Mitsuru Yokoyama

Finite-difference time-domain simulation of two-dimensional photonic crystal surface-emitting laser

Using the three-dimensional (3D) finite-difference time-domain (FDTD) method, we have investigated in detail the optical properties of a two-dimensional (2D) photonic crystal (PC) surface-emitting laser having a square-lattice structure. In this study we perform the 3D-FDTD calculation for the structure of an actual fabricated device. The device is based on bandedge resonance, and four band edges are present at the corresponding band edge point. For these band edges, we calculate the quality (Q) factor. The results show that the Q factor of a resonant mode labeled A1 is larger than that of other resonant modes; that is, lasing occurs easily in mode A1. The device can thus achieve single-mode lasing oscillation. To increase the Q factor, we also consider the optimization of device parameters. The results provide important guidelines for device fabrication.

Two-dimensional photonic crystal lasers

In this article, we report on a two-dimensional (2D) photonic crystal (PhC) laser with a surface-emitting function. First of all, 2D PhC laser with triangular-lattice structure is described. A uniform 2D coherent lasing oscillation based on coupling of lightwaves propagating to six equivalent Gamma-X directions is successfully demonstrated. A large area 2D lasing oscillation over 300 micrometers in diameter and correspondingly a very narrow divergence angle less than 2 degree are observed. Then, the properties of a square-lattice PhC laser are described where the geometry of unit cell structure is designed appropriately to control the polarization mode of emitted light of the device. As the band diagram of the square-lattice photonic crystal is influenced by the unit cell structure, the electromagnetic field distributions at individual band edges are strongly modified and become unified or linear by changing the structure from a circular to an elliptical geometry. We fabricate a laser with a square-lattice PhC of which unit cell structures are elliptical. In spite of the very large diameter, single wavelength at 1.3micrometers and linear polarization mode are observed at representative positions. These results encourage us to realize lasers with desirable features such as perfect single-mode emission over a large area, high output power, and surface emission with a very narrow divergence angle.

Density Measurement of Negative Hydrogen Ions by DC Laser Photodetachment Method

The change of the electron saturation current of a Langmuir probe in a hydrogen discharge due to irradiation of Ar+ laser has been measured as a function of the distance between the probe and the laser beam. The increase of the probe current caused by the photodetachment of negative ions of hydrogen ( H-) induced by the laser is inversely proportional to the distance. The H- density can be determined from this dependence of the increase of the probe current upon the laser-probe distance. However, a simplified treatment of the measured characteristics results in an overestimation of the H- density, and a correction to account for the effect of sheath formation around the probe is made. The H- density determined after the correction ranges from 15% to 50% of the electron density.

Wafer Fusion Condition for GaAs/AlGaAs System and Its Application to Laser Diode

The wafer fusion technique is applied to a GaAs/GaAs system and the current–voltage (I–V) characteristics of the fused interface are investigated. A voltage barrier is observed at the n-GaAs/n-GaAs fused interface, but not at the p-GaAs/p-GaAs fused interface. The wafer fusion technique is also applied to laser fabrication, and a Fabry-Perot-type semiconductor laser using the GaAs/AlGaAs system is fabricated. The lasing oscillation at a wavelength of 785 nm is confirmed. The threshold current density is estimated to be as low as 2.1 kA/cm2.

Optical functional devices based on photonic crystals:laser and add-drop devices

Two optical functional devices based on 2D photonic crystal are investigated. First of all, add-drop filtering devices based on single defects in 2D photonic crystal slabs are characterized. After describing basic characteristics of channel-drop function, we show the improvement of device characteristics through defect engineering. We also demonstrate the channel-add function of the device. Next, we describe a 2D large-area surface-emitting laser exhibiting single-longitudinal and single-lateral mode oscillation with narrow divergence angle based on the 2D coupling effect of lightwave in 2D photonic crystals. A method of controlling the polarization mode by changing the shape of the unit cell is also presented. These results indicate that 2D photonic crystals are useful for the realization of novel optical functional devices.