The population and coherent dynamics of excitons in InAs quantum dots were investigated using transient pump-probe and four-wave mixing spectroscopies in the telecommunications wavelength range. The sample
was fabricated on an InP(311)B substrate using strain compensation to control the emission wavelength. This technique also enabled us to stack over a hundred QD layers, which resulted in a significant enhancement of nonlinear signals. By controlling the polarization directions of incident pulses, we precisely estimated the radiative and non-radiative lifetimes, the transition dipole moment, and the dephasing time while taking into account their anisotropic properties. The measured radiative lifetime and dephasing time shows large anisotropies with respect to the crystal axes, which results from the anisotropic nature of the transition dipole moment. The
anisotropy is larger than that for InAs quantum dots on a GaAs(100) substrate, which seems to reflect a lack of symmetry on an (311)B substrate. A quantitative comparison of these anisotropies demonstrates that nonradiative population relaxation and pure dephasing are quite small in our QDs.
Lasing and sharp line emission in the 1.55-μm wavelength region is demonstrated from ensembles and single InAs
quantum dots (QDs) embedded in InGaAsP on InP (100) by metalorganic vapor phase epitaxy (MOVPE). Wavelength tuning of the QDs is achieved through the insertion of ultra-thin (1-2 monolayers) GaAs interlayers underneath the
InAs QDs. To increase the active volume widely-stacked QD layers are identically reproduced. Closely-stacked QDs
reveal unpolarized emission from the cleaved side due to vertical electronic coupling which is important for polarization
insensitive semiconductor optical amplifiers. Fabry-Perot narrow ridge-waveguide lasers implementing five layers of
widely-stacked QDs as gain medium operate in continuous wave mode at room temperature with low threshold current,
low transparency current density of 6 A/cm2 per QD layer, and low loss of 4.2 cm-1, which are accompanied by a 80 nm
wide gain spectrum. Device performance does not suffer from sidewall recombination in deeply-etched QD lasers which
possess similar threshold currents as shallowly-etched ones and do not deteriorate with time. This allows the fabrication
of mono-mode and compact devices with small bending radii, as demonstrated by the operation of a QD ring laser with
40-GHz free spectral range. Micro-PL of single QDs exhibits sharp exciton-biexciton emission around 1.55 μm
persisting to temperatures above 70 K; the prerequisite for single photon sources working at liquid nitrogen temperature
for fiber-based quantum information and cryptography systems.
Low dimensional structures (LDS) form a major new branch of physics research. They are semiconductor structures, which have such a small scale in one or two spatial dimensions that their
electronic properties are significantly different from the same material in bulk form. These properties are changed by quantum effects. Throughout the world there is increasing interest in the preparation, study and application of LDS. Their investigation has revitalised condensed matter science, in particular semiconductor materials. These complex LDS offer device engineers new design
opportunities for tailor-made new generation electronic and photonic devices. New crystal growth techniques such as molecular beam epitaxy (MBE) and metal-organic chemical vapour (MOCVD) deposition have made it possible to produce such LDS in practice. These sophisticated technologies for the growth of high quality epitaxial layers of compound semiconductor materials on single crystal semiconductor substrates are becoming increasingly important for the development of the semiconductor electronics industry. This article is intended to convey the flavour of the subject by focussing on the technology and
applications of self-assembled quantum dots and to give an elementary introduction to some of the essential characteristics.
We review recent progress in the development of nanophotonic devices using the optical near-field interaction. ZnO nanocrystallites are potentially ideal components for realizing room-temperature operation of such devices due to their high exciton-binding energy and great oscillator strength. To confirm this promising optical property of ZnO, we examined the near-field time-resolved spectroscopy of ZnO nanorod double-quantum-well structures (DQWs). First, we
observed the nutation of the population between the resonantly coupled exciton states of DQWs, in which the coupling
strength of the near-field interaction was found to decrease exponentially as the separation increased. Furthermore, we
successfully demonstrated the switching dynamics of a
dipole-forbidden optical energy transfer among resonant exciton
states. Our results provide criteria for designing nanophotonic devices. The success of time-resolved near-field
spectroscopy of isolated DQWs described here is a promising step toward realizing a practical nanometer-scale photonic
switch and related devices.
We have investigated the molecular beam epitaxial growth and characteristics of long wavelength InAs pseudomorphic and metamorphic quantum dot lasers grown on GaAs. Utilizing the techniques of tunnel injection and acceptor-doping of quantum dots, we have achieved high performance 1.3 μm InAs quantum dot lasers on GaAs, which exhibit Jth=180 A/cm2, T0=∞, dg/dn≈1×10-14 cm2, f-3dB =11 GHz, chirp of 0.1 Å and zero α-parameter. By detailed investigation of the growth kinetics and characteristics of metamorphic quantum dot heterostructures on GaAs, we have demonstrated high performance 1.5 μm InAs metamorphic quantum dot lasers on GaAs that are characterized Jth~60A/cm2, T0≈620K, and near-zero α-parameter and chirp (~ 0.1 Å).
Novel optoelectronic systems based on ensembles of semiconductor are described. We will present here the optical and electronic properties of organic-inorganic hybrid structures that enable integration of useful organic and inorganic characteristics for novel sensing applications. Several semiconductor nanostructures with both direct band gap and indirect band gap will be discussed in a few different polymer and biological matrices. A number of these colloidal
semiconductor quantum dots and related quantum-wire structures have been characterized using absorption, photoluminescence, and Raman measurements; these Raman measurements include those made on self-assembled monolayers of DNA molecules terminated on one end with a common substrate and on the other end with TiO2 quantum
dots. The electronic properties of these structures are modeled and compared with experiment. Devices fabricated with these materials as well as their potential for sensing will be discussed.
