Abstract
Improving the coupling efficiency of tapered metallic gaps using spatial amplitude modulation is theoretically investigated. The influences of the critical parameters on the coupling efficiency, such as incident beam width, incident wavelength, and numerical aperture of coupling lens, are analyzed, respectively, and a coupling efficiency increase of about 16.43-fold is obtained by optimizing these parameters. The physical mechanism of the coupling efficiency improvement is further discussed. The substantial improvement of the coupling efficiency via spatial amplitude modulation shows the potential in designing tapered metal-insulator-metal waveguides for field enhancement and nanofocusing.
Highlights
Obtaining a small-size transmission light field of high-power density is a crux of nanophotonics
The results demonstrate that the tightly focused optical field is strongly dispersed in free space, so higher order propagating modes are mainly excited in the tapered metallic gap (TMG) and most optical energy is reflected back or absorbed by the metallic walls
We demonstrate that the light field coupling efficiency of a TMG with a large taper angle and input entrance size can be markedly improved via spatial amplitude modulation of the incident beam
Summary
Obtaining a small-size transmission light field of high-power density is a crux of nanophotonics. GSPs in tapered MIM plasmonic waveguides are generally excited by the lowest propagating (TM0) mode using the fire-end coupling method through bulk light or guided modes coupling.[14,15] To reduce the reflection and scattering losses and to transfer most of incident optical energy to the TM0 mode, the taper angles and the input entrance sizes of tapered MIM structures are generally smaller than the critical taper angle We propose a spatial amplitude modulation method to improve the light field coupling efficiency of a tapered metallic gap (TMG) with a large taper angle and large-scale input entrance. The optical FE of the TMG is improved greatly and an FE of about 3318 can be obtained through optimizing the relevant parameters (compared with the situation without amplitude modulation, the light coupling efficiency is increased by 16.43-fold)
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