Showing papers by "Keith A. Nelson published in 2003"

Spatiotemporal coherent control of lattice vibrational waves.

Abstract: We achieved automated optical control over coherent lattice responses that were both time- and position-dependent across macroscopic length scales. In our experiments, spatiotemporal femtosecond pulse shaping was used to generate excitation light fields that were directed toward distinct regions of crystalline samples, producing terahertz-frequency lattice vibrational waves that emanated outward from their multiple origins at lightlike speeds. Interferences among the waves resulted in fully specified far-field responses, including tilted, focusing, or amplified wavefronts. Generation and coherent amplification of terahertz traveling waves and terahertz phased-array generation also were demonstrated.

Field expulsion and reconfiguration in polaritonic photonic crystals.

Abstract: We uncover a rich set of optical phenomena stemming from the incorporation of polar materials exhibiting transverse phonon polariton excitations into a photonic crystal structure. We identify in the frequency spectrum two regimes in which the dielectric response of the polaritonic medium can induce extreme localization of the electromagnetic energy. Our analysis of the effect of polarization and the interaction between the polariton and photonic band gaps on the Bloch states leads to a pair of mechanisms for sensitive frequency-controlled relocation and/or reconfiguration of the fields.

Phonon-polariton excitations in photonic crystals

Abstract: The incorporation of materials which exhibit transverse phonon-polariton excitations into a photonic crystal produces an intricate optical system possessing unique and varied photon phenomena. In particular, we demonstrate theoretically that such a system will exhibit both near-dispersionless bands with field localization in the polaritonic material and metalliclike bands with complete flux expulsion in an extremely small frequency interval around the characteristic phonon frequency. Moreover, when the fundamental resonances of the polaritonic rods overlap with the bands of a geometrically identical metallodielectric crystal, nearby states will couple to produce a band in which the localized field varies continuously between two distinct nodal patterns, in an exceedingly small frequency range. We also discuss the implications of losses on these phenomena and verify that our results can be realized experimentally.

Integrated diffractive terahertz elements

Abstract: 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.

Automated spatiotemporal diffraction of ultrashort laser pulses.

Abstract: We exploit the close similarities between time-frequency and position-wave-vector correspondences to control the spatiotemporal diffraction pattern of ultrashort laser pulses. This approach permits novel, automated generation of sophisticated two-dimensional femtosecond waveforms. A two-dimensional space-time version of a Gerchberg-Saxton algorithm is used to iteratively determine the phase pattern in position-frequency space that produces a user-defined intensity profile in wave-vector-time space.

Acoustic waves at interfaces studied by laser ultrasonics (invited)

Abstract: When energy is deposited at the interface between two materials, then acoustic waves that are confined in the interface region and traveling in the plane of the interface may arise. In this article, we illustrate the feasibility and versatility of interface wave laser ultrasonics by presenting a selection of recently developed laser ultrasonic configurations for detecting acoustic waves at the free surface of a solid and at a liquid–solid interface in a variety of applications, such as nonlinear wave propagation, characterization of coatings, porous materials and substrates, and acoustic wave imaging.

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

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 μm 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 LiNbO3 Crystals

Abstract: 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.

Phase Transition Dynamics Studied by Coherent Phonon Excitation with Ultrashort Laser Pulses

Abstract: The low-frequency light scattering spectroscopy always encounters experimentally a difficulty near the phase transition point due to the overlapping between the soft mode spectra and the strong elastic-scattering component originated from the surface and/or imperfections irradiated by the laser light. A time-resolved phonon spectroscopy, the coherent phonon excitation method, has been introduced as a new method to compliment the light scattering method. It observe directly the time dependence of the soft phonons excited artificially in terms of pico or femtosecond laser pulses. Several cases of the studies on TGS, TGSe, KDP, DKDP and LiNbO 3 are briefly mentioned.

Two-dimensional Arbitrary THz Waveform Generation and Integrated Waveguide Propagation

Abstract: Spatially shaped femtosecond optical fields are used to generate spatially and temporallyspecified THz responses in electrooptic media. Mode-selective propagation through integrated THz waveguide structures is demonstrated.

Spatial, temporal, and spatiotemporal femtosecond pulse shaping and coherent control

Abstract: Coherent optical control over THz-frequency lattice waves that move at light-like speeds is achieved in an automated fashion through spatial-only, temporal-only, and spatiotemporal femtosecond pulse shaping. These methods are used to generate complex excitation light fields for THz wave amplification, timed array generation, and other objectives.

Terahertz polaritonics: spectroscopy and technology

Abstract: 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.