Ryoichi Akimoto

Ultrafast All-Optical Refractive Index Modulation in Intersubband Transition Switch Using InGaAs/AlAs/AlAsSb Quantum Well

Large, very fast phase modulation was observed in transverse electric (TE) probe light when an intersubband transition (ISBT) switch module using an InGaAs/AlAs/AlAsSb quantum well was pumped by transverse magnetic (TM) light. The phase shift amounted to 1.88 rad for a pump pulse energy of 4 pJ (fiber input), with very fast response. This phenomenon is explained by the change in plasma dispersion caused by the redistribution of electrons among subbands having different effective masses. This phase modulation will enable us to realize various novel ultrafast all-optical devices for signal processing.

Magnetic and transport properties of III–V diluted magnetic semiconductor Ga1−xCrxAs

Magnetic and transport properties of Cr-based III–V diluted magnetic semiconductor Ga1−xCrxAs Cr concentrations up to x=0.10 have been investigated. For all the films, no long-range magnetic order was observed down to 2 K. A sign of paramagnetic Curie temperature is positive, indicating that the dominant magnetic interaction between Cr atoms is ferromagnetic. The effective number of Bohr magneton is estimated to be 5.1±0.4, which is close to that of Cr2+ ion. The magnetization curve shows superparamagnetic behavior at low temperatures. The radius of the local spin order in x=0.034 is estimated to be 1.5–2.0 nm at T=5 K and it decreases with increasing temperature. From Hall effect measurements at room temperature, the magnitude of mobility is estimated to be less than 0.5 cm2/V s. This low mobility strongly suggests that the hopping conductivity is dominant in Ga1−xCrxAs films.

Cross Phase Modulation Efficiency Enhancement in In0.8Ga0.2As/Al0.5Ga0.5As/AlAs0.56Sb0.44 Coupled Double Quantum Wells by Tailoring Interband Transition Wavelength

The cross-phase modulation (XPM) efficiency in new coupled double quantum wells (CDQWs) was examined. Since the XPM efficiency was enhanced under the resonant condition of interband transition (IBT), the new CDQWs were designed to control the IBT absorption edge wavelength (IBT-EW) while the intersubband transition (ISBT) wavelength was maintained at around the optical communication wavelength. By controlling the coupling strength between two wells and the bandgap energy of the wells, the new CDQWs can have a possibility of individual control of the IBT and ISBT wavelengths. In the new CDQWs, a high XPM efficiency close to 0.5 rad/pJ was observed; this value was three times higher than that observed in previous studies.

Monolithically integrated all-optical gate switch using intersubband transition in InGaAs/AlAsSb coupled double quantum wells

We have developed a compact all-optical gate switch with a footprint less than 1 mm2, in which an optical nonlinear waveguide using cross-phase-modulation associated with intersubband transition in InGaAs/AlGaAs/AlAsSb coupled double quantum wells and a Michelson interferometer (MI) are monolithically integrated on an InP chip. The MI configuration allows a transverse magnetic pump light direct access to an MI arm for phase modulation while passive photonic integrated circuits serve a transverse electric signal light. Full switching of the π-rad nonlinear phase shift is achieved with a pump pulse energy of 8.6 pJ at a 10-GHz repetition rate. We also demonstrate all-optical demultiplexing of a 160-Gb/s signal to a 40-Gb/s signal.

Optically induced long-lived electron spin coherence in ZnSe∕BeTe type-II quantum wells

The spin coherence of photoexcited electrons in ZnSe∕BeTe type-II quantum wells has been investigated by the time-resolved Kerr rotation technique. Fast and efficient escape of photoexcited holes from the ZnSe layers to the BeTe layers suppresses the electron-hole recombination and their exchange interaction. This effect leads to the formation of dense electrons in ZnSe layers and long electron spin dephasing time reaching a value of 6.1ns at 1.4K.

