| In this paper we investigate the role of the metallic surface with periodic subwavelength patterns in electromagnetic enhancement and in extraordinary optical transmission (EOT).Surface enhanced Raman scattering (SERS) can greatly enhance the Raman signal, which fulfills the requirement of single molecule detection. Further more, the physical mechanism behind this phenomenon shines new light on the basic science, which makes SERS arouse intense interests. The enhanced spectra signal is mainly due to the enhanced electric field at the structure surface where molecules are located. Thus the electromagnetic enhancement of the nano structure has been investigated to enhance the SERS signal. At present, researches on the electromagnetic enhancement are commonly focused on the "hot spot", which appears in specific tiny regions. This leading to a rigorous requirement of nano-location techniques, and limits its application in SERS. In this paper, a large-area electromagnetic enhancement (LAEE) phenomenon is proposed, which relieves the requirement of accurate location of sample molecules. A semianalytic model is built up to achieve a quantitative understanding of this LAEE phenomenon, and to guide the design.In this paper, a large-area electromagnetic enhancement (LAEE) is observed on a metallic surface patterned with periodic subwavelength indentations. We find that nearly50%of the overall substrate area can act as the field-enhancement area, which relieves the requirement of accurate location of sample molecules, and increases the total emission of spectra signals on the whole surface. The LAEE is shown to be a quite general property that exhibits similar characteristics for different types of indentations (such as apertures or bumps). To achieve an understanding of the phenomenon for guiding the design, we build up microscopic models that consider the excitation and multiple scattering of surface waves on the periodically corrugated metallic surfaces. Two distinct surface waves are involved in this microscopic dynamics, the surface plasmon polariton (SPP) and another quasi-cylindrical wave (QCW), which always appear with a fixed mixing ratio and thus form a new hybrid wave (HW). Analysis of the model shows that the LAEE is due to a resonant excitation of surface waves (SPP and QCW). Under the resonance condition derived from the model, the surface waves will sum up constructively and propagate on the metal surface over a long distance, which greatly enlarges the area of field enhancement. Inspired by the analysis, gain medium is introduced into indentations to compensate the scattering loss of surface waves, which further enhances the resonance and the enhancement factor (EF) of electric-field intensity (～2000), which is almost one order of magnitude higher than that without gain.We theoretically study the compensation of propagation loss of surface plasmon polaritons (SPPs) with the use of a finite-thickness dielectric layer with optical gain. Impacts of the gain coefficient ngain", of the gain-layer thickness h and of the wavelength λ on the loss compensation and on the field distribution of the SPP mode are systematically explored. With a gradual increase of the gain-layer thickness, the SPP propagation loss first increases unexpectedly and then decreases as expected. This is explained as a trade off between two contradictive effects, the effect of a larger refractive index of the gain layer than air and the loss compensation effect of the gain layer.Based on the theoretical analysis of LAEE, we design and perforate square hole array on the gold substrate for LAEE. The electromagnetic field on the surface of the structures are measured by the near field scanning optical microscope (NSOM). An optical system is set up based on the IX71microscope. The reflectance of the structures are measured, and the role of the surface wave resonance in the LAEE is verified.Another phenomenon related to the subwavelength structure is the extraordinary optical transmission (EOT). This refers to the strongly enhanced transmission of light through the subwavelength holes comparing with the value of classical theory. The EOT phenomenon has aroused intense interests due to the physical mechanism behind it, and becomes the landmark event in the plasmonics. Two explanations have been proposed to interpret the EOT phenomenon, the macroscopic Bloch-mode picture and the microscopic surface-wave description. In this paper, by building up a microscopic description of the surface Bloch mode (SBM) with the use of surface waves on flat metallic surfaces, we establish explicit relations between the macroscopic SBM picture and the microscopic surface-wave picture for explaining the EOT. In addition, the microscopic description of the SBM shines new light on the design of subwavelength metallic hole arrays as plasmonic metamaterials.In this paper, we build up a microscopic description of the SBM that consider the excitation and multiple scattering of HWs on metallic surfaces patterned with a periodic array of subwavelength holes. Both the complex propagation constant kSBM and the field distribution of the SBM can be analytically described by the microscopic HW model. We establish explicit relations between the macroscopic SBM picture and the microscopic surface-wave picture for explaining the EOT. The microscopic description shows that the SBM is tightly related with the EOT through a complex pole of the in-plane wave vector kx of the periodically perforated metallic surface, at which the SBM and the EOT possess similar field distributions expressed as pseudo-periodic superposition of HWs that originate from every holes in the array.Within the the microscopic surface-wave picture for explaining the EOT, we theoretically calculate and analyze the EOT phenomenon of the subwavelength hole array perforated on the superconducting NbN at THz frequencies. We explain the experimental data offered by the Jin Biaobing group of Nanjing University, and show the physical mechanism of the EOT for superconducting at THz frequencies. |
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