Fig. 4.

Coupling efficiency and electric near-field enhancement in Al₂₀₁. a Spatial map of the coupling efficiency (normalized mode energy density, calculated using classical electromagnetism) shows that maximum coupling occurs if the benzene molecule is inside the Al₂₀₁ particle and decreases rapidly away from its surface. At the position of benzene the coupling efficiency is about 30–50% of the maximum value. The contours are spaced every 0.2. b–d Field enhancement from time-dependent density-functional theory calculations at (b) the Al₂₀₁ resonance and at the Al₂₀₁-benzene (c) lower and (d) upper polaritons (with benzene located at the center of the facet). Benzene focuses the electric field, perturbing the nanoplasmonic cavity and increasing the coupling efficiency beyond the expectation based on the field at the bare resonance⁵²