Nikolay S. Stoyanov

Heterodyned impulsive stimulated Raman scattering of phonon–polaritons in LiTaO3 and LiNbO3

Phonon-polariton dispersion is characterized in ferroelectric lithium tantalate and lithium niobate through femtosecond time-resolved impulsive stimulated Raman scattering (ISRS). An improvement in the ISRS setup permits optical heterodyne detection of the signals. In addition to substantially increasing the sensitivity and accuracy of the measurements, the phase sensitivity of heterodyne detection makes it possible to fully characterize the polariton wave after it has propagated outside of the excitation region. The detection of propagating responses with heterodyned ISRS is explored theoretically and experimentally. Discrepancies in earlier results reported for these materials are resolved.

Integrated diffractive terahertz elements

Femtosecond laser machining with high-energy pulses is used for fabrication of diffractive elements in LiNbO3 crystalline samples. This permits terahertz generation, frequency dispersion, detection, and analysis within a single integrated platform that is well suited for applications in terahertz spectroscopy or signal processing.

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.

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.

Direct visualization of phonon-polariton focusing and amplitude enhancement

We report a new method to generate focused phonon polaritons inside ferroelectric crystals. Using an excitation pulse in the form of a ring we generate a phonon-polariton wave packet that propagates inward toward a focal point at the center. This results in a sharp increase in the polariton intensity. It also permits direct visualization and quantitative measurements of the Gouy phase shift and of anisotropy in polariton propagation. Since vibrational and electromagnetic modes are coupled, coherent vibrational amplitudes can be increased substantially through exploitation of electromagnetic wave propagation.

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.

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.

Terahertz polaritonics: spectroscopy and technology

Summary form only given. Coherent phonon-polaritons are generated by femtosecond pulses in electro-optic crystals and exploited for THz spectroscopic and signal processing applications. The use of spatially and temporally shaped excitation fields to generate specified polariton responses the integration of THz waveguides and other functional elements into the polariton host crystals and the use of polaritons for linear and nonlinear THz spectroscopy are discussed.