370 research outputs found
Generation of Attosecond Pulses with Controllable Carrier-Envelope Phase via High-order Frequency Mixing
Advancing table-top attosecond sources in brightness and pulse duration is of
immense interest and importance for an expanding sphere of applications. Recent
theoretical studies [New J. Phys., 22 093030 (2020)] have found that high-order
frequency mixing (HFM) in a two-color laser field can be much more efficient
than high-order harmonic generation (HHG). Here we study the attosecond
properties of the coherent XUV generated via HFM analytically and numerically,
focusing on the practically important case when one of the fields has much
lower frequency and much lower intensity than the other one. We derive simple
analytical equations describing intensities and phase locking of the HFM
spectral components. We show that the duration of attosecond pulses generated
via HFM, while being very similar to that obtained via HHG in the plateau, is
shortened for the cut-off region. Moreover, our study demonstrates that the
carrier-envelope phase of the attopulses produced via HFM, in contrast to HHG,
can be easily controlled by the phases of the generating fields
Macroscopic effects in generation of attosecond XUV pulses via high-order frequency mixing in gases and plasma
We study the generation of attosecond XUV pulses via high-order frequency
mixing (HFM) of two intense generating fields, and compare this process with
the more common high-order harmonic generation (HHG) process. We calculate the
macroscopic XUV signal by numerically integrating the 1D propagation equation
coupled with the 3D time-dependent Schr\"odinger equation. We analytically find
the length scales which limit the quadratic growth of the HFM macroscopic
signal with propagation length. Compared to HHG these length scales are much
longer for a group of HFM components, with orders defined by the frequencies of
the generating fields. This results in a higher HFM macroscopic signal despite
the microscopic response being lower than for HHG. In our numerical
simulations, the intensity of the HFM signal is several times higher than that
for HHG in a gas, and it is up to three orders of magnitude higher for
generation in plasma; it is also higher for longer generating pulses. The HFM
provides very narrow XUV lines ()
with well-defined frequencies, thus allowing for a simple extension of optical
frequency standards to the XUV range. Finally, we show that the group of HFM
components effectively generated due to macroscopic effects provides a train of
attosecond pulses such that the carrier-envelope phase of an individual
attosecond pulse can be easily controlled by tuning the phase of one of the
generating fields.Comment: 14 pages, 7 figure
Quasi-specular albedo of cold neutrons from powder of nanoparticles
We predicted and observed for the first time the quasi-specular albedo of
cold neutrons at small incidence angles from a powder of nanoparticles. This
albedo (reflection) is due to multiple neutron small-angle scattering. The
reflection angle as well as the half-width of angular distribution of reflected
neutrons is approximately equal to the incidence angle. The measured reflection
probability was equal to ~30% within the detector angular size that corresponds
to 40-50% total calculated probability of quasi-specular reflection
Phase-matching gating for isolated attosecond pulse generation
We investigate the production of an isolated attosecond pulse~(IAP) via the
phase-matching gating of high-harmonic generation by intense laser pulses. Our
study is based on the integration of the propagation equation for the
fundamental and generated fields with nonlinear polarisation found via the
numerical solution of the time-dependent Schr\"odinger equation. We study the
XUV energy as a function of the propagation distance (or the medium density)
and find that the onset of the IAP production corresponds to the change from
linear to quadratic dependence of this energy on the propagation distance (or
density). Finally, we show that the upper limit of the fundamental pulse
duration for which the IAP generation is feasible is defined by the temporal
spreading of the fundamental pulse during the propagation. This nonlinear
spreading is defined by the difference of the group velocities for the neutral
and photoionised medium
Temperature dependence of the probability of "small heating" and total losses of ucns on the surface of fomblin oils of different molecular mass
We measured the temperature dependence of the probability of small heating
and total losses of UCNs on the PFPE Fomblin Y surface with various molecular
masses Mw=2800, 3300, 6500 amu in the temperature range of 100-300 K. The
probability of small heating sharply decreases with increasing Mw and
decreasing temperature. The probability of total loss weakly decreases with
decreasing temperature and takes the minimum value at Mw=3300 amu. As this oil
provides a homogeneous surface with minimal probabilities of small heating and
total losses of UCNs, it is the preferred candidate for experiments on
measuring the neutron lifetime
Storage of very cold neutrons in a trap with nano-structured walls
We report on storage of Very Cold Neutrons (VCN) in a trap with walls
containing powder of diamond nanoparticles. The efficient VCN reflection is
provided by multiple diffusive elastic scattering of VCN at single
nanoparticles in powder. The VCN storage times are sufficiently long for
accumulating large density of neutrons with complete VCN energy range of up to
a few times 10(-4) eV. Methods for further improvements of VCN storage times
are discussed
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