[Paper Review] Quantum-Spillover-Enhanced Surface-Plasmonic Absorption at the Interface of Silver and High-Index Dielectrics

Dafei Jin, Qing Hu+7 more|arXiv (Cornell University)|Apr 29, 2015

Gold and Silver Nanoparticles Synthesis and Applications5 references3 citations

TL;DR

This paper demonstrates that quantum spillover of conduction electrons at silver/high-index dielectric interfaces enables interfacial electron-hole pair production, which dominates surface plasmon decay and leads to significantly enhanced, broadened plasmonic absorption—exceeding classical predictions. The effect is strongest in Ag-TiO2, where e-h pair loss dominates across the entire visible spectrum, enabling efficient light absorption and hot-electron generation beyond classical limits.

ABSTRACT

We demonstrate an unexpectedly strong surface-plasmonic absorption at the interface of silver and high-index dielectrics based on electron and photon spectroscopy. The measured bandwidth and intensity of absorption deviate significantly from the classical theory. Our density-functional calculation well predicts the occurrence of this phenomenon. It reveals that due to the low metal-to-dielectric work function at such interfaces, conduction electrons can display a drastic quantum spillover, causing the interfacial electron-hole pair production to dominate the decay of surface plasmons. This finding can be of fundamental importance in understanding and designing quantum nano-plasmonic devices that utilize noble metals and high-index dielectrics.

Motivation & Objective

• To investigate non-classical plasmonic absorption mechanisms at silver/high-index dielectric interfaces that deviate from classical surface plasmon theory.
• To determine the role of quantum electron spillover and interfacial electron-hole pair production in enhancing plasmon decay and absorption.
• To establish a theoretical and experimental framework linking dielectric work function, electron spillover depth, and plasmonic response.
• To demonstrate experimentally that high-index dielectrics like TiO2 induce strong, broadened absorption in silver films due to quantum effects.

Proposed method

• Employed a generalized jellium model with Kohn-Sham and generalized Poisson equations to simulate electron density and effective potential at Ag/dielectric interfaces.
• Calculated space- and frequency-dependent permittivity ε(z;ω) and generalized Green’s function to model dynamic plasmonic response.
• Used density-functional theory (DFT) with exchange-correlation and electron affinity corrections to predict quantum spillover depth (Im d⊥) and interfacial electron behavior.
• Conducted UV-Vis spectrophotometry on 20 nm Ag films with 20 nm ALD-deposited dielectrics (SiO2, Al2O3, HfO2, TiO2) on quartz substrates to measure reflection spectra.
• Applied classical transfer-matrix and statistical-roughness theory (Kretschmann method) to isolate Drude loss contributions and compare with experiment.
• Performed independent transmission measurements on dielectric-on-quartz samples to confirm dielectric losslessness in the visible range.

Experimental results

Research questions

• RQ1 Can quantum spillover of electrons at Ag/high-index dielectric interfaces lead to enhanced plasmonic absorption beyond classical predictions?
• RQ2 To what extent does interfacial electron-hole pair production dominate plasmon decay compared to phonon scattering (Drude loss) or interband transitions?
• RQ3 How does the dielectric's work function and electron affinity influence the depth of electron spillover and the resulting plasmonic absorption spectrum?
• RQ4 Why do high-index dielectrics like TiO2 induce significantly broader and stronger absorption dips than low-index dielectrics like SiO2 in Ag-based heterostructures?

Key findings

• Ag-TiO2 exhibits fully e-h-pair-dominant plasmonic absorption across the entire visible spectrum (350–700 nm), with the strongest and broadest reflection dip observed experimentally.
• The measured absorption in Ag-HfO2 and Ag-TiO2 deviates significantly from classical theory, with experimental dips being broader and more intense than predicted by Drude loss models.
• The quantum spillover depth (Im d⊥) increases with dielectric index: ~1.1 Å for SiO2, ~1.5 Å for Al2O3, ~2.1 Å for HfO2, and ~3.0 Å for TiO2, correlating with enhanced absorption.
• Electron-hole pair production becomes the dominant decay channel for surface plasmons at high-index interfaces, surpassing both Drude and dielectric losses.
• The experimental reflection spectra for high-index dielectrics (Al2O3, HfO2, TiO2) show strong, broad dips not predicted by classical models, confirming non-classical absorption.
• Independent transmission measurements confirm that the dielectrics themselves are lossless in the visible range, ruling out interband absorption as the cause of observed dips.