Masanori Nagase

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.

All-optical demultiplexing of 160–10Gbit∕s signals with Mach-Zehnder interferometric switch utilizing intersubband transition in InGaAs∕AlAs∕AlAsSb quantum well

We have developed a Mach-Zehnder interferometric all-optical switch employing intersubband transition in an InGaAs∕AlAs∕AlAsSb-coupled double quantum well waveguide. The recently discovered cross-phase modulation phenomenon was utilized as the switching mechanism; the nonlinear index of refraction for transverse electric polarized light is induced by intersubband optical excitation using transverse magnetic pump light. We demonstrate the demultiplexing operation of 160Gbit∕s data signals to 10Gbit∕s using this switch. At the input control pulse energy of 8pJ, the demultiplexed signals showed an extinction ratio better than 10dB, and an error-free demultiplexing was achieved.

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.

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.

Bistability Characteristics of GaN/AlN Resonant Tunneling Diodes Caused by Intersubband Transition and Electron Accumulation in Quantum Well

The bistability characteristics of GaN/AlN resonant tunneling diodes (RTDs) grown on a sapphire substrate were investigated. The RTDs exhibit bistability characteristics with high and low resistivity switched by varying the polarity of the bias voltage. Negative differential resistance is realized in the current-voltage (I-V) characteristics after low resistivity is achieved by application of a negative bias. The calculated I-V characteristics based on self-consistent methods indicate that the bistability characteristics are caused by the accumulation of electrons in the quantum well due to intersubband transitions. Also, the bistability characteristics of GaN/AlN RTDs were discussed toward the application to the ultrafast nonvolatile memory.

Investigating the bistability characteristics of GaN/AlN resonant tunneling diodes for ultrafast nonvolatile memory

The bistability characteristics of GaN/AlN resonant tunneling diodes (RTDs) grown on a sapphire substrate by metalorganic vapor phase epitaxy (MOVPE) were investigated to better understand their physical origin and explore their use in nonvolatile memories. The bistability current–voltage (I–V) characteristics of GaN/AlN RTDs, which were due to intersubband transitions and electron accumulation in the quantum well, were clearly observed over a wide temperature range between 50 and 300 K. However, the I–V characteristics sometimes degraded at temperatures above 250 K. Complex staircase structures were observed in the voltage region showing a negative differential resistance in the I–V curve, and the forward current increased or decreased rapidly as the forward-bias voltage increased. Repeated measurements of the I–V characteristics over the wide temperature range between 50 and 300 K revealed that the bistability characteristics of GaN/AlN RTDs degraded owing to the leakage of electrons accumulating in the quantum well through a deep level in the AlN barrier associated with crystal defects such as dislocations and impurities. Therefore, reduction in crystal defect and impurity densities in the AlN barrier, and a careful design that considers deep levels are important for realizing realize ultrafast nonvolatile memories based on the bistability characteristics of GaN/AlN RTDs.

Selective-Area Growth of Thick Diamond Films Using Chemically Stable Masks of Ru/Au and Mo/Au

Selective-area growth of diamond films in microwave-plasma chemical vapor deposition was performed using newly developed masks. By forming chemically stable masks made of Ru/Au or Mo/Au, which have high melting points, good adhesion to diamond, and difficulty in forming carbide compounds, patterned diamond films with a large thickness of 50 µm, a large area of 5 mm2, and a high orientation in the [001] direction were successfully grown on (001) diamond substrates without degradation of the crystal quality of masked areas.

Peak Width Analysis of Current–Voltage Characteristics of Triple-Barrier Resonant Tunneling Diodes

We studied the peak width of current vs voltage (I–V) characteristics of triple-barrier resonant tunneling diodes (TBRTDs) experimentally and theoretically. A GaInAs/InP TBRTD was fabricated by organo metallic vapor phase epitaxy (OMVPE). A theory of I–V characteristics of TBRTDs was developed by taking the structural inhomogeneity into account to explain the experimental peak width. The fluctuation of the well width in a TBRTD grown by OMVPE was estimated as two atomic layers.

Ultrafast All-Optical Gating Operation Using Michelson Interferometer for Hybrid Integration of Intersubband Transition Switch on Si Platform

The direct butt-joint coupling between the Si-wire-based Michelson interferometer (MI) and the InGaAs/AlAsSb quantum-well waveguide was performed to realize the hybrid integrated ultrafast intersubband transition switch. By attaining the narrow gap between the waveguides, we have observed the Michelson interference spectra between the two signal lights: one is the phase modulated light from the InGaAs/AlAsSb quantum-well waveguide and the other is the light returned from the Si loop mirror of MI. Furthermore, by controlling the phases and powers of two signal lights with the thermooptic phase shifter and variable coupler in MI, we have demonstrated the ultrafast all-optical gating operation with a response time of 2 ps. The all-optical gating switch with a small size of 0.3 mm 2.35 mm was successfully fabricated by the hybrid integration technologies of III-V and Si materials.

Diamond Vertical Schottky Barrier Diode with Al2O3 Field Plate

A field-plate structure is applied to vertical diamond Schottky barrier diode. A sputtered Al2O3 with 0.2 µm thickness is utilized for field-plate insulator. Fabricated diamond VSBD shows low leakage characteristics. Accordingly, the breakdown voltage is improved from 900V to 1,800V.