Taro Arakawa

Fabrication of vertical‐microcavity quantum wire lasers

A strained InGaAs quantum wire laser with a vertical microcavity structure was fabricated for the first time. In this laser structure, quantum wires with a lateral width of about 10 nm were grown by a selective metalorganic chemical vapor deposition technique. The length of the microcavity was 4λ(λ=883 nm), with AlAs/AlGaAs distributed Bragg reflectors. The cavity effect was demonstrated by the measurement of photoluminescence with and without the cavity. Lasing oscillation was observed at 77 K by optical pumping.

Photoluminescence studies of GaAs quantum wires with quantum confined Stark effect

We investigated the quantum confined Stark effect in GaAs quantum wires formed in a V-groove structure, demonstrating observation of a blueshift of the photoluminescence peak with the increase of electric fields at 50 K. This blueshift is attributed to the fact that the change in enhanced binding energy of excitons due to the electric field is larger than that in quantized energy levels of electrons and holes. Time-resolved photoluminescence was also measured. The photoluminescence decay time is decreased in small quantum wires of 8 nm width with the increase of electric fields, while the decay time is increased in the quantum wires with a size of 35 nm. These results indicate that the escaping of carriers is more dominant in smaller structures than reduction of the oscillator strength due to the electric fields.

Fabrication of InGaAs strained quantum wire structures using selective-area metal-organic chemical vapor deposition growth

We fabricated InxGa1-xAs strained quantum wire structures with various In compositions using a selective-area metal-organic chemical vapor deposition growth technique. Photoluminescence (PL) measurements at 14 K demonstrated that strained quantum wires of high quality were obtained when x is less than 0.35. Change of the full width at half-maximum of the PL peaks indicates that the structural dimensions of the quantum wires exceeded the critical thickness at around x=0.4.

Microring Resonator Wavelength Tunable Filter Using Five-Layer Asymmetric Coupled Quantum Well

We propose and demonstrate a semiconductor single microring resonator wavelength tunable filter utilizing InGaAs/InAlAs five-layer asymmetric coupled quantum wells for a fast low-power operation. A directional coupler with a shallow gap is used for coupling between busline and microring waveguides to control precisely the coupling efficiency. A wafer is grown by molecular beam epitaxy, and the waveguides are fabricated by two-step inductively coupled plasma reactive ion etching. The resonant wavelength shift of 0.9 nm is obtained under the reverse bias of 13 V.

Hitless wavelength-selective switch based on quantum well second-order series-coupled microring resonators

A hitless wavelength-selective switch (WSS) based on InGaAs/InAlAs multiple quantum well (MQW) second-order series-coupled microring resonators is proposed and fabricated. In the core layer, a five-layer asymmetric coupled quantum well (FACQW) structure is employed. The WSS is driven by the electrorefractive index change in the FACQW core layer caused by the quantum-confined Stark effect (QCSE). The wafer for the WSS is grown by molecular beam epitaxy and waveguide structures are formed by dry etching. Boxlike spectrum responses and hitless switching characteristics of the WSS are successfully demonstrated for the first time. The change in coupling efficiency at a coupler between a ring and a busline and between rings and its effect on the switching characteristics are also discussed.

InGaAs/InAlAs Five-Layer Asymmetric Coupled Quantum Well Exhibiting Giant Electrorefractive Index Change

An InGaAs/InAlAs five-layer asymmetric coupled quantum well (FACQW) exhibiting a giant electrorefractive index change was proposed and has been studied theoretically and experimentally. Giant electrorefractive sensitivity |dn/dF| (4.4×10-4 cm/kV) at a wavelength range with a width of over 100 nm can be expected at an electric field of approximately F=-30 to -60 kV/cm. The FACQW structure was successfully fabricated using molecular beam epitaxy (MBE). The results of photoabsorption current measurements are consistent with the theory. The giant electrorefractive index change of the FACQW is very promising for realizing low-voltage and high-speed compact Mach–Zehnder modulators and switches.

