Soichi Otsuki

A novel fiber-optic gas-sensing configuration using extremely curved optical fibers and an attempt for optical humidity detection

Abstract A novel configuration for optochemical sensors is proposed using a single optical fiber which is extremely curved at an angle of 180° and turned back. Although the curvature of optical fiber caused some loss in the transmission of the incident light, this loss was not very large. To examine the utility of a sensor head using the curved fiber, all or part of the curved region was coated with a Rhodamine B/hydroxypropyl cellulose humidity sensing film instead of the normal cladding. When the curved fiber was used instead of a straight fiber, the modulation of the incident light by the sensing film was dramatically enhanced, and the change with humidity in the transmission of the incident light through the sensor became much larger. The light signal of the humidity sensor using the curved fiber exhibited a short response time, a high reproducibility, and a smooth dependence on humidity with little hysteresis effect in a wide humidity range.

Wavelength-scanning surface plasmon resonance imaging for label-free multiplexed protein microarray assay

We presented wavelength-scanning surface plasmon resonance (SPR) imaging as an optical method for label-free, multiplexed assay of protein microarray. The image of reflected light was measured while the wavelength was scanned in steps at a fixed incident angle. The thickness of the gold layer deposited on the substrate was mainly ∼50 nm for collecting sample data, whereas it was ∼200 nm on a minor part for collecting reference signals. The intensity-versus-wavelength data was doubly compensated for the wavelength dependence and the temporal fluctuation. The wavelength affording minima of the reflected intensity based on SPR (SPR wavelength) were automatically determined at each pixel of the measured image. A gold-deposited substrate was firstly modified with a functional thiol and lastly immobilized with biotin. We estimated the thickness of the thiol and biotin layers as well as that of a spacer layer using our new SPR imaging. Next we monitored the specific binding of avidin to biotin immobilized on the substrate in the flow of a buffer solution. As the measurement was repeated at intervals of 10s, the SPR wavelength determined at each pixel was averaged in real-time for 20 selected areas consisting 314 pixels. The limit of detection was 20 pm in SPR wavelength corresponding to 5 pm in thickness.

Two-dimensional thickness measurements based on internal reflection ellipsometry

An imaging ellipsometer technique on internal reflection geometry that can measure the thickness distribution of a thin film possessing an assumed refractive index is described. Because a prism is used for the internal reflection geometry, it was theoretically predicted that angular derivation from the normal incidence on the prism surface affects only the psi value by a factor of 0.97 at maximum. Measurements were carried out for an optical system of silica substrate-TiO2 layer-silica layer-protein film-air, with a thin-film array of dried protein as the sample film. Thickness of the protein films was two-dimensionally estimated only from the measured map of the delta value by use of the simulated relationship between the thickness and the delta value. The thickness map obtained was coincident on the whole with the results according to a mechanical scanning. The detection limit was approximately +/- 0.2 nm. These findings validate the optical effect of a high-index additional layer to improve the sensitivity and precision of thickness measurements of the sample film on transparent substrates.

Multiple scattering of polarized light in turbid infinite planes: Monte Carlo simulations.

Monte Carlo simulations were performed for infinite plane media containing spherical particles of different sizes. Most of the features of the surface plots for the elements of the effective scattering Mueller matrices are explained by the azimuthal dependence of the matrix predicted according to the theory of Raković et al. [Appl. Opt.38, 3399 (1999)10.1364/AO.38.003399APOPAI1559-128X]. The reduced effective scattering Mueller matrices calculated according to the theory have eight nonzero elements, which are only dependent on the distance from the illumination point. The reduced matrices are factorized approximately into products of a depolarizer and retarding diattenuators. The turbid infinite plane media nearly behave as a pure depolarizer at long distances and become more diattenuating and birefringent with decreasing distance.

Symmetry relationships for multiple scattering of polarized light in turbid spherical samples: theory and a Monte Carlo simulation.

This paper presents a theory describing totally incoherent multiple scattering of turbid spherical samples. It is proved that if reciprocity and mirror symmetry hold for single scattering by a particle, they also hold for multiple scattering in spherical samples. Monte Carlo simulations generate a reduced effective scattering Mueller matrix, which virtually satisfies reciprocity and mirror symmetry. The scattering matrix was factorized by using the symmetric decomposition in a predefined form, as well as the Lu-Chipman polar decomposition, approximately into a product of a pure depolarizer and vertically oriented linear retarding diattenuators. The parameters of these components were calculated as a function of the polar angle. While the turbid spherical sample is a pure depolarizer at low polar angles, it obtains more functions of the retarding diattenuator with increasing polar angle.

Multiple scattering of polarized light in turbid birefringent media: a Monte Carlo simulation.

