Phonon Polaritonics in Broad Terahertz Frequency Range with Quantum Paraelectric SrTiO 3 .

Photonics in the frequency range of 5 to 15 terahertz (THz) potentially open a new realm of quantum materials manipulations and biosensing. This range, sometimes called "the new terahertz gap", is traditionally difficult to access due to prevalent phonon absorption bands in solids. Low-loss phonon-polariton materials may realize sub-wavelength, on-chip photonic devices, but typically operate in mid-infrared frequencies with narrow bandwidths and are difficult to manufacture in large scale. Here for the first time, quantum paraelectric SrTiO 3 enables broadband surface phonon-polaritonic devices in 7-13 THz. As a proof of concept, polarization-independent field concentrators are designed and fabricated to locally enhance intense, multicycle THz pulses by a factor of 6 and increase the spectral intensity by over 90 times. The time-resolved electric field inside the concentrators are experimentally measured by THz-field induced second harmonic generation. Illuminated by a table-top light source, the average field reaches 0.5 GV/m over a large volume resolvable by far-field optics. Our results potentially enable scalable THz photonics with high breakdown fields made of various commercially available phonon-polariton crystals for studying driven phases in quantum materials and nonlinear molecular spectroscopy. This article is protected by copyright. All rights reserved.