Phonon-Polariton Propagation, Guidance, and Control in Bulk and Patterned Thin Film Ferroelectric Crystals

TL;DR

This paper demonstrates control and guidance of phonon-polaritons in ferroelectric crystals using ultrafast spectroscopy, laser patterning, and optical shaping, enabling functional THz waveguide and resonator structures.

Contribution

It introduces methods for guiding and controlling phonon-polaritons in bulk and patterned ferroelectric crystals, including fabrication of functional THz devices.

Findings

Ferroelectric crystals act as slab waveguides for phonon-polaritons.

Ultrafast laser ablation enables patterning of crystals for guiding structures.

Spatial and temporal shaping enhances electric field in phonon-polariton resonators.

Abstract

Using time resolved ultrafast spectroscopy, we have demonstrated that the far infrared (FIR) excitations in ferroelectric crystals may be modified through an arsenal of control techniques from the fields of guided waves, geometrical and Fourier optics, and optical pulse shaping. We show that LiNbO3 and LiTaO3 crystals of 10-250 micron thickness behave as slab waveguides for phonon-polaritons, which are admixtures of electromagnetic waves and lattice vibrations, when the polariton wavelength is on the order of or greater than the crystal thickness. Furthermore, we show that ferroelectric crystals are amenable to processing by ultrafast laser ablation, allowing for milling of user-defined patterns designed for guidance and control of phonon-polariton propagation. We have fabricated several functional structures including THz rectangular waveguides, resonators, splitters/couplers, interferometers, focusing reflectors, and diffractive elements. Electric field enhancement has been obtained with the reflective structures, through spatial shaping, of the optical excitation beam used for phonon-polariton generation, and through temporal pulse shaping to permit repetitive excitation of a phonon-polariton resonant cavity.