Yuuki Sato

Polarization Bistable Characteristics of 1.55 µm Vertical-Cavity Surface-Emitting Lasers

We have demonstrated polarization bistability in 1.55 µm vertical-cavity surface-emitting lasers (VCSELs). InAlGaAs/InP VCSELs with square mesa structures oscillated in the lowest-order transverse mode and in one of the two orthogonal linear polarization states. Highly stable all-optical flip-flop operation was performed at the telecommunication wavelength for the first time by injecting two orthogonally polarized light pulses.

Polarization Bistable Characteristics of Mesa Structure 980 nm Vertical-Cavity Surface-Emitting Lasers

The polarization bistable characteristics of 980 nm vertical-cavity surface-emitting lasers (VCSELs) with a mesa structure were experimentally studied. To achieve this, a distributed Bragg reflector mirror was processed into a rectangular mesa structure waveguide using inductively coupled plasma reactive ion etching and was buried with a polyimide layer. The VCSELs oscillated with single frequency and the lowest order transverse mode under CW operation at room temperature. Additionally, the VCSELs with a nearly-square mesa structure oscillated in either of the two orthogonal linear polarization states. Bidirectional polarization switching was demonstrated by two injection lights for the orthogonal linear polarization states. These results, together with the former results of polarization bistablity, which were observed in another type of VCSEL [Kawaguchi et al.: Electron. Lett. 31 (1995) 109], suggest that polarization bistability is a basic characteristic of square-shaped waveguide VCSELs.

Timing Jitter Reduction by All-Optical Signal Regeneration Using a Polarization Bistable VCSEL

All-optical signal regeneration is experimentally demonstrated using a polarization bistable vertical-cavity surface-emitting laser. The retiming operation of signal regeneration is performed by using an AND gate operation and a reset operation. An optical clock pulse and input data signal are used for the AND gate operation. The timing jitter of the regenerated signal is reduced by optimizing the injection power ratio of the clock pulse and the data signal. The retiming operation is analyzed using a simple model that includes random fluctuation of the polarization switching threshold and bandwidth limitation of the response to the injection light.

Polarization Switching in Vertical-Cavity Surface-Emitting Lasers by Asymmetrical Current Injection

Polarization switching between two orthogonal linear polarizations was experimentally demonstrated in newly designed 980-nm vertical-cavity surface-emitting lasers ( VCSELs ). A square-shaped mesa structure was prepared in a p-type distributed Bragg reflector mirror. Four p-side electrodes were placed in contact with the four sides of the square mesa. When the same current was injected into the four electrodes, the VCSEL oscillated with 0 or 90 linear polarizations. When the current was asymmetrically injected (the same current for two electrodes on the opposite sides of the square, but different for neighboring electrodes), the direction of the linear polarization became perpendicular to the direction of the line connecting the two electrodes injected with the larger current. It was confirmed that the VCSELs oscillated with a single frequency and the lowest order transverse mode in all cases.

Evaluation of TiO2 Nanoparticle Thin Films Prepared by Electrophoresis Deposition

The absence of cracks and a high optical transparency are critical factors for obtaining high performance when TiO2 thin films are used as photocatalysts and as the cathode material in dye-sensitized solar cells. Synthesized TiO2 nanoparticles were deposited by constant-current electrophoresis in ethanol. TiO2 nanoparticle thin films deposited at a low current density had no apparent cracks and a high optical transparency. Small TiO2 nanoparticles deposited are thought to be transported at low current densities. This enables TiO2 nanoparticle chains to form by the oriented attachment mechanism and thereby increases the electron diffusion length.

Fabrication of Al2O3 Films Using Aerosol Deposition Method and Their Characterization

An aerosol deposition method (ADM) apparatus is newly designed and fabricated. Heat-dried-alumina (Al2O3) particles of an average diameter of ~500 nm with narrow dispersion were used as starting materials. These were mixed with N2 carrier gas, and formed a colloiding aerosol. These were accelerated to several hundred m/s through a fixed narrow slit nozzle, and ejected to the substrate. The Al2O3 film was deposited by scanning the substrate with 125 μm/s at room temperature. The deposited Al2O3 films were highly transparent. It is confirmed that there was no primary but crushed particles in the film. Roughness of thin film with 1 μm thick was approximately 22 nm. It is expected that highly dense Al2O3 films could be deposited. Deposition on other materials is also available at room temperature.

Optical buffer memory using a polarization bistable VCSEL

A novel optical buffer memory with a shift register function using a two-dimensional array of polarization bistable VCSELs is proposed. 1-bit optical buffering, which is an essential part of the memory, has been experimentally demonstrated.

Preparation and Evaluation of TiO2 Nanoparticle Thin Films using Electrophoresis Deposition Method

Thin films of synthesized TiO2 nanoparticles were deposited by constant-current electrophoresis deposition in ethanol using colloid of various concentration of TiO2 nanoparticles. Thin films deposited in colloid of high concentration had the high density, no apparent cracks and the higher optical transparency. The eficiency of the fabricated dye-sensitized solar cells was improved using thin film of TiO2 nanoparticle deposited in colloid with high concentration. It was suggested that the thin film with high quality could be deposited by making the velocity of a TiO2 nanoparticle low because the linkage between TiO2 nanoparticles were promoted by oriented attachment mechanism.

Effect of Ba Addition on Electrical Characteristics of Bi-Based ZnO Varistors

With the goal of fabricating low-breakdown-voltage varistors, the effect of adding Ba to ZnO varistors on the ZnO grain size was investigated. Grain growth of ZnO could be markedly promoted by adding both Ba and Bi. The maximum grain size was approximately 150 μm and the minimum varistor voltage was approximately 12 V/mm. However, it had relatively poor tolerance characteristics for electrical degradation. It is speculated that when adding both Ba and Bi to a Mn–Co-added ZnO varistor, it is necessary to form the molten phases of Ba and Bi to promote grain growth of ZnO. It is also conjectured that the growth of ZnO grains is not promoted when Ba and Bi do not coexist in the molten phase because Ba forms compounds with Mn independently with the addition of small amounts of Bi.

Preparation and Evaluation of TiO2 Nanoparticle Thin Films by Electrophoresis Deposition

The transparency and uniformity of thin films of titanium oxide (TiO2) are important factors in determining the characteristics of photocatalysts and dye-sensitized solar cells. Synthesized TiO2 nanoparticles were deposited by constant-current electrophoresis deposition in ion-free water and ethanol. The number of cracks was reduced when ethanol was used instead of water. TiO2 nanoparticle thin films produced using a short deposition time had no apparent cracks and a high optical transparency. Moreover, thick TiO2 nanoparticle thin films, which have high optical trans-parency and no apparent cracks, could be deposited by repeating short-time deposition in ethanol.