The quantum dots have added great benefits to the photonic activity, among them the decoupling between the lattice parameter of the substrate and the dot has opened the way to enlarge the spectral windows which can be accessible on different substrates. For example on a GaAs substrate a long wavelength laser emission of 1.46 μm
has been demonstrated at room temperature. The specific properties like: large material gain, large spectral
bandwidth, high speed carrier dynamics, have improved device performances. The minimum threshold current
densities of laser devices, the large spectral bandwidth of semiconductor optical amplifiers and the very high
repetition rate and very short pulse width on mode locked lasers are other benefits.
The concept of randomly-oriented semiconductor nanowires formed on non-single-crystal substrates is introduced and compared with semiconductor nanowires synthesized on single-crystal-substrates in the framework of epitaxial growth. In principle, epitaxial growth of semiconductor nanowires with the presence of metal-catalysts requires no single-crystal substrates owing to the small size of nanowires. A segment on a substrate from which crystallographic information is
transferred to a single nanowire would only need to be as larger as the cross-section of a nanowire if a specific geometrical alignment for a group of nanowires is not required, suggesting that randomly-oriented semiconductor nanowires be formed on a surface that is characterized with short-range atomic order in contrast to long-range atomic order that exists on the surface of single-crystal substrates. The surfaces exhibiting short-range atomic order can be prepared on non-single-crystal substrates, further suggesting functional devices that utilize randomly-oriented semiconductor nanowires be fabricated on non-single-crystal substrates. Design, fabrication and characteristics of a photoconductor that utilizes an ensemble of randomly-oriented indium phosphide nanowires are described.
We demonstrate quantum dot (QD) based continuous-wave photonic crystal (PhC) nanocavity laser operates at room temperature with a very low effective threshold power of ~ 375 nW. The continuous-wave lasing was achieved at 1.3 μm with InAs/GaAs self-assembled QDs and high quality PhC nanocavity with a quality factor of 87,000. The light-in versus light-out curve shows no sharp threshold unlike conventional lasers with pronounced kinks around the thresholds.
This near-thresholdless behavior with smooth transition from thermal to coherent light region indicates that this nanocavity laser has a very high spontaneous emission coupling efficiency. The temporal coherence of such a high-β laser was studied by interference measurements. We also discuss the characteristics of background noise of QD based PhC nanocavity lasers compared with quantum well based PhC nanocavity lasers.
We have developed an ultra compact dispersion compensator based on multiple one dimensional coupled-defect-type
photonic crystals, utilizing large optical group velocity dependence on the wavelength without polarization mode
dependence. The photonic crystal of the compensator consists of a SiO2/Ti2O5 multi-layer thin-film structure and SiO2
defect layers and was designed for a 1.55-μm, 40-Gbit/s optical communication system. The thin-film structure is
substrate-free, which enables the compensator to be small, that is, a 1.4-mm-edge cube. To obtain a large group-velocity
difference, 60 substrate-free films are stacked to form the compensator. The passband is 2 nm, and the group delay time-difference
within the band is more than 100 ps. A 40-Gbit/s non-return-to-zero optical transmission experiment was
carried out with the compensator, demonstrating dispersion-compensation operation over a 10-km standard single-mode
fiber, which corresponds to dispersion of 170 ps/nm.
Organic nanofibers from semiconducting conjugated molecules are well suited to meet refined demands for advanced
applications in future optoelectronics and nanophotonics. In contrast to their inorganic counterparts,
the properties of organic nanowires can be tailored at the molecular level by chemical synthesis. Recently we
have demonstrated the complete route from designing hyperpolarizabilities of individual molecules by chemically
functionalizing para-quaterphenylene building blocks to the growth and optical characterization of nonlinear,
optically active nanoaggregates. For that we have investigated nanofibers as grown via organic epitaxy. In the
present work we show how chemically changing the functionalizing end groups leads to a huge increase of second
order susceptibility, making the nanofibers technologically very interesting as efficient frequency doublers. For
that the nanofibers have to be transferred either as individual entities or as ordered arrays onto specific target
substrates. Here, we study the applicability of contact printing as a possible route to non-destructive nanofiber
HAFs and HFs (MOFs) having unique dispersion properties in near infrared and visible regime will be discussed. In
particular, we focus on the dispersion control in a 1.0μm band which have gathered rapidly increasing interests for the
aspects of both telecom and non-telecom applications. Furthermore, dispersion shift towards visible wavelengths using a
holey fiber technology has tried and the results will be explained. Finally, we will briefly discuss about the potential
usage of the MOFs for short wavelength transmission.
We demonstrate an InP nanowire based photodetector laterally integrated between two (111)-oriented vertical silicon surfaces. The nanowires are grown through a simple single step chemical vapor deposition (CVD) process using gold nanoparticles as catalyst with
in-situ p-doping and have been heteroepitaxially bridged between a pair of prefabricated p-doped Si electrodes. Nonlinear current-voltage characteristics are observed. Although this nonlinearity resembles a back-to-back rectifying profile it originates from space-charge limited conductivity of the nanowires. DC photoelectric characteristics of the device were measured under optical illumination (λ=630 nm) above the bandgap energy (1.34 eV or ~925 nm at room temperature) of InP. The variation in photoconductance with varying input optical power demonstrates high sensitivity of the device to optical illumination.