Influence of One Monolayer Thickness Variation in GaAs/AlGaAs Five-Layer Asymmetric Coupled Quantum Well upon Electrorefractive Index Change

The five-layer asymmetric coupled quantum well (FACQW) is a new potential-tailored quantum well (QW) for ultrafast and low-voltage optical modulators and switches. First, the influence of one monolayer (ML) thickness variation of a single layer in the GaAs/AlGaAs FACQW on the electrorefractive index change Δn is theoretically studied. The thickness variation of two thicker GaAs layers has a considerable influence on Δn of the FACQW, while the thickness variation of thin AlAs and AlGaAs barrier layers has a smaller influence on Δn. The ratio of the thicknesses of the two GaAs well layers significantly affects the Δn characteristics of the FACQW. The change Δn does not vary appreciably as long as the ratio is kept constant. Second, the influence of the statistical fluctuation of the layer thickness by 1 ML in all of the layers on the Δn characteristics of the FACQW is also discussed. Even when Δn decreases with the increase of the occurrence probability of a layer being thicker or thinner by 1 ML, the FACQW still has a much larger Δn than conventional rectangular quantum wells do.

Observation of Giant Electrorefractive Effect in Five-Layer Asymmetric Coupled Quantum Wells (FACQWs)

The five-layer asymmetric coupled quantum well (FACQW) is a new potential-tailored quantum well (QW) that is promising for ultrafast and ultralow-voltage optical modulators and switches. We fabricated a waveguide phase modulator with a core layer of GaAs/AlGaAs multiple FACQWs, and a giant electrorefractive (ER) effect in the FACQW was measured for the first time. The ER sensitivity Δn/ΔF measured by the Fabry-Perot resonance method was as large as 1.7 ×10-4 cm/kV at around an electric field of 40 kV/cm. This giant sensitivity is in fair agreement with the theory. This result shows that the FACQW is promising for realizing ultrawide-band, ultrafast and low-voltage optical modulators and switches.

Fabrication and Optical Characterization of Five-Layer Asymmetric Coupled Quantum Well (FACQW)

The five-layer asymmetric coupled quantum well (FACQW) is a new potential-tailored quantum well (QW) that is promising for ultrafast and ultralow-voltage optical modulators and switches. We succeeded in fabricating GaAs/AlGaAs FACQW with monolayer accuracy by the molecular beam epitaxy (MBE) method by monitoring reflection high-energy electron diffraction (RHEED) specular beam intensity oscillation. Photoabsorption current measurements of the FACQW sample showed good agreement with theoretical results, and a potential for much lower voltage operation. In addition, we studied the growth sequences of GaAs/AlGaAs QWs in the migration-enhanced epitaxy (MEE) method in order to fabricate the FACQW with steeper and flatter heterointerfaces. The sequence of supplying materials for Al0.3Ga0.7As growth, on which there is no report, was modified and optimized, and the QWs of higher quality were obtained at a growth temperature of 490°C using the optimized sequence. The results of photoluminescence measurements show that the MEE method modified as mentioned above is a promising growth technique for the fabrication of FACQWs of higher quality.

Anomalous Sharp Dip of Large Field-Induced Refractive Index Change in GaAs/AlGaAs Five-Layer Asymmetric Coupled Quantum Well

The five-layer asymmetric coupled quantum well (FACQW) is a new potential-tailored quantum well for ultrafast and low-voltage optical modulators and switches. Almost linear and large electrorefractive index change can be obtained in the transparency wavelength regions. In the GaAs/AlGaAs FACQW, an abrupt change in refractive index change Δn due to an applied electric field F occurs at a certain electric field range, which results in an anomalous sharp dip of Δn versus F. The physical origin and the elimination of the dip are discussed in detail. The abrupt change of refractive index is caused by significant changes of the wavefunction overlap integrals (and exciton binding energies) of transitions between the ground states for an electron (e1) and a heavy hole (hh1), and transitions between e1 and the first excited state for a heavy hole (hh2). The overlap changes are mainly due to shifts of the wavefunction distribution of hh1 and hh2, respectively. The dip can be eliminated by changing the position or Al content of the AlGaAs barrier layer in the FACQW. In addition, the larger negative index change in a modified FACQW structure is demonstrated.