Kei Kaneko

Ultrafast intersubband relaxation (<= 150 fs) in AlGaN/GaN multiple quantum wells

The ultrafast intersubband relaxation in GaN quantum wells has been verified. Al0.65Ga0.35N/GaN multiple quantum wells, with as many as 200 wells, were grown by optimizing the barrier thickness and introducing GaN intermediate layers. The intersubband absorption is sufficiently strong for the relaxation time to be measured. A pump–probe measurement is performed to investigate the relaxation. An ultrashort relaxation time of less than 150 fs is obtained at a wavelength of 4.5 μm. The transient time is shorter than that of InGaAs quantum wells by approximately an order of magnitude. This result is promising for realizing ultrafast optical switches.

Near-infrared intersubband absorption in GaN/AlN quantum wells grown by molecular beam epitaxy

GaN/AlN multiple-quantum-well structures were grown by molecular beam epitaxy. Abrupt interfaces and good periodicity were confirmed. Absorption measurements indicated that intersubband absorptions occurred at wavelengths of 1.3–2.2 μm. Spectral fits by Lorentzians suggested that the well thicknesses fluctuated by two monolayers. The linewidths of the individual fits were as narrow as 80–120 meV. The characteristics of the absorption saturation were investigated at a wavelength of 1.46 μm. A relaxation time of 400 fs and saturation energy density of 0.5 pJ/μm2 were obtained. These results are promising for realizing ultrafast optical switches with energy consumption of the picojoule order.

All-optical switch utilizing intersubband transition in GaN quantum wells

Intersubband transition (ISBT) in GaN quantum wells (QWs) was investigated from the viewpoint of application to ultrafast all-optical switches. The effect of crystalline quality on the absorption saturation characteristics was examined and reduction of edge dislocations was found to be a crucial factor in decreasing operation energy. Then, the switching performance was investigated for devices with improved crystalline quality. Modulation of signal pulses with a pulse interval of less than 1 ps was confirmed. Another device displayed gate-switch operation with an extinction ratio of greater than 10 dB. Comparison of the absorption recovery process in both devices suggested that the process is strongly affected by the QW structure

Sub-picosecond all-optical gate utilizing aN intersubband transition.

All-optical gate-switch operation utilizing GaN intersubband-transition has been achieved by reducing edge dislocation density in the epitaxial layers. The diminution of dislocation was accomplished by MBE regrowth on an MOCVD-grown layer. Excess propagation loss for transverse magnetic polarization decreased due to the reduction of the dislocation. By the improvement of the propagation property, sub-picosecond all-optical gate with an extinction ratio of more than 10 dB was accomplished with an input pulse energy of 150 pJ. Moreover, the insertion loss with the switch on was improved.

Polarization dependent loss in III-nitride optical waveguides for telecommunication devices

Excess polarization dependent loss (PDL) was investigated for GaN waveguide devices grown by molecular beam epitaxy (MBE). The loss for transverse magnetic polarization strongly depended on the edge dislocation density in the crystal, because the dislocations capture electrons and act like a wire-grid polarizer. By means of MBE regrowth on GaN grown with metal-organic chemical vapor deposition (MOCVD), the PDL was reduced to 1∼2dB∕mm with an edge dislocation density of 3×109cm−2, whereas it was approximately 10dB∕mm for an all-MBE-grown sample. An ultrafast all-optical switch utilizing the intersubband transition was fabricated with a multiple quantum well structure that was regrown with MBE on MOCVD-grown GaN. An extinction ratio of as high as 11.5dB was achieved with a control pulse energy of 150pJ, which is attributable to the reduction of the excess PDL.

Calculation of Near-Infrared Intersubband Absorption Spectra in GaN/AlN Quantum Wells

Intersubband transitions in GaN multiple quantum wells (MQW) are expected to be applicable to 1-Tb/s all-optical switches. The near-infrared (1.33–2.15 µm) intersubband absorption spectra of GaN/AlN MQW samples have been theoretically investigated. The measured spectra have been well explained by a model in which zero-voltage drop across one quantum well period and 0.5- to 0.75-nm-thick interface graded layers were assumed. The effective electric field in the well was estimated to be greater than 5 MV/cm. For thick wells, absorption spectra are sensitive to the electric field, and hence to the strain, barrier thickness, and doping level. In thin wells, the effect of the electric field is negligible, but the quality of the interface affects the spectra. The calculation model has been applied to the design of coupled quantum wells (CQW), where faster optical response is expected. Higher-order intersubband absorption at wavelengths of less than 1 µm is also expected to be enhanced in CQWs.

Simulation of Ultrafast GaN/AlN Intersubband Optical Switches

A one-dimensional finite-difference time-domain (FDTD) simulator for ultrafast optical switches based on intersubband transition (ISBT) in GaN/AIN waveguide is described. Influences of the inhomogeneous broadening and the 2D mode profile have been taken into consideration. The ultrafast optical response (τ ∼ 185 fs) measured in a GaN/AIN waveguide was successfully reproduced by the simulator. At present, however, the saturation characteristics of the fabricated device are mainly limited by the excess TM loss caused by the dislocation in MBE-grown nitride layers. When the dislocation density is reduced and the structure is optimized, the switching pulse energy will be improved to about 10 pJ. Further reduction (∼ 1 pJ) will be possible when low-loss submicron waveguides with spot-size converters are developed.

Two-Dimensional Growth of AlN and GaN on Lattice-Relaxed Al0.4Ga0.6N Buffer Layers Prepared with High-Temperature-Grown AlN Buffer on Sapphire Substrates and Fabrication of Multiple-Quantum-Well Structures

Lattice mismatch effects on AlN and GaN growth were studied, aiming at the realization of multiple-quantum-well (MQW) structures. Lattice-relaxed AlN, GaN and Al0.4Ga0.6N were prepared as buffer layers. Microcrystal islands were observed for AlN and GaN respectively grown on the GaN and AlN buffer layers, due to lattice mismatch. However, two-dimensional growth was observed for both layers on the Al0.4Ga0.6N buffer layer. This growth-mode change was ascribable to the fact that a lateral lattice constant for the Al0.4Ga0.6N surface, with residual in-plane compression, is almost the center between those of AlN and GaN. For the MQW structures grown on Al0.4Ga0.6N, it was thought that the AlN and GaN grew two-dimensionally and coherently without significant dislocation generation.

Ultrafast all optical modulation based on intersubband transition in semiconductor quantum wells

Ultrafast modulation of interband-resonant light by intersubband-resonant light in n-doped GaAs/AlGaAs and GaN/AlGaN quantum wells was investigated by femtosecond pump-probe technique. A planar-type AlGaAs/GaAs modulation device shows a modulation speed of ∼1 ps at room temperature. The observed modulation efficiency indicates that 99% modulation can be achieved with a control pulse energy of ∼1 pJ when a waveguide-type device structure is utilized. The feasibility of the all-optical modulation in GaN/AlGaN quantum wells is also investigated. The intersubband carrier relaxation time, which mainly determines the modulation speed, is measured and is found to be extremely fast (130–170 fs). The results indicate that the optical modulation at a bit rate of over 1 Tb/s will be possible by utilizing the intersubband transition in GaN/AlGaN quantum wells. The modulation efficiency in GaN/AlGaN quantum wells is also discussed in comparison with that in GaAs/AlGaAs quantum wells.

Time-Resolved Characterization of All-Optical Switch Utilizing GaN Intersubband Transition

All-optical gate switching with a high extinction ratio was characterized. A faster and a slower decay component were observed. The slower one will be suppressed with optimized quantum well structures and properly selected operation wavelengths.