Solvated Electrons in Polar Liquids as ϵ -Near-Zero Materials Tunable in the Terahertz Frequency Range

Electrons in polar liquids give rise to a polaron resonance at a terahertz (THz) frequency ${\ensuremath{\nu}}_{0}$ depending on electron concentration. The impact of this resonance on light propagation is studied in experiments, where a femtosecond pump pulse generates electrons via multiphoton ionization and a THz probe pulse propagated through the excited sample is detected in a phase-resolved way. We observe a behavior characteristic for $ϵ$-near-zero materials with strongly modified phase and group velocities around ${\ensuremath{\nu}}_{0}$, and a broadening of the THz pulse envelope below ${\ensuremath{\nu}}_{0}$. Calculations based on a local-field approach reproduce the $ϵ$-near-zero behavior.