Shin-ichiro Gozu

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.

Ultrafast all-optical switch with cross-phase modulation by area-selective ion implantation in InGaAs/AlAsSb coupled double quantum wells.

We have developed a compact gate switch with monolithic integration of all-optical cross-phase modulation (XPM) in a Mach-Zehnder interferometer (MZI). XPM is caused by intersubband transition (ISBT) in InGaAs/AlAsSb coupled double quantum wells (CDQWs) by area-selective silicon ion implantation and rapid thermal annealing (RTA). While injecting pump light through a transverse electric/transverse magnetic (TE/TM) beam combiner, XPM is induced in one MZI arm and gating operation can be realized. The RTA condition is optimized, and the sample is annealed at 780 °C for 8 s with an implantation dose of 5 × 10(13) cm(-2). Dependence of XPM efficiency on the length of the implanted mesa is also analyzed, and there exists an optimum implantation length to fulfill both high efficiency of ISBT modulation and low loss of the probe and pump signals.

Band edge tailoring of InGaAs/AlAsSb coupled double quantum wells for a monolithically integrated all-optical switch.

We demonstrate a compact all-optical Michelson interferometer (MI) gating switch with monolithic integration of two different bandgap energies. Based on the ion-induced intermixing in InGaAs/AlAsSb coupled double quantum wells, the blueshift of the band edge can be tailored. Through phosphorus ion implantation with a dose of 5 × 10(14) cm(-2) and subsequent annealing at 720 °C for 60 s, an implanted sample can acquire a high transmittance compared with the as-grown one. Meanwhile, the cross-phase modulation (XPM) efficiency of a non-implanted sample undergoing the same annealing process decreases little. An implanted part for signal propagation and a non-implanted section for XPM are thus monolithically integrated for an MI switch by an area-selective manner. Full switching of a π-rad nonlinear phase shift is achieved with pump pulse energy of 5.6 pJ at a 10-GHz repetition rate.

Ultrafast electron dynamics of intersubband excitation concerning cross-phase modulation in an InGaAs/AlAs/AlAsSb coupled double quantum well

The ultrafast electron dynamics accompanying intersubband excitation in an InGaAs/AlAs/AlAsSb double quantum well has been investigated by femtosecond pump-probe spectroscopy. The photoinduced changes in the refractive index that causes cross-phase modulation is dominated by a decreased number as well as increased temperature of the electrons in the lower conduction subbands. The change in the number of conduction electrons decays monotonically, whereas the change in the electron temperature exhibits a maximum at around 0.9 ps after the photoexcitation. Heating of the conduction electrons is caused by their excess energy after intersubband longitudinal-optical phonon scattering.

Refractive index of Si-doped n-InGaAs

The dependence of the refractive index of Si-doped n-InGaAs on carrier density was investigated. The shift in the refractive index from that of undoped InGaAs was found to be mainly caused by the band-filling effect, the band-gap shrinkage effect, and the plasma effect. Model calculations agreed better with the measured data when the conduction band nonparabolicity effect was taken into account. Nevertheless, the calculated refractive index values were slightly lower than the experimental values. Because the difference was larger for photon energies close to the band-gap energy of InGaAs, the shift in the refractive index is attributed to the absorption tail effect that appears near the band-gap energy due to Si doping.

All-optical wavelength conversion at 40Gb/s with enhanced XPM by facet reflection using intersubband transition in InGaAs/AlAsSb quantum well waveguide

XPM efficiency associated with intersubband transition in InGaAs/AlAsSb quantum well waveguide is enhanced by use of facet reflection. The probe experiences the pump-induced index change twice, thereby increasing the XPM efficiency two times. XPM-based wavelength conversion at 40Gb/s is demonstrated.

All-optical XOR logic gate using intersubband transition in III-V quantum well materials

A monolithically integrated all-optical exclusive-OR (XOR) logic gate is experimentally demonstrated based on a Michelson interferometer (MI) gating device in InGaAs/AlAsSb coupled double quantum wells (CDQWs). The MI arms can convert the pump data with return-to-zero ON-OFF keying (RZ OOK) to binary phase-shift keying (BPSK) format, then two BPSK signals can interfere with each other for realizing a desired logical operation. All-optical format conversion from the RZ OOK to BPSK is based on the cross-phase modulation to the transverse electric (TE) probe wave, which is caused by the intersubband transition excited by the transverse magnetic (TM) pump light. Bit error rate measurements show that error free operation for both BPSK format conversion and XOR logical operation can be achieved.

Effects of shutter transients in molecular beam epitaxy.

We have studied the effects of shutter transients (STs) in molecular beam epitaxy (MBE). Two series of samples were grown by MBE and evaluated by X-ray diffraction (XRD) and X-ray reflectivity (XRR) measurements. The effects of STs were evaluated by growth rate (GR) analysis using a combination of growth time (GT) and thickness evaluated by XRD and XRR measurements. We revealed two opposite effects of STs: (1) overshoot of GR and (2) increase in GR with GT and subsequent saturation. Each effect was consistent with the previous studies; however, the previous studies showed no relationships between these two effects. By considering closing time of the shutter, the two opposite effects were well understood.

Exciton spin relaxation in In0.53Ga0.47As/AlAs0.56Sb0.44 quantum wells

The spin relaxation process of In0.53Ga0.47As/AlAs0.56Sb0.44 quantum wells is investigated by spin-dependent pump and probe reflectance measurements with a high time resolution of 200u2009fs. The observed spin relaxation time of 17.5u2009ps at 20u2009K indicates high potential for applications to high-speed optical devices. A positive temperature dependence of the spin relaxation time due to the unique band structure is observed at 30–100u2009K. The spin relaxation is found to be mainly governed by the Bir-Aronov-Pikus process [Sov. Phys. JETP 42, 705 (1976)] below 100u2009K and by the D’yakonov-Perel’ process above 100u2009K.