Koichi Awazu

Photonic crystal and quantum dot technologies for all-optical switch and logic device

Nano-photonic technologies of GaAs-based two-dimensional photonic crystal (2DPC) slab waveguides (WGs) and InAs-based quantum dots (QDs) are reviewed for a symmetrical Mach?Zehnder (SMZ) type, ultra-small and ultra-fast all-optical switch (PC-SMZ) and logic device. As the first phase, ultra-fast (~ps) and ultra-low energy (~100?fJ) switching has been demonstrated using a chip 600??m?300??m in size. The second phase is to create a PC-SMZ-based ultra-fast photonic logic switch with a latch function for a future ultra-fast photonic digital processor. One of the priority subjects is to establish a new design method, i.e., topology optimization (TO) method of 2DPC-WGs with wide/flat bandwidth, high transmittance and low reflectivity. Another one is to develop selective-area-grown, high-density and highly uniform InAs QDs with large optical nonlinearity (ONL) by using a metal-mask (MM) molecular beam epitaxy (MBE) growth method. Recent results regarding these two subjects encourage us to reach the final goal.

Colorimetric detection of controlled assembly and disassembly of aptamers on unmodified gold nanoparticles.

Abstract Aptamers are nucleic acid ligands that are generated artificially by in vitro selection and behave similar to antibodies. The development of aptamer-based sensing systems or strategies has been in vogue for the past few decades, because aptamers are smaller in size, stable, cheaper and undergo easier modifications. Owing to these advantages, several facile aptamer-based colorimetric strategies have been created by controlling the assembly and disassembly of aptamers on unmodified gold nanoparticle probes. As these kinds of assay systems are rapid and can be visualized unaided by instruments, they have recently become an attractive method of choice. The formation of purple-colored aggregates (attraction) from the red dispersed (repulsion) state of GNPs in the presence of mono- or divalent ions is the key principle behind this assay. Due to its simplicity and versatility, this assay can be an alternative to existing diagnostic assays. Here, we have investigated the critical elements involved in colorimetric assays, and have screened different proteins and small ligands to evaluate biofouling on GNPs.

Strained Si–O–Si bonds in amorphous SiO2 materials: A family member of active centers in radio, photo, and chemical responses

Amorphous SiO 2  (a- SiO 2 ), such as bulk silicaglasses and thin films has been one of the key materials in modern optoelectronic industries. These materials are currently used in communication technologies as optical fibers, thin films for electrical insulation in dynamic random access memories (DRAM), and optical lenses for excimer laserlithography, for example. The property essential for these applications is the wide band gap amounting to ∼9 eV. However, bulk silicaglasses commercially available and silica thin films show photoresponses to subband gap lights in the vicinity of 5 eV and unexpected trapping of charges, and the behavior has a strong dependency on the preparation history. A number of studies were carried out to clarify the relationship between the properties and structural imperfections in the materials and the formation mechanisms of the defects. There are two categories of the imperfections: one is dopant- or impurity-related imperfections and the other is nonstoichiometry related defects. These defects constitute gap states in a- SiO 2 . The structural identification was usually performed by absorption and emission spectroscopy in the visible–ultraviolet (UV) region and electron spin resonance(ESR). The experimentally proposed models were compared with the predictions by theoretical calculations of energy levels. Recent development of the excimer laserlithography technique led us to recognize that a latent member, which has been unnoticed because of no response to the optical absorption or emission in the visible-UV range and ESRabsorption, exists in the family of active centers in a- SiO 2 , that is a strained Si–O–Si bond originating from the planar three membered ring. In contrast, the puckered four membered ring is unstrained. Although it has been pointed out that there was a wide distribution in Si–O–Si bond angle from 90° to 180° by x-ray analysis or 29 Si solid state nuclear magnetic resonance, the physical, and chemical responses of the Si–O–Si bonds with a particular bond angle could not be differentiated. Very recently it was clarified that a strained Si–O–Si bond, in other words chemically excited bonds, has an optical absorption locating on the band edge. The chemically excited bond can be scavenged by fluorine doping, because it is chemically reactive. In the present review we show that the unresolved optical and electric responses of silicaglasses can be comprehensively understood by taking the presence of the strained bonds into consideration.

Photoluminescence Centers in VAD SiO2 Glasses Sintered under Reducing or Oxidizing Atmospheres

Two types of diamagnetic luminescence centers were found in VAD SiO2 glass sintered under reducing atmospheres. One of the PL centers, which had an excitation band at 5.17 eV (240 nm) and two emission bands, at 4.2 eV (295 nm) and 3.1 eV (400 nm), was mainly detected in the glasses sintered under H2/He atmosphere. The intensity of the luminescence increased with the partial pressure of hydrogen of the atmosphere. The other PL center, which had an excitation band at 5.06 eV (245 nm) and at least two emission bands at 4.2 eV (295 nm) and around 3.0 eV (413 nm), has been predominantly observed in the sample melted under vacuum. On the basis of the atmosphere dependence of PL and optical absorption measurements in the VUV region, the former was assigned to Si (II) dissolved in a silica network. The Si cluster, in which the number of direct Si-Si bonds exceeded 4, was proposed as a possible candidate for the latter.

