Scientists have discovered a groundbreaking method to manipulate light propagation by temporarily transforming common liquids into epsilon-near-zero (ENZ) materials using intense femtosecond laser pulses.
Real part of the dielectric function of isopropanol (IPA) without optical excitation (black line) and with optical excitation leading to the three different electron concentrations of 50, 100 and 200 µM (colored lines). The frequency at which the curves intersect with the zero line gives the polaron resonance frequency. Image Credit: Max Born Institute
In conventional optical media, both the phase and group velocity of light cannot exceed the speed of light in a vacuum. However, in epsilon-near-zero (ENZ) materials, light exhibits unique behavior: at a specific frequency, the phase velocity becomes infinite while the group velocity vanishes.
Until now, such phenomena have only been observed in select solids and nano-engineered materials. This study introduces a novel approach that transiently transforms common liquids, like water and alcohol, into ENZ materials at terahertz (THz) frequencies using intense femtosecond laser pulses.
When a polar molecular liquid is ionized by femtosecond laser pulses, it generates free electrons that quickly localize or "solvate." Over femtosecond timescales, these electrons occupy spaces within the molecular network, a disordered structure of electric dipoles.
The binding energy of the electron in its final position is largely influenced by the electric interaction between the electron and the liquid's molecular dipoles. This ultrafast localization process triggers collective oscillations between the electron and nearby liquid molecules, forming a many-body excitation known as a polaron. This excitation has a distinct frequency in the THz range, which depends on the electron concentration in the liquid.
At this polaron frequency, the liquid's dielectric function and refractive index cross zero, as shown in the figure above. Essentially, the phase velocity of light at this frequency tends to infinity, while the group velocity of light pulses approaches zero—behavior typical of an ENZ material.
For practical applications, the ability to shift the polaron frequency by altering the electron concentration is a particularly attractive feature. This offers a way to control and tailor the material's ENZ properties across a broad frequency range, from about 0.1 to 10 THz. These findings open up new possibilities for manipulating light propagation in liquids, which could lead to significant advancements in optical sensing and communication technologies.
Journal Reference:
Runge, M. et. al. (2025) Solvated Electrons in Polar Liquids as ε-Near-Zero Materials Tunable in the Terahertz Frequency Range. Physical Review Letters. doi.org/10.1103/PhysRevLett.134.056901