Search

We present an electrostatic-field-tunable ferroelectric template to produce photoreduced silver nanostructures for Raman scattering enhancement. The intensity and distribution of the surface electrostatic field in the ferroelectric template determine the morphology of the photoreduced silver nanostructures and thus the degree of the Raman signal enhancement. The surface electrostatic field is produced by periodically proton-exchanged (PPE) regions in LiNbO3 and is tuned by thermal annealing to obtain the favorable photoreduced silver nanostructure. The variation of surface electrostatic properties by thermal annealing is simulated using the finite element method and measured by electrostatic force microscopy. The mechanism of silver nanostructure formation affected by the electrostatic field distribution is discussed. The formed silver nanostructures are functionalized by R6G dye to enable Raman signal measurement. The proposed method is demonstrated to be effective in tuning the surface electrostatic field distribution and produces a 4.13 times higher silver nanostructure and a 2.51 times larger Raman intensity in comparison with the conventional PPE sample.

Increased detection sensitivity was achieved by adjusting the incident angle on periodic gold nanostructures that induced a resonant coupling between surface and substrate surface plasmon modes. For 500 nm-period gold nanoslits, a small incident angle, 7°, resulted in 2.64 times narrower linewidth and a 1.8 times increase in the figure of merit as compared to normal incidence. Furthermore, the intensity sensitivity was increased 4.5 times due to the change in the resonant coupling and redshift of the surface plasmon mode.

The evanescent tails of a guiding mode as well as its first and second derivatives were measured by a modified end-fire coupling method. The effective index of the waveguide can be obtained by simultaneously fitting these three fields using single parameter. Combined with an inverse calculation algorithm, the fields with fitted evanescent tails showed great improvement in the refractive index profiling of the optical waveguide, especially at the substrate region. Single-mode optical fibers and planar waveguides of proton-exchanged (PE) and titanium-indiffusion (Ti:LiNbO3) on lithium niobate substrates with different refractive index profiles were measured for the demonstration.

In this paper we report that nanoacoustic pulses can modulate the extraordinary optical transmission (EOT) in nanogratings with a high frequency bandwidth. This study was performed on gold nanogratings on top of a GaN crystal by combining a near-field scanning optical microscope with a femtosecond nanoultrasonic system. Experimental results indicate that the propagating longitudinal nanoacoustic pulses changed the refractive index of a GaN crystal and therefore modulated the near-field cavity mode behavior. Our finding suggests that the temporal modulation with a >11GHz bandwidth can be achieved, with a high potential for future temporal and high speed control on the EOT behavior in nanostructures.

A cost-effective, stable and ultrasensitive localized surface plasmon resonance (LSPR) sensor based on gold nanoparticles (AuNPs) partially embedded in transparent substrate is presented. Partially embedded AuNPs were prepared by thermal annealing of gold thin films deposited on glass at a temperature close to the glass transition temperature of the substrate. Annealed samples were optically characterized by using spectroscopic ellipsometry and compare with theoretical modeling to understand the optical responses from the samples. By combining the partially-embedded AuNPs substrate with a microfluidic flow cell and dove prism in an ellipsometry setup, an ultrasensitive change in the LSPR signal can be detected. The refractive index sensitivity obtained from the phase measurement is up to 1938 degrees/RIU which is several times higher than that of synthesized colloidal gold nanoparticles. The sample is further used to investigate the interactions between primary and secondary antibodies. The bio-molecular detection limit of the LSPR signal is down to 20 pM. Our proposed sensor is label free, non-destructive, with high sensitivity, low cost, and easy to fabricate. These features make it feasible for commercialization in biomedical applications.

Chip-based biosensors for sensitive label-free detection were fabricated and tested by using Fano-type resonant nanostructures. The sensor was composed of a 190 nm-thick gold nanoslit surrounded by 600-nm-period grooves. Transverse-magnetic polarized wave in these gold nanostructures generated asymmetrical resonant spectra due to the interference of broad-band cavity resonance in the single slit and narrow-band surface plasmon resonance on the periodic grooves. Compared to nanoslit arrays, such Fano-type sensor has a sharper resonance which yields a figure of merit up to 48. In addition, the crossed talk between sensing elements is reduced due to the Bragg reflection of the periodic grooves. A smaller detection separation down to 10 μm width was achieved. An antigen-antibody interaction experiment in aqueous environment verified the detection sensitivity in surface binding event.

We present a modified end-fire coupling method to reconstruct two-dimensional index profiles of optical waveguides. The reconstruction is based on the measured differential optical fields and an inverse algorithm for the Helmholtz equation. The differential fields are obtained from spatially perturbed images recorded by a CCD camera. A closed-loop piezoelectric stage given a sinusoidal wave and an oil-immersion objective lens for high-resolution images are used in the measurement. A single-mode fiber and a Ti:LiNbO(3) waveguide with known index profile at 632.8 nm are measured for demonstration. The measured index profiles agree quite well with core regions of known index profiles.

The near-field radiation pattern of a long thin slit (with a width much smaller than the excitation wavelength) in a uniform aluminum surface was measured and modeled by numerical computation. In particular, the interplay between the incident light polarization and the slit width is found to play an essential role in the near-field profile on the back side of the nanoslits. Two-dimensional finite-difference time-domain computer simulations were performed to calculate the near-field intensity profile for different slit widths and metal thicknesses. This method will allow the optimization of three-dimensional near-field radiation patterns for a variety of near-field molecular scanning schemes.

Optical contrast on the surface of mesoscale air–dielectric structures was evaluated with collection-mode near-field scanning optical microscopy. When air holes (300 nm) were smaller than the incident wavelength (400 or 600 nm), the optical field was concentrated in the dielectric region. When air holes (1000 nm) were larger than the incident wavelength (400 or 600 nm), there was topographic edge enhancement. After confirmation with finite-difference time-domain simulation, we conclude that light diffracted off topographic edges is a major contributor to contrast in collection-mode near-field optical images.

The propagation and the distribution of the optical near field in nanometallic slits are measured by a near-field scanning optical microscope. The optical near field for the p-polarized wave is confined to the middle of the slit. In contrast, the near field for the s-polarized wave is located at the edges. A simulation by the finite-difference time-domain method verifies that the near-field distribution for the s-polarized wave is due to the propagation of the surface plasmon wave (SPW) at the air-metal surface. The existence of the SPW also accounts for the extraordinary transmittance of s-polarized light, which is one order of magnitude larger than that of p-polarized light.

We report the studies of conjugated polymers, polyaniline thin films, with a near-field scanning optical microscope. Because of the absorption variation in different oxidation states, transmission-mode near-field scanning optical microscope images were employed to map out the distribution of the oxidation states on a submicrometer scale. When the near-field wavelength is varied (between 632.8 and 543.5 nm), the phase separation between the oxidized and the reduced repeated units, with domain sizes on a nanometer-length scale, is observed.

The near-field intensity distribution perpendicular to the light-propagation direction was measured by photochemical processes on conjugated-polymer thin films. The shape of the distribution is elliptical, with the long axis along the direction of the incident polarization. The results are compared with calculations based on the realistic tapered probe geometry. The asymmetry distribution is due to the simultaneous presence of horizontal and vertical electric fields in the near field.