PLASMONIC METASURFACES FOR EXTRA-ORDINARY TRANSMISSION AND ABSORPTION OF LIGHT IN VISIBLE TO IR REGIME

Abstract

In the field of optics and photonics, plasmonic metasurfaces have been one of the rapidly growing research fields. For the incidence of electromagnetic radiation, a plasmonic metasurface shows excellent light-controlling properties, which are not easy to obtain using conventional optical elements such as optical lenses, spectral filters, polarizers, etc. The optical properties of the metasurfaces are specially controlled by the geometrical period and the geometrical parameters of the subwavelength sized unit metal/ dielectric nanostructures. The optical response of plasmonic metasurfaces is generally analyzed either in transmission mode or reflection mode. The transmission of light through a plasmonic metasurface is controlled by the excitation of surface plasmon modes which funnel energy through the metasurface; and the total power of the light transmitted through the plasmonic structure is much larger than the power of the light falling only on the open portion of the structure. This phenomenon is called extra-ordinary transmission (EOT) of light. EOT of visible or NIR light has already been experimentally demonstrated through a single set of metal nanoslits or nanoholes array having a horizontal spacing between them. However, their transmission efficiency was very poor which can be improved by selecting the specific plasmonic nanostructure. This thesis presents the simulation study for the giant extra-ordinary transmission of near infrared light through a seemingly opaque plasmonic metasurface consisting of two sets of metal nanoslits arrays. Metal nanoslits are arranged in such a manner that the opening of upper metal nanoslits array is covered by the closed portion of lower metal nanoslits array, and vice versa. A giant extraordinary transmission of more than 90 % is possible because of the perfect coupling between the surface plasmon modes generated at the different interfaces of upper and lower metal nanoslits arrays. Because of the excitation of surface plasmon modes at metal dielectric interfaces, such vi types of metasurfaces can be used in refractive index sensing. Further optimization of the geometrical parameters of the proposed metasurface leads to the excitation of two surface plasmon modes. One of the surface plasmon modes is excited at the metal-analyte interface, while the other one is excited at the metal-substrate interface. For the change of refractive index of the analyte medium, the first one shows a redshift and the second one, none. Such type of metasurface sensor provides self-referenced refractive index sensing ability. The optimization of the geometrical parameters of the plasmonic metasurface also makes it possible to enhance the figure of merit of the sensor. Surface plasmon modes generated at the metal dielectric interface of a plasmonic metasurface can transmit light through an apertureless plasmonic metasurface. Experimental demonstration of the extraordinary transmission of visible light through an apertureless plasmonic metasurface consisting of two sets of metal gratings overlapped with each other has also been presented in this thesis. The transmission through such apertureless plasmonic metasurface is attributed to the coupling of the electromagnetic field of the surface plasmon modes excited at the upper interface with that on the lower interface. The coupling of the electromagnetic field is possible because of the penetration of the electric field due to the surface plasmon modes generated at the metal-air interface through the plasmonic metasurface and its coupling with the surface plasmon modes generated at the metal-substrate interface. This study enriches the understanding of EOT, and opens avenues of cost-effective photonic devices such as sensors, spectral filters, polarizers, etc.