432 research outputs found
Pronounced grain boundary network evolution in nanocrystalline Cu subjected to large cyclic strains
The grain boundary network of nanocrystalline Cu foils was modified by the
systematic application of cyclic loadings and elevated temperatures having a
range of magnitudes. Most broadly, the changes to the boundary network were
directly correlated to the applied temperature and accumulated strain,
including a 300% increase in the twin length fraction. By independently varying
each treatment variable, a matrix of grain boundary statistics was built to
check the plausibility of hypothesized mechanisms against their expected
temperature and stress/strain dependences. These comparisons allow the field of
candidate mechanisms to be significantly narrowed. Most importantly, the effect
of temperature and strain on twin length fraction were found to be strongly
synergistic, with the combined effect being ~150% that of the summed individual
contributions. Looking beyond scalar metrics, an analysis of the grain boundary
network showed that twin related domain formation favored larger sizes and
repeated twin variant selection over the creation of many small domains with
diverse variants. Taken together, the evidence indicates that shear-coupled
boundary migration twinning is the most likely explanation for grain boundary
engineering in nanocrystalline Cu.Comment: 9 figure
Segmented subwavelength silicon gratings manufactured by high productivity microelectronic technologies for linear to radial/azimuthal polarization conversion
International audienceA polarization rotation is realized by subwavelength binary gratings, where the round trip phases of thesmallest grating modes are fixed to the smallest possible integer numbers of 2π allowing a phase difference of πbetween TE and TM polarizations and almost 100% transmission. The principle is applied to a polarization transformationin the 1030 to 1064-nm wavelength range, using a segmented polarization rotating element convertinga linearly polarized incidence to a radial or azimuthal polarization distribution. The elevated costs of such kindsof polarization transformers based on assembled birefringent crystals are avoided by using mass-fabricationcompatible silicon-on-insulator technology on a wafer scale. It shows the general potential of microelectronictechnology, concerning the batch manufacturing of wavelength-scale diffractive, grating-based elements forprocessing free space waves
Shaping, imaging and controlling plasmonic interference fields at buried interfaces
Filming and controlling plasmons at buried interfaces with nanometer (nm) and
femtosecond (fs) resolution has yet to be achieved and is critical for next
generation plasmonic/electronic devices. In this work, we use light to excite
and shape a plasmonic interference pattern at a buried metal-dielectric
interface in a nanostructured thin film. Plasmons are launched from a
photoexcited array of nanocavities and their propagation is filmed via
photon-induced near-field electron microscopy (PINEM). The resulting movie
directly captures the plasmon dynamics, allowing quantification of their group
velocity at approximately 0.3c, consistent with our theoretical predictions.
Furthermore, we show that the light polarization and nanocavity design can be
tailored to shape transient plasmonic gratings at the nanoscale. These results,
demonstrating dynamical imaging with PINEM, pave the way for the fs/nm
visualization and control of plasmonic fields in advanced heterostructures
based on novel 2D materials such as graphene, MoS, and ultrathin metal
films.Comment: 16 pages, 5 figures, 3 supplementary figure
Smallest aspect-ratio form-birefringence half-wave plate
International audienceSubwavelength 0th order gratings permit to create a phase-shift between the polarized 0th order grating modes propagating down the slits and grooves of a binary corrugation, and to transform the polarization of an incident beam. The phase-shift per unit height of the grating is an increasing function of the refractive index difference between ridges and grooves. If the ridges are made by photolithography in a resist or by polymer embossing, the low refractive index leads to a very large corrugation aspect ratio (approx. 4 for a half-wave phase-shift) that is difficult to fabricate and/or provides insufficient mechanical stability. If the ridges are made in a high index non-organic material (e.g. a semiconductor) the needed depth is reduced (although still notably larger than 1 for a half-wave phase-shift). However, in this case due to a more significant Fabry-Perot effect between the upper and lower boundaries of the 0th order grating, high transmission is guaranteed only if its resonance condition is ensured for both polarizations simultaneously. Using an inventive design by phase management of the involved grating modes we have found that all three conditions (pi phase-shift between TE and TM and both Fabry-Perot resonances) can indeed be satisfied in a binary grating of reasonable aspect ratio when the substrate has a refractive index notably smaller than the ridges
Stroke saturation on a MEMS deformable mirror for woofer-tweeter adaptive optics
High-contrast imaging of extrasolar planet candidates around a main-sequence
star has recently been realized from the ground using current adaptive optics
(AO) systems. Advancing such observations will be a task for the Gemini Planet
Imager, an upcoming "extreme" AO instrument. High-order "tweeter" and low-order
"woofer" deformable mirrors (DMs) will supply a >90%-Strehl correction, a
specialized coronagraph will suppress the stellar flux, and any planets can
then be imaged in the "dark hole" region. Residual wavefront error scatters
light into the DM-controlled dark hole, making planets difficult to image above
the noise. It is crucial in this regard that the high-density tweeter, a
micro-electrical mechanical systems (MEMS) DM, have sufficient stroke to deform
to the shapes required by atmospheric turbulence. Laboratory experiments were
conducted to determine the rate and circumstance of saturation, i.e. stroke
