We have developed a new detection scheme based on a scanning near-field optical microscope to image both the linear and the non-linear (e.g. second harmonic generation (SHG)) on surfaces with sub-wavelength resolution. The microscope we used here scatters the evanescent waves that contain sub-wavelength information with the apex of a metallic tip. The resolution of this microscope is directly given by the size of the radius of curvature of the metal tip end
(about 5 nm). Our set-up is applied to the optical study of crystalline films and random metal surfaces. Using thin SBN films
(Strontium Barium Niobate) we demonstrate that near-field optics is a surface sensitive measurement. The ability to perform high quality and highly resolved images is transposed to increase the resolution of imaging in the THz domain. It is also used in the visible domain on random metal films. Several studies have demonstrated that random metal surfaces show localization of the electric field on small area called "hot spots" where the electric field can exceed the applied field by several orders of magnitude. Position of the hot spots depends on film structure, on the polarization and wavelength of the illuminating laser beam. In addition, these random metal films are known to be the source of nonlinear optical effects. We are currently working to precisely locate the respective position of the linear and non-linear hot spots on silver.
Near-field Raman spectroscopy using an apertureless metallic probe has attracted much attention owing to its capability of chemical analysis with high spatial resolution far beyond the diffraction limit. The local plasmon excited at the probe tip amplifies optical near-field in the vicinity of the tip apex, and the local field is used to enhance Raman scattering. The metallic probe contributes not only to the enhancement of the Raman scattering, but also to spectral changes due to the chemical and mechanical interaction between the metal and the molecules. We experimentally and theoretically investigated these two effects in this study. These effects selectively provide vibrational information of the molecules directly adsorbed on the metal, and, therefore, have a potential to improve the spatial resolution. In addition, the metallic probe has also been applied for enhancement of nonlinear Raman scattering. Coherent anti-Stokes Raman scattering (CARS) has been strongly enhanced by the probe, and has provided molecular-vibration images of deoxyribo nucleic acid (DNA) with high spatial resolution.
A performance enhancement to planar lens lithography (PLL) through the use of i-line narrowband exposures has been investigated. Experimental results show that for a 50nm silver layer the image fidelity of narrowband exposures out performs broadband exposures. This is due to the removal of off-plasmonic-resonance wavelengths, which cause unwanted background exposure and a loss of image fidelity. Dense gratings have been resolved down to 145nm periods, as well as line-pairs down to separation distances of 117nm. These results out perform the diffraction-limits that restrict traditional optical-system resolution limits.
We demonstrate that a nanostructured plasmonic composite material
can show negative index of refraction at infrared and optical frequencies. In contrast to conventional negative refraction materials, our design does not require periodicity and thus is highly tolerant to fabrication defects. Moreover, since the proposed material is intrinsically non-magnetic (μ ≡ 1), its performance is not limited to proximity of a resonance so that the resulting structure has relatively low loss. We develop the analytical description of the relevant electromagnetic phenomena and justify our analytic results via numerical solutions of Maxwell equations.
Index-guide of surface plasmon polariton in negative dielectric gap waveguides is studied theoretically. Concepts of low-dimensional optical waves and wavenumber surface are introduced for simplicity. Propagation properties of two-dimensional optical waves in a metal gap waveguide are calculated by means of an effective index method and FDTD calculations. A wave front of two-dimensional optical waves expands as cylindrical waves and optical beams confined in the gap have the two-dimensional diffraction limit. It is shown that the beam width of two-dimensional optical waves can be squeezed to nanometer order by decreasing the gap distance. A dielectric core embedded in the gap can be used as an efficient nano-optical waveguide.
The phenomenon of resonant tunneling through thin metal films with periodic narrow grooves is attributed to excitation of the surface plasmon (SP) via the periodic groove structure coupler at the metal surface. In this paper, we will use the particle-in-cell (PIC) plasma simulation method to study this SP-mediated optical tunneling. The PIC method is a time-domain scheme to calculate self-consistently the interaction between the electromagnetic fields and the plasma particles. At the beginning of simulation, the mobile electrons and immobile positive ions are uniformly distributed in the thin Gaussian-shaped-grooved silver film with the plasma density calculated from silver's plasma frequency. The momentum collision-frequency method is employed to model the collision dissipation. For normally incident TM-polarized wave, the transmission coefficients have the maximum values at the LSP resonant modes, similar to the results predicted by Drude model, except for with lower transmission coefficients. Due to the electron dynamics considered in the PIC method, the plasma energy and the trajectories can be monitored during the simulation. The change of the averaged plasma energy with time exhibits some ripple-like patterns, which comes from various competing processes of heating and cooling. But the temperature of the plasma has little effect on the transmission coefficient and the wave tunneling.
We have studied the electromagnetic field distribution around metallic nanostructures using a scanning near-field optical microscope. The probe is a small fluorescent particle settled at the extremity of a tungsten tip. If the sample is illuminated, the fluorescence emitted by the particle varies with its position on the surface, giving a representation of the electromagnetic field distribution. The nanostructures, which are gold nanoparticles and nanoapertures in a thin gold film have been imaged as a function of the incident light polarization direction. In the case of gold nanoparticles, an increase of the field intensity is visible when the probe is localized on the nanostructure. This field distribution is elongated in the direction of the incident light polarization. In the case of nanoholes, the images show that, in addition to the transmission through the holes, some light is localized between adjacent holes. This light localization, which is strongly dependent on the polarization direction, is attributed to plasmon polariton waves emitted by the holes in the metal film.
