348 research outputs found
Oscillations above sunspots from the temperature minimum to the corona
Context. An analysis of the oscillations above sunspots was carried out using
simultaneous ground-based and Solar Dynamics Observatory (SDO) observations
(SiI 10827A, HeI 10830A, FeI 6173A, 1700A, HeII 304A, FeIX 171A).
Aims. Investigation of the spatial distribution of oscillation power in the
frequency range 1-8 mHz for the different height levels of the solar
atmosphere. Measuring the time lags between the oscillations at the different
layers.
Methods. We used frequency filtration of the intensity and Doppler velocity
variations with Morlet wavelet to trace the wave propagation from the
photosphere to the chromosphere and the corona.
Results. The 15 min oscillations are concentrated near the outer penumbra in
the upper photosphere (1700 A), forming a ring, that expands in the transition
zone. These oscillations propagate upward and reach the corona level, where
their spatial distribution resembles a fan structure. The spatial distribution
of the 5 min oscillation power looks like a circle-shape structure matching the
sunspot umbra border at the photospheric level. The circle expands at the
higher levels (HeII 304A and FeIX 171A). This indicates that the low-frequency
oscillations propagate along the inclined magnetic tubes in the spot. We found
that the inclination of the tubes reaches 50--60 degrees in the upper
chromosphere and the transition zone. The main oscillation power in the 5-8 mHz
range concentrates within the umbra boundaries at all the levels. The highest
frequency oscillations (8 mHz) are located in the peculiar points inside the
umbra. These points probably coincide with umbral dots. We deduced the
propagation velocities to be 28+-15 km/s, 26+-15 km/s, and 55+-10 km/s for the
SiI 10827A-HeI 10830A, 1700A-HeII 304A, and HeII 304A-FeIX 171A height levels,
respectively
Quantum limits of super-resolution in reconstruction of optical objects
We investigate analytically and numerically the role of quantum fluctuations
in reconstruction of optical objects from diffraction-limited images. Taking as
example of an input object two closely spaced Gaussian peaks we demonstrate
that one can improve the resolution in the reconstructed object over the
classical Rayleigh limit. We show that the ultimate quantum limit of resolution
in such reconstruction procedure is determined not by diffraction but by the
signal-to-noise ratio in the input object. We formulate a quantitative measure
of super-resolution in terms of the optical point-spread function of the
system.Comment: 23 pages, 7 figures. Submitted to Physical Review A e-mail:
[email protected]
Sub-wavelength lithography over extended areas
We demonstrate a systematic approach to sub-wavelength resolution
lithographic image formation on films covering areas larger than a wavelength
squared. For example, it is possible to make a lithographic pattern with a
feature size resolution of by using a particular -photon, multi-mode entangled state, where , and banks of birefringent
plates. By preparing a statistically mixed such a state one can form any pixel
pattern on a pixel grid occupying a square
with a side of wavelengths. Hence, there is a trade-off between
the exposed area, the minimum lithographic feature size resolution, and the
number of photons used for the exposure. We also show that the proposed method
will work even under non-ideal conditions, albeit with somewhat poorer
performance.Comment: 8 pages, 8 figures, 1 table. Written in RevTe
A possible mechanism of ultrafast amorphization in phase-change memory alloys: an ion slingshot from the crystalline to amorphous position
We propose that the driving force of an ultrafast crystalline-to-amorphous
transition in phase-change memory alloys are strained bonds existing in the
(metastable) crystalline phase. For the prototypical example of GST, we
demonstrate that upon breaking of long Ge-Te bond by photoexcitation Ge ion
shot from an octahedral crystalline to a tetrahedral amorphous position by the
uncompensated force of strained short bonds. Subsequent lattice relaxation
stabilizes the tetrahedral surroundings of the Ge atoms and ensures the
long-term stability of the optically induced phase.Comment: 6 pages, 3 figure
Entangled-State Lithography: Tailoring any Pattern with a Single State
We demonstrate a systematic approach to Heisenberg-limited lithographic image
formation using four-mode reciprocal binominal states. By controlling the
exposure pattern with a simple bank of birefringent plates, any pixel pattern
on a grid, occupying a square with the side half a
wavelength long, can be generated from a -photon state.Comment: 4 pages, 4 figure
Characteristics of the electric field accompanying a longitudinal acoustic wave in a metal. Anomaly in the superconducting phase
The temperature dependence of the amplitude and phase of the electric
potential arising at a plane boundary of a conductor when a longitudinal
acoustic wave is incident normally on it is investigated theoretically and
experimentally. The surface potential is formed by two contributions, one of
which is spatially periodic inside the sample, with the period of the acoustic
field; the second is aperiodic and arises as a result of an additional
nonuniformity of the electron distribution in a surface layer of the metal. In
the nonlocal region the second contribution is dominant. The phases of these
contributions are shifted by approximately \pi /2. For metals in the normal
state the experiment is in qualitative agreement with the theory. The
superconducting transition is accompanied by catastrophically rapid vanishing
of the electric potential, in sharp contrast to the theoretical estimates,
which predict behavior similar to the BCS dependence of the attenuation
coefficient for a longitudinal sound.Comment: 9 pages, 6 figure
Quantum Imaging
We provide a brief overview of the newly born field of quantum imaging, and
discuss some concepts that lie at the root of this field.Comment: 8 pages, 19 figure
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