Ultrastrong plasmon–phonon coupling via epsilon-near-zero nanocavities

Abstract

Vibrational ultrastrong coupling (USC), where the light-matter coupling strength is comparable to the vibrational frequency of molecules, presents new opportunities to probe the interactions of molecules with zero-point fluctuations, harness cavitymodified chemical reactions, and develop novel devices in the mid-infrared spectral range. Here we use epsilon-near-zero nanocavities filled with a model polar medium (SiO 2 ) to demonstrate USC between phonons and gap plasmons. We present classical and quantum mechanical models to quantitatively describe the observed plasmon-phonon USC phenomena and demonstrate a modal splitting of up to 50% of the resonant frequency (normalized coupling strength η > 0.25). Our wafer-scale nanocavity platform will enable a broad range of vibrational transitions to be harnessed for USC applications.Here we demonstrate vibrational USC within nanocavities, drastically reducing the size of the system and thus the amount of material involved in achieving MIR USC. Specifically, we use coaxial nanocavities, which exhibit strong transmission resonances and field enhancements close to the cutoff frequency of the TE 11 -like mode; these can be understood as resulting from excitation of the zeroth-order Fabry-Perot resonance 34 or, alternatively, as arising from an effective epsilon-near-zero (ENZ) effect [35][36][37] . We couple this nano-coax ENZ mode to the lattice vibrations of SiO 2 and demonstrate MIR USC with a mode splitting exceeding 50% of the vibrational transition frequency (η > 0.25) within a solid-state system.

Collective Vibrational Strong Coupling Effects on Molecular Vibrational Relaxation and Energy Transfer: Numerical Insights via Cavity Molecular Dynamics Simulations**

“…A recent experimental work [24] shows that the observed vibrational relaxation rate of the “dark reservoir” (or the hot molecules) is not modified in a Fabry–Pérot microcavity (where the molecular number can reach 10 9 ∼10 12 ), which is consistent with our simulation results. For plasmonic cavities—an emerging platform for studying VSC, [25, 26] since the effective cavity volume is much less than Fabry–Pérot microcavities, the slower‐than‐ O (1/ N sub ) scaling could be observed in experiments.…”

Collective Vibrational Strong Coupling Effects on Molecular Vibrational Relaxation and Energy Transfer: Numerical Insights via Cavity Molecular Dynamics Simulations**

“…Ar ecent experimental work [24] shows that the observed vibrational relaxation rate of the "dark reservoir" (or the hot molecules) is not modified in aF abry-PØrot microcavity (where the molecular number can reach 10 9~1 0 12 ), which is consistent with our simulation results.F or plasmonic cavities-an emerging platform for studying VSC, [25,26] since the effective cavity volume is much less than Fabry-PØrot microcavities,t he slower-than-O(1/N sub )s caling could be observed in experiments.…”

Probing nanoparticle substrate interactions with synchrotron infrared nanospectroscopy: Coupling gold nanorod Fabry-Pérot resonances with SiO2 and h−BN phonons

“…At maximal mixing (θ = π/4), R is minimized and equal to 1.71 and 2.51, respectively. Additionally, with¯ 2 = 3 = 0 at maximal mixing, the splitting can be simplified via a power series to + (π/4) − − (π/4) ≈ g/ 0 √ m 2 m 3 = σ , with σ the form of the coupling strength that appears in quantum-optical treatments of interacting cavities [74][75][76]. This simplification produces strong coupling ratios R(π/4) ≈ 2|σ |/(γ 2 + γ 3 ) = 1.71 (SiO 2 ) and 2.48 (hBN), as well as values of 0.24 (SiO 2 ) and 0.31 (hBN) for the ultrastrong coupling condition |σ |/ 0 .…”