Perovskite Phonon Polaritonics in the Terahertz Gap

Photonics in the “new terahertz (THz) gap” between 5 and 15 THz have numerous applications from molecular sensing to ultrafast materials manipulations. However, there are few choices of photonic materials in this frequency range due to prevalent phonon absorption in solids. Recent progress in phonon-polariton microstructures and metamaterials demonstrated low-loss and sub-wavelength photonic devices in the mid-infrared frequencies, but many require crystalline thin films and have small bandwidths. Crystalline ternary oxides were predicted to have great potential as broadband phonon-polaritonic materials in mid- and far-infrared frequencies, but the difficulty in fabrication has impeded further development. Here we report surface phonon-polaritonics based on single-crystalline SrTiO3 that realizes planar ultrafast concentrators with a tunable resonance in 7–13 THz. Our design principle allows simple fabrication without damaging the crystalline quality of the materials. Experimentally, we achieved polarization-independent field amplification by a factor of 5 and average spectral enhancement over 30 times. We measured the resonance spectra and transient electric fields of the concentrators by THz-field induced second harmonic generation (TEFISH), showing up to 0.5 GV/m field strength in a large, optically resolvable area with a table-top source. The ability to generate strong, multicycle, variable-polarized THz fields in the new terahertz gap offers a convenient platform for studying driven phases and nonlinear phononics in quantum materials.