Photoluminescence in VAD SiO2:GeO2 Glasses Sintered under Reducing or Oxidizing Conditions

The atmosphere dependence of photoluminescence (PL) centers produced in SiO2:GeO2 glasses was examined by preparing VAD SiO2:GeO2 preform rods with changing the sintering atmosphere systematically from reducing to oxidizing. Four types of luminescence centers could be detected for samples sintered under H2/He=1/20, He or O2/He=1/10 atmospheres. The atmosphere dependence of luminescence intensity suggests that reduced species of Ge and/or Si are responsible for the luminescence centers. Ge(II) dissolved in a SiO2 network, which was characterized as a lone pair of electrons located on a Ge atom with 2 or 3 oxygen neighbors. Si(II) dissolved in a silica network, and the Si and/or Ge cluster and [chemical formula] or [chemical formula] were proposed for the possible structural models of PL centers.

Silica-based monolithic sensing plates for waveguide-mode sensors

We developed a monolithic sensing plate for a waveguide-mode sensor. The plate consists of a SiO(2) glass substrate and a thin silicon layer the surface of which is thermally oxidized to form a SiO(2) glass waveguide. We confirmed that the sensing plate is suitable for high-sensitivity detection of molecular adsorption at the waveguide surface. In addition, a significant enhancement of the sensitivity of the sensor was achieved by perforating the waveguide with holes with diameters of a few tens of nanometers by selective etching of latent tracks created by swift heavy-ion irradiation. Possible strategies for optimizing the plate are discussed.

High sensitivity sensors made of perforated waveguides

Sensors based on surface plasmons or waveguide modes are at the focus of interest for applications in biological or environmental chemistry. Waveguide-mode spectra of 1 mum-thick pure and perforated silica films comprising isolated nanometric holes with great aspect ratio were measured before and after adhesion of streptavidin at concentrations of 500 nM. The shift of the angular position for guided modes was nine times higher in perforated films than in bulk films. Capturing of streptavidin in the nanoholes is at the origin of that largely enhanced shift in the angular position as the amplitude of the guided mode in the waveguide perfectly overlaps with the perturbation caused by the molecules. Hence, the device allows for strongly confined modes and their strong perturbation to enable ultra-sensitive sensor applications.

Monitoring biomolecular interactions on a digital versatile disk: a BioDVD platform technology.

A spinning-disk biosensor utilizing optical interference of reflected light from a multilayered structure, consisting of dielectric, metal, and optical phase-change thin films, is shown to have the potential to monitor various interactions on its surface. We refer to this platform as a BioDVD, since it utilizes the optical system of a digital versatile disk (DVD) to measure changes in reflected light intensity. Here, we demonstrated that nucleic acid hybridization and RNA-protein interactions can be analyzed efficiently, in a label-free environment, by measuring the reflected light intensity using a DVD-like mechanism. Moreover, our studies revealed that the detection sensitivity for the interactions on the BioDVD can be altered by shifting the state of the phase-change materials, where the amorphous state can be used for analysis and another state (crystalline) can be used both for recording information and selectively masking areas of the disk.

Alternative modification methods for sol–gel coatings of silica, titania and silica–titania using ultraviolet irradiation and water vapor

Abstract We investigated relatively new techniques for densification and crystallization of sol–gel silica, titania and silica–titania coatings at relatively low temperatures. Irradiation of ultraviolet photons above the band-gap energy was found to lead shrinkage of the sol–gel films through electronic excitations. Crystallization of sol–gel titania coatings was achieved by irradiation of ultraviolet laser (4.7 eV) beam. Exposure to water vapor was shown to be effective in rearrangement of sol–gel films causing densification of silica, crystallization of titania and phase separation of silica–titania around 100°C. These techniques are useful for preparation of hard silica coatings and crystalline titania films on low-heat-resistant substrates, such as polymer plates.

A sensitive multilayered structure suitable for biosensing on the BioDVD platform.

Several technologies are currently available for the analysis of biomolecular interactions with high sensitivity and efficiency. However, these instruments are invariably expensive and, thus, are not suitable for bedside analyses. To circumvent this issue, we have previously reported a BioDVD platform that allowed us to use a DVD mechanism to monitor various biomolecular interactions [Gopinath et al., 2008, ACS Nano 2, 1885-1895]. In the present study, to improve the sensitivity of the BioDVD platform for various analyses, we have performed computer simulations to optimize the ZnS-SiO(2) layer thicknesses and determined an optimized optical interferometric response after adjusting the ZnS-SiO(2) layer thickness to 65 and 60 nm for the inner and outer layer thicknesses, respectively. Biomolecular interaction analyses performed with the optimized BioDVD disks revealed a 3-fold improvement in the sensitivity, compared to our previously reported multilayered structure. In this study, we have also shown that the BioDVD platform is suitable not only for analyzing nucleic acid hybridization and interactions between RNA-small ligands and RNA-proteins, but also for antigen-antibody interactions. Furthermore, our evaluations revealed that each sample required no more than 10 tracks of data to analyze the biomolecular interactions on the BioDVD platform, which permits a greater number of spots per BioDVD disk and also reduces the time needed to measure the biomolecular interactions.