A new type of red light-emitting diode (LED) has been developed using Eu-doped GaN (GaN:Eu) as an
active layer. The LED can emit characteristic emission due to the intra-4f shell transitions in Eu3+ ions
doped in GaN at room temperature. By optimizing organometallic vapor phase epitaxial growth
conditions of the GaN:Eu and the device structure, the output light power has increased signifi cantly up
to sub-milliwatts. For the more improved output light power, energy-transfer mechanism from the GaN
host to Eu ions and effects of impurity codoping are also discussed in GaN:Eu.
According to the promotion of recent ubiquitous society, it is predicted that the transaction and
transmission of information in supercomputers and data centers will rapidly increase. However, the
problem of future capability in transaction and transmission of information is emerging by the extension
of conventional LSIs’ progress. In this paper, we proposed Photonics-Electronics Convergence System
for solving the future bottleneck of wiring among LSI chips and introduced the trend of high-density
silicon photonics integrated technologies for the Photonics-Electronics Convergence System.
Si photonics technologies enable dense integration of such photonic devices as passive devices, p/n
modulators, and Ge photo-diodes. The CMOS-compatible foundry services of Si photonics achieve
wafer-scale production of photonic integrated circuits with sophisticated functions as well as low cost by
a multi-project-wafer process. Recently, they also include the fabrication of complicated photonic
crystals and related nanostructures in their recipes. Thus, the research and development of photonic
devices and integrated circuits are changing rapidly and becoming more design- and system-oriented.
This paper reports such new trends and some example demonstrations.
High-speed silicon optical modulators are reviewed with respect to applications of optical signal
generation in two major modulation formats in optical-fiber telecommunications: on-off keying as a
common modulation format in commercialized 10-Gbit/s optical-fi ber networks and quadrature phaseshift
keying as an advanced modulation format in cutting-edge 64-Gbit/s and 128-Gbit/s coherent
optical-fiber networks. The silicon optical modulators in this review consist of Mach-Zehnder
interferometer waveguides having lateral PN-junction rib-waveguide phase shifters and are suitable to
highly integrated monolithic optical modulators with small footprint and low optical insertion loss. We
present the performance of a 10-Gbit/s on-off keying silicon optical modulator in a fiber-pig-tailed
package with a bit error rate as low as that for a commercialized lithium-niobate optical modulator. We
describe a nested silicon Mach-Zehnder optical modulator for 64-Gbit/s quadrature phase-shift keying
and characterize it with a constellation diagram and bit-error-rate characteristics. The progress and the
prospects for 128-Gbit/s dual-polarization quadrature phase-shift keying are presented.
Germanium (Ge) on silicon (Si) is an enabler of electronic and photonic convergence on the Si CMOS
platforms. Strained Ge has opened up a brand new fi eld of silicon photonic devices such as the longer
wavelength detection of photodetector and Ge lasers, i.e., indirect bandgap semiconductors. The
selective epitaxy of Ge has been studied because of its capability for limited area growth, low damage
during post-processing, and fewer threading dislocations in the Ge epilayers. The present paper
describes our recent discovery of the strain tunability of Ge by the selective growth mask.
Heterogeneous integration technologies that use wafer bonding are explained. The key points of several
bonding methods are revealed, including direct bonding and resin bonding. Direct bonding enables tight
strength at the interface without any glue, although maintaining a very fl at surface is crucial to achieve
good bonding. Surface Activated Bonding, which is direct bonding method, achieved a low threshold
current density of hybrid lasers. Resin bonding needs proper procure conditions before bonding to
achieve a good interface without any voids. This bonding method creates a laser structure with a high
index contrast to achieve an ultra-low threshold current of hybrid lasers.
Photonic integration by hybrid silicon/silica waveguide systems is attractive for realizing various optical
devices and optical integrated circuits. Silicon waveguides and silica-based waveguides have
complementary characteristics, although those waveguides can be formed on identical SOI wafers by a
CMOS compatible process. Therefore, photonic integration by a hybrid silicon/silica waveguide system
was proposed. We introduce the concept and a photonic integration method with these waveguide
systems. We also investigated both the low-loss waveguide junction and the hetero waveguide crossing
between Si-wire waveguides and silica-based waveguides. Finally, we introduce optical devices and
circuits with the hybrid silicon/silica waveguide system.
Continuous-wave Raman lasing in the silicon rib waveguide was presented in 2005 as the long-awaited
silicon laser. However, the required miniaturization to micrometer dimensions and reduction of the
threshold to microwatt energies had not advanced sufficiently since the initial discovery. Such lasers
have remained limited to cm-sized cavities with thresholds higher than 20 mW, even with the assistance
of reverse-biased p-i-n diodes. In this paper, we have report a continuous-wave Raman silicon laser
using a photonic-crystal high-quality (Q) factor nanocavity without any p-i-n diode, which yield a device
with a cavity size of less than 10 micrometers and an ultralow threshold of 1 μW. We contrived a unique
design of the high-Q nanocavity to bring out the tremendous potential derived from the simple principle
that light-matter interactions are proportional to the ratio of Q and the volume of the cavity. Our
demonstration represents a milestone in solid-state optics and may pave the way to the construction of
practical silicon lasers and amplifiers for large-scale-integration in photonic circuits.
Enhancing radiative transitions including spontaneous emission and scattering of silicon (Si) has been of
interest for realizing light emitting and quantum optical devices. Here, we demonstrate enhancement of
Si spontaneous emission using ultrasmall and high-Q Si photonic crystal (PhC) cavities via the Purcell
effect because the emission of bulk Si is quite weak due to its indirect energy band structure. The
photoluminescence of electron-hole droplets in Si PhC cavities is resonantly increased by a factor of
140. In addition, we observe enhancement of spontaneous Raman scattering of carbon nanotubes by Si
PhC cavities. These approaches for cavity-enhanced radiative transitions show another useful way for
realizing Si-based optical devices.
We numerically demonstrated all-optical logic operation based on microring resonators. The designed
systems have the same input and output wavelength, which make these logic gates much easier to
implement for a practical use. Coupled mode theory is used to study the operation of this circuit.
Polarization control techniques in silicon photonic wire waveguide systems have been studied
experimentally. A simple directional coupler with an oblong silicon core works as a polarization splitter,
which can separate two orthogonal modes with a polarization extinction ratio (PER) of over 10dB for the
C-band. An asymmetric core configuration produces polarization rotation during light propagation.
Double- and dual-core polarization rotators have been designed, fabricated and characterized. The dualcore
type was found to have some advantages regarding fl exibility of design and material absorption.
The PER of the dual-core rotator is also more than 10dB. These polarization control devices have
potential to be used in ultrahigh-speed data transmission with polarization multiplexing technique.