Paper concerns experimental and numerical investigation on supercontinuum emission in the process of femtosecond beam filamentation under different geometrical focusing in presence of anomalous group velocity dispersion in fused silica. It was shown that energy of supercontinuum visible part increases discretely with increasing of input laser pulse energy. It is connected with light bullets formation. Also it was found that energy of supercontinuum visible part connected with each bullet doesn’t depend on focusing geometry.
The analysis of the possibilities of microstructuring (micromachining) of various solids by sharply focused femtosecond
laser (fsl) capable of providing for a high intensity for moderate pulse energies remains quite topical. Usually the process
takes place immediately at the focal spot and an extended structure can only be formed by scanning the sample. In our
experiments focusing fsl pulses into a vacuum setup we produced extended microstructures (microdamages) on a few
tens nm thickness gold foil deposited onto a fused-quartz substrate. The structure length has been an order of magnitude
larger than the typical size of the irradiated region. The detection of Au+ photoions ensured a highly-sensitivite on-line
diagnostics of this phenomenon and in the limit can be used for the probing of the picostructuring of solids irradiated by
a laser. In the next step we are "cloning" the microstructure (microdamage) from one surface to another when there is a
few tens microns vacuum-spacing between them and only one surface with a metal film is "directly" irradiated by fsl
pulses. For the expertiment we prepared two similar fused-quartz prisms and pressed one sample to another. The
clearance is determined by the thickness of a spacer (Cu-foil). The possible explanations are given.
The pathways of excitation energy transfer (EET) via pigments of the light-harvesting antenna are still in discussion. The
bacteriochlorophyll fluorescence of peripheral light-harvesting complexes (LH2) from purple bacteria can be observed
upon two-photon excitation (TPE) within 1200-1500 nm spectral range (a broad band near 1300 nm). Earlier the
occurrence of this band was taken as an evidence for the participation of "dark" carotenoid S1 state in EET processes (see
[Walla et al., Proc. Nat. Acad. Sci. U.S.A. 97, 10808-10813 (2000)] and references in it). However we showed that TPE
spectrum of LH2 fluorescence within 1200-1500 nm is not associated with carotenoids [Stepanenko et al., J. Phys.
Chem. B. 113(34), 11720-11723 (2009)]. Here we present TPE spectra of fluorescence for chromatophores and lightharvesting
complexes LH2 and LH1 from wild-type cells and from carotenoid-depleted or carotenoidless mutant cells of
several purple bacteria. The broad band within 1300-1400 nm was found for all preparations. Absorption pump-probe
femtosecond spectroscopy applied to LH2 complex from Rb. sphaeroides revealed the similar spectral and kinetic
patterns for TPE at 1350 nm and one-photon excitation at 675 nm. Analysis of pigment composition of this complex by
high-pressure liquid chromatography showed that even under mild isolation conditions some bacteriochlorophyll
molecules were oxidized to 3-acetyl-chlorophyll molecules having the long-wavelength absorption peak in the 650-700
nm range. It is proposed that these 3-acetyl-chlorophyll molecules are responsible for the broad band in TPE spectra
within the 1200-1500 nm region.
The ultrafast e--bunches produced by femtosecond laser (fsl) radiation are powerful tool in modern physics to observe
different ultrashort processes induced by fsl pulses and to reach high spatio-temporal resolution. In Letokhov's
projection microscope such a beam transfers an information from a tip with a sample illluminated by fsl pulses to a
detector. Here the fsl radiation provides for an effective photoelectron multiphoton emission without significant heating
of the sample. Two organic samples, formed from dye molecules and the organic conducting polymers has been
vizualized using the nanocapillary as a tip. The advantages of the nanocapillary tip are described in the report.
The temporal resolution depends upon the pulse duration of the e--beam, τe. One of the most powerful way to
measure τe is to use the Gaponov-Miller force (GMf), or the ponderomotive force, which the electrons experience in the
inhomogeneous field of a focused laser pulse. Such a force helped us to demonstrate the «instantaneousness» of
multiphoton emission process from solid targets. As the beam propagated, it spread in time. Using GMf we temporally
characterized the e--pulse transmitted through microcapillary (which can be the basis of the promising scanning
microscope) and combined spatial nanoresolution and picosecond temporal resolution. Also the ultrafast microlocalized
e--beam is an ideal tool to measure the GMf created by focused fsl pulses and to characterize very intense laser beam
profile in-situ. In principal, such approach may allow for direct subwave spatio-temporal probing of superintense laser
beam profiles.
Ultrafast dynamics of bismuth excited by femtosecond laser pulses was studied varying wavelength of pump and probe
pulses from 400 to 2500 nm and from 400 to 900 nm respectively. Three relaxational components with decay rates of 1
ps, 7 ps and ~ 1 ns were found in the photoinduced response. The fastest process is ascribed to the excited electrons at
T6+ point of the Brillouin zone, that are responsible for coherent A1g phonon generation and its frequency softening.
Intermediate component is due to crystal heating via electron-phonon interaction. Finally, a long-lived nanosecond signal
is linked to electrons near the Fermi level at Ls point.
The supercontinuum conical emission of a 50-fs laser pulse focused into a Κ 108 glass is studied experimentally and numerically. It
is found that, as the pulse energy was increased from 2 to 30 uj, the continuous picture of conical emission decomposed into speckles
upon focusing with a lens and split into narrow rings upon focusing with an axicon. Preserving of distinct interference picture under
more than 1000 shots exposure evidences in favor of much more stable positioning of microfilament in the case of axicon.
