48 research outputs found
Ionization-induced Susceptibility by Nearly-free Electrons in Gases Influenced by the Coulomb Potential
In the present paper we study the influence of the Coulomb potential on the
real and imaginary parts of the plasma-induced susceptibility in a photoionized
gas. We show that the real part of the susceptibility is more than one order of
magnitude larger due to the action of a Coulomb potential. Surprisingly, the
long-range Coulomb potential of the atomic core leads to an additional
contribution to the imaginary part of the susceptibility which has no
counterpart in the case of a short-range potential. We demonstrate that the
origin of this behavior are electrons in states very close to the continuum
(nearly-free electrons), and analyze the dependence of the susceptibility on
the intensity and wavelengths.Comment: 6 pages, 7 figure
Ultrafast Nonlinear Optical Effects of Metal Nanoparticles Composites
We present a theoretical method for the calculation of the transient nonlinearity in dielectric composites doped with metal nanoparticles and demonstrate some applications of this approach. First, we describe the theoretical basis of the linear and nonlinear properties of metal nanoparticles by using the time-domain discrete-dipole approximation. By using the two-temperature model for the description of the electron-electron and electron-lattice interaction, we derive an equation for the transient third-order nonlinear susceptibility. Based on this method and the effective medium approximation, we present numerical results for the nonlinear optical susceptibility for different nanocomposites media consisting of noble metal nanoparticles surrounded by a dielectric host. With increasing pump intensities, the plasmon resonance is shifted which leads to a saturation of the absorption. We present a theory of mode-locking of solid-state and semiconductor disk lasers using metal nanocomposites as saturable absorbers. Finally, we consider a novel slow-light device based on metal nanocomposites
High-power Soliton-induced Supercontinuum Generation and Tunable Sub-10-fs VUV Pulses from Kagome-lattice HC-PCFs
We theoretically study a novel approach for soliton-induced high-power
supercontinuum generation by using kagome lattice HC-PCFs filled with a noble
gas. Anomalous dispersion and broad-band low loss of these fibers enable the
generation of two-octave broad spectra by fs pulses, with high coherence and
high spectral peak power densities up to five orders of magnitude larger than
in standard PCFs. In addition, up to 20 percents of the output radiation energy
forms a narrow UV/VUV band, which can be tuned by contolling the pressure in
the range from 350 nm to 120 nm. In the temporal domain this corresponds to
sub-10-fs UV/VUV pulses with pulse energy of few tens of microjoule, caused by
the formation of a high-order soliton emitting non-solitonic radiation.Comment: 4 pages, 4 figure
Frequency-selective self-trapping and supercontinuum generation in arrays of coupled nonlinear waveguides
We study spatiotemporal dynamics of soliton-induced twooctave-
broad supercontinuum generated by fs pulses in an array of coupled
nonlinear waveguides. We show that after fission of the input pulse into
several fundamental solitons, red and blue-shifted nonsolitonic radiation, as
well as solitons with lower intensity, spread away in transverse direction,
while the most intense spikes self-trap into spatiotemporal discrete solitons
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Non-instantaneous third-order optical response of gases in low-frequency fields
It is commonly assumed that for low-intensity short optical pulses far from resonance, the third-order optical nonlinear response is instantaneous. We solve the three-dimensional time-dependent Schrödinger equation for the hydrogen atom and show that this is not the case: the polarization is not simply proportional to the cube of the electric field even at low intensities. We analyze the fundamental-frequency and third-harmonic nonlinear susceptibilities of hydrogen, investigate their dependence on intensity, and find that the delays in the Kerr response rapidly approach the femtosecond time-scale at higher intensities, while the delays in the third harmonic generation remain much lower. We also propose an experimental scheme to detect and characterize the above effects
Metallic nanostructures as electronic billiards for nonlinear terahertz photonics
Optical properties of metallic nanoparticles are most often considered in
terms of plasmons, the coupled states of light and quasi-free electrons. Here
we predict that confinement of electrons inside the nanostructure leads to
another, very different, type of resonance, which determines the optical
properties in the frequency range significantly below the plasmonic resonance.
We demonstrate that closely placed confinement-induces resonances typically
join into a single composite "super-resonance" which produces giant
nonlinearity at low frequencies. Our simulations show how such nonlinearities
can be used for efficient down-conversion of optical pump to terahertz and
mid-infrared frequencies in sub-micrometer devices based on nanoparticle
composites. We discuss the interaction of these quantum-confinement-induced
resonances with the conventional plasmonic ones, as well as the unusual quantum
level statistics, adapting here the paradigms of the electronic billiard
theory.Comment: This is updates and improved version after a response to reviewer
comment
Tailoring THz radiation by controlling tunnel photoionization events in gases
Applications ranging from nonlinear terahertz spectroscopy to remote sensing require broadband and intense THz radiation which can be generated by focusing two-color laser pulses into a gas. In this setup, THz radiation originates from the buildup of the electron density in sharp steps of attosecond duration due to tunnel ionization, and subsequent acceleration of free electrons in the laser field. We show that the spectral shape of the THz pulses generated by this mechanism is determined by superposition of contributions from individual ionization events. This provides a straightforward analogy with linear diffraction theory, where the ionization events play the role of slits in a grating. This analogy offers simple explanations for recent experimental observations and opens new avenues for THz pulse shaping based on temporal control of the ionization events. We illustrate this novel technique by tailoring the spectral width and position of the resulting radiation using multi-color pump pulses
Unified Model for a Nonlinear Pulse Propagation in Composites and Optimization of THz Generation
We describe a unified numerical model which allows fast and accurate
simulation of nonlinear light propagation in nanoparticle composites, including
various effects such as group velocity dispersion, second- and third-order
nonlinearity, quasi-free-carrier formation and plasma contribution, exciton
dynamics, scattering and so on. The developed software package SOLPIC is made
available for the community. Using this model, we analyze and optimize
efficient generation of THz radiation by two-color pulses in ZnO/fused silica
composite, predicting an efficiency of 3\%. We compare the role of various
nonlinear effects contributing to the frequency conversion, and show that
optimum conditions of THz generation differ from those expected intuitively.Comment: 8 pages, 4 figures, corrected several typos and missing "a"/"the",
make unified reference style, introduced one abbreviation to improve
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