1,042 research outputs found
Spatial evolution of short pulses under coherent population trapping
Spatial and temporal evolution is studied of two powerful short laser pulses
having different wavelengths and interacting with a dense three-level
Lambda-type optical medium under coherent population trapping. A general case
of unequal oscillator strengths of the transitions is considered. Durations of
the probe pulse and the coupling pulse () are assumed to be
shorter than any of the relevant atomic relaxation times. We propose analytical
and numerical solutions of a self-consistent set of coupled Schr\"{o}dinger
equations and reduced wave equations in the adiabatic limit with the account of
the first non-adiabatic correction. The adiabaticity criterion is also
discussed with the account of the pulse propagation. The dynamics of
propagation is found to be strongly dependent on the ratio of the transition
oscillator strengths. It is shown that envelopes of the pulses slightly change
throughout the medium length at the initial stage of propagation. This distance
can be large compared to the one-photon resonant absorption length. Eventually,
the probe pulse is completely reemitted into the coupling pulse during
propagation. The effect of localization of the atomic coherence has been
observed similar to the one predicted by Fleischhauer and Lukin (PRL, {\bf 84},
5094 (2000).Comment: 16 pages revtex style, 7 EPS figures, accepted to Physical Review
Geometric phase and o-mode blue shift in a chiral anisotropic medium inside a Fabry-P\'erot cavity
Anomalous spectral shift of transmission peaks is observed in a
Fabry--P\'erot cavity filled with a chiral anisotropic medium. The effective
refractive index value resides out of the interval between the ordinary and the
extraordinary refractive indices. The spectral shift is explained by
contribution of a geometric phase. The problem is solved analytically using the
approximate Jones matrix method, numerically using the accurate Berreman method
and geometrically using the generalized Mauguin--Poincar\'e rolling cone
method. The -mode blue shift is measured for a
4-methoxybenzylidene-4'--butylaniline twisted--nematic layer inside the
Fabry--P\'erot cavity. The twist is electrically induced due to the
homeoplanar--twisted configuration transition in an ionic-surfactant-doped
liquid crystal layer. Experimental evidence confirms the validity of the
theoretical model.Comment: the text is available both in English (Timofeev2015en.tex) and in
Russian (download: other formats - source - Timofeev2015ru.tex,
Timofeev2015rus.pdf
Theory of second-harmonic generation in a chirped 2D nonlinear optical superlattice under nonlinear Raman-Nath diffraction
We analyze second-harmonic generation (SHG) in a two-dimensional nonlinear optical superlattice (NLOS) with
its modulation period being chirped in the propagation direction and constant in the transverse direction.
This results in efficient multiple SHG via nonlinear RamanβNath diffraction. We obtain exact analytical expressions
for a SH amplitude generated in chirped 2D NLOSs and for its quasi-phase-matching bandwidth. The
results of analytical calculations are in excellent agreement with the numerical ones. We show that the process
is robust to angular deviations of NLOS and it can be applied to enable tunable and broadband frequency
conversion
Ultranarrow resonance peaks in the transmission and reflection spectra of a photonic crystal cavity with Raman gain
The Raman gain of a probe light in a three-state -scheme placed
into a defect of a one-dimensional photonic crystal is studied theoretically.
We show that there exists a pump intensity range, where the transmission and
reflection spectra of the probe field exhibit \textit{simultaneously} occurring
narrow peaks (resonances) whose position is determined by the Raman resonance.
Transmission and reflection coefficients can be larger than unity at pump
intensities of order tens of W/cm. When the pump intensity is
outside this region, the peak in the transmission spectrum turns into a narrow
dip. The nature of narrow resonances is attributed to a drastic dispersion of
the nonlinear refractive index in the vicinity of the Raman transition, which
leads to a significant reduction of the group velocity of the probe wave.Comment: 9 pages, 3 figure
Coherent control of light-pulse propagation in a Raman induced grating
We study light-pulse propagation in a dynamically controllable periodic structure (grating)
resulting from Raman interaction of a weak probe pulse with a standing-wave pump and a
second control laser field inN-type four-level atomic media. The grating is induced due to
periodic spatial modulation of the Raman gain in a standing pump field (