Ding-Zheng Lin

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.

Subwavelength nondiffraction beam generated by a plasmonic lens

We experimentally examined the near-field and far-field optical properties of a subwavelength annular aperture (SAA) made on silver film and undertook finite difference time domain simulations. In our near-field measurements, an interference pattern with a period very close to half of the surface plasmon (SP) wavelength (λSP∕2) was found on the surface of the silver film. Moreover, we observed that the transmitted light of the 442nm incident laser was focused at several micrometers behind the silver SAA structure at a tiny spot (354nm) and with a remarkable 31μm depth of focus. This implies that the silver SAA structure can generate a nondiffraction beam and can be used to fabricate high aspect ratio subwavelength structures.

Experimental analysis of surface plasmon behavior in metallic circular slits

Using a focused ion beam, the authors fabricated metallic circular slits onto a glass substrate coated with silver film. The influence of the number of slits and the focusing light phenomena was investigated by capturing the light transmitted through the circular slits. They demonstrate experimentally that the circular grating formed by a set of periodical slits can excite both stronger surface plasmon (SP) and localized surface plasmon as the number of slits increases. They found that the SP tended to congregate at the center of the circular grating, and that the reemitted light could be used to achieve a focusing phenomenon.

Effect of subwavelength annular aperture diameter on the nondiffracting region of generated Bessel beams.

A subwavelength annular aperture (SAA) made on metallic film and deposited on a glass substrate was fabricated by electron-beam lithography (EBL) and which was followed by a metal lift-off process to generate a long propagation range Bessel beam. We propose tuning the focal length and depth of focus (DOF) by changing the diameter of the SAA. We used finite-difference time domain (FDTD) simulations to verify our experimental data. We found that the position of the Bessel Beam focus spot (i.e. focal length) will be farther away from the SAA plane as the diameter of the SAA increases. In addition, the depth of focus (DOF) which is the length of the Bessel beam non-diffracting area, also increases as the diameter of the SAA expands.

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.

A Study of the Long Propagation Range Bessel Beam Generated by a Subwavelength Annular Aperture Structure

The subwavelength annular apertures (SAA) made on metallic layers by electron beam lithography with metal lift-off processes have been demonstrated. We have experimentally enhanced the Bessel beam distance through the SAA structure with large diameters.