Eric R. Statz

Tracking the motion of charges in a terahertz light field by femtosecond X-ray diffraction

In condensed matter, light propagation near resonances is described in terms of polaritons, electro-mechanical excitations in which the time-dependent electric field is coupled to the oscillation of charged masses. This description underpins our understanding of the macroscopic optical properties of solids, liquids and plasmas, as well as of their dispersion with frequency. In ferroelectric materials, terahertz radiation propagates by driving infrared-active lattice vibrations, resulting in phonon-polariton waves. Electro-optic sampling with femtosecond optical pulses can measure the time-dependent electrical polarization, providing a phase-sensitive analogue to optical Raman scattering. Here we use femtosecond time-resolved X-ray diffraction, a phase-sensitive analogue to inelastic X-ray scattering, to measure the corresponding displacements of ions in ferroelectric lithium tantalate, LiTaO3. Amplitude and phase of all degrees of freedom in a light field are thus directly measured in the time domain. Notably, extension of other X-ray techniques to the femtosecond timescale (for example, magnetic or anomalous scattering) would allow for studies in complex systems, where electric fields couple to multiple degrees of freedom.

Direct Visualization of a Polariton Resonator in the THz Regime.

We report fabrication of a THz phonon-polariton resonator in a single crystal of LiNbO3 using femtosecond laser machining with high energy pulses. Fundamental and overtone resonator modes are excited selectively and monitored through spatiotemporal imaging. The resonator is integrated into a single solid-state platform that can include THz generation, manipulation, readout and other functionalities.

Terahertz wave generation and propagation in thin-film lithium niobate produced by crystal ion slicing

Terahertz phonon-polariton generation and real-space imaging with femtosecond optical pulses are demonstrated in a 10‐μm-thick film of single-crystalline lithium niobate that was generated through crystal ion slicing. The film dispersion properties were characterized throughout the polariton wavelength range of 5–100μm, revealing substantial slab waveguide behavior at the longer wavelengths.

Coherent control of phonon-polaritons in a terahertz resonator fabricated with femtosecond laser machining.

Using femtosecond laser machining, we fabricated a terahertz resonant cavity in LiNbO3. Optical pulse sequences with variable repetition rates, generated through a novel pulse-shaping method, are used for characterization of the cavity resonances and for amplification of terahertz phonon-polaritons in the cavity.

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

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.

Demonstration of terahertz frequency-dependent field transformation in an irregular waveguide structure with direct measurement of the internal electric fields.

We demonstrate irregular scattering structure frequency-dependent field control at terahertz frequencies by means of a TM(10) to TM(30) mode converter designed for operation near 300 GHz and fabricated out of lithium niobate. Imaging of the electric fields in the sample, with a Fourier analysis of the time domain signal, yielded the performance as a function of frequency.

Polaritonics in complex structures: Confinement, bandgap materials, and coherent control

We report on the design, fabrication, and testing of ferroelectric patterned materials in the guided-wave and polaritonic regime. We demonstrate their functionality and exploit polariton confinement for amplification and coherent control using temporal pulse shaping.

Simulation of Phonon-Polariton Generation and Propagation in Ferroelectric LiNbO 3 Crystals

We simulate propagation of phonon-polaritons (admixtures of polar lattice vibrations and electromagnetic waves) in ferroelectric LiNbO3 with a model that consists of a spatially periodic array of harmonic oscillators coupled to THz electromagnetic waves through an electric dipole moment. We show that when this model is combined with the auxiliary differential equation method of finite difference time domain (FDTD) simulations, the salient features of phonon-polaritons may be illustrated. Further, we introduce second order nonlinear coupling to an optical field to demonstrate phonon-polariton generation by impulsive stimulated Raman scattering (ISRS). The phonon-polariton dispersion relation in bulk ferroelectric LiNbO3 is determined from simulation.

PHONON-POLARITON EXCITATION IN COUPLED FERROELECTRIC WAVEGUIDES

ABSTRACT Unique THz phonon-polariton properties in coupled LiNbO3 rectangular waveguides are examined experimentally and compared to theoritical expectations.

Terahertz polaritonics: High-field THz coherent control and spectroscopy

The tools of THz polaritonics are extended for improved polariton amplification and for nonlinear THz spectroscopy using amplified and focused polariton waveforms. THz coherent spectroscopy and control applications are enabled