Michael J. Fitch

Terahertz spectroscopy techniques for explosives detection.

Spectroscopy in the terahertz frequency range has demonstrated unique identification of both pure and military-grade explosives. There is significant potential for wide applications of the technology for nondestructive and nonintrusive detection of explosives and related devices. Terahertz radiation can penetrate most dielectrics, such as clothing materials, plastics, and cardboard. This allows both screening of personnel and through-container screening. We review the capabilities of the technology to detect and identify explosives and highlight some of the critical works in this area.

High-fidelity quantum logic operations using linear optical elements.

Knill, Laflamme, and Milburn [Nature (London) 409, 46 ((2001))]] have shown that quantum logic operations can be performed using linear optical elements and additional ancilla photons. Their approach is probabilistic in the sense that the logic devices fail to produce an output with a failure rate that scales as 1/n, where n is the number of ancilla. Here we present an alternative approach in which the logic devices always produce an output with an intrinsic error rate that scales as 1/n(2), which may have several advantages in quantum computing applications.

Microwave and millimeter-wave ranging for coherent distributed RF systems

Microwave and millimeter-wave ranging systems, waveforms, and experimental results are described for coherent distributed RF systems applications. Measured results show that coherent distributed systems operating at carrier frequencies with coherence at λ/10 are possible well into the millimeter-wave regime by using widely separated two-tone ranging waveforms. The two-tone waveform in the context of continuous-wave ranging is introduced. A method for overcoming the range-ambiguous output of the matched filter processing is described. Microwave and millimeter-wave ranging measurements are shown and compared to the Cramer-Rao lower bound for range accuracy.

Propagation of Terahertz Radiation in Porous Polymer and Ceramic Materials

In this work we investigate the propagation of terahertz radiation through polyurethane foam and porous alumina ceramics to understand the effects of structure on the optical properties of these materials at terahertz frequencies. A terahertz time domain system with a GaAs photoconductive emitter and a ZnTe electro‐optic crystal was used to generate and detect the transmitted terahertz signal. Using the amplitude and phase characteristics of these signals, the thickness, index of refraction, and other physical and optical properties of the materials were determined.

Demonstration of Multi-Band Frequency-Selective Surfaces Using Split-Ring Triangular Resonators

Design, experimental and modeling studies have been undertaken on periodic, multi-frequency two-dimensional arrays of four differently sized metallic split-ring triangular resonators fabricated on a dielectric substrate. A sample consisting of quadrants of four differently dimensioned resonators showed only a broadened, apparently multimodal feature in the transmission spectrum. Four discrete resonances were achieved using a number of permutations of a primitive unit cell containing each of the triangular resonators. For each of the unit-cell permutations, four discrete resonances are predicted based on numerical simulation; however, the spectral intensities are shown to be markedly different in a subset of cases.

Comparison of Cross-Polarization Characteristics of Frequency-Selective Surfaces Composed of Square Split-Ring and Spiral Resonators

This paper reports on the design, measurement and modeling of frequency-selective surfaces in the microwave regime using arrays of spiral resonators. A distinct and unusual variation in the transmission properties of the frequency-selective surface composed of spiral resonators was observed. This variation may be a function of the geometry of the spiral resonator used and/or the use of slant/cross-polarization characterization. The cross-polarization results obtained for both orientations imply that the spiral resonators behave differently than the circular split-ring resonators. These results may be explained by an approximately 50deg shift in electromagnetic axis, reflecting the differences in the geometries of the two structures

Terahertz Technology for Defense Related Applications

Terahertz technology for security and defense related applications has recently experienced an increase in interest with an emphasis on imaging of concealed explosives. Our research has focused on spectroscopy of explosives and novel THz taggants.