J. T. Yeh

Beaming light from a subwavelength metal slit surrounded by dielectric surface gratings.

In this article, we demonstrate that a subwavelength metal slit surrounded by dielectric surface gratings possesses a directional beaming effect. We propose a surface plasmon diffraction scheme to explain the three kinds of beaming conditions. The numerical simulations of the illustrative structures undertaken used a Finite Difference Time Domain (FDTD) Method and a Rigorous Coupled Wave Analysis (RCWA) Method. Our simulations were found to be consistent and in agreement with the experimental results. In comparison with other metal structures, we find that dielectric metal structures offer better performance as well as the advantage of being able to be efficiently mass produced for large volume industrial applications.

Directional light beaming control by a subwavelength asymmetric surface structure

We propose a direct experimental set-up to observe the directional beaming effect of surface plasmon. A single diffracted beam from an asymmetric-sided surface corrugation is demonstrated. A single subwavelength slit with an asymmetric structure was fabricated using a focused ion beam (FIB) onto a metal surface with a glass substrate. By means of surface plasmon (SP) diffraction, the directionality of the light can be changed by the period of the metallic gratings. We show corresponding numerical simulations achieved by a Rigorous Coupled-Wave Analysis (RCWA) method and a Finite-Difference Time-Domain (FDTD) method. The simulation results were in agreement with the experimental data.

Optimizing electromagnetic enhancement of flexible nano-imprinted hexagonally patterned surface-enhanced Raman scattering substrates

The production of inexpensive, large-scale, uniform substrates for surface-enhanced Raman scattering (SERS) is a key to popularize its usage in chemical and biological detection. We demonstrate a flexible nano-imprinted hexagonally patterned SERS-active substrate. Its electromagnetic enhancement factor was optimized by the thickness adjustment of its silver over-coated film. The experimental data show a good correspondence with the theoretical prediction. Such substrate was shown to exhibit high uniformity and reproducibility with a variation of less than 2%, offering a potential of greatly exploiting such substrate in infield biocide monitoring.

Similarities and differences for light-induced surface plasmons in one- and two-dimensional symmetrical metallic nanostructures

Two types of double-sided nanostructure, one possessing a slit aperture with parallel grooves and the other possessing a circular aperture with concentric grooves, were fabricated to examine the similarities and differences of their diffraction behavior in one-dimensional (1-D) and two-dimensional (2-D) nanostructures. Based on the projection-slice theory, we conjecture that the surface plasmons in these two different nano-scale grooves possess similar modes. A localized surface plasmon (LSP) was used to examine the transmission characteristics induced by the apertures. The transmission characteristics of the slitted nanostructure and the circular nanostructure aperture were then measured. We coupled the transmission spectra measured from these two apertures with a 1-D parallel groove transmission curve simulated by a 1-D rigorous coupled wave analysis. Measured spectra results show reasonable agreement with the simulated data. We propose that the apparent blueshift observed in the peak frequency of a 2-D nanostructure is due to the difference in the shape of the aperture and the spot transmission characteristics of 1-D and 2-D systems as induced by a LSP.

Enhancing intensity of emitted light from a ring by incorporating a circular groove

We fabricated a ring containing a single circular groove (RCG) on silver film and which was supported on a glass substrate. We found that by changing the mean radius of the circular groove, the light intensity emitted from the RCG can be modulated by using the scattering light from the circular groove. In addition, we also fabricated circular grooves with the same depth but of different widths so that we could examine the scattering light behavior of the grooves. Herein, we propose a theoretical model which takes into account the amplitude modulation of the cylindrical waves. Our results showed that our proposed model agreed well with the experimental results.

Propagation characteristics of silver and tungsten subwavelength annular aperture generated sub-micron non-diffraction beams.

We examined the optical properties such as propagation modes, focal length, side lobes, etc. of metallic subwavelength annular apertures (SAA) and used finite-difference time-domain (FDTD) simulation to compare our experimental findings. Using two different metals, silver and tungsten, we examined the different optical transmission properties of the two metallic SAA structures. The far-field propagation of the silver SAA structure was found to be a type of quasi-Bessel beam when compared with a quasi-Bessel beam generated by a perfect axicon. The propagation characteristics of these two beams were found to match qualitatively. The far-field transmitted light generated by the silver SAA structure was found to possess a 390 nm sub-micron focal spot with a 24 microm depth of focus, which was much smaller than the focal spot generated by a perfect axicon. We also found that a silver SAA structure can generate a sub-micron quasi- Bessel beam that has a much lower far-field side-lobe when compared to that of non-diffraction beams generated by a tungsten SAA structure.

Fourier analysis of surface plasmon waves launched from single nanohole and nanohole arrays: unraveling tip-induced effects

The authors report the investigation of surface plasmon waves (SPW) generated by single nanohole and nanohole arrays. Scattering-type scanning near-field microscopy is used to directly observe near-field distribution. The images after Fourier transformation display characteristic patterns that match with the derived analytic formula. The correspondence helps to identify the role of the scanning tip in generating SPW, making possible of the removal of this tip-induced effect. This study provides a means to perform in-depth investigation on surface plasmon polaritons.

Recording Bessel-like beam shapes generated by plasmonics lens.

In our study, we spun a negative photoresist layer on top of a plasmonic lens which was formed by adopting a metallic ring structure with a nano-scale width opening. We recorded the beam shape of the Bessel-like beam emitting from the plasmonic lens which formed a high aspect ratio structure. We found that the high aspect ratio structure was higher after exposure as the inner and outer diameter had increased. In addition, we used an oblique incidence on the negative resist metallic ring structure to produce an inclined micro-structure. Different exposure results were obtained with the two different metal thicknesses. Therefore, in our study, we not only proved that it is possible to record the shape of a Bessel-like beam, but we also demonstrated that it is possible to create a plasmonic lens which is capable of creating a high aspect ratio structure through exposure.

Near field imaging of subwavelength plasmonic structures

This report presents an overview of our recent near-field investigation of both local and surface plasmon resonance (SPR) with a scattering-type scanning near-field optical microscope (s-SNOM) which has sub-10 nanometer resolution. With the ability to perform near-field optical experiments at multiple excitation wavelengths simultaneously, this instrument has recorded near-field intensity and phase images of a wide range of subwavelength plasmonic structures: single nanohole and nanoslit, circular and elliptical hole arrays, etc. The near-field results obtained with different excitation wavelengths were confirmed by numerical calculation and were made direct correspondence with far-field observations by comprehensive models. The multi-wavelength s-SNOM proves to be an essential tool to unravel many interesting plasmonic phenomena in nanometer scale. This work investigates the nature of subwavelength plasmon optics which potentially will play an important role in the development of many innovative highly efficient optoelectronic devices (light-emitting devices and solar cells) and highly sensitive sensors based on SPR and surfaceenhanced Raman scattering.