Phonon Polaritonics in Broad Terahertz Frequency Range with Quantum Paraelectric SrTiO₃

Abstract

Photonics in the frequency range of 5–15 terahertz (THz) potentially open a new realm of quantum materials manipulation 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 on a large scale. Here, for the first time, quantum paraelectric SrTiO3 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 is experimentally measured by THz‐field‐induced second harmonic generation. Illuminated by a table‐top light source, the average field reaches 0.5 GV m−1 over a large volume resolvable by far‐field optics. These 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.

Subwavelength, Phase-Sensitive Microscopy of Third-Order Nonlinearity in Terahertz Frequencies

“…When the materials’ χ (3) is known, the TEFISH signal can tell the strength of the enhanced local THz field, or alternatively, a larger “effective” χ (3) in the resonator if we assume the free-space THz field strength to calculate the TEFISH signal. We chose the bull’s eye resonator geometry, first introduced in the context of surface plasmon polaritons, and applied the same principle to SPhP. , We implemented the methodology of numerical optimization and fabrication previously reported for SrTiO 3 SPhP resonators with another ionic crystal CaF 2 , whose dielectric function is negative between 8 and 13 THz and can support SPhP in this frequency range . Briefly, we simulated the electric field response of the resonator with a finite-difference time-domain method (Lumerical).…”

“…We chose the bull's eye resonator geometry, first introduced in the context of surface plasmon polaritons, and applied the same principle to SPhP. 68,69 We implemented the methodology of numerical optimization and fabrication previously reported for SrTiO 3 SPhP resonators with another ionic crystal CaF 2 , whose dielectric function is negative between 8 and 13 THz and can support SPhP in this frequency range. 69 Briefly, we simulated the electric field response of the resonator with a finitedifference time-domain method (Lumerical).…”

“…68,69 We implemented the methodology of numerical optimization and fabrication previously reported for SrTiO 3 SPhP resonators with another ionic crystal CaF 2 , whose dielectric function is negative between 8 and 13 THz and can support SPhP in this frequency range. 69 Briefly, we simulated the electric field response of the resonator with a finitedifference time-domain method (Lumerical). The field enhancement with respect to the incoming THz pulses with a Gaussian beam profile is optimized by three geometrical parameters: the periodicity of the concentric ring grating, the diameter of the central disk, and the thickness of the SU-8 layer.…”