Tomonari Sato

Photonic crystal lasers using wavelength-scale embedded active region

Lasers with ultra-low operating energy are desired for use in chip-to-chip and on-chip optical interconnects. If we are to reduce the operating energy, we must reduce the active volume. Therefore, a photonic crystal (PhC) laser with a wavelength-scale cavity has attracted a lot of attention because a PhC provides a large Q-factor with a small volume. To improve this devices performance, we employ an embedded active region structure in which the wavelength-scale active region is buried with an InP PhC slab. This structure enables us to achieve effective confinement of both carriers and photons, and to improve the thermal resistance of the device. Thus, we have obtained a large external differential quantum efficiency of 55% and an output power of ?10?dBm by optical pumping. For electrical pumping, we use a lateral p?i?n structure that employs Zn diffusion and Si ion implantation for p-type and n-type doping, respectively. We have achieved room-temperature continuous-wave operation with a threshold current of 7.8??A and a maximum 3?dB bandwidth of 16.2?GHz. The results of an experimental bit error rate measurement with a 10?Gbit?s?1 NRZ signal reveal the minimum operating energy for transferring a single bit of 5.5?fJ. These results show the potential of this laser to be used for very short reach interconnects. We also describe the optimal design of cavity quality (Q) factor in terms of achieving a large output power with a low operating energy using a calculation based on rate equations. When we assume an internal absorption loss of 20?cm?1, the optimized coupling Q-factor is 2000.

Room-temperature continuous-wave operation of lateral current injection wavelength-scale embedded active-region photonic-crystal laser

We have developed a wavelength-scale embedded active-region photonic-crystal laser using lateral p-i-n structure. Zn diffusion and Si ion implantation are used for p- and n-type doping. Room-temperature continuous-wave lasing behavior is clearly observed from the injection current dependence of the output power, 3dB-bandwidth of the peak, and lasing wavelength. The threshold current is 390 μA and the estimated effective threshold current is 9.4 μA. The output power in output waveguide is 1.82 μW for a 2.0-mA current injection. These results indicate that the embedded active-region structure effectively reduce the thermal resistance. Ultrasmall electrically driven lasers are an important step towards on-chip photonic network applications.

Ultralow-energy and high-contrast all-optical switch involving Fano resonance based on coupled photonic crystal nanocavities

We experimentally and theoretically clarified that a Fano resonant system based on a coupled optical cavity has better performance when used as an all-optical switch than a single cavity in terms of switching energy, contrast, and operation bandwidth. We successfully fabricated a Fano system consisting of doubly coupled photonic-crystal (PhC) nanocavities, and demonstrated all-optical switching for the first time. A steep asymmetric transmission spectrum was clearly observed, thereby enabling a low-energy and high-contrast switching operation. We achieved the switching with a pump energy of a few fJ, a contrast of more than 10 dB, and an 18 ps switching time window. These levels of performance are actually better than those for Lorentzian resonance in a single cavity. We also theoretically investigated the achievable performance in a well-designed Fano system, which suggested a high contrast for the switching of more than 20 dB in a fJ energy regime.

20-Gbit/s directly modulated photonic crystal nanocavity laser with ultra-low power consumption

We have demonstrated an ultracompact buried heterostructure photonic crystal (PhC) laser, consisting of an InGaAsP-based active region (5.0 x 0.3 x 0.15 μm3) buried in an InP layer. By employing a buried heterostructure with an InP layer, we can greatly improve thermal resistance and carrier confinement. We therefore achieved a low threshold input power of 6.8 μW and a maximum output power in the output waveguide of -10.3 dBm by optical pumping. The output light is effectively coupled to the output waveguide with a high external differential quantum efficiency of 53%. We observed a clear eye opening for a 20-Gbit/s NRZ signal modulation with an absorbed input power of 175.2 μW, resulting in an energy cost of 8.76 fJ/bit. This is the smallest reported energy cost for any type of semiconductor laser.