Holograms recorded in planar optical waveguides by 30 fs pulses from the second harmonic of a Ti:Sapphire laser (λ is about 400 nm) are investigated. The 20 μm thick films of dichromated gelatin (n2 = 1 .54) deposited on a polished quartz substrate (n1 = 1 .456) is used as a planar waveguide model. The recording pulses enter the planar waveguide through its upper surface. Reconstruction in the waveguide regime is investigated for the cases when λ ≅633 nm and λ lies between 1150 and 1250 nm.
Investigation of photo-induced processes in Sn nanocrystals covered by a submonolayer of C60 anions was performed with femtosecond pump-probe method. Samples in thin films were excited by 150 fs laser pulse at 400 nm (109 W/cm2).
Dynamics of difference transmission and reflection has been measured in the spectral range of 1 100-1700 nm. The dynamics of relaxation are quite different for various nanostructures depending on the deposition mode and the ratio of tin and fullerene content. Relaxation, observed in the samples, is explained by electron transfer from excited anions to metal followed by energy transfer from excited C60 molecules to anions in the ground state.
It is shown that the high-resolution laser photoelectron microscope with subwavelength-spatial resolution can be used for an absolute values of the two-photon external photoelectric effect measurements with high (a few nm-scale) localization. The spatial distribution of light intensity in the near field is studied by observing the photoelectron projection images of a subwavelength nanoaperture. The imaging electrons are obtained as a result of two-photon external photoelectric effect induced in the aperture formed at the end of an optical fiber by femtosecond pulses of the second-harmonic radiation (410 nm) of a Ti:sapphire laser. The light-field distribution in the aperture is not distorted by any near-by object, which allows the first nonperturbing measurement of such a distribution.
The effect of geometric-optical wave front reconstruction is investigated. Femtosecond holograms, which demonstrate both diffraction and geometric-optical mechanisms of reconstruction, were obtained. The waveguide variant of the geometric-optical wavefront reconstruction and the ultrashort pulses temporal reconstruction are discussed.
Investigation of the photoinduced processes in pure C60 films was performed by means of sub-100 fs resolution spectrometer. Excitation by 100 fs laser pulses with photon energy above and below the mobility edge revealed that both charged and neutral components appear during the excitation pulse. At both excitation wavelengths electrons and holes are generated by direct optical excitation and not due to singlet-singlet annihilation. Anions are created with a delay of 10-13 - 10-11s in a result of electrons capture by C60 molecules.
The recording geometry and recording media for the method of achromatic wavefront reconstruction are discussed. The femtosecond recording on the thick slabs of dichromated gelatin and the samples of silver-containing porous glass was obtained. The applications of the method to ultrafast laser spectroscopy and to phase conjugation were suggested.
Investigation of the photoinduced processes in pure C60 films and in C60 films doped by Ti or Sn was performed by means of sub-100 fs resolution spectrometer. Excitation by 100 fs laser pulses with photon energy above and below the mobility edge revealed that both charged and neutral components appear during the excitation pulse. At both excitation wavelengths electrons and holes are generated by direct optical excitation and not due to singlet-singlet annihilation. Anions are created with a delay of 10-13 - 10-11 s in a result of electrons capture by C60 molecules. Compared to the pure fullerene this process is faster for Sn-doped and slower for Ti-doped sample. Oxygen uptake in the film is mainly responsible for the observed characteristic features of the relaxation in Ti-doped C60. For Sn-doped C60 the yield of charge carriers photogeneration is noticeably larger than for pure C60. Non-monotonic relaxation, observed in the former case, is explained by the charge exchange between C60 anion and metal.
The new nanolithography technique realized experimentally using the local field of femtosecond laser pulses enhanced in nanoscale region due to lighting rod effect and to the excitation of local resonances in STM tip--substrate system. Surface topography analysis in STM mode demonstrates the controlled surface modification in a suitable regime of intensity parameters and femtosecond laser pulses focusing in the STM tip region.
Photopolymerization and photoinduced diffusion of the molecular oxygen in oxygen bearing C60 films by (lambda) equals 459, 514 nm continuous laser radiation and by 100-fs, (lambda) equals 395, 410 nm pulses were compared by visual observation of photodarkening spots and by recording the Raman spectra, in the range 200 - 1700 cm-1, of the irradiated regions. It was found out that in the case of femtosecond pulses, the efficiencies of both photoinduced processes are significantly lower than those in the case of continuous radiation.
A new method of achromatic wavefront reconstruction by geometric-optical reflection of the reconstructing radiation from surfaces with constant phase difference between the object and reference waves is theoretically described and experimentally realized. Method's distinction from holographic one is discussed. Femtosecond laser pulses are used for recording in the experiment.
Relaxation of fullerenes in solids and solutions was investigated by femtosecond pump-probe technique at various experimental setups. Intensity of excitation pulse was varied at 1010 - 1012 W/cm2, energies of pump and probe quanta were changed from 1.57 to 3.14 eV, pulse duration -- from 60 to 350 fs. An increase of absorption was observed in solutions with a time scale of 1.2 ps for C60 and 0.15 ps for C70 after excitation of HOMO-LUMO transition. It was attributed to symmetry breaking of excited molecules. Observed for solid C60 dependence of relaxation rate on the density of excited molecules and on the wavelengths of the pump and probe pulses is explained by 'hot' singlet-singlet annihilation the rate of which is proportional to R-3. Comparison of photoinduced absorption and bleaching decays allowed to conclude that only a small fracture of excited molecules produce localized charge carriers, mainly by excitonic annihilation proces and these carriers are responsible for long-lived residual absorption observed in photoinduced absorption kinetics. Difference absorption spectra observed near zero delay time were similar to electroabsorption ones superimposed on excited state absorption of C60 film. This electroabsorption feature disappeared in phase with pumping pulse. This phenomenon is connected with Stark shift in strong electric field of the powerful pumping pulse.
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