstrated. In this device, 450-nm-thick GaInAsP/InP layers with lateral-current-injection structure prepared by a two step OMVPE regrowth-method is bonded on a host substrate by using Benzocyclobutene bonding process. A threshold current of Ith=11 mA is obtained with a cavity length of 300 μm and a stripe of 1 μm.

10 Gbit/s data transmission through an optical link formed on a Si substrate was carried out by using GaInAsP/InP membrane-based DFB laser and PIN photodiode. The 3dB bandwidth of 11.3 GHz and the BER of 6×10−7 at 10 Gbit/s operation were obtained.

A vertical trident coupler for 3D optical circuit was demonstrated. This is novel interlayer directional-type coupler between crystalline Si and amorphous-Si:H wire waveguides. The trident structure can manage both easiness of fabrication and high coupling even with relatively thick interlayer distances. In experiment, this trident coupler realized low coupling loss (coupling efficiency) of 0.58dB (87%) at 1550 nm wavelength and almost constant coupling loss of around 0.5dB within the wavelength range from 1530 nm to 1610 nm.