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II12.03 - Dielectric Constant Contrast-Enabled Field Enhancement in Plasmonic Nanofocusing 
April 25, 2014   9:15am - 9:30am

We have investigated a metal-insulator-metal (MIM) waveguide with a horizontal slot (HS) embedded in the intermediate insulator for stronger nanoscale field enhancement. The HS has a lower refractive index than the surrounding insulator and creates a high contrast in the dielectric-constant profile of the intermediate insulator. To examine the effect of the contrast in the dielectric constant on the field enhancement, we have applied the HS-MIM approach to our previously reported highly efficient three-dimensional (3D) nanofocusing photon compressor (3D NPC). The compressor consists of a MIM waveguide that tapers three-dimensionally into a deep sub-wavelength-scale tip. We targeted the telecom C-band (~1.55μm), and simulated a 3D-tapered SiO2-slot-embedded Ag-Si-Ag waveguide on a fused silica substrate. The dimensions of the Si layer (index n = 3.5) varied linearly from h=200 and w=300 nm (body) to h=30 and w=45 nm (tip), while the thickness of the silica (index n = 1.45) slot remained constant at a given value below 10 nm. As light propagates through an HS-MIM-based 3D NPC, the electric field intensity in the embedded slot increases due to the presence of a contrast in the dielectric constant profile. And, the enhancement in the HS becomes significantly more noticeable as the dimensions of the waveguide decrease, especially inside the nanofocusing tip. When compared to the previous MIM counterpart, the HS-MIM-based 3D NPC shows more than a ten-fold and twenty-fold improvement in field intensity enhancement for a 3-nm-thick and 1-nm-thick slots, respectively. And, a thinner HS yielded an enhancement-improvement factor closer to the upper limit given by the material properties [n/n]. Interestingly, the field enhancement is highly dependent on the slot thickness yet almost independent of its position inside the insulator. The focusing efficiency (the percentage of the energy delivered into the nanofocusing tip) remained almost unchanged from the value for the MIM counterpart (~85%). As we explored the fundamental mode of the HS-MIM waveguide, we have discovered that the performance improvement of the HS-MIM-based 3D NPC originates from more desirable mode properties influenced by the presence of the HS. We observed that a thinner HS produced a shorter characteristic propagation length. In addition, compared to the MIM counterpart, the optimized taper angle for the HS-MIM-based 3D NPC is smaller because it needs to compensate relatively larger mode mismatches that occur in the taper region. We also simulated different index configurations for the intermediate insulator, including the cases in which the layer was completely made of either low or high index material. Among the examined configurations, the HS-MIM approach shows the most significant improvement in field enhancement with almost no loss in focusing efficiency. In our presentation, we will discuss detailed simulation techniques, results, and potential plasmonic applications.

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