Silicon photonic receiver for satellite laser communication terminals.

We report the optical performance of a photonic receiver for laser communication applications. The receiver is composed of 14 × 12 grating coupler arrays. The received optical signal power will be combined electrically via germanium photodiodes. The photonic receiver is designed for 20-µm to 30-µm mode field diameter (MFD) input sources. To maximize the fill factor of the 200 µm × 200 µm light-receiving area, a design strategy has been proposed. (1) Grating couplers are customized for compactness. (2) Periods of grating couplers are designed to work as end-fire and back-fire grating couplers for the same incident angle of the input laser source. (3) Different widths of waveguides are routed to minimize cross talk. The photonic receiver is evaluated with a 10-µm MFD source. As a result of the evaluation, the receiving area considering the minimum efficiency of -10.5 dB is 95% of the designed area when illuminating 20-µm to 300-µm MFD laser sources.

Angled-MMI-based wavelength splitters on silicon nitride waveguide platforms for fluorescence sensing.

Compact wavelength splitters based on angled multimode interferometers (AMMIs) on silicon nitride platforms working in visible lights are reported for fluorescence sensing applications. A diplexer and triplexer with different footprints are designed and experimentally demonstrated. The diplexer and triplexer have the insertion loss of ∼1.7 and ∼2.7 dB/channel with cross talks of less than -22 dB and -17 dB on target wavelengths, respectively. These splitters are used to distinguish the signals collected from two fluorescent dyes that give different emission spectra when excited with an excitation source, due to their different Stokes shifts. In the case of the triplexer, a third port is to collect the excitation light, both to monitor the remaining excitation power and to reduce the interference at the signal ports. A termination structure at the end of the AMMIs and input and output tapering waveguides as a part of the wavelength splitters are designed and their performances are presented.

Nanometer-Scale Phase Transformation Determines Threshold and Memory Switching Mechanism.

Creation of nanometer-scale conductive filaments in resistive switching devices makes them appealing for advanced electrical applications. While in situ electrical probing transmission electron microscopy promotes fundamental investigations of how the conductive filament comes into existence, it does not provide proof-of-principle observations for the filament growth. Here, using advanced microscopy techniques, electrical, 3D compositional, and structural information of the switching-induced conductive filament are described. It is found that during in situ probing microscopy of a Ag/TiO2 /Pt device showing both memory- and threshold-switching characteristics, a crystalline Ag-doped TiO2 forms at vacant sites on the device surface and acts as the conductive filament. More importantly, change in filament morphology varying with applied compliance currents determines the underlying switching mechanisms that govern either memory or threshold response. When focusing more on threshold switching features, it is demonstrated that the structural disappearance of the filament arises at the end of the constricted region and leads to the spontaneous phase transformation from crystalline conductive state into an initial amorphous insulator. Use of the proposed method enables a new pathway for observing nanosized features in a variety of devices at the atomic scale in three dimensions.

Polarization-independent nonuniform grating couplers on silicon-on-insulator.

Grating couplers are proposed for polarization-independent coupling of light between a single-mode fiber and a 220-nm-thick channel waveguide on silicon-on-insulator. The grating couplers have nonuniform grating periods that are composed of the intersection or union of a set of two near-optimal TE- and TM-grating periods. The proposed grating couplers have a coupling efficiency greater than 20% and polarization dependent loss (PDL) lower than 0.5 dB within 3-dB bandwidth in design. For the evaluation of the design concept, a fabricated intersection grating coupler has the PDL of less than 0.8 dB within the wavelength range of 1540 to 1560 nm, and the coupling efficiency is ∼18%.

Glycothermal Synthesis and Photocatalytic Properties of Highly Crystallized Anatase TiO2 Nanoparticles.

Highly crystallized anatase TiO2 nanoparticles were synthesized at a temperature as low as 120 °C through a glycothermal reaction using amorphous titanium hydrous gel as precursor and 1,4-butanediol and water as solvent. X-ray diffraction (XRD) and transmission electron microscopy (TEM) data support that the glycothermal processing method provides a simple low-temperature route for producing highly crystallized anatase TiO2 nanoparticles without pH adjustment. It is demonstrated that the shape and dispersability of TiO2 nanoparticles can be controlled by the reaction conditions, such as the reaction temperature and variation of the volume ratio of 1,4- butanediol/water (B/W). It was observed that TiO2 samples glycothermally prepared at 220 °C and the B/W ratio of 8/0 showed excellent photocatalytic behavior. The high activity is attributed to the high crystallinity and bipyramidal shape of the particles, which have fewer defects and more active {101} surfaces.

Fabrication of nano-LaCrO3 receptor by polymeric precursor method and its impedancemetric NOx sensing properties.

