David W. Ward

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.

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.

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.

On the physical origins of the negative index of refraction

The physical origins of negative refractive index are derived from a dilute microscopic model, producing a result that is generalized to the dense condensed phase limit. In particular, scattering from a thin sheet of electric and magnetic dipoles driven above resonance is used to form a fundamental description for negative refraction. Of practical significance, loss and dispersion are implicit in the microscopic model. While naturally occurring negative index materials are unavailable, ferromagnetic and ferroelectric materials provide device design opportunities. An erratum page was added to the end of the published paper on 11 November 2005.

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.

Performance of BGP among mobile military networks

As tactical military networks deploy new IP-capable radios (e.g., JTRS), it is expected that network connectivity will increase both within and among Service and Coalition networking domains (e.g., Army, Navy, NATO). However, unmanaged use of this increased connectivity may result in violation of some mission-critical operational limits. One mechanism known to (coarsely) manage large-scale IP networks is BGP routing policy. As such, this paper begins the study of BGPs applicability to manage an evolution of the GIG in which IP-based tactical radios proliferate (i.e., the Future GIG). To this end, this paper first presents a modification to BGP that allows for dynamic management of its peering sessions to accommodate network node mobility. Next, since it is known that BGP can have performance issues (e.g., slow convergence), this paper uses network emulation1 to perform a performance assessment of our modified BGP protocol. Specifically, for ten independent realizations of a mobile wireless networking model, our modified BGP protocol is evaluated with respect to its generated protocol overhead, its ability to develop valid routes to destinations (e.g., reachability), and its influence on networks outage events. Furthermore, our modified BGP protocol is compared to the OSPF and OSPF-MDR routing protocols for mobile networks with increasing number of nodes. Our results show that our modified BGPs overhead growth is significantly higher than both OSPF variations, but the networks average reachability and median outage times are similar. To decrease the significant overhead seen from BGP, this paper also presents a second modification to the BGP protocol based on the use of a connected dominating set (CDS) backbone. Here, the emulation results show that the use of a CDS backbone can significantly decrease BGPs overhead with little impact on the networks average reachability and median duration of its outage events. Lastly, it was found that the maximum network outage times for both modified BGP protocols can be several percent greater than that experienced by either OSPF-based routing protocol. However, the occurrence rate of such long network outage events (i.e., those greater than 60 secs) was also seen to be infrequent for networks whose average shortest paths was low (i.e., less than 3 hops).

Finite‐difference time‐domain (FDTD) simulations of electromagnetic wave propagation using a spreadsheet

We describe a simple and intuitive implementation of the method of finite‐difference time‐domain (FDTD) simulations for propagating electromagnetic waves using Microsoft Excel. The method overcomes the usual obstacles of unfamiliarity with programming languages as it relies on little more than the cut and paste features of Excel.

Phonon-Polariton Excitation in Ferroelectric Slab Waveguides and Photonic Crystals

Unique THz phonon-polariton properties in LiNbO3 planar waveguides and photonic crystals are examined experimentally and compared to theoretical expectations.