1,129 research outputs found
Imaging of THz waves in 2D photonic crystal structures embedded in a slab waveguide
No full textWe present space- and time-resolved simulations and measurements of single-cycle terahertz (THz) waves propagating through two-dimensional (2D) photonic crystal structures embedded in a slab waveguide. Specifically, we use a plane wave expansion technique to calculate the band structure and a time-dependent finite-element method to simulate the temporal evolution of the THz waves. Experimentally, we measure the space–time evolution of the THz waves through a coherent time-resolved imaging method. Three different structures are laser machined in LiNbO3 crystal slabs and analyzing the transmitted as well as the reflected THz waveforms allows determination of the bandgaps. Comparing the results with the calculated band diagrams and the time-dependent simulations shows that the experiments are consistent with 3D simulations, which include the slab waveguide geometry, the birefringence of the material, and a careful analysis of the excited modes within the band diagrams.Swiss National Science Foundation (project no. 200020-119934
Focus on nonlinear terahertz studies
No full textResulting from the availability of improved sources, research in the terahertz (THz) spectral range has increased dramatically over the last decade, leading essentially to the disappearance of the so-called 'THz gap'. While most work to date has been carried out with THz radiation of low field amplitude, a growing number of experiments are using THz radiation with large electric and magnetic fields that induce nonlinearities in the system under study. This 'focus on' collection contains a number of articles, both experimental and theoretical, in the new subfield of THz nonlinear optics and spectroscopy on various systems, among them molecular gases, superconductors, semiconductors, antiferromagnets and graphene
Knowledge about knowledge since Nelson & Winter: a mixed record
No full textProgress in our understanding of the role of knowledge in the economy, based on Nelson and Winter's book published in 1982, has been mixed. It has been greatest when their concepts have been enriched by empirical evidence, often coming from outside evolutionary economics. It has been least when discussions have been mainly theoretical, and constrained within evolutionary economics.knowledge, innovation, technical change
Nanoconcentration of terahertz radiation in plasmonic waveguides
No full textHere, we establish the principal limits for the nanoconcentration of the THz radiation in metal/dielectric waveguides and determine their optimum shapes required for this nanoconcentration. We predict that the adiabatic compression of THz radiation from the initial spot size of R[subscript 0] ~ λ[subscript 0] to the final size of R = 100 - 250 nm can be achieved with the THz radiation intensity increased by a factor of ×10 to ×250. This THz energy nanoconcentration will not only improve the spatial resolution and increase the signal/noise ratio for the THz imaging and spectroscopy, but in combination with the recently developed sources of powerful THz pulses will allow the observation of nonlinear THz effects and a carrying out a variety of nonlinear spectroscopies (such as two-dimensional spectroscopy), which are highly informative. This will find a wide spectrum of applications in science, engineering, biomedical research, environmental monitoring.National Science Foundation (U.S.). (Grant Number CHE-0507147
The jingling Geordie: community arts and the regional culture of the North East of England
No full textIn the light of the massive economic and social changes which have affected the North East of England over the last 25 years, the author assesses the vitality of the indigenous culture and reflects upon current cultural trends and the North East’s future, particularly in relation to a regional Europe. He traces the folk-tradition of the region and looks at ways in which this can be drawn upon to develop a meaningful link between past and present. He looks closely at the changing nature of class-relationships in the North East and reflects upon how a valid local culture can survive in a multi-cultural society. He draws upon his own extensive experience in Community Arts, looking at definitions of the term in the new political climate and arguing for its positive contribution to the cultural debate. He dwells on the issue of regionalism and devolution in a new Europe, comparing the situation in the North East of England with political and cultural changes in Scotland and other parts of the United Kingdom
Commensurate Two-Quantum Coherences Induced by Time-Delayed THz Fields
