Enhancement of Refractive Index Sensitivity Using Small Footprint S-Shaped Double-Spiral Resonators for Biosensing.

We demonstrate an S-shaped double-spiral microresonator (DSR) for detecting small volumes of analytes, such as liquids or gases, penetrating a microfluidic channel. Optical-ring resonators have been applied as label-free and high-sensitivity biosensors by using an evanescent field for sensing the refractive index of analytes. Enlarging the ring resonator size is a solution for amplifying the interactions between the evanescent field and biomolecules to obtain a higher refractive index sensitivity of the attached analytes. However, it requires a large platform of a hundred square millimeters, and 99% of the cavity area would not involve evanescent field sensing. In this report, we demonstrate the novel design of a Si-based S-shaped double-spiral resonator on a silicon-on-insulator substrate for which the cavity size was 41.6 µm × 88.4 µm. The proposed resonator footprint was reduced by 680 times compared to a microring resonator with the same cavity area. The fabricated resonator exposed more sensitive optical characteristics for refractive index biosensing thanks to the enhanced contact interface by a long cavity length of DSR structures. High quality factors of 1.8 × 104 were demonstrated for 1.2 mm length DSR structures, which were more than two times higher than the quality factors of microring resonators. A bulk sensitivity of 1410 nm/RIU was calculated for detecting 1 µL IPA solutions inside a 200 µm wide microchannel by using the DSR cavity, which had more than a 10-fold higher sensitivity than the sensitivity of the microring resonators. A DSR device was also used for the detection of 100 ppm acetone gas inside a closed bottle.

1 MW peak-power subpicosecond optical pulse source based on a gain-switched laser diode.

We have generated optical pulses of 1.2 MW peak power and 0.6 ps duration using a 1060 nm band gain-switched laser diode pulse oscillator. Optical pulses are amplified by three-stage ytterbium-doped fiber amplifiers, and remarkable reductions of amplified spontaneous emission noise and temporal duration have been accomplished based on self-phase modulation in the middle-stage amplifier. After the main amplifier, optical pulses were temporally compressed by a grating pair, and this enabled generation of subpicosecond optical pulses with over 1 MW peak power.

Narrow-spectral-linewidth silicon photonic wavelength-tunable laser with highly asymmetric Mach-Zehnder interferometer.

We propose a narrow-spectral-linewidth silicon photonic wavelength-tunable laser with a novel external wavelength-tunable filter, which consists of two silicon ring resonators with different circumferences and a highly asymmetric Mach-Zehnder interferometer (MZI), the two optical paths of which have significantly different lengths. Calculations and experimental results indicated that the gain difference between longitudinal modes was increased by the highly asymmetric MZI. Consequently, a narrow spectral linewidth of 12 kHz and a stable single-mode oscillation were obtained.

Near-infrared imaging of liquid mixtures utilizing multi-channel photonic crystal wavelength filters.

We demonstrate a near-infrared (NIR) spectroscopy-based real-time imaging system of aqueous alcohol solutions (ethanol containing water). The system obtains a set of NIR images by the combination of an InGaAs area sensor and four-channel patterned photonic crystal wavelength filters. Acquired pictures were decomposed into a set of NIR images and processed by principal component analysis (PCA). According to a preliminary experiment for water/ethanol mixture samples, we confirmed that the system was capable of identifying the mixture ratio with accuracy of the order of a few percentage points at a frame rate of approximately 24 frames per second (fps).

Multichannel bandpass filters utilizing multilayer photonic crystal.

We demonstrate multichannel optical bandpass filters utilizing alternating dielectric multilayers stacked on a patterned substrate. A microstructured Si/SiO(2) multilayer was formed by rf biased magnetron sputtering process on a fused silica substrate with surface gratings. The bandwidths of each filter channel were measured to be 50~175 nm in the near infrared region. Control of center wavelength and bandwidth by the grating pitch was verified.

Resonantly guided modes in microstructured optical fibers with a circular array of high-index rods.

A microstructured optical fiber with a new type of waveguiding mechanism is proposed. The fiber consists of a circular rod array of high index material (n=3.48) embedded in a low index background (n=1.44). The rod array exhibits guided-mode resonance (GMR) for cylindrical waves arriving from inside the array, and thus functions as a highly reflective circular wall. Through finite-difference time-domain (FDTD) simulations, we confirmed light confinement and guidance near the GMR wavelength. Basic optical characteristics such as dispersion relations, loss spectra, and mode field profiles were calculated.

Vertical-coupling optical interface for on-chip optical interconnection.

We present a vertical-coupling optical interface with a grating coupler for transmitting and receiving optical signals between single-mode optical fibers and microphotonic waveguides with a view to realize on-chip optical interconnection. The optical interface consisting of a simple grating structure with a reflective mirror and an optical power combiner exhibits high optical coupling efficiency and wide tolerance range for the misalignment of optical fibers. The optical interface exhibits high coupling efficiency even if the optical input is almost vertical to the chip surface.

Guided-mode resonance in curved grating structures.

The guided-mode resonance phenomenon in curved grating structures was studied. By using finite-difference time-domain simulation in the cylindrical coordinate system, we investigated the dependence of the peak reflectivity and bandwidth of the resonance upon the curvature radius. We clarified that the reflectivity and bandwidth were similar to those of flat grating structures for a finite range of curvature. We also discussed the key factor that determines the performance of reflection.

Design and fabrication of multichannel Si/SiO2 autocloned photonic crystal edge filters.