insufficiency. A 1024-actuator 1.5-um-stroke MEMS device was empirically tested
with software Kolmogorov-turbulence screens of r_0=10-15cm. The MEMS when
solitary suffered saturation ~4% of the time. Simulating a woofer DM with ~5-10
actuators across a 5-m primary mitigated MEMS saturation occurrence to a
fraction of a percent. While no adjacent actuators were saturated at opposing
positions, mid-to-high-spatial-frequency stroke did saturate more frequently
than expected, implying that correlations through the influence functions are
important. Analytical models underpredict the stroke requirements, so empirical
studies are important.Comment: 16 pages, 10 figure
Euler, Jacobi, and Missions to Comets and Asteroids
Whenever a freely spinning body is found in a complex rotational state, this
means that either the body is a recent victim of an impact or a tidal
interaction, or is a fragment of a recently disrupted progenitor. Another
factor (relevant for comets) is outgassing. Due to impacts, tidal forces and
outgassing, the asteroidal and cometary precession must be a generic
phenomenon: while some rotators are in the state of visible tumbling, a much
larger amount of objects must be performing narrow-cone precession not so
easily observable from the Earth. The internal dissipation in a freely
precessing top leads to relaxation (gradual damping of the precession) and
sometimes to spontaneous changes in the rotation axis. Recently developed
theory of dissipative precession of a rigid body reveals that this is a highly
nonlinear process: while the body is precessing at an angular rate ,
the precession-caused stresses and strains in the body contain components
oscillating at other frequencies. Dependent upon the spin state, those
frequencies may be higher or, most remarkably, lower than the precession rate.
In many states dissipation at the harmonics is comparable to or even exceeds
that at the principal frequency. For this and other reasons, in many spin
states the damping of asteroidal and cometary wobble happens faster, by several
orders, than believed previously. This makes it possible to measure the
precession-damping rate. The narrowing of the precession cone through the
period of about a year can be registered by the currently available
spacecraft-based observational means. However, in the near-separatrix spin
states a precessing rotator can considerably slow down its relaxation.Comment: 21 pages, 1 figur
Observation of a new light-induced skyrmion phase in the Mott insulator Cu2OSeO3
We report the discovery of a novel skyrmion phase in the multiferroic
insulator Cu2OSeO3 for magnetic fields below the equilibrium skyrmion pocket.
This phase can be accessed by exciting the sample out of equilibrium with
near-infrared (NIR) femtosecond laser pulses but can not be reached by any
conventional field cooling protocol. From the strong wavelength dependence of
the photocreation process and via spin dynamics simulations, we identify the
magnetoelastic effect as the most likely photocreation mechanism. This effect
results in a transient modification of the magnetic interaction extending the
equilibrium skyrmion pocket to lower magnetic fields. Once created, the
skyrmions rearrange and remain stable over a long time, reaching minutes. The
presented results are relevant for designing high-efficiency non-volatile data
storage based on magnetic skyrmions.Comment: 11 pages, 5 figure
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Nanosecond Time Resolved Electron Diffraction Studies of the (Alpha) to (Beta) Transition in Pure Ti Thin Films using the Dynamic Transmission Electron Microscope (DTEM)
The transient events of the {alpha}-{beta} martensitic transformation in nanocrystalline Ti films were explored via single shot electron diffraction patterns with 1.5 ns temporal resolution. The diffraction patterns were acquired with a newly constructed dynamic transmission electron microscope (DTEM), which combines nanosecond pulsed laser systems and pump-probe techniques with a conventional TEM. With the DTEM, the transient events of fundamental material processes, that are far too fast to be studied by conventional bulk techniques, can be captured in the form of electron diffraction patterns or images with nanosecond temporal resolution. The transient phenomena of the martensitic transformations in nanocrystalline Ti is ideally suited for study in the DTEM, with their rapid nucleation, characteristic interface velocities {approx}1 km/s, and significant irreversible microstructural changes. Free-standing 40-nm-thick Ti films were laser-heated at a rate of {approx}10{sup 10} K/s to a temperature above the 1155 K transition point, then probed at various time intervals with a 1.5-ns-long, intense electron pulse. Diffraction patterns show an almost complete transition to the {beta} phase within 500 ns. Postmortem analysis (after the sample is allowed to cool) shows a reversion to the {alpha} phase coupled with substantial grain growth, lath formation, and texture modification. The cooled material also shows a complete lack of apparent dislocations, suggesting the possible importance of a ''massive'' short-range diffusion transformation mechanism
Practical use of variational principles for modeling water waves
This paper describes a method for deriving approximate equations for
irrotational water waves. The method is based on a 'relaxed' variational
principle, i.e., on a Lagrangian involving as many variables as possible. This
formulation is particularly suitable for the construction of approximate water
wave models, since it allows more freedom while preserving the variational
structure. The advantages of this relaxed formulation are illustrated with
various examples in shallow and deep waters, as well as arbitrary depths. Using
subordinate constraints (e.g., irrotationality or free surface impermeability)
in various combinations, several model equations are derived, some being
well-known, other being new. The models obtained are studied analytically and
exact travelling wave solutions are constructed when possible.Comment: 30 pages, 1 figure, 62 references. Other author's papers can be
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