Multiple scattering of polarized light in a birefringent turbid plane medium was studied using a Monte Carlo simulation. The reduced effective scattering Mueller matrix obtained in the simulation was factorized in two dimensions using the Lu-Chipman decomposition, yielding polarization parameters that exhibited dependences on the azimuth and the radial distance around the illumination point. We propose a double-scattering model for the propagation of polarized photons in turbid infinite plane media. When the birefringence slow axis is along the azimuth of 90° on the plane surface, the retardance becomes the largest negative along the azimuth of 0° and the largest positive along the azimuth of 90° and increases with increasing the azimuth from 0° to 90°. This azimuthal dependence may result from the overlap of the contributions from the light propagations vertical to, and lateral along, the plane surface. Thus, the dependences on the azimuth and the radial distance of the polarization parameters, such as the retardance, its orientation, optical rotation, and the depolarization coefficients, are correctly predicted.

Solvent-dependent photophysical properties of aminophenoxazone dyes as optical probes

Abstract Photophysical properties of three aminophenoxazone (APO) dyes were investigated in many kinds of pure and mixed solvents to assess the possibility for application as an optical probe. The absorption band underwent a significant red shift which was accompanied by a slight change in the spectral profile when increasing the polarity of the solvent. The wavenumber showed a linear correlation with the solvent polarity/polarizability π ∗ in aprotic solvents, indicating that the APO dyes are only proton-accepting. The results in dioxane-water deviated downward from this correlation, suggesting that the hydrogen-bonding interaction with water molecules enhances the red shift. The fluorescence spectrum also exhibited a marked red shift and a change in spectral profile when increasing the polarity of the solvent. The relationship between the wavenumber and π ∗ was nonlinear in aprotic solvents and dioxane-water. A decrease in the rate constant of nonradiative fluorescence decay ( k nr ) abruptly changed to increase when π ∗ increased in dioxane-water. This is presumably because two kinds of nonradiative decay process are involved. The alteration of the chemical structure, i.e., the exchange of ethyl to methyl of the substituent at the 7-amino group and the introduction of a methyl group to the 1 position of the phenoxazone ring caused an increase in the Stokes shift and moved the point of the abrupt change in k nr to a larger π ∗ value.

Thickness measurements on transparent substrates based on reflection ellipsometry. I. Optical effects of high-refractive-index additional layers.

Theoretical studies were conducted for thickness measurements using transparent substrates on the external and internal reflection configurations. For three-phase systems consisting of ambient, film, and substrate, the refractive index of the substrate could be optimized to obtain the high sensitivity of an ellipsometric quantity delta to the film thickness and the small susceptibility of delta to errors in the incident angle. It was shown that the combination of an ordinary glass substrate and an additional dielectric layer with an appropriate layer thickness works as a synthetic high-index single substrate (SHIS). The optical effect of the combination was approximately described by use of the effective refractive index of SHIS. A method to select the refractive index of the additional layer was also given.

Naphthalenesulphonyl groups as fluorescence probes for examining the conformational behaviour of polyallylamine

Abstract The optical absorption and fluorescence properties of polyallylamine (PAA), modified with 1- and 2-naphthalenesulphonyl (1NS and 2NS) groups, were examined in aqueous solution. The monomer emissions of both chromophores increased on addition of ethanol or HCl to the solutions. The addition of K2HPO4 caused a decrease in the monomer emissions for PAA containing 1.92 mol.% of INS and for PAA containing 4.2 mol.% of 2NS. This probably results from the contraction of the polymer conformation due to electrostatic crosslinking by phosphate ions. A similar result was obtained on addition of KCl to the same polymers and was also attributed to polymer contraction due to enhanced intramolecular interactions. However, the monomer emissions remained unchanged or increased for PAA polymers with higher chromophore contents. It is suggested that these highly modified polymers possess a compact conformation without salt, so that little conformational change is exhibited on addition of these salts. The fluorescence behaviour of the 1NS and 2NS groups was compared with that of the 5-dimethylamino-1-naphthalenesulphonyl (DNS) group. The sensitivity of 1NS and 2NS as optical probes towards the above species was higher or as high as that of DNS. In addition, the 1NS group was sensitive to the polarity of the microenvironment around the chromophore in a similar manner to the DNS group. The maximum wavelength of the monomer emission decreased as the polarity decreased.

Internal reflection ellipsometry in air and water ambient

We measured a multilayer sample in air and water ambient below the critical angle of incidence based on internal reflection ellipsometry (IRE). Measurements with varying incident angle and varying wavelength both provided values by the fitting for refractive indices and thicknesses of layers consistent with design or theoretical ones. This verified that IRE is useful in measurement and analysis of thin films on transparent substrates and especially effective to study thin films at transparent substrate-liquid interfaces.