3,841 research outputs found
Simultaneous electronic and the magnetic excitation of a ferromagnet by intense THz pulses
The speed of magnetization reversal is a key feature in magnetic data
storage. Magnetic fields from intense THz pulses have been recently shown to
induce small magnetization dynamics in Cobalt thin film on the sub-picosecond
time scale. Here, we show that at higher field intensities, the THz electric
field starts playing a role, strongly changing the dielectric properties of the
cobalt thin film. Both the electronic and magnetic responses are found to occur
simultaneously, with the electric field response persistent on a time scale
orders of magnitude longer than the THz stimulu
Air nonlinear dynamics initiated by ultra-intense lambda-cubic THz pulses
Air turns into a nonlinear medium for electromagnetic waves under
exceptionally strong fields. However up to present, its minuscule nonlinear
response has limited the exploration to the optical frequency regime owing to
the availability of intense near-infrared lasers. Here, we report on the
observation of large-amplitude nonlinearity in air induced by an extremely
intense light bullet at Terahertz frequencies (0.1-10 THz) provoking strong air
birefringence. The observed nonlinearity manifests itself as third order
susceptibility. The presented nonlinear observations break the barrier for the
entire exciting THz-induced nonlinear phenomena in air ranging from THz-induced
self-focusing and self-phase modulation to THz solitons and filamentation
Performance Evaluation of Inverted Tee (IT) Bridge System
The Inverted Tee (IT) girder bridge system was originally developed in 1996 by the University of Nebraska–Lincoln (UNL) researchers and Nebraska Department of Transportation (NDOT) engineers. This bridge system currently accounts for over 110 bridges in Nebraska used for both state highways and local county roads. Extensive longitudinal and transverse deck cracking have been observed and noted in numerous bridge inspection reports. Since the IT girder bridge system is relatively new, limited data and knowledge exist on its structural performance and behavior. This study evaluates the IT girder bridge system by conducting twenty field observations as well as recording accelerometer, strain gauge, and LVDT time histories and lidar scans for a selected subset of these bridges and then a three-dimensional finite element analysis (FEA) was conducted. The field observations included visual inspection for damage and developing deck crack maps to identify a trend for the damage. System identification of the bridge deck and girders helped investigate the global and local structural responses, respectively. Operational modal analysis quantified the natural frequencies, damping ratios, and operational deflected shapes for the instrumented IT girder bridges. These results helped diagnose the reason for the longitudinal deck cracking. The IT girders respond non- uniformly for the first operational deflected shape and independently for higher modes. Two comparable bridges, namely one slab and one NU girder bridge, were instrumented to verify and demonstrate that the IT girder behavior is unique. An advanced geospatial analysis was conducted for the IT girder bridges to develop lidar depth maps of the deck and girders elevations. These depth maps help identify locations of potential water/chloride penetration and girders set at various elevations and/or where the deck thickness is non-uniform. Live load tests helped quantify the transverse dynamic behavior of the bridge girders. Quantifying the transverse dynamic behavior helped validate the source of longitudinal deck cracking in IT girder bridges, which was determined to be the differential deflection between adjacent IT girders. The FEA analysis was conducted to evaluate the live load moment and shear distribution factors and compare that to the predicted values calculated from the AASHTO Standard and LRFD bridge design specifications. The comparison indicated that the predicted distribution factors were conservative. Also, interviews with IT bridge producers and contractors were conducted to determine production and construction advantages and challenges of this bridge system
Subwavelength atom localization via amplitude and phase control of the absorption spectrum
We propose a scheme for subwavelength localization of an atom conditioned
upon the absorption of a weak probe field at a particular frequency.
Manipulating atom-field interaction on a certain transition by applying drive
fields on nearby coupled transitions leads to interesting effects in the
absorption spectrum of the weak probe field. We exploit this fact and employ a
four-level system with three driving fields and a weak probe field, where one
of the drive fields is a standing-wave field of a cavity. We show that the
position of an atom along this standing wave is determined when probe field
absorption is measured. We find that absorption of the weak probe field at a
certain frequency leads to subwavelength localization of the atom in either of
the two half-wavelength regions of the cavity field by appropriate choice of
the system parameters. We term this result as sub-half-wavelength localization
to contrast it with the usual atom localization result of four peaks spread
over one wavelength of the standing wave. We observe two localization peaks in
either of the two half-wavelength regions along the cavity axis.Comment: Accepted for publication to Physical Review
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