LaCrO3 was prepared by using the polymeric precursor method for use as a receptor material and its NOx sensing characteristics were investigated. Nano-LaCrO3 powders were synthesized at the optimum compositions of the mole ratio of [La-, Cr-source]:[EG]:[AcAc] = [a, a]:[160 a]:[8 a] with 1 wt% polyvinylpyrrolidone (PVP) using ethylene glycol (EG) as a solvent, acetyl acetone (AcAc) as a chelating agent, and PVP as a polymer additive. The thermal decomposition behavior, crystal structure, morphology, and particle sizes of nano powders were characterized by a thermal analysis (TG-DTA), X-ray diffraction (XRD), a field emission scanning electron microscopy (FE-SEM), and a particle size analyzer, respectively LaCrO3 powders were mainly orthorhombic in structure and the primary particle size was 30 nm according to the XRD results. All solid-state compact impedancemetric-type sensor devices composed of Li1.5Al0.5Ti1.5(PO4)3 (LATP) as a transducer and a perovskite-type LaCrO3 nano powder as a receptor, have been investigated for their ability to detect NOx (NO and NO2) in the range of 1-250 ppm at 400 degrees C. The sensor device showed high gas sensitivities at NO gas, but relatively low gas sensitivities for NO2 gas.

Fabrication of a pure TiO2 thin film using a self-polymeric titania nano-sol and its properties.

A pure TiO2 thin film without adding any organic binder was fabricated by using a self-polymeric titania nano-sol (14 mass%), which was prepared by the acid peptization method. The particle size distribution in the 14 mass% TiO2 sol, in which almost of particles had a size below 10.2 nm and the crystal phase confirmed by X-ray diffraction analysis was anatase. The diluted nano-sol had a capability to form a thin film at a low temperature (100-400 degrees C) on the slide glass by dipping method. The average thickness of a coating film was measured to be about 0.25-0.30 microm. A coated film had a high refractive index over 1.88 at least irrespective of the heat-treatment even at room temperature drying and showed a super-hydrophilicity (< 5 degrees) after 20 minutes under Ultra Violet light irradiation, and it sustained in the darkness during a long period over 7 days depending on the heat-treatment conditions. Atomic Force Microscopic observation shows that the morphology of a heat-treated film had a relationship with the long-term hydrophilicity in the darkness.

Silicon polarization independent microring resonator-based optical tunable filter circuit with fiber assembly.

We report the design, fabrication, photonic packaging and the characterization of a silicon polarization independent optical tunable filter circuit with fiber assembly. We demonstrate the polarization transparent filter characteristics with an insertion loss of ~13.4 dB, an extinction ratio of ~20 dB, and a 3 dB bandwidth of 0.2 nm. The tuning range is of ~11.72 nm, along with the tuning speed of less than 400 µs. The tuning efficiency is ~0.23 nm/mW. The use of polarization diversity scheme and the silicon photonic packaging bridges the gap between the silicon photonic circuits and the real applications.

Si-photonics based passive device packaging and module performance.

We report a fully packaged silicon passive waveguide device designed for a tunable filter based on a ring-resonator. Polarization diversity circuits prevent polarization dependant issues in the silicon ring-resonator. For the device packaging, the YAG laser welding technique has been used for pigtailing both of the input and output fibers. Post welding misalignment was compensated by mechanical fine tuning using the seesaw effect via power monitoring. Packaging loss less than 1.5 dB with respect to chip measurement has been achieved using 10 µm-curvature radius lensed fibers. In addition, the packaging process and the module performance are presented.

Optimal laser welding assembly sequences for butterfly laser module packages.

We present an optimal laser welding assembly sequence for butterfly laser packages: 1) initial shift, 2) front welding, 3) rear welding, 4) joint gripper releasing, 5) mechanical fine tuning of horizontal misalignment. This sequence has been optimized significantly by modeling the initial shift and experimental investigations of three assembly sequences. Our results show that misalignment from the Post-Weld-Shift (PWS) can be compensated by accurately estimating the initial shift in the vertical direction. Furthermore, the laser hammering procedure, to compensate for misalignment of the vertical direction, can be eliminated by proper package design. Using only final mechanical tuning for horizontal misalignments, optical coupling efficiencies of 73-99% have been achieved for lasers packaged in butterfly modules.

Direct observation of internal fluidity in a water droplet during sliding on hydrophobic surfaces.

In the current study, we used a high-speed camera system with particle image velocimetry to observe the internal fluidity of water droplets during sliding. The droplets' velocity during sliding was controlled by slipping and rolling motions. On the superhydrophobic surface, with a contact angle of 150 degrees, the droplet fell at high velocity by slipping. However, on a normal hydrophobic surface whose water contact angle was around 100 degrees, both slipping and rolling controlled the droplet's velocity during sliding. In addition, the advancing velocity might be large when the slip velocity is large and the contact area is small.