Pressure-Tunable Hyperbolic Plasmons in Black Phosphorus Films

High-pressure environments provide a unique platform for tuning quantum phenomena, yet their applications in plasmonics remain underexplored. Here, we investigate the pressure-induced evolution of plasmons in black phosphorus films using infrared spectroscopy. Continuous pressure tuning of anisotropic plasmon resonances reveals the existence of in-plane hyperbolic plasmons, whose regime blueshifts into the mid-infrared range in the low-pressure A17 phase and collapses in the high-pressure A7 phase due to the suppressed anisotropy. Notably, we observed an exceptionally broad spectral range tuning of plasmon resonance frequency, ranging from 214  cm^{-1} to a maximum of 4751  cm^{-1}, along with significant modulation in absorption intensity and anisotropy. Pronounced plasmonic anomalies at the transition point reveal phase coexistence, while the abrupt low-pressure plasmon onset signals a Lifshitz transition. Our findings establish high pressure as a novel method for tailoring plasmonic properties and unlocking new possibilities for reconfigurable nanophotonic devices.