187 research outputs found
Ultra-fast magnetisation rates within the Landau-Lifshitz-Bloch model
The ultra-fast magnetisation relaxation rates during the laser-induced
magnetisation process are analyzed in terms of the Landau-Lifshitz-Bloch (LLB)
equation for different values of spin . The LLB equation is equivalent in
the limit to the atomistic Landau-Lifshitz-Gilbert (LLG)
Langevin dynamics and for to the M3TM model [B. Koopmans, {\em et al.}
Nature Mat. \textbf{9} (2010) 259]. Within the LLB model the ultra-fast
demagnetisation time () and the transverse damping ()
are parameterized by the intrinsic coupling-to-the-bath parameter ,
defined by microscopic spin-flip rate. We show that for the phonon-mediated
Elliott-Yafet mechanism, is proportional to the ratio between the
non-equilibrium phonon and electron temperatures.
We investigate the influence of the finite spin number and the scattering
rate parameter on the magnetisation relaxation rates. The relation
between the fs demagnetisation rate and the LLG damping, provided by the LLB
theory, is checked basing on the available experimental data. A good agreement
is obtained for Ni, Co and Gd favoring the idea that the same intrinsic
scattering process is acting on the femtosecond and nanosecond timescale.Comment: 9 pages, 7 figure
Electron-lattice kinetics of metals heated by ultrashort laser pulses
We propose a kinetic model of transient nonequilibrium phenomena in metals
exposed to ultrashort laser pulses when heated electrons affect the lattice
through direct electron-phonon interaction. This model describes the
destruction of a metal under intense laser pumping. We derive the system of
equations for the metal, which consists of hot electrons and a cold lattice.
Hot electrons are described with the help of the Boltzmann equation and
equation of thermoconductivity. We use the equations of motion for lattice
displacements with the electron force included. The lattice deformation is
estimated immediately after the laser pulse up to the time of electron
temperature relaxation. An estimate shows that the ablation regime can be
achieved.Comment: 7 pages; Revtex. to appear in JETP 88, #1 (1999
Nonequilibrium Magnetization Dynamics of Nickel
Ultrafast magnetization dynamics of nickel has been studied for different
degrees of electronic excitation, using pump-probe second-harmonic generation
with 150 fs/800 nm laser pulses of various fluences. Information about the
electronic and magnetic response to laser irradiation is obtained from sums and
differences of the SHG intensity for opposite magnetization directions. The
classical M(T)-curve can be reproduced for delay times larger than the electron
thermalization time of about 280 fs, even when electrons and lattice have not
reached thermal equilibrium. Further we show that the transient magnetization
reaches its minimum approx. 50 fs before electron thermalization is completed.Comment: 8 pages, 5 figures, revte
MHz Unidirectional Rotation of Molecular Rotary Motors
A combination of cryogenic UV-vis and CD spectroscopy and transient absorption spectroscopy at ambient temperature is used to study a new class of unidirectional rotary molecular motors. Stabilization of unstable intermediates is achieved below 95 K in propane solution for the structure with the fastest rotation rate, and below this temperature measurements on the rate limiting step in the rotation cycle can be performed to obtain activation parameters. The results are compared to measurements at ambient temperature using transient absorption spectroscopy, which show that behavior of these motors is similar over the full temperature range investigated, thereby allowing a maximum rotation rate of 3 MHz at room temperature under suitable irradiation conditions
Generation of strong-field spectrally tunable terahertz pulses
The ideal laser source for nonlinear terahertz spectroscopy offers large versatility delivering both ultra-intense broadband single-cycle pulses and user-selectable multi-cycle pulses at narrow linewidths. Here we show a highly versatile terahertz laser platform providing single-cycle transients with tens of MV/cm peak field as well as spectrally narrow pulses, tunable in bandwidth and central frequency across 5 octaves at several MV/cm field strengths. The compact scheme is based on optical rectification in organic crystals of a temporally modulated laser beam. It allows up to 50 cycles and central frequency tunable from 0.5 to 7 terahertz, with a minimum width of 30 GHz, corresponding to the photon-energy width of ΔE=0.13 meV and the spectroscopic-wavenumber width of Δ(λ-1)=1.1 cm-1. The experimental results are excellently predicted by theoretical modelling. Our table-top source shows similar performances to that of large-scale terahertz facilities but offering in addition more versatility, multi-colour femtosecond pump-probe opportunities and ultralow timing jitter
Targeted knock-down of miR21 primary transcripts using snoMEN vectors induces apoptosis in human cancer cell lines
