Modes Manipulation Within Subwavelength Metallic Gratings

Surface plasmon polariton (SPP) and cavity modes supported by the plasmonic gratings are often of hybrid nature in ruling light-nanostructure interactions. The coupling of incident light to SPP waves through the excitation of SPP modes usually traps the light at the air-metal interface, while the coupling to standing waves through cavity modes confines the light within the cavities. However, in a specific application, the highly selective coupling of incident light to either SPP or cavity mode is always required. We present here, for a single metal-insulator-metal (MIM) groove, the coupling strength of incident light to a certain mode can be engineered by varying the groove width and depth. To further enhance the coupling selectivity, the periodicity was introduced to the MIM grooves: When the period approaches (deviates from) the wavelength of SPPs, the incident light highly couples to SPP (cavity) mode, due to the constructive (destructive) interference between the propagating SPPs at metal-air interfaces. Two types of metallic gratings, aluminum grating with its period approaching the λSPP and iridium grating with its period far away deviating from the λSPP, were fabricated and measured. The almost null TM transmission through aluminum grating and detection of surface enhanced Raman scattering (SERS) signal from iridium grating solidly guarantee the validity of the proposed method in manipulating the modes, through which the energy flow can be guided to its supposed-to-appear regions.