The nonlinear electromagnetic response is one of the foundations of modern technology and it arises in natural
materials at the atomic scale. We briefly present some of the fundamentals of nonlinearity in natural materials
and then we present experimental studies of analogous behavior in meta-atoms, the fundamental building block
of metamaterials. Specifically tunnel-diode loaded, microwave split-ring resonators are shown to enable various
nonlinear phenomena including self-sustained oscillation, harmonic/comb generation, frequency locking/pulling,
and quasi-chaos generation. We discuss the possible adaptation of these unit cells to create bulk nonlinear metamaterials.
We report the results of a study to model the behavior of nonlinear metamaterials in the microwave frequency range
composed of arrays of split-ring resonators combined with nonlinear circuit elements. The overall model consists of an
array of coupled damped oscillators whose inter-element coupling is a function of signal amplitude, similar to that which
exists in the Fermi-Pasta-Ulam system.  We note the potential occurrence of classical nonlinear effects including
parametric coupling, FPU recurrence and chaos. These effects lead to nonlinear waves on the array which are a type of
soliton particular to the form of nonlinearity that has been incorporated. We have studied, in particular, the nonlinear
effects that arise from tunnel diodes embedded in the resonant circuits. We carry out simulations of the resulting circuit
Terahertz (THz) impulse ranging is used to examine ceramic ball bearings for fractures. THz radiation is demonstrated to transmit through both unfired and fired ceramic targets. The electromagnetic scattering signature of commercial aluminum oxide bearings is measured and compared to identical bearings damaged by thermal stress. Two separate methods are used to determine the presence of fractures: late time impulse response and time domain angular modulation. Late time impulse response detects changes in temporally shifted scattering mechanisms, while time domain angular modulation allows rapid detection of fractures. This evaluation technique is non-contact, nondestructive, requires no liquid medium, and is insensitive to ambient temperatures.
Over the past decade the experimental technique of THz time domain spectroscopy (THz-TDS) has proved to be a versatile method for investigating a wide range of phenomena in the THz or far infrared spectral region from 100 GHz to 5 THz. THz-TDS has wide potential for sensing and imaging. The experimental technique is described along with recent results on THz beam propagation for long base-line THz measurements. THz imaging has been demonstrated using both quasi-optical and synthetic aperture approaches, results are presented including images of scatterers as well as non-destructive evaluation of ceramics. Two potential sensing applications of THz-TDS are discussed, thin film characterization and use of waveguides for sensing.
We have designed and demonstrated two wide bandwidth passive frequency doublers capable of second harmonic generation over the entire 100 nm cavity optics bandwidth of a passively modelocked Ti:Al<SUB>2</SUB>O<SUB>3</SUB> laser with minimal loss in conversion efficiency. In addition, we demonstrate that focussing effects are extremely important in determining the bandwidth in sum frequency mixing of ultrashort pulses in the near VUV region. This effect is demonstrated with noncollinear sum frequency mixing performed between the fundamental and the third harmonic subpicosecond pulses of this laser source.