Koji Ishida

Design, fabrication, and characterization of coupling-strength-controlled directional coupler based on two-dimensional photonic-crystal slab waveguides

A directional coupler (DC) with a coupling-strength control defect (CCD) based on a two-dimensional photonic-crystal slab waveguide is numerically and experimentally studied. The structure consists of two parallel-coupled single-defect waveguides separated by a triple-line air-hole array, with some missing air holes in the middle of the triple line. Using a two-dimensional finite-difference time-domain method, a 50% coupler was designed and almost 50% coupling performance was experimentally confirmed at a wavelength of ∼1.3 μm with a coupling length of about 7 μm, about 1/3 in length as compared to the DC without the CCD.

SIMS Study of Compositional Disordering in Si Ion Implanted AlGaAs?GaAs Superlattice

Compositional disordering in Si ion implanted AlGaAs–GaAs superlattices (SLs) has been studied by SIMS. It is found that Si diffuses fast when the Si concentration exceeds 3×1018 cm-3. The disordering of the SLs always occurs with this fast Si diffusion. It is suggested that the disordering of the SLs is induced by the substitutional exchange of (SiIII–SiV) pairs with the matrix vacancies. The Si diffusion in the SL layer is not enhanced by the presence of the GaAlAs/GaAs interface.

Ultra-Small GaAs-Photonic-Crystal-Slab-Waveguide-Based Near-Infrared Components: Fabrication, Guided-Mode Identification, and Estimation of Low-Loss and Broad-Band-Width in Straight-Waveguides, 60°-Bends and Y-Splitters

We designed and fabricated ultra-small photonic-crystal (PC)-waveguide (WG)-based components of straight-WGs, 60°-bends and Y-splitters, utilizing single-line-defect in the air-bridge type GaAs PC, and revealed their optical properties in detail by observing a transmittance (T) spectrum over a broad wavelength-region from 850 to 1600 nm. For this purpose, we developed a halogen-lamp-based spectrometer for an ultra-thin slab-sample, which has turned out very useful. We also calculated the corresponding T-spectrum by using three-dimensional finite-difference-time-domain method. In all cases where comparison is possible, the observed spectrum agrees essentially with the calculated one. Owing to these broad spectra, we have unambiguously identified the high-T region due to single guided-mode in the respective components. Comparisons of the spectrum between different samples or components, such as the straight-WGs with much different lengths, or the straight-WG and the sharp bend have also helped to reasonably interpret the observed spectra, allowing a comprehensive understanding of optical properties of those components. The propagation loss and the band width in each component could also be estimated from the comparison. The loss is estimated as 1.5 ±0.5 dB/mm for the straight-WG, while less than 1 dB/bend, if it exists, for the bend WG, and both band-widths are broader than 40 nm. We also find that the Y-splitter is capable of dividing light into two ports almost equally in intensity with a total transmittance of more than 85%; the band-width is broader than 40 nm. As a result, we have found that all those should be useful as key components in future ultra-fast planar optical integrated circuits.

InAs quantum-dot laser utilizing GaAs photonic-crystal line-defect waveguide

We have observed laser action from optically-pumped InAs-quantum-dots embedded in a line-defect waveguide in an air-bridge type GaAs-photonic-crystal slab (an array of air-holes). The lasing is found to occur without any optical cavity such as a set of Fabry-Perot mirrors. Comparison of the observed transmittance spectrum with the calculated band dispersion of the W3 defect-mode enables us to specify the lasing wavelength as that at the band edge. From this fact it follows that distributed feedback mechanism at the band edge with a vanishingly small group-velocity should be responsible for the present lasing. Usefulness of this kind of compact laser in a future ultrafast planar photonic integrated circuit is discussed.

SIMS Study of Si–Be Co-doping Effects for Suppression of Compositional Disordering in AlGaAs–GaAs Superlattices

We have studied the Si–Be co-doping effect for the suppression of the compositional disordering of AlGaAs–GaAs superlattices (SLs) by SIMS. The Si–Be co-doping is found to suppress the fast Si diffusion which is ascribed to the formation of (SiIII–SiV) pairs. The possible mechanism is that the formation of (BeIII–SiIII) pairs prevents that of (SiIII–SiV) pairs. The suppression of the Si fast diffusion impedes the SL disordering. The threshold concentration of Be required to suppress the SL (300 A periodicity) disordering is around 2×1018 cm-3 in the case of 1.5×1019 cm-3 Si concentration.

Fabrication of Index-Guided AlGaAs MQW Lasers by Selective Disordering Using Be Focused Ion Beam Implantation

AlGaAs multiquantum well (MQW) lasers with buried optical guide (BOG) were fabricated using Be focused ion beam (FIB) implantation. The process is based on the effect wherein the compositional disordering of Si doped MQW is suppressed by Be ion implantation and subsequent annealing. The proposed process is very simple and does not require any lithographic procedure. It is confirmed that the fabricated laser acts as an index-guided laser. This technique is a promising new step toward optoelectronic integrated circuits.

Fabrication of Submicron Grating Patterns Usinc Compositional Disordering of AlGaAs-GaAs Superlattices by Focused Si Ion Beam Implantation

Submicron grating patterns (0.4 µm period) were fabricated in AlGaAs-GaAs superlattices (SLs) using compositional disordering of SLs by focused Si ion beam implantation and subsequent annealing. The grating structure which is composed of preserved SL and mixed (disordered) regions was confirmed by scanning electron microscopic (SEM) and scanning Auger microscopic (SAM) observations.

Ultra-Small Photonic-Crystal-Waveguide-Based Y-Splitters Useful in the Near-Infrared Wavelength Region

We report on the successful development of an ultra-small photonic-crystal(PC)-waveguide-based 120° Y-splitter operating at 1310 nm, which consists of mutually intersecting single-line-defects in the Γ-K direction with an additional air-hole in the middle of the intersection, based on the air-bridge-type GaAs PC of an array of air-holes. The transmittance spectra observed by using a halogen-lamp-based spectrometer newly developed for thin PC-waveguide samples, reveal that a total transmittance of 85% at two output ports is achieved with a band-width as broad as 50 nm. This result indicates that the present Y-splitter should be useful as a key component in future ultraminiature planar optical circuits.

Scan speed effects on enhanced disordering of GaAs‐AlGaAs superlattices by focused Si ion beam implantation

We studied compositional disordering of GaAs‐AlGaAs superlattices induced by focused Si ion beam implantation and subsequent annealing with different doses and scan speeds by secondary ion mass spectrometry and Rutherford backscattering spectrometry. The results indicate that the Al‐Ga intermixing is enhanced by substitutional exchange of Si‐Si pairs with vacancies as in the case of the unfocused implantation. However, with a slow scan implantation, it is confirmed that the implantation‐induced defects suppress the Si diffusion and consequently Al‐Ga interdiffusion.

Fabrication of Index-Guided AlGaAs Multiquantum Well Lasers with Buried Optical Guide by Si-Induced Disordering

Transverse mode controlled AlGaAs–GaAs multiquantum well(MQW) lasers with buried MQW optical guide (MQWBOG) have been developed using compositional disordering induced by Si ion implantation and subsequent annealing. It is confirmed that the MQW-BOG lasers fabricated by these simple and completely planar processes operate in a fundamental transverse and single longitudinal mode.