We have previously reported an antisense technology, 'snoMEN vectors', for targeted knock-down of protein coding mRNAs using human snoRNAs manipulated to contain short regions of sequence complementarity with the mRNA target. Here we characterise the use of snoMEN vectors to target the knock-down of micro RNA primary transcripts. We document the specific knock-down of miR21 in HeLa cells using plasmid vectors expressing miR21-targeted snoMEN RNAs and show this induces apoptosis. Knock-down is dependent on the presence of complementary sequences in the snoMEN vector and the induction of apoptosis can be suppressed by over-expression of miR21. Furthermore, we have also developed lentiviral vectors for delivery of snoMEN RNAs and show this increases the efficiency of vector transduction in many human cell lines that are difficult to transfect with plasmid vectors. Transduction of lentiviral vectors expressing snoMEN targeted to pri-miR21 induces apoptosis in human lung adenocarcinoma cells, which express high levels of miR21, but not in human primary cells. We show that snoMEN-mediated suppression of miRNA expression is prevented by siRNA knock-down of Ago2, but not by knock-down of Ago1 or Upf1. snoMEN RNAs colocalise with Ago2 in cell nuclei and nucleoli and can be co-immunoprecipitated from nuclear extracts by antibodies specific for Ago2
Spallative ablation of dielectrics by X-ray laser
Short laser pulse in wide range of wavelengths, from infrared to X-ray,
disturbs electron-ion equilibrium and rises pressure in a heated layer. The
case where pulse duration is shorter than acoustic relaxation time
is considered in the paper. It is shown that this short pulse may cause
thermomechanical phenomena such as spallative ablation regardless to
wavelength. While the physics of electron-ion relaxation on wavelength and
various electron spectra of substances: there are spectra with an energy gap in
semiconductors and dielectrics opposed to gapless continuous spectra in metals.
The paper describes entire sequence of thermomechanical processes from
expansion, nucleation, foaming, and nanostructuring to spallation with
particular attention to spallation by X-ray pulse
Subwavelength anti-diffracting beams propagating over more than 1,000 Rayleigh lengths
Propagating light beams with widths down to and below the optical wavelength require bulky large-aperture lenses and remain focused only for micrometric distances. Here, we report the observation of light beams that violate this localization/depth- of-focus law by shrinking as they propagate, allowing resolution to be maintained and increased over macroscopic propagation lengths. In nanodisordered ferroelectrics we observe a non-paraxial propagation of a sub-micrometre-sized beam for over 1,000 diffraction lengths, the narrowest visible beam reported to date. This unprecedented effect is caused by the nonlinear response of a dipolar glass, which transforms the leading opticalwave equation into a Klein-Gordon-type equation that describes a massive particle field. Our findings open the way to high-resolution optics over large depths of focus, and a route to merging bulk optics into nanodevices
ИССЛЕДОВАНИЯ БИОЛОГИЧЕСКИХ ОБЪЕКТОВ НА КЛЕТОЧНОМ И СУБКЛЕТОЧНОМ УРОВНЕ С ПОМОЩЬЮ ФЕМТОСЕКУНДНОГО ЛАЗЕРНОГО ОПТИЧЕСКОГО ПИНЦЕТА-СКАЛЬПЕЛЯ
The aim of this work was developing of elements of the precise three-dimensional positioning technology of one or several micron and submicron size biological objects. Thereto a laboratory unit of hardware-software complex of optical femtosecond laser tweezers-scalpel was developed and constructed in the Joint institute for high temperatures RAS using material resources of Russia. Experimental data concerning a maximal manipulation speed of CHO and cells, produced from mammalian spinal ganglia (using protocols for producing pure culture of Schwann cells) was received. Besides facts of interaction of laser radiation with intracellular structures that lead to unexpected behavior of cell in the zone of optical trap and change of maximal speed of cell manipulation were determined. Целью данной работы является разработка элементов технологии прецизионного трехмерного позицио- нирования одного или нескольких биологических объектов микронного и субмикронного размеров. Для этого в Объединенном институте высоких температур РАН разработан и изготовлен лабораторный обра- зец программно-аппаратного комплекса оптического фемтосекундного лазерного пинцета-скальпеля на основе приборной базы, производимой в России. Получены экспериментальные результаты о максималь- ной скорости манипулирования CHO и клетками культуры, полученной из спинального ганглия млекопи- тающего (по протоколам получения очищенных культур шванновских клеток), а также о взаимодействии излучения с внутриклеточными структурами, которое приводит к изменению предполагаемого поведе- ния клетки в области оптической ловушки и максимальной скорости манипулирования последней.
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