Jun-ichi Kasai

Cross-phase-modulation-based wavelength conversion using intersubband transition in InGaAs/AlAs/AlAsSb coupled quantum wells

We report an ultrafast cross phase modulation (XPM) effect in intersubband transition (ISBT) of InGaAs/AlAs/AlAsSb coupled quantum wells, where the ISBT absorption of a transverse-magnetic mode pump signal induces phase modulation of a transverse-electric mode probe signal. Using waveguide-type ISBT devices, we have achieved XPM-based 10 Gbit/s wavelength conversion with a power penalty of 2.53 dB. Also, we propose XPM-based signal processing circuits for gate switching and modulation format conversion.

Absorption Dynamics in All-Optical Switch Based on Intersubband Transition in InGaAs–AlAs–AlAsSb Coupled Quantum Wells

We developed a pigtailed fiber module including an intersubband transition all-optical switch using InGaAs-AlAs-AlAsSb coupled double quantum wells and studied the absorption saturation dynamics. An increased extinction ratio was obtained by optimizing the delay time between control and signal pulses due to the reduced effect of two-photon absorption. An extinction ratio of 10 dB was obtained for a fiber input pulse energy of 16 pJ (200-fs pulsewidth). The absorption recovery time varied from 600 fs to 2 ps depending on the control pulse energy

Threading dislocations in GaAs on pre-patterned Si and in post-patterned GaAs on Si

Abstract A detailed correlation has been made between the threading dislocation density, the residual stress and the growth area for striped and square mesas of molecular beam epitaxially grown ∼ 3 μm thick GaAs on pre-patterned Si, and of post-patterned ∼ 3 μm thick GaAs on Si. The morphology and distribution of the dislocations are also compared for samples of various sizes between 1 and 100 μm in width, which are observed before and after ex situ annealing, using cross-sectional transmission electron microscopy. Micro-photoluminescence measurements reveal that the residual stress of the patterned GaAs decreases with a decrease in the mesa size, especially for sizes of less than 25 μm, independent of the patterning method and the shape of the samples. However, no clear reduction in the dislocation density is observed in as-grown layers on pre-patterned stripe and square Si mesa smaller than 10 μm. On the contrary, the defect density in the layers having pattern sizes less than 4 μm is higher than that in layers having sizes larger than 10 μm, due to the additional generation of microtwins/stacking faults from the {111} faceted regions which are formed at the sides along the 〈110〉 directions of the grown films. Annealing treatment at 900°C for 10 s effectively reduces the density of these latter defects in patterned films by both pre- and post-patterning procedures, irrespective of the pattern sizes. However, no distinct decrease in the dislocation density is detected in small size patterns of less than 10 μm. This feature is equally observed for post-patterned samples which include thin Si insertion layers in GaAs films. The obtained results strongly suggest that thermal-expansion mismatch-induced stress does not cause any further generation of dislocations during cooling from the growth temperature to room temperature. The dislocations are generated during growth due to lattice mismatch-induced stress.

Green-to-Yellow Continuous-Wave Operation of BeZnCdSe Quantum-Well Laser Diodes at Room Temperature

We have grown laser diode structures using highly strained BeZnCdSe quantum wells by molecular beam epitaxy and successfully obtained continuous-wave lasing in the green-to-yellow spectral region (543–570 nm) at room temperature. The green-to-yellow lasing color was tuned by simply varying the Cd content of the quaternary BeZnCdSe quantum well. The threshold current densities of 20-µm-wide lasers were found to be sufficiently low (less than 0.85 kA/cm2). This result demonstrates that BeZnCdSe is a promising material for use as the active layer in high-performance green-to-yellow laser diodes.

Ultralow Intersubband Absorption Saturation Intensity at Communication Wavelength Achieved in Novel Strain Compensated InGaAs/AlAs/AlAsSb Quantum Wells Grown by Molecular Beam Epitaxy

We have markedly improved the optical properties of extremely thin quantum wells (QWs) required for ISBT devices operating at optical communication wavelengths using novel InGaAs/AlAs/AlAsSb QW structures with 4–7 monolayers (MLs) of AlAs. The intersubband saturation intensity (Is) was reduced to 3 fJ/µm2. This represented an IS reduction of nearly 3 orders of magnitude relative to that of the previous samples, whether or not the sample had a 1-ML AlAs interface. In this paper, we report the properties of novel InGaAs/AlAs/AlAsSb quantum wells grown by molecular beam epitaxy, and discuss the linear and nonlinear optical responses of ISBT.

