338,982 research outputs found
Observation of Terahertz Radiation via the Two-Color Laser Scheme with Uncommon Frequency Ratios
In the widely-studied two-color laser scheme for terahertz (THz) radiation
from a gas, the frequency ratio of the two lasers is usually fixed at
1:2. We investigate THz generation with uncommon frequency
ratios. Our experiments show, for the first time, efficient THz generation with
new ratios of 1:4 and 2:3. We observe that the THz
polarization can be adjusted by rotating the longer-wavelength laser
polarization and the polarization adjustment becomes inefficient by rotating
the other laser polarization; the THz energy shows similar scaling laws with
different frequency ratios. These observations are inconsistent with multi-wave
mixing theory, but support the gas-ionization model. This study pushes the
development of the two-color scheme and provides a new dimension to explore the
long-standing problem of the THz generation mechanism.Comment: 6 pages, 3 figure
Efficient implementation of the adaptive scale pixel decomposition algorithm
Context. Most popular algorithms in use to remove the effects of a
telescope's point spread function (PSF) in radio astronomy are variants of the
CLEAN algorithm. Most of these algorithms model the sky brightness using the
delta-function basis, which results in undesired artefacts when used on image
extended emission. The adaptive scale pixel decomposition (Asp-Clean) algorithm
models the sky brightness on a scale-sensitive basis and thus gives a
significantly better imaging performance when imaging fields that contain both
resolved and unresolved emission.
Aims. However, the runtime cost of Asp-Clean is higher than that of
scale-insensitive algorithms. In this paper, we identify the most expensive
step in the original Asp-Clean algorithm and present an efficient
implementation of it, which significantly reduces the computational cost while
keeping the imaging performance comparable to the original algorithm. The PSF
sidelobe levels of modern wide-band telescopes are significantly reduced,
allowing us to make approximations to reduce the computing cost, which in turn
allows for the deconvolution of larger images on reasonable timescales.
Methods. As in the original algorithm, scales in the image are estimated
through function fitting. Here we introduce an analytical method to model
extended emission, and a modified method for estimating the initial values used
for the fitting procedure, which ultimately leads to a lower computational
cost.
Results.The new implementation was tested with simulated EVLA data and the
imaging performance compared well with the original Asp-Clean algorithm. Tests
show that the current algorithm can recover features at different scales with
lower computational cost.Comment: 6 pages; 4 figure
Topological phase transitions in small mesoscopic chiral p-wave superconductors
Spin-triplet chiral p-wave superconductivity is typically described by a
two-component order parameter, and as such is prone to unique emergent effects
when compared to the standard single-component superconductors. Here we present
the equilibrium phase diagram for small mesoscopic chiral p-wave
superconducting disks in the presence of magnetic field, obtained by solving
the microscopic Bogoliubov-de Gennes equations self-consistently. In the
ultra-small limit, the cylindrically-symmetric giant-vortex states are the
ground state of the system. However, with increasing sample size, the
cylindrical symmetry is broken as the two components of the order parameter
segregate into domains, and the number of fragmented domain walls between them
characterizes the resulting states. Such domain walls are topological defects
unique for the p-wave order, and constitute a dominant phase in the mesoscopic
regime. Moreover, we find two possible types of domain walls, identified by
their chirality-dependent interaction with the edge states
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