Innovating ENZ Materials for Next-Generation Photonics

Scientists have discovered a groundbreaking method of manipulating light emission by temporarily transforming ordinary liquids into epsilon-near-zero (ENZ) materials using intense femtosecond laser pulses.

In ordinary optical media, both the phase and group velocities of light cannot exceed the speed of light in a vacuum. However, in epsilon near-zero (ENZ) materials, light exhibits unique behavior: at a certain frequency, the phase velocity becomes infinite, while the group velocity vanishes.

Until now, such phenomena have only been observed in solids and nanoengineered materials. This study presents a novel approach that temporarily converts common liquids, such as 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 produces free electrons that are rapidly localized or “solvated.” On femtosecond timescales, these electrons occupy spaces in the molecular lattice, a disordered structure of electric dipoles.

The binding energy of an electron in its final position is largely influenced by the electrical interaction between the electron and the molecular dipoles of the liquid. This ultrafast localization process induces collective oscillations between the electron and nearby liquid molecules, giving rise to a many-body excitation called a polaron. This excitation has a specific frequency in the THz range that depends on the electron concentration in the liquid.

At this polaron frequency, the dielectric function and refractive index of the liquid cross zero, as shown in the figure above. The phase velocity of light at this frequency essentially tends to infinity, while the group velocity of the light pulses approaches zero – typical behavior of an ENZ material.

For practical applications, the ability to shift the polaron frequency by changing the electron concentration is a very attractive feature. This provides a way to control and tune the ENZ properties of the material over a wide 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 advances in optical communication and sensing technologies.