Kyozo Kanamoto

Ultra-fast photonic crystal/quantum dot all-optical switch for future photonic networks

We demonstrated a novel two-dimensional photonic-crystal based Symmetric Mach Zehnder type all-optical switch (PC-SMZ) with InAs quantum dots acting as a nonlinear phase-shift source. The 600-mum-long PC-SMZ exhibited a 15-ps-wide switching-window at sufficiently low optical-energy of ~100 fJ.

Electron traps in AlGaAs grown by molecular‐beam epitaxy

Using deep‐level transient capacitance spectroscopy we have investigated deep electron traps in n‐AlGaAs grown by molecular‐beam epitaxy (MBE). The thermal activation energies of seven traps, labeled ME1–ME7, observed in this study increase with increasing Al content(x) up to the direct‐indirect crossover point (x∼0.42), but show only a small change with further increases in Al content. Traps ME4–ME7 are dominant in samples with x≤0.2. Traps ME4–ME6 strongly depend on the growth ambient. The concentration of ME7 is almost independent of the ambient in the growth chamber but decreases rapidly with increasing growth temperature. ME7 is a native defect and can almost certainly be identified with the trap EL2 observed in bulk and vapor‐phase epitaxially grown GaAs. Traps ME4–ME6 are probably formed by impurities involving oxygen such as CO, H2O, and AsO in the growth ambient. All of the traps, ME5–ME7 are clearly responsible for a decrease in the photoluminescence intensity of MBE grown AlGaAs.

Classification and origins of GaAs oval defects grown by molecular beam epitaxy

Abstract By extensive surface and cross sectional microscopic observations, oval defects on single GaAs and multiple GaAs/AlGaAs epilayers grown by molecular beam epitaxy (MBE) were investigated. It is found that there are at least seven different representative kinds of oval defects which possess distinct morphological characteristics. Among six different types of oval defects without macroscopic core particulates, four types are ascribed to surface microscopic contaminations, while other two types to the Ga source material. However, the majority of oval defects with our MBE system were usually the type with macroscopic core particulates and were found to originate at the epilayer/substrate interface. They are, therefore, attributed to surface macroscopic contaminations.

Nonlinear optical phase shift in InAs quantum dots measured by a unique two-color pump/probe ellipsometric polarization analysis

The nonlinear optical phase shift in self-assembled InAs quantum dots (QDs) under resonant excitation in a ground-state transition was measured by a unique two-color pump∕probe ellipsometric polarization analysis. This ellipsometric analysis makes use of the large optical birefringence of SK-QD [(SK) — Stranski-Krastanov] originating from the asymmetric structure. A phase shift of 0.5π rad was obtained at an input pump pulse energy density of 30pJ∕μm2, a detuning of 11meV, and a time delay of 20ps in a 1mm long waveguide having QDs with a peak wavelength of 1290nm, a volume density of 4×1015cm−3, and inhomogeneous broadening of 35meV. Analysis revealed that the phase shift is mainly attributed to the absorption saturation for TE-polarized light, though other mechanisms also could contribute at higher pumping. The calculation, based on the two-level approximation, revealed that the minimum energy density for π shift is 240fJ∕μm2, calculated under ideal conditions.

Spectroscopic Characterization of Low-Temperature Grown GaAs Epitaxial Films

Femtosecond time-resolved reflectance and Raman scattering studies have been made on GaAs epitaxial layers grown at temperatures between 200 and 300° C and subsequently annealed. A subpicosecond carrier lifetime (∼0.25 ps) has been measured for a sample grown at 250° C and annealed at 600° C. Raman measurements using a back scattering geometry show a strong TO phonon band for samples grown at 200° C and 250° C, while it is absent for samples grown at 275 and 300° C. The band width of the LO band increases with decreasing growth temperature. A phonon band corresponding to As precipitates is also observed at 200 cm-1 for samples grown at 200 and 250° C. A strong correlation is found between the measured carrier lifetime and Raman profile.

Emission spectra of single quantum well lasers with inhomogeneous current injection

Emission spectra of a tandem‐type GaAs single quantum well laser diode were investigated under pulsed operating conditions. By controlling the two injection current levels, we could force the device to operate not only at the lowest (n=1) quantized state transition but also at the second (n=2) transition. Anomalous pulse responses, moreover, were found for the two lasing modes which were simultaneously observed in time‐integrated spectra. The dynamic emission behavior was utilized to achieve a variety of intensity combinations of the two radiation modes at the widely different wavelengths.

Photoluminescence line shape of excitons in GaAs single‐quantum wells with and without heterointerface ordering

The photoluminescence line shape of excitons at low temperatures is investigated in GaAs single‐quantum wells grown by molecular‐beam epitaxy with and without intentional heterointerface ordering. From the study of the excitation density dependence in the range between 1015 and 1017 cm−3, it is found that the inhomogeneous linewidth is significantly affected by band‐filling effects of intrinsic defect states spatially localized in the wider wells. In the sample with heterointerface ordering the band‐filling effects are directly identified by observation of saturation of the localized excitonic emissions, which are split as a result of increased spatial coherence of the excitonic states.

Fabrication and characterization of photonic crystal-based symmetric Mach-Zehnder (PC-SMZ) structures based on GaAs membrane slab waveguides

We have experimentally demonstrated two- dimensional (2-D) photonic crystal (PC) waveguides (WGs) such as straight, bent, Y-branch, directional coupler (DC), and quantum dots (QDs)-based nonlinear optical (NLO) WGs using a GaAs air-bridge membrane with triangular-lattice line-defect WGs. These WGs are necessary for a 2-D PC-based symmetric Mach-Zehnder (PC-SMZ) type ultrafast all-optical switch. Experimental results on the clear observation of the optical interference at the Y-junction DC, nonlinearity-induced phase shift at the NLO waveguide, and transmission spectra in the total PC-SMZ WG configuration will be addressed, as well as measured results on each WG element.

Ultrafast response of photonic crystal atoms with Kerr nonlinearity to ultrashort optical pulses

We investigate the response of photonic crystal (PC) atoms with Kerr nonlinearity to the excitation of ultrashort optical pulses. Surprisingly, it is found that PC atoms with sharp resonant peaks in frequency spectrum (or with large quality factors) can respond to ultrashort pulses very rapidly when Kerr nonlinearity is introduced. More interestingly, the response time under high-power densities can be even shorter than the duration of the pump pulse. It is revealed that the nonlinearity induced broadening of dynamical frequency spectrum is responsible for the ultrafast response time, implying the possibility of ultrafast all-optical switching.

Distinct observation of interwell coupling effect on optical transitions in double quantum wells in an electric field

We present results of an experimental study of interwell coupling effects using asymmetric double quantum well structures under electric fields. Data were obtained by means of photocurrent spectroscopy. It was concluded that remarkable variations in both energy and intensity of excitonic transitions, which were observed for the strongly coupled quantum wells, originate from the intersection of two quantized levels in the conduction band.