No full textThe interaction of carbonyl sulfide dipolar gas molecules with two time-delayed, single-cycle THz pulses is shown both experimentally and theoretically to induce two-quantum rotational coherences that are significantly enhanced with respect to those induced by one THz pulse, depending on the relative delay of the pulses. The underlying phenomenon is quite general in that it can occur even after a single THz pulse if more than one molecular species is present, since the free induction decay emitted by one species (demonstrated here by atmospheric water vapor) can provide the second field interaction for the other.United States. Office of Naval Research (Grant N00014- 09-1-1103)National Science Foundation (U.S.) (Grant CHE-1058709
Influence of multi-exciton correlations on nonlinear polariton dynamics in semiconductor microcavities
No full textUsing two-dimensional spectroscopy, we resolve multi-polariton coherences in quantum wells embedded inside a semiconductor microcavity and elucidate how multi-exciton correlations mediate polariton nonlinear dynamics. We find that polariton correlation strengths depend on spectral overlap with the biexciton resonance and that up to at least four polaritons can be correlated, a higher-order correlation than observed to date among excitons in bare quantum wells. The high-order correlations can be attributed to coupling through the cavity mode, although the role of high-order Coulomb correlations cannot be excluded
Onset of nondiffusive phonon transport in transient thermal grating decay
No full textThe relaxation of a spatially sinusoidal temperature perturbation in a dielectric crystal at a temperature comparable to or higher than the Debye temperature is investigated theoretically. We assume that most phonons contributing to the specific heat have a mean free path (MFP) much shorter than the thermal transport distance and can be described by the thermal diffusion model. Low-frequency phonons that may have MFP comparable to or longer than the grating period are described by the Boltzmann transport equation. These low-frequency phonons are assumed to interact with the thermal reservoir of high-frequency phonons but not with each other. Within the single mode relaxation time approximation, an analytical expression for the thermal grating relaxation rate is obtained. We show that the contribution of “ballistic” phonons with long MFP to the effective thermal conductivity governing the grating decay is suppressed compared to their contribution to thermal transport at long distances. The reduction in the effective thermal conductivity in Si at room temperature is found to be significant at grating periods as large as 10 μm.Solid-State Solar-Thermal Energy Conversion CenterUnited States. Dept. of Energy. Office of Basic Energy Sciences (Award No. DE-SC0001299
Direct visualization of terahertz electromagnetic waves in classic experimental geometries
No full textWe used newly developed experimental methods to collect educational video clips of electromagnetic waves propagating at the speed of light. The terahertz frequency waves were generated and detected in LiNbO3 crystals structured to reproduce classic optical geometries and experiments, such as two-slit interference and diffraction from a grating. Direct visualization of the phase fronts as the electromagnetic pulses propagate, reflect, diffract, and interfere helps develop intuition and understanding about the fundamental behavior of light and waves. We believe these videos will be a valuable addition to lectures on introductory optics and physics, because they bring contemporary research to the classroom and provide clear and direct experimental demonstrations of light and wave behavior.National Science Foundation (U.S.) (Grant No. ECCS-0824185)National Science Foundation (U.S.) (Grant No. CHE-0616939)National Science Foundation (U.S.) (Graduate Research Fellowship
Femtosecond surface plasmon interferometry
No full textWe demonstrate femtosecond plasmonic interferometry with a novel geometry. The plasmonic microinterferometer consists of a tilted slit-groove pair. This arrangement allows for (i) interferometric measurements at a single wavelength with a single microinterferometer and (ii) unambiguous discrimination between changes in real and imaginary parts of the metal dielectric function. The performance is demonstrated by monitoring the sub-picosecond dynamics of hot electrons in gold.Deutsche Forschungsgemeinschaft (TE770/1)National Science Foundation (U.S.) (CHE-0616939)National Science Foundation (U.S.) (CHE-0507147)European Commission (EU Network of Excellence ”PhOREMOST”)European Commission (EU Network of Excellence NMP3-SL-2008-214107-Nanomagma)European Commission (FUNCOAT CONSOLIDER INGENIO 2010 CSD2008-00023)European Commission (MAGPLAS MAT2008-06765-C02-01/NAN)European Commission (NANOMAGNETCMS- 0505/MAT/0194)European Commission (MICROSERES S-0505/TIC/0191
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