Vibrational ultrastrong coupling, where the light–matter coupling strength is comparable to the vibrational frequency of molecules, presents new opportunities to probe the interactions between molecules and zero-point fluctuations, harness cavity-modified 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 (SiO₂) to demonstrate ultrastrong coupling between phonons and gap plasmons. We present classical and quantum-mechanical models to quantitatively describe the observed plasmon–phonon ultrastrong coupling 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 ultrastrong coupling applications.

振动超强耦合，其光-质耦合强度可与分子的振动频率相媲美，为探测分子与零点波动之间的相互作用，利用腔体修饰的化学反应以及开发中红外的新型装置提供了新的机会光谱范围。在这里，我们使用填充有模型极性介质（SiO ₂）的ε-近零纳米腔来演示声子与间隙等离子体激元之间的超强耦合。我们提出了经典模型和量子力学模型来定量描述观察到的等离激元-声子超强耦合现象，并演示了高达50％共振频率的模态分裂（归一化耦合强度η> 0.25）。我们的晶圆级纳米腔平台将能够利用各种振动转变来实现超强耦合应用。