Silicon photonic wire waveguides, featuring very strong optical confinement and compatibility with silicon electronics,
provide a compact photonic platform on which passive, dynamic, and active photonic devices can be integrated. We have
already developed a low-loss waveguide platform and integrated various photonic devices. For passive devices, we have
developed polarization-independent wavelength filters using a monolithically integrated polarization diversity circuit, in
which waveguide-based polarization manipulation devices are implemented. The polarization-dependent loss of a ring
resonator wavelength filter with polarization diversity is less than 1 dB. For dynamic devices, we have developed
compact carrier-injection-type variable optical attenuators (VOAs). The length of the device is less than one millimeter,
and the response time is nanosecond order. The device has already been made polarization independent. We have
recently monolithically integrated these fast VOAs with low-dark-current germanium photodiodes and achieved
synchronized operation of these devices. For nonlinear devices, a free-carrier extraction structure using a PIN junction
implemented in the waveguide can increase the efficiency of nonlinear functions. For example, in a wavelength
conversion based on the Four-wave-mixing effect, the conversion efficiency can be increased by 6 dB.
We devised a silicon photonic circuit with polarization diversity. The circuit consists of polarization splitters and
rotators. The splitter is based on simple 10-micrometer-long directional couplers. The polarization extinction ratio is 23
dB and excess loss is less than 0.5 dB. The rotator consists of a silicon waveguide embedded in an off-axis siliconoxynitride
waveguide. A 35-micrometer-long rotator gives a rotation angle of more than 72 degrees and excess loss of
about 1 dB. Both devices can be made by using planar fabrication technology and do not require a complex structure
such as three-dimensional forming. Using these devices, we developed a polarization diversity circuit for a ringresonator
wavelength filter. The polarization dependent loss of the filter with polarization diversity is about 1 dB. A 10-
Gbps data transmission with scrambled polarization is demonstrated.
We observed a propagation of surface plasmon polaritons (SPPs) along bent Au-wires on a thin SiO2-coat-InP substrate with the bending radius R from 20 to 1000 μm and evaluated the bending-part attenuation coefficients, i.e., the bending losses of propagating SPPs along them with R from 20 to 400 μm by measuring the transmittance of SPPs. We discussed about the SPP propagation and the applicability to an optical electronic device and circuit of SPPs.
We demonstrate efficient nonlinear functions using silicon nanophotonic structures. In the ultrasmall core of the
waveguides and cavities, nonlinear phenomena are significantly enhanced. Applying the two-photon absorption effect,
we have confirmed all optical modulation, in which the modulation speed is improved to around 50 ps by eliminating
free carriers. Applying the four-wave-mixing effect, we have achieved high-efficiency wavelength conversion. The
conversion efficiency is -11 dB, and the efficiency will be further improved by eliminating free carriers. Using the four-wave-
mixing effect, we have also realized a low-noise entangled photon pair source. The source does not need a
refrigeration system for noise reduction, which is a great advantage for practical application.
This paper presents our recent progress in the development of a Si wire waveguiding system for microphotonics devices. We have developed function devices that integrate several fundamental components and confirmed that they exhibited excellent characteristics due to the accuracy of the Si microfabrication. The propagation loss of the waveguide is less than 1.2dB/cm, and branching devices and basic filters show good characteristics. Using the fundamental microfabrication technique, we have developed other passive and dynamic functional devices. As an example of our recent advances using passive devices, we present a polarization diversity system consisting of a separator and a rotator. As a component of a dynamic functional device, we show a low-loss rib-type silicon wire waveguide with low-impedance p-i-n structure and its optical attenuation characteristics.
We determined the annealing dynamics of AsGa antisite defects in As ion-implanted GaAs based on a model where AsGa antisite defects trap photo-excited carriers. An Arrhenius plot of the carrier decay rate vs. annealing temperature in the high temperature regime gave an energy EPA, which was different from true activation energy. The annealing time dependence of EPA obtained by the two diffusion models (self diffusion and VGa vacancy assisted diffusion of defects) were compared with EPA's obtained form already published works, which proved that the density of VGa vacancy was high enough to assist the diffusion of AsGa antisite defects and that the annealing dynamics of AsGa antisite defects was VGa vacancy assisted diffusion.
We measured the size and temperature dependence of the optical nonlinear cross section (σeff), the carrier recombination time (τ), and the dephasing time T2 using CdS0.12Se0.8 microcrystallites embedded in alkaline multicomponent glasses and CdSe microcrystallites embedded in SiO2 thin film using the folded-boxcar configuration of degenerate-four-wave-mixing. As the average radius of a CdS0.12Se0.8 microcrystallite decreased from 10 to 1 nm, the values of σeff and the carrier recombination time changed from 2.6x10-16 to 1.1x10-16 cm2 and from 70 to 2 psec, respectively. The smaller a microcrystallite was, the faster the carrier recombination time became. The size dependence of the carrier recombination time showed that an energy level structure of microcrystallites with a radius of less than a few nanometers is a two-level system, for which it was theoretically known that σeff was proportional to T2. The size and temperature dependence of T2 for the CdS0.12Se0.8 microcrystallites revealed the presence of the acoustic-phononassisted relaxation processes different from the pure-dephasing processes in the dephasing processes. The dependency indicated that longer T2 might enhance σeff. We investigated the enhancement for a nonlinear cross section in CdSe microcrystallites embedded in SiO2 thin film by becoming longer T2. The carrier recombination time, σeff, and T2 of the 3-nm-average-radius CdSe microcrystallites embedded in a SiO2 thin film were 40 psec, 4.5x10-15 cm2, and 150 fsec. The σeff was ten times as large as σeff for the 3-nm-average-radius CdS0.12Se0.88 microcrystallites embedded in alkaline multi-component glasses. The T2 was ten times as long as the extrapolated value for the same size CdS0.12Se0.88 microcrystallites doped in the glasses. We conclude that the longer T2, originated from the changing of the surface and interface, enables the enhancement.
Silicon-based electronics has matured and holds a dominant position in critical technologies for computing systems. Advances in micro-miniaturization techniques enable us to fabricate nanometric devices with novel functions based on mesoscopic physics, and we expect that such devices will innovate on existing systems. Optics has also made tremendous progress since the first laser to generate quasi-coherent light was developed. Lasers are now widely used in basic science and in practical applications such as information processing and communications systems. Many studies have demonstrated novel functions in logics based on not only non-linear effects of media but also quantum-optic effects in nanometer-scaled structures. However, how to overcome the diffraction limit remains an unsolved fundamental problem how to break down the diffraction limit. Here, we report some ideas for nanophotonics and present a future picture of computing systems.
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