Universal Features of Terahertz Absorption in Disordered Materials

Using an analytical theory, experimental terahertz time-domain spectroscopy data and numerical evidence, we demonstrate that the frequency dependence of the absorption coupling coefficient between far-infrared photons and atomic vibrations in disordered materials has the universal functional form, C(omega) = A + B*omega^2, where the material-specific constants A and B are related to the distributions of fluctuating charges obeying global and local charge neutrality, respectively.Comment: 5 pages, 3 fig

Self-focusing of light by potassium vapor

Raman-shifted ruby-laser light on the high-frequency side of the 2P3/2 resonance line (7665 A) of potassium was self-focused by a dilute potassium gas.Peer reviewedElectrical and Computer Engineerin

Did the Selective Breeding of a Non-native Grass Promote Invasiveness?

Concerns with invasive species have included exotic forage grasses that have undergone extensive breeding through development programs for greater success of establishment. These grasses have been widely distributed for soil erosion control and livestock forage production throughout the United States and many of these species have become invasive in native habitats. To determine whether plant breeding procedures increased the invasiveness of these exotic grasses relative to their native counterparts, we assessed the competitiveness of an old world bluestem (Bothriochloa ischaemum; OWB) a known invader of the Great Plains in the United States, relative to its "wildtype" from the Czech Republic, and two native tallgrass prairie grasses (big bluestem [Andropogon gerardii] and little bluestem [Schizachyrium scoparium]). To assess inter-and intraspecific competition a substitutive design greenhouse competition experiment was conducted in Stillwater, Oklahoma with partners from the Czech University of Life Sciences. Growth of the invasive B. ischaemum was enhanced when grown with the native S. scoparium. However, the Czech Republic wildtype did not exhibit increased production when grown in competition with natives compared to conspecifics, suggesting the extensive breeding of B. ischaemum may have enhanced its competitive abilities. To further assess plant-soil feedbacks we conducted a second greenhouse study which examined soil biotic communities using soil inoculums associated with either wildtype or invasive B. ischaemum cultivars. Growth and reproduction of native grass species, the wildtype, and invasive B. ischaemum were assessed 16 weeks following inoculation with soil microbial communities. Phospholipid and neutral lipid fatty acids were quantified at the end of the study to determine microbial biomass associated with each soil inoculum and species or cultivar combination. Total biomass production and abundance of AMF was enhanced when the wildtype was grown with inoculum associated with the invasive cultivar compared to wildtype inoculum, indicating the wildtype favors soil biotic communities associated with the invasive cultivar. When assessing responses of native grasses, A. gerardii illustrated greater production and abundance of AMF compared to S. scoparium suggesting seeding to A. gerardii may lead to greater success in grassland restorations following eradication of B. ischaemum, compared to restorations seeded to S. scoparium.Natural Resources and Ecology Managemen

Theory of the dispersive modulator

The recent observations by Loy of amplitude modulation and compression of infrared laser pulses caused by passage through the dispersive modulator (a cell containing dilute NH3 vapor with a Stark-modulated resonant frequency) are explained. It is shown that there is a formal equivalence between (i) the passage of a near-resonant light beam through a vapor with a time-dependent resonant frequency, and (ii) the passage of a frequency-modulated light beam through a vapor with a constant resonant frequency. For low-intensity light this equivalence reduces the problem to the optical analog of chirp radar.Peer reviewedElectrical and Computer Engineerin

Parallel plate THz transmitter

A THz transmitter that directly excites the guided wave modes of a dielectric filled parallel plate waveguide is demonstrated. When coupled to free space, the transmitter yields a large peak time-domain THz signal. The device yields significant signal amplitudes with varying output spectra with and without a bias field applied. This transmitter provides powerful direct excitation of guided wave modes and is the next step toward an integrated guided wave transverse-electromagnetic mode THz bandwidth device.Peer reviewedElectrical and Computer Engineerin

THz transverse electromagnetic mode two-dimensional interconnect layer incorporating quasi-optics

We report the demonstration of a planar THz interconnect layer capable of transmitting subpicosecond pulses in the transverse electromagnetic (TEM) mode over arbitrarily long paths with low absorption and no observable group velocity dispersion. Quasioptical elements are incorporated within the interconnect layer forming a configurable THz bandwidth TEM-mode planar interconnect with negligible group velocity dispersion and low loss. For a 146 mm guided path length, including four reflections, the pulses are broadened by the frequency dependent absorption of the interconnect layer from 0.28 to 0.32 ps, and attenuated by the factor 0.2.Peer reviewedElectrical and Computer Engineerin

Reduced conductivity in the terahertz skin-depth layer of metals

The terahertz conductivities of plates of Cu and Al were measured to remain the same at 295 and 77 K using waveguide terahertz time domain spectroscopy (THz-TDS). This result was true for a variety of commercial alloys and surface preparations. Consequently, carrier scattering by lattice defects within the 100 nm THz skin depth is much larger than scattering by phonons at room temperature. However, an exception was found to be the THz skin-depth layer of an evaporated 300 nm Al film in contact with a polished Si surface. For this interface Al layer, the conductivity increased by a factor of 4 when cooled to 77 K.Peer reviewedElectrical and Computer Engineerin