InGaAs-AlAs-AlAsSb coupled quantum well intersubband transition all-optical switch with low switching energy for OTDM systems

We report a novel intersubband transition all-optical switch with low switching energy using InGaAs-AlAs-AlAsSb coupled double quantum wells (C-DQWs) with AlAs spacer layers. Very low saturation energy density of 34 fJ//spl mu/m/sup 2/ was observed for bulk transmittance with wavelength of 1.62 /spl mu/m. Using a waveguide, whose core is 93 periods of the C-DQWs, an all optical switch with a low switching energy of 10 pJ for 10-dB extinction ratio was realized.

LOW-TEMPERATURE MICRO-PHOTOLUMINESCENCE USING CONFOCAL MICROSCOPY

We describe an optical setup for low‐temperature micro‐photoluminescence measurements using confocal microscopy. The spatial resolution of the setup was estimated by observing photoluminescence images of a luminescent line formed in a cleaved face of a quantum‐well crystal. A high spatial resolution of 0.55 μm was obtained at a 457.9 nm excitation wavelength. Measurements of photoluminescence excitation spectra are also possible with a high spatial resolution of less than 1 μm. As an application of low‐temperature micro‐photoluminescence measurements, photoluminescence images of a dot array comprising buried quantum wells have been obtained. These images show the distribution due to emissions from single dots with diameters down to 0.3 μm.

545 nm Room-Temperature Continuous-Wave Operation of BeZnCdSe Quantum-Well Green Laser Diodes with Low Threshold Current Density

We report the growth of BeZnCdSe quantum-well laser diode (LD) structures with a short-period superlattice cladding layer and demonstrate continuous-wave lasing in the pure-green spectral region (545 nm) at room temperature. The threshold current density and voltage of a 5-?m-wide gain-guided laser were found to be 1.7 kA/cm2 and 10.4 V, respectively. This threshold current density is sufficiently low compared with that of InGaN/GaN green LDs.

Optical properties of high-quality InAlAs/InGaAs SQWs grown by MBE using specially refined In and Al sources

Abstract Extremely high quality InAlAs/InGaAs single quantum wells (SQWs) were grown by MBE using specially refined In and Al sources. Samples grown using the new sources demonstrated high photoluminescence (PL) intensities as well as narrow peaks at 4.2 K, without any significant variation in a wide growth temperature range from 510 to 620° C. For instance, PL linewidths at 4.2 K were 12.3 and 3.9 meV for well widths ( L z ) of 16 and 54 A, respectively. The linewidth at L z A was reduced to 8.9 meV for a 2 min growth interruption at the heterointerfaces, without any PL intensity drop. Undoped InAlAs also showed very narrow PL linewidth of 10.5 meV at 4.2 K. These PL linewidths are narrower than any previously reported for this material system. Moreover, electron mobilities of undoped InGaAs or selectively doped InAlAs/InGaAs structures remain high (10000–11900 cm 2 /V· at room temperature and 58400–65600 cm 2 /V·s at 77 K in the dark) regardless of the growth temperature in the 510–620° C range.

Morphological Evolution of the InGaN-Based Quantum Well Surface due to a Reduced Density of Threading Dislocations in the Underlying GaN through Higher Growth Pressure

We have examined how fewer threading dislocations (TDs) in an underlying GaN epilayer affects the surface morphology of InGaN/InGaN quantum well (QW) structures grown by low-pressure metalorganic vapor phase epitaxy. When the GaN growth pressure was raised from 100 Torr to 400 Torr, the density of pure edge and mixed character TDs was reduced due to the low-density formation and increase in the size of initially grown GaN islands. However, the density of pure screw TDs simultaneously increased. The QW surface morphology strongly depended on the densities of mixed or screw TDs in the underlying GaN epilayer. We found that the pit density in the surface of the QW structure decreased with a lower density of mixed TDs, and that the QW surface morphology then exhibited a pattern of large nanoscale islands if the density of screw TDs simultaneously increased, or a pattern of heavily bunched steps if it was unchanged.