Observation of the enhancement of electric fields normal to the surface using mid-infrared slot antennas and an atomic layer deposition technique

• Dumbbell-shaped mid-infrared slot antennas were formed on the Al2O3/Si substrate.
• A few nm thick Al2O3 layer was deposited using the atomic layer deposition method.
• Slot antennas produced an enhancement of an electric field parallel to the surface.
• Restrahlen bands from the Al2O3 layer and the natural oxide of Si could be observed.
• An electric field localization in the normal direction was experimentally confirmed.

Optical electric field enhancement in the normal direction was experimentally investigated using mid-infrared slot antennas that were formed on a thin Al2O3 layer/Si substrate. The Al2O3 layer thicknesses could be controlled to an accuracy of a given atomic layer through the use of atomic layer deposition, and varied from 0 nm to 60 nm. An in-depth probe of the electric field was performed by observing the change in the reflection signal arising from the Restrahlen band of the natural oxide of Si formed on the surface of a Si substrate. In contrast to dipole nanoantennas, we could clearly observe Restrahlen bands of Al2O3 as well as the native Si oxide film. This was because the direction of the enhanced electric field was primarily parallel to the substrate surface in the slot antennas, which was different from the dipole nanoantenna having strong normal electric fields at the antenna ends. The atomic layer deposition technique provides versatile information on the electric field distribution within the depth direction, being considered complementary to the electromagnetic simulation of nanoantennas.