Hisaya Oda

Light amplification by stimulated Raman scattering in AlGaAs-based photonic-crystal line-defect waveguides

The authors performed an amplification experiment of optical signal around 1550 nm by stimulated Raman scattering in a photonic-crystal slab waveguide (WG) of single-line defect. They adopted an air-bridge type AlGaAs-based WG sample to avoid two-photon absorption. Short light pulses of 5 ps in duration operated at 20 MHz and a cw laser were employed for pump (excitation) and probe (signal) lights, respectively. As a result, amplification of a net gain of ∼3 dB (maximum) is achieved with pulse pump energy of 22 pJ for a sample of 1.0 mm in length.

Self-phase modulation in photonic-crystal-slab line-defect waveguides

The authors observed self-phase modulation in spectral domain at 1.50μm in AlxGa1−xAs-based photonic-crystal-slab line-defect waveguides. Adopting a sample with x=0.18 has enabled the authors to avoid two-photon absorption, causing undesirable free-carrier-induced refractive-index change. Variation of the observed spectrum with input pulse peak power is well reproduced by theoretical simulation. They find that a 1.6ps light pulse of 2.5pJ suffices for causing a phase shift of π in a 1mm long sample owing to tight light confinement and nonlinear refractive index n2 due to instantaneous optical Kerr effect is (3.8±1.2)×10−13cm2∕W. A possibility of developing ultrafast all-optical switch due to n2 is discussed

Observation of Raman scattering in GaAs photonic-crystal slab waveguides

We have observed Raman spectra of optical phonons at 1.34 mum in an air-bridge type of GaAs photonic-crystal (PC) slab waveguides (WGs) of 0.6 mm in length. Unlike the bulk GaAs case, both longitudinal (LO)- and transverse (TO) phonons were observed not only in the forward direction, but also in the backward direction. This anomalous feature can be well interpreted as arising from strong confinement of light in PC WGs. The scattering efficiency of LO phonon at 292 cm(-1) is estimated as 1.9 x 10(-7) cm(-1)-sr(-1) in the forward scattering. Based on the present information, the possibility of developing an optical amplifier with use of stimulated Raman scattering in a future ultrafast and ultrasmall PC WGs-based optical circuit is discussed.

Studies on key nano-fabrication processes for GaAs-based air-bridge-type two-dimensional photonic-crystal slab waveguides

We report successful studies on improved key nano-fabrication processes for AlGaAs-based air-bridge-type two-dimensional photonic-crystal (2DPC) slab waveguides. The first topic is proximity-effect correction in electron beam lithography for the definition of a uniform and precise periodic/non-periodic 2DPC waveguide pattern. The second topic is optimization of the selective wet-etching condition in an air-bridge fabrication process. The resultant nano-fabrication processes have been proved to be quite effective for the realization of 2DPC slab waveguides such as modified and topology-optimized bend waveguides having broad-band and high-transmittance spectra in excellent agreement with the calculated ones.

Monolithically grown multi-color InAs quantum dots as a spectral-shape-controllable near-infrared broadband light source

We have grown monolithic multi-color InAs quantum dots (QDs) for use as a near-infrared (NIR) broadband light source. Our previously developed rotational metal mask enables selective-area growth of QD ensembles with emission peak wavelengths controlled by strain-reducing layers thicknesses, varying the emission approximately 1.2–1.3 μm. The peak shift value was up to 120 nm, and a combined 160-nm-bandwidth emission spectrum was achieved. Further, the emission spectra shapes were controlled using specific optical excitation powers for each QD ensemble. These results demonstrate the effectiveness of multi-color QDs as a spectrum-shape-controllable NIR broadband light source, particularly suited for optical coherence tomography.

Operation of an InAs quantum-dot embedded GaAs photonic crystal slab waveguide laser by using two-photon pumping for photonics integrated circuits

The development of small sized laser operating above room temperature is important in the realization of optical integrated circuits. Recently, micro-lasers consisting of photonic crystals (PhCs) and whispering gallery mode cavities have been demonstrated. Optically pumped laser devices could be easily designed using photonic crystal-slab waveguides (PhC-WGs) with an air-bridge type structure. In this study, we observe lasing at 1.3μm from two-photon pumped InAs-quantum-dots embedded GaAs PhC-WGs above room temperature. This type of compact laser shows promise as a new light source in ultra-compact photonics integrated circuits.

Ultra-small near-infrared multi-wavelength light source using a heterojunction photonic crystal waveguide and self-assembled InAs quantum dots

We herein propose and verify an ultra-small near-infrared (NIR) multi-wavelength light source using a heterojunction photonic crystal waveguide (PC-WG) and quantum dots (QDs). A heterojunction two-dimensional PC-WG, which consists of multiple PC-WGs with sequentially shifted structural parameters, is fabricated on a GaAs-slab including InAs QDs. Spontaneous emission (SE) from embedded InAs QDs was enhanced at multiple wavelengths resonating with slow-light regions of the PC-WG modes. The enhanced SE was propagated and detected through the heterojunction PC-WG. These results indicate the feasibility of the proposed light source.

NONLINEAR OPTICAL EFFECTS IN PHOTONIC-CRYSTAL SLAB LINE-DEFECT WAVEGUIDE FOR ULTRAFAST ALL-OPTICAL SWITCH

In order to develop an ultra fast all-optical switch, we have studied optical Kerr effect and two photon absorption in photonic crystal line defect waveguides. Based on these results, we have succeeded in all-optical Mach-Zehnder interferometer switching.

InAs quantum-dots laser utilizing GaAs W1 type photonic-crystal slab line-defect waveguide

Photonic crystals are very suitable for controlling radiation field and propagation characterization of light. As an important application, it is ultra compact and ultrafast optical integrated circuits (OIC) based on photonic crystals slab waveguides (PC-WGs) composing of the line-defects [1, 2]. The PC-WG is also attractive for laser lasing, because very small group velocity of near the Brillouin zone (BZ) edge should enhance interactions between the radiation field and matter. Indeed, the lasing spectrum has been observed from optically pumped InAs-quantum dots (InAs-QDs) embedded in PC-WG of the multimode W3 type (three rows missing line-defect) [3]. In the present study, we observe laser action in InAs-QDs PC-WG of the single-mode W1 type (single row missing line-defect).

Blue light emission from trivalent cerium doped in sol-gel silica glass

Rare earths in glass matrices are promising for active optical devices as amplifiers and lasers. Emission originating from d-f transitions in sol-gel glass has not been studied very often, while those based on f-f transitions were widely utilized. However, d-f emission in rare earths is very important because of their strong oscillator strength and broad emission widths suitable for the application to scintillators and solid-state lasers. Co-doping of aluminum in sol-gel synthesis was known to be effective for the emission enhancement of trivalent terbium and europium. Recently, we applied aluminum co-doping to cerium and europium systems in sol-gel glass to succeed in the observation of strong blue light emission originating from d-f transitions. Glass samples were prepared with conventional sol-gel process where tetramethylorthosilicate was hydrolyzed in the mixture of water, ethanol and dimethylformamide with nitric acid catalyst. After adding cerium nitrate and aluminum nitrate, the solution experienced drying followed by calcination at 1,050°C under air environment. When molar ratio of cerium to silicon was adjusted at 0.1% and Al concentration was varied in 0.1 ~ 2.0%, transparent glass products showed bright and broad blue photoluminescence under UV illumination. The fluorescence lifetimes were found to be about 50 ~ 90 ns, indicating that the emission was due to d-f transitions. Considering the simplicity of the process, blue phosphors based on sol-gel glass will be very promising for future applications.