An Si/SiO2 multilayer with zigzag layer interfaces is fabricated on a patterned silica substrate using the autocloning method. The multilayer is designed to function as multichannel long-wave pass type edge filters with various cutoff wavelengths. A cutoff wavelength shift of the order of 190 nm in the near infrared region (1300-1500 nm) is experimentally demonstrated.

Antireflection coatings for multilayer-type photonic crystals.

The possibility of antireflection (AR) coatings on a dielectric multilayer having sub-wavelength deep structural modification is investigated. We numerically surveyed the effect of reflectivity reduction attained by double-layer AR coatings for a wavy multilayer on a patterned substrate. It was clarified that double-layer AR coatings for wavy multilayer is possible with a similar performance level as conventional flat multilayer. Also, it was demonstrated that a pair of AR layers effectively works for a wide range of the horizontal pitch.

Construction of database for three-dimensional human tooth models and its ability for education and research--Carious tooth models -.

To construct a human teeth database which is freely available to researchers and students, three-dimensional human tooth models were generated in a previous study, by means of micro-CT, from 35 human teeth extracted during orthodontic treatment. In this study, X-ray images of 55 extracted human teeth were acquired using three-dimensional micro-CT at a resolution of 50x50x50 microm, and then visualized using a numerical data visualization software. These carious tooth models provided insight into the morphology and progression of carious defects as well as a rare insight into the morphology of carious tooth pulp, therefore rendering them as a useful tool and efficient method for dental students' learning. Moreover, these three-dimensional models could be simultaneously observed and used by many students and researchers at any one time, which was a superior advantage than having only one actual tooth for learning and study by many.

Alveolar bone regeneration of subcutaneously transplanted rat molar.

Regeneration of alveolar bone is essential for periodontal treatment. Recently, cell replacement therapy has been focused on periodontal disease, but the source of the cells that regenerate alveolar bone is still uncertain. Therefore, to clarify the source of these bone-regenerating cells, we transplanted GFP-transgenic rat molars into the subcutaneous tissues of wild-type rats. Five days after transplantation, the tooth was surrounded by connective tissue containing many blood vessels. At 10 days, bone-like tissue was formed in the connective tissue between the branches of the bifurcated root. This hard tissue expanded to almost all of this bifurcation area without osseous ankylosis after 20 days. All osteoblast-like cells in the newly formed matrix were immunopositive for GFP. In addition, these cells and the peripheral cells of the matrix showed intense immunoreactivity for BMP4, Runx2, BSP, and OPN. These results demonstrate that periodontal ligament tissue contains osteoprogenitor cells that have the ability to regenerate alveolar bone. Our model suggests that these regeneration processes might be similar to normal alveolar bone formation.

Immunohistochemical localization of alpha-Smooth muscle actin during rat molar tooth development.

The dental follicle contains mesenchymal cells that differentiate into osteoblasts, cementoblasts, and fibroblasts. However, the characteristics of these mesenchymal cells are still unknown. alpha-Smooth muscle actin (alpha-SMA) is known to localize in stem cells and precursor cells of various tissues. In the present study, to characterize the undifferentiated cells in the dental follicle, immunohistochemical localization of alpha-SMA was examined during rat molar tooth development. Rat mandibles were collected at embryonic days (E) 15-20 and postnatal days (P) 7-28. Immunohistochemical stainings for alpha-SMA, periostin, Runt-related transcription factor-2 (Runx2), tissue nonspecific alkaline phosphatase (TNAP), and bone sialoprotein (BSP) were carried out using paraffin-embedded sections. alpha-SMA localization was hardly detected in the bud and cap stages. At the early bell stage, alpha-SMA-positive cells were visible in the dental follicle around the cervical loop. At the late bell to early root formation stage (P14), these cells were detected throughout the dental follicle, but they were confined to the apical root area at P28. Double immunostaining for alpha-SMA and periostin demonstrated that alpha-SMA-positive cells localized to the outer side of periostin-positive area. Runx2-positive cells were visible in the alpha-SMA-positive region. TNAP-positive cells in the dental follicle localized nearer to alveolar bone than Runx2-positive cells. BSP was detected in osteoblasts as well as in alveolar bone matrix. These results demonstrate that alpha-SMA-positive cells localize on the alveolar bone side of the dental follicle and may play a role in alveolar bone formation.

Temporal behaviour of field in high quality factor photonic crystal microcavity structure.

Temporal behaviour of incident pulse in high-quality (Q) factor photonic crystal microcavities are studied by two dimensional finite difference time domain calculations. For high-Q mode excitation, two periods of oscillation are observed in addition to the exponential decay corresponding to the cavity mode photon life time. Long and short period oscillations correspond to beats with low-Q mode (for short pulse widths) and to lattice periodicity for all pulse widths, respectively. For low-Q mode and off-resonant excitations, long period oscillations correspond to coupling to bandedge states.

Compact 1 x N thermo-optic switches based on silicon photonic wire waveguides.

Using silicon photonic wire waveguides, we constructed compact 1 x 1, 1 x 2, and 1 x 4 Mach-Zehnder interferometer type optical switches on a silicon-on-insulator substrate and demonstrated their switching operations through the thermo-optic effect. These switches were smaller than 140 x 65, 85 x 30, and 190 x 75 mum, respectively. At a 1550-nm wavelength, we obtained an extinction ratio larger than 30 dB, a switching power as low as 90 mW, and a switching response time of less than 100 mus. Furthermore, switching operations were successfully demonstrated for the 1 x 4 switch.