In this paper, we investigated the electric field profiles and phase distribution at the metal interfaces of the structure, and then analyzed their dependence on the groove depth and distance between slit and grooves though finite-difference time-domain (FDTD) simulation. Calculated results show that variant groove depth generates phase difference periodically, which indicates the existence of standing wave in the groove. The results also show that the phase of the emission at each groove is proportional to the distance travelled by the surface wave in one period. Based on these facts, a simple process of the transmission model is illuminated.
Elastic scattering and emission spectra simultaneously observed with enormous SERS (Surface Enhanced Raman Scattering) signal from dye and adenine on hot or blinking silver nanoparticles are discussed to get insight into the enhancement mechanism. Huge SERS signal from touching Ag nanoparticles is extinguished by duration of measurement possibly due to thermal diffusion or desorption of adsorbed molecules. Simultaneously, elastic scattering peak at ca. 630 nm disappeared. Three-dimensional Finite Difference Time Domain (FDTD-3D) method manifests this scattering peak originates from enhanced coupling of localized surface plasmon (LSP) on adjacent Ag particles through absorption of adspecies located at their junction. Simultaneously with vast SERS signal, distinct emission peaks were observed at 550-600 nm and 600-750 nm. Based on the spectral variations for different surface coverage and for different dye species, the shorter and longer wavelength peaks were attributed to excited electron on metal, and from fluorescence of molecules, respectively. Furthermore, we found the shorter wavelength peak shows invariant Stokes shift irrespective of excitation wavelengths and adsorption species, indicating it arises from inelastic scattering of excited electron by adsorbed molecules. Nanosphere lithography was exploited to fabricate metal nanostructure with single molecule sensitivity in SERS, which results in much higher probability of the blinking compared to continuous Ag films.
We have found that spherical gold nanoparticles immobilized on a gold substrate with a gap of a few nanometers, which is supported by self-assembled monolayers, show large activity of second-harmonic generation (SHG). Spectroscopic SHG measurements were performed with a Ti:Sa laser in order to investigate the origin of the intense SHG. It was found that the SHG intensity increases with shorter wavelength region, indicating that the enhancement originates from localized surface plasmon resonance in the system. We also fabricated microarrays of the surface immobilized gold nanoparticles through photoregistration of the self-assembled monolayers used to support the nanogap. The nanoparticle microarrays were characterized by SHG microscopy. The microarrays can be applied to multichannel biological sensors using linear optical spectroscopy.
Metallic Nanostructures are giving rise to a great deal of attention from a broad scientific community, ranging from physicist and electrical engineers to biologists. The interest is growing rapidly in finding novel devices for future applications that allow using metallic waveguides for optical signal transmission and processing. In this contribution, we investigate some of the fundamental phenomena that take place in these systems. Also the extraordinary transmission of light though sub-wavelength holes in a metal is investigated, keeping in mind various potential biophotonics applications. In this paper, we demonstrate an optical nano-imaging technique that is particularly well suited to characterize the near-field interaction of light with metallic nanostructures: coherent near-field microscopy. This technique allows the total characterization of the near-field by giving full access to its amplitude and its phase. Its application to the characterization and study of plasmonic nanostructures is illustrated using several systems, the coherent near-field optical measurements of light transmission though sub-wavelength holes drilled in a gold thin film and surface plasmons propagating on a metal film and its interaction at a metal-air interface.
There are two modes of surface plasmon, radiative and nonradiative. Theoretically, the evanescent surface electric fileds (nonradiative surface plasmon) have their maximum on the surface and exponentially decaying field perpendicular to it. Though the evanescent surface wave can not radiate, the enhancement of the surface electric field has become an important feature in plasmonics. Especially, if one can control the enhancement factor of a plasmonic device, that is, if the intensity of the optical near field which is excited on a plasmonic object can be adjusted exactly, there will be many novel applications implemented. Based on three-dimensional finite-difference time-domain method, we analyze the interactions of the two identical spherical silver particles with optical wave. Our results show that the electric amplitudes at the midpoint of the two particles are dependent on the direction of the alignment. We convince that by changing the angle between the alignment of the particles and the polarization, the local electric intensity would be tuned well.
We have performed numerical analysis of localized surface plasmons (LSP) at a nano silver particle on a glass substrate using the finite-difference time-domain method, taking into account the size dependence of the dielectric constants of silver. It was found that the characteristics of LSP at a nano metal particle depend on both shapes of the particles and a dielectric constant of the substrate. In calculations, we employed the geometry in which a nano-particle was located on a glass substrate, the area of calculations. The three types of particles were assumed: a sphere, a spheroid and a hemisphere. In spheroid, the aspect ratio of particles R, is changed from 1.0 to 2.0. For the normal incidence to the spheroid, i.e., the long-axis, the characteristics of LSP are insensitive to R compared. It was observed the red-shift of LSP resonance wavelength and the field enhancement due to the mirror image in the substrate. For spheres and spheroid, the strong enhancement of the z-component field was observed on the substrate. For the hemisphere, we have found the strong localization of the field along the edge of the hemisphere and the strong enhancement of the x-component field was observed on the surface of the substrate.