Quantum Hydrodynamic Theory for Sub-Nanometer Gaps: Atomic Protrusions Govern Near-Field Enhancement and Tunneling...

Area of Science:

• Plasmonics and Nanophotonics
• Quantum Mechanics and Electrodynamics
• Materials Science and Engineering

Background:

• Advanced nanofabrication enables control of surface morphology at the atomic level.
• Classical electrodynamics fails to capture quantum effects in sub-nanometer plasmonic nanogaps.
• Atomic-scale protrusions and separations are critical in extreme nanogap devices.

Purpose of the Study:

• To compute and analyze the optical response of metallic sub-nanometer nanogaps with atomic protrusions.
• To compare quantum hydrodynamic theory (QHT) predictions with the classical local-response approximation (LRA).
• To investigate the impact of atomic morphology on near-field nanofocusing and spectral evolution.

Main Methods:

• Utilized quantum hydrodynamic theory (QHT) to model optical response.
• Benchmarked QHT predictions against the classical local-response approximation (LRA).
• Analyzed the effects of atomic-scale variations in electrode morphology on optical properties.

Main Results:

• Atomic variations significantly reshape near-field nanofocusing while minimally affecting far-field scattering.
• QHT predicts a tunable redshift-to-blueshift inflection point in spectral evolution with decreasing gap size.
• LRA fails to capture this non-monotonic behavior, predicting continuous redshift and field enhancement.

Conclusions:

• QHT provides a reliable framework for understanding quantum effects in extreme plasmonic nanogaps.
• Tunable inflection points offer a method for optically diagnosing and engineering quantum phenomena.
• Results guide the design of molecular-scale optoelectronic and nanophotonic devices.