Gate control of spin transport in multilayer graphene

We experimentally studied the gate voltage dependence of spin transport in multilayer graphene (MLG) using the nonlocal spin detection technique. We found that the spin signal is a monotonically decreasing linear function of the resistance of MLG, which is characteristic of the intermediate interfacial transparency between the MLG and the ferromagnetic electrodes (Co). The linear relation indicates a large spin relaxation length significantly exceeding 8μm. This shows the superiority of MLG for the utilization of the graphite-based spintronic devices.

Semiconductor Double-Ring-Resonator-Coupled Tunable Laser for Wavelength Routing

A monolithic widely tunable semiconductor laser based on a double-ring resonator is developed for use in a wavelength-routing switch. By using the double-ring resonator as a wavelength-selective filter, operation over a wide wavelength tuning range is achieved with a low tuning current. This low-tuning-current operation makes the laser very promising as a high-speed tunable light source for a wavelength-routing-based switch by effectively suppressing the thermal wavelength drift induced by current injection. In addition, the laser fabrication process is simpler compared to conventional distributed Bragg reflector tunable lasers. A tuning range of 50.0 nm, covering the entire C-band, is successfully demonstrated with an injection current of less than 5.2 mA. The wavelength drift caused by the thermal transients is less than 5 GHz.

Directly modulated buried heterostructure DFB laser on SiO 2 /Si substrate fabricated by regrowth of InP using bonded active layer

We describe the growth of InP layer using an ultrathin III-V active layer that is directly bonded to SiO₂/Si substrate to fabricate a buried heterostructure (BH) laser. Using a 250-nm-thick bonded active layer, we succeeded in fabricating a BH distributed feedback (DFB) laser on SiO₂/Si substrate. The use of a lateral current injection structure is important for forming a p-i-n junction using bonded thin film. The fabricated DFB laser is directly modulated by a 25.8-Gbit/s NRZ signal at 50°C. These results indicate that our fabrication method is a promising way to fabricate high-efficiency lasers at a low cost.

Surfactant-mediated growth of InGaAs multiple-quantum-well lasers emitting at 2.1μm by metalorganic vapor phase epitaxy

We report on the effect of antimony surfactant on the growth of strained InGaAs multiple-quantum-well (MQW) structure by metalorganic vapor phase epitaxy and the application of the structure to buried-heterostructure (BH) lasers. For a 1.85%-strained MQW, supplying a small amount of antimony during well growth is effective in suppressing the three-dimensional growth and increasing the photoluminescence peak intensity at a wavelength of 2.09 μ m . The secondary ion mass spectroscopy measurement reveals that hardly any antimony is incorporated into the wells. The fabricated BH laser has an emission wavelength of 2.103 μ m under continuous-wave operation at 25 °C.

Tunable Distributed Amplification (TDA-) DFB Laser with Asymmetric Structure

The wavelengths of tunable distributed amplification (TDA) distributed feedback (DFB) lasers consisting of multiple units including an active region and a tuning region can be changed without mode hopping. To expand the tuning range of TDA-DFB lasers, an asymmetric periodic structure is proposed where the units in the cavity have different lengths. It is shown theoretically that the asymmetric periodic structure is effective for expanding the mode-hop-free tuning range. In addition, a tunable laser array was fabricated consisting of six TDA-DFB lasers with an asymmetric periodic structure. A total wavelength tuning range of 44 nm was successfully obtained by expanding the tuning range of each laser, which corresponded to 110 channels with a 50-GHz grid. Moreover, the channel switching speed was investigated in relation to thermal drift. A switching time of less than 40 μs with a frequency deviation of less than 1 GHz was achieved with a thermal drift suppression technique that used the tuning regions of non-lasing lasers as heaters for temperature compensation.

Heterogeneously integrated photonic-crystal lasers on silicon for on/off chip optical interconnects

We demonstrate the continuous-wave operation of lambda-scale embedded active-region photonic-crystal (LEAP) lasers at room temperature, which we fabricated on a Si wafer. The on-Si LEAP lasers exhibit a threshold current of 31 μA, which is the lowest reported value for any type of semiconductor laser on Si. This reveals the great potential of LEAP lasers as light sources for on- or off-chip optical interconnects with ultra-low power consumption in future information communication technology devices including CMOS processors.