A Three-Dimensional Silicon Nitride Polarizing Beam Splitter

A 3-D polarizing beam splitter based on a silicon nitride (Si3N4) vertical directional coupler is experimentally demonstrated. A new planarization technique by incorporating conventional chemical-mechanical lapping with a dry-etching process is developed, in order to obtain a flat film surface for the second Si3N4 core deposition after the first-layer waveguide is formed. Both the Si3N4 layer thicknesses are 200 nm. As there is a material refractive index mismatch between the vertically separated waveguides, the polarization splitter can be realized with a bottom waveguide width of 1.55 μm and a top core width of 1.35 μm. The transverse electric (TE) polarized light can be transmitted completely to the cross-layer output-port, whereas the transverse magnetic (TM) polarized wave outputs mostly from the port at the input layer. A high-extinction ratio and a wide operation bandwidth can be achieved. An extinction ratio of 26 dB for the cross-layer output-port at 1550-nm wavelength and that of 16 dB for the input-layer output-port are obtained. There is an excess coupling loss of for the TE light, but a 1-dB loss for the TM wave.

High-speed all-optical modulation using an InGaAs/AlAsSb quantum well waveguide.

All-optical cross-phase modulation through the intersubband transition in an InGaAs/AlAsSb coupled quantum well was examined by using a sinusoidal intensity-modulated pump. For probe waves in the wavelength range of 1330-1620 nm and a pump modulation at a repetition rate of 76 GHz, cross-phase modulation (XPM) occurred with an efficiency eta, of 0.653-0.142 rad/W. Based on the efficiency we derived a transfer function for the intersubband transition (ISBT) modulator that can be conveniently used in simulating the characteristics of the modulated output (probe) for long excitation pulses with arbitrary waveforms.

Enhancement of All-Optical Cross Phase Modulation in InGaAs/AlAsSb Coupled Quantum Wells Using InAlAs Coupling Barriers

Ultrafast all-optical cross phase modulation (XPM) was enhanced in the InGaAs/AlAsSb coupled quantum wells using new InAlAs coupling barrier. Furthermore, a high XPM efficiency of 0.15 rad/pJ, which is approximately three times as large as that of a previous sample with the AlAs coupling barrier, was obtained by realizing the strong optical confinement in a narrower waveguide. From the analysis of the quantum levels and the measurement of the absorption spectra, the enhanced XPM efficiency was suggested to be contributed by the refractive index dispersion of the interband transition that was modulated by the intersubband transitions (ISBTs). This large XPM efficiency is expected to give a higher performance in Mach-Zehnder interferometer-type ultrafast all-optical switch.

Intersubband transition based on a novel II-VI quantum well structure for ultrafast all-optical switching

We have achieved intersubband absorption as short as 1.58 µm in (CdS/ZnSe)/BeTe quantum wells (QWs) by employing relatively thicker BeTe barriers of 10 monolayers, covering an optical communication wavelength of 1.55 µm within its absorption band. The ISB carrier relaxation was investigated by means of femtosecond one-color pump and probe measurement at the ISB absorption peak position for samples with various well widths. The ISB carrier relaxation time of 270 fs was observed in the sample with the absorption peak at 1.82 µm. The slow decay component with a time constant of 2–4 ps, which has been observed in ZnSe/BeTe QWs, was not observed in the (CdS/ZnSe)/BeTe QWs, indicating that the Γ(ZnSe)-X(BeTe) electron transfer is suppressed as expected from the band alignment. For applications toward ultrafast all-optical switching devices, a ridge-waveguide structure composed of (CdS/ZnSe)/BeTe QWs active and ZnMgBeSe cladding layers is proposed. Refractive indices of these layers have been examined and these data are fed to design ridge-waveguide structures. As a first step, a slab waveguide structure has been successfully grown.

Low-threshold-current yellow BeZnCdSe quantum-well ridge-waveguide laser diodes under continuous-wave room-temperature operation

Low-threshold-current yellow BeZnCdSe single-quantum-well (SQW) laser diodes (LDs) have been developed by using a ridge-waveguide structure. The top p-cladding layer was etched to suppress the leakage current that flowed laterally outside of the electrode. Ridge waveguides were covered with a SiO2 layer and planarized by chemical–mechanical polishing and reactive ion etching. Room-temperature lasing under continuous-wave condition was achieved with the laser cavity formed by cleaved waveguide facets coated with high-reflectivity dielectric films. Two types of LDs with different SQW thicknesses and Cd contents were developed and compared at various waveguide widths and lengths. Yellow LDs with sub-10 mA threshold current were obtained.