Terahertz conductivity of thin metal films

The conductivities of thin Al, Au, and Ag films were measured via their transmission at terahertz frequencies. The conductivities of all the films, particularly the thinner films and Al films, were much smaller than their bulk dc values. This reduced conductivity can be quantitatively understood in terms of an increased scattering rate from defects. The transmission is consistent with a frequency independent conductivity, implying a very fast electron scattering time.Peer reviewedElectrical and Computer Engineerin

Superluminal optical pulse propagation in nonlinear coherent media

The propagation of light-pulse with negative group-velocity in a nonlinear medium is studied theoretically. We show that the necessary conditions for these effects to be observable are realized in a three-level $\Lambda$-system interacting with a linearly polarized laser beam in the presence of a static magnetic field. In low power regime, when all other nonlinear processes are negligible, the light-induced Zeeman coherence cancels the resonant absorption of the medium almost completely, but preserves the dispersion anomalous and very high. As a result, a superluminal light pulse propagation can be observed in the sense that the peak of the transmitted pulse exits the medium before the peak of the incident pulse enters. There is no violation of causality and energy conservation. Moreover, the superluminal effects are prominently manifested in the reshaping of pulse, which is caused by the intensity-dependent pulse velocity. Unlike the shock wave formation in a nonlinear medium with normal dispersion, here, the self-steepening of the pulse trailing edge takes place due to the fact that the more intense parts of the pulse travel slower. The predicted effect can be easily observed in the well known schemes employed for studying of nonlinear magneto-optical rotation. The upper bound of sample length is found from the criterion that the pulse self-steepening and group-advance time are observable without pulse distortion caused by the group-velocity dispersion.Comment: 16 pages, 7 figure

Technical Report: Final project report for Terahertz Spectroscopy of Complex Matter

This project designed characterization techniques for thin films of complex matter and other materials in the terahertz spectral region extending from approximately 100 GHz to 4000 GHz (4 THz) midway between radio waves and light. THz has traditionally been a difficult region of the spectrum in which to conduct spectroscopic measurements. The “THz gap” arises from the nature of the sources and detectors used in spectroscopy both at the optical (high frequency) side and electronic (low frequency) side of the gap. To deal with the extremely rapid oscillations of the electric field in this frequency region this research project adapted techniques from both the electronics and optics technologies by fabricating microscopic antennas and driving them with short optical pulses. This research technique creates nearly single cycle pulses with extremely broad spectral bandwidth that are able to cover the THz spectral range with a single measurement. The technique of THz time domain spectroscopy (THz-TDS) has seen increasing use and acceptance in laboratories over the past fifteen years. However significant technical challenges remain in order to allow THz-TDS to be applied to measurement of solid materials, particularly thin films and complex matter. This project focused on the development and adaptation of time domain THz measurement techniques to investigate the electronic properties of complex matter in the terahertz frequency region from 25 GHz to beyond 5 THz (165 inv. cm). This project pursued multiple tracks in adapting THz Time Domain Spectroscopy (THz-TDS) to measurement of complex matter. The first, and most important, is development of a reliable methods to characterize the complex dielectric constant of thin films with high accuracy when the wavelength of the THz radiation is much longer than the thickness of the film. We have pursued several techniques for measurement of thin films. The most promising of these are waveguide spectroscopy and THz interferometry. Since THz spectroscopy measures the changes of the transmitted spectra, any noise on the THz signal contributes to measurement errors. The dynamic range—defined as the RMS noise of the THz detector compared to the peak THz signal—of THz spectroscopy using photoconductive antennas is extremely high, typically over 10,000. However the precision with which spectroscopic data can be measured is limited by the noise on the laser source which is typically 0.1% to 1%. For low values of the sample absorbance and for values of optical thickness less than approximately 0.01, the change in transmission approaches the measurement accuracy. The sample refractive index can be measured with better accuracy since the index causes a temporal shift of the THz pulse by an amount time shift of nd/c where n is the refractive index, d the sample thickness, and c the speed of light. Time shifts of tens of femtoseconds can generally be resolved so that index-thickness values of nd > ten microns can be accurately measured. Waveguide spectroscopy is a way to increase the path length in thin film by several orders of magnitude, and thus have a large interaction length even when the film is much less than a wavelength in thickness. Film thicknesses of 10’s of nm have been measured. THz interferometry cancels out many of the noise sources of THz spectroscopy and can thus result in measurements of films of several hundred nm in thickness and is additionally suitable for optical pump, THz probe spectroscopic techniques. A large amount of additional work was performed in support of the main project direction or to explore promising alternative avenues for research. This report discussed work on the the confinement of low density species for measurement of nanogram or picogram quantities of material. Whispering gallery mode resonators to achieve long path lengths were also investigated as were imaging techniques for sub-wavelength imaging of thin films. The report concludes with a report on investigations of fundamental issues in THz beam propagation and coupling that impact THz spectroscopy and the other spectroscopic techniques developed during this project