Zhuangqi Cao

Surface-Enhanced Raman Scattering of Biological Molecules on Metal Colloid II: Effects of Aggregation of Gold Colloid and Comparison of Effects of pH of Glycine Solutions between Gold and Silver Colloids

As the second paper of a series of our studies on surface-enhanced Raman scattering (SERS) of biological molecules on metal colloid, this paper reports effects of aggregation of gold colloid on the SERS intensity of glycine (Gly) in aqueous solutions and comparison of effects of pH of the Gly solutions between gold and silver colloids. In order to investigate the relation between the SERS effect and colloidal aggregation, we have developed a spectroscopic system by which we can measure simultaneously time-resolved SERS spectra and a time-dependent intensity change in a surface plasmon absorption of a metal colloid system. By use of this instrument we monitored time-dependent intensity changes in the SERS signals and in an absorption band at 802 nm of Gly on gold colloid at pH 3.9 and 6.0. For both pH values, the intensities of the SERS and the absorption band change in concert with each other at the emergence of the SERS effect. The intensity changes in the SERS signals are rather slow for the solution of pH 6.0 while rapid for that of pH 3.9. The coagulation proceeds slowly near the isoelectric point of Gly, whereas the rapid coagulation happens in the acidic region. SERS spectra of Gly adsorbed on silver colloid were measured at various pH values, and it was found that, in sharp contrast to the gold colloid system, which we reported in our previous paper, the SERS intensity of Gly on the silver colloid decreases markedly at pH 2.0 where most of Gly in the solution has a COOH group instead of a COO− group. It seems that the coagulation of the silver colloid particles does not occur very much below pK1 (2.35) because the COOH groups do not interact strongly with the positively charged silver particles.

Free-space coupling of a light beam into a symmetrical metal-cladding optical waveguide

We report on a coupling method, which transfers energy of a light beam from free space into a symmetrical metal-cladding optical waveguide from the top surface without employing the prism, grating, tapered film, and other coupling components. In the experiment, a glass flat with a thickness about 100 μm, sandwiched between two gold films is used as a guiding layer. A top gold film of 46 nm thick serves as a coupling layer as well as the cladding and a base gold film of 200 nm serves as the substrate of the waveguide. Up to 50% of the incident light energy fed into the waveguide has been demonstrated.

Observation of large positive and negative lateral shifts of a reflected beam from symmetrical metal-cladding waveguides.

Both large positive and negative lateral shifts were observed for the reflected light beam on a symmetrical metal-cladding waveguide. The positive and negative shifts approach about 480 and 180 microm, respectively, which to our knowledge are the largest experimental results ever reported. The experiment also proves that the positive or the negative shift depends on sign of the difference between the intrinsic and radiative damping.

Study of ultrahigh-order modes in a symmetrical metal-cladding optical waveguide

We report theory and experiment on ultrahigh-order modes in a symmetrical metal-cladding thick optical waveguide. The waveguide consists of a LiNbO3 slab or a glass slab with a thickness greater than 0.1mm, two gold films deposited on the upper and bottom sides of the slab serving as the cladding of the waveguide. By using the free-space coupling technique, ultrahigh-order modes (m>2000) are excited, and its high sensitivity to both refractive index and thickness of the guide, as well as the polarization independence are demonstrated.

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.

Oscillating wave sensor based on the Goos–Hänchen effect

Instead of the evanescent field sensors, an oscillating wave sensor based on the Goos–Hanchen effect is proposed in this letter. It is demonstrated that as the intrinsic damping is well-matched with the radiative damping of the ultrahigh-order modes in a symmetrical metal-cladding waveguide with submillimeter scale, the enormously enhanced lateral beam shift results in a very high sensitivity of the sensor.

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.

Mechanism of giant Goos–Hänchen effect enhanced by long-range surface plasmon excitation

The giant Goos–Hanchen shift on a long-range surface plasmon (LRSP) configuration is examined theoretically. This lateral shift at the resonance of the LRSP can be two orders of magnitude greater than a wavelength. The analytical formulas of the radiative damping and the intrinsic damping of the LRSP are derived. It is found that the thicknesses of the second medium and the metal film determine the radiative damping and the intrinsic damping is illuminated by the imaginary part of the dielectric constant of the metal. Negative lateral beam shift occurs when the intrinsic damping is larger than the radiative damping. The theoretical results show good agreement with the predictions of the formulas.

Oscillating wave displacement sensor using the enhanced Goos–Hänchen effect in a symmetrical metal-cladding optical waveguide

An oscillating wave displacement sensor based on the enhanced Goos-Hänchen (G-H) effect in a symmetrical metal-cladding optical waveguide is proposed. Since the detected signal is irrelevant to the power fluctuation of the incident light and the magnitude of the G-H shift is enhanced to hundreds of micrometers, a 40 pm resolution is demonstrated in our experiment without employing any complicated optical equipment and servo techniques.