Xiaoming Dou

Surface-enhanced Raman scattering studies on immunoassay

Surface-enhanced Raman scattering (SERS) has recently been a matter of keen interest from the points of both basic science and applications because by using the SERS effect one can obtain Raman signals even from a single molecule. Immunoassay is one of the most promising fields in the applications of SERS, and the purpose of this review paper is to discuss the potential of SERS in immunoassay. This paper consists of four parts work on the indirect and direct methods of immunoassay via SERS. These methods provide the laboratorial attempts on biomedical diagnostic applications of SERS.

Improved attenuated-total-reflection technique for measuring the electro-optic coefficients of nonlinear optical polymers

A new technique for measuring the electro-optic coefficient of nonlinear optical polymer is described. In an attenuated-total-reflection experiment the refractive index of the poled polymer is changed because of the Pockels effect. This change causes a shift of the angular position of the surface plasmon modes that corresponds to a change in reflectivity at a fixed angle. By measuring the change of the light reflectivity at the properly chosen angle one can calculate the electro-optic coefficient of the poled polymer. Compared with other, conventional methods, here the electro-optic coefficients are given in a simpler form and the required parameters are easier to measure. The commonly used lock-in amplifier is not required. This technique is a highly sensitive method for measuring the electro-optic coefficient because of the newly chosen working interior angle for which a tiny change in the refractive index leads to a large change in reflectivity.

Low voltage electro-optic polymer light modulator using attenuated total internal reflection

Abstract An improved electro-optic (EO) polymer light modulator based on attenuated-total-internal-reflection (ATR) is demonstrated. The modulator consists of a prism–metal–EO-polymer–metal multilayer structure. An applied electric field across the EO polymer layer electrically modulates the energy coupling efficiency of incoming light into guided wave resonance at fixed angles. Compared with conventional ATR modulators based on surface plasmon resonance, the driving voltage for this modulator has been greatly reduced because of the newly chosen working interior angle. It also offers advantages in terms of insertion loss and aperture size over other techniques for the amplitude modulation of a collimated light beam.

Temperature-dependent near-infrared spectra of bovine serum albumin in aqueous solutions: spectral analysis by principal component analysis and evolving factor analysis.

Fourier transform near-infrared (FT-NIR) spectra have been measured for bovine serum albumin (BSA) in an aqueous solution (pH 6.8) with a concentration of 5.0 wt % over a temperature range of 45–85 °C. Not only conventional spectral analysis methods, such as second-derivative spectra and difference spectra, but also chemometrics, such as principal component analysis (PCA) and evolving factor analysis (EFA), have been employed to analyze the temperature-dependent NIR spectra in the 7500–5500 and 4900–4200 cm−1 regions of the BSA aqueous solution. Intensity changes of bands in the 7200–6600 cm−1 and 4650–4500 cm−1 regions in the difference spectra indicate variations of the hydration and secondary structure of BSA in the aqueous solution, respectively. The plot of a band intensity at 7080 cm−1 in the different spectra shows a clear turning point at 63 °C, revealing that a significant change in the hydration occurs at about 63 °C. The forward and backward eigenvalues (EVs) from EFA suggest that marked changes in the hydration and secondary structure of BSA take place in the temperature ranges of 61–65 °C and 59–63 °C, respectively. In addition, the temperature of 71 °C marked in the EFA plots may correspond to the onset temperature of increase in the intermolecular β-sheet structure.

Exact analytical method for planar optical waveguides with arbitrary index profile

We present a novel matrix method that allows the straightforward determination of exact propagation constants as well as the field configuration in the region that is beyond the turning point at the substrate side for arbitrarily graded-index planar waveguides.

Phase shift at a turning point in a planar optical waveguide

We present a novel matrix approach to proving that the phase shift at a turning point in a planar optical wave-guide is exactly equal to pi rather than to pi/2 or to some other value. We also show the existence of phase contributions from reflected subwaves, which to our knowledge have never been taken into account previously.

Tunnelling coefficients across an arbitrary potential barrier

A simple, yet accurate, method of solving the Schrodinger equation across an arbitrary potential barrier is described, based on the analytic transfer-matrix technique. This paper gives the final analytical expression for the transmission probability. Numerical calculations for four typical potential barrier structures are compared with the traditional WKB, modified WKB and the modified Airy function (MAF) methods. Excellent agreement with an exact example is demonstrated. Most importantly, in cases where other potential-barrier methods fail, our method performs well.

Analysis of planar waveguides with index discontinuity or index slope discontinuity

In this paper, we propose a novel analytic transfer matrix method (ATMM) to the analysis of planar waveguides with index discontinuity or index slope discontinuity, for the cases when the conventional WKB method is no longer valid. We also analyze the physical insight of the approximations in WKB theory, and according to our research, the phase shift at the turning point is not π/2, but exactly π. Test calculations are done for an index profile with a known solution and the comparison shows that our method gives extremely accurate propagation constant.

Simplified Analysis for Leaky Modes in Planar Optical Waveguides

In combination with the transfer matrix method and the concept of an equivalent attenuated coefficient, a first-order perturbation theory is proposed to study the leaky modes in planar optical waveguides. Simplified analytical expressions of the perturbed propagation constants are presented in a more explicit and general manner compared to the existing theory.

Electro-optic polymer modulator based on guided-wave resonance

A new type of electro-optic polymer modulator based on guided modes of the Attenuated Total Reflection (ATR) spectrum is described and fabricated. Using a prism to couple a laser beam into a poled polymer thin film, it is found in its ATR spectrum that the fall-offs of the absorbance peaks corresponding to the guided modes can be considered linear. The angular positions of these fall-offs are sensitive to the dielectric coefficient of the poled polymer. If the operating interior angle of the modulator is properly chosen at the midst of these fall-offs, the intensity of the reflected light can be directly modulated by the applied electric field due to the electric-field-induced dielectric coefficient change in the poled polymer film. Compared with the conventional Electro-optical (EO) modulator based on waveguide technology, the insertion loss of the device can be greatly reduced; Compared with the EO modulator based on surface plasmon resonance, the driven voltage can be lessen because that guided wave resonance is much sharper than the surface plasmon resonance.