Tracing light: on Linear and Nonlinear Terahertz Polaritonics in Quantum Paraelectrics

The advancement of high-speed data processing and signal manipulation technologies has pushed the frontier into the terahertz (THz) frequency range, raising a fundamental challenge: how to efficiently control electromagnetic fields at these frequencies. This issue, often referred to as the "THz gap" in optical and electronic engineering, remains a major obstacle to practical applications. A promising strategy is to manipulate THz electromagnetic waves through polaritons—hybrid light-matter excitations—that harness the intrinsic nonlinear response of matter.

In this talk, I will introduce Quantum Paraelectric (QP) solids as a novel platform for THz phonon-polaritonics, leveraging the extreme nonlinearity of SrTiO₃ in its QP phase, which arises due to its proximity to the incipient ferroelectric state. This strong nonlinearity enables efficient self- and cross-coupling between polaritons, paving the way for all-optical, field-programmable THz polariton circuits.

In the second part of the talk, I will turn to the basic problem of linear propagation of THz electromagnetic waves in SrTiO₃ and KTaO₃. As it turns out, our experimental data challenges long-held assumptions about light propagation in dispersive dielectrics—an area considered settled since the foundational works of Lorentz, Sommerfeld, and Brillouin — bringing a fresh perspective on a fundamental problem in wave physics.