Abstract
We experimentally demonstrate a three-dimensional plasmonic terahertz waveguide by lithographically patterning an array of sub-wavelength pillars on a silicon substrate. Doped silicon can exhibit conductive properties at terahertz frequencies, making it a convenient substitute for conventional metals in plasmonic devices. However, the surface wave solution at a doped silicon surface is usually poorly confined and lossy. Here we demonstrate that by patterning the silicon surface with an array of sub-wavelength pillars, the resulting structure can support a terahertz surface mode that is tightly confined in both transverse directions. Further, we observe that the resonant behavior associated with the surface modes depends on the dimensions of the pillars, and can be tailored through control of the structural parameters. We experimentally fabricated devices with different geometries, and characterized the performance using terahertz time-domain spectroscopy. The resulting waveguide characteristics are confirmed using finite element numerical simulations, and we further show that a simple one-dimensional analytical theory adequately predicts the observed dispersion relation.
Highlights
We report here that by structuring the silicon surface using conventional deep reactive ion etching, one can achieve resonant guided-mode behavior as in metals
The numerical simulations and experiments are in agreement across the range of frequencies measured
One may note that there is small discrepancy between measurements and simulations above the resonant frequency. We believe this is because the mesh size in the numerical simulations in figure 3 was chosen to be approximately 40 μm, which is sufficient to model the behavior at lower THz frequencies, but can lead to quantization errors at higher frequencies
Summary
A periodic sequence of silicon pillars can have bound resonant modes that are highly confined to the surface both in transverse and lateral direction. We experimentally and numerically examine terahertz wave propagation on a heavily boron-doped silicon surface structured with pillars. The structured silicon surface acts as an effective medium which can lead to highly confined terahertz guided mode propagation. We describe how the geometrical properties of the structure relate to the resonant behavior of the fundamental mode, and compare the results with a simple analytical model originally developed for two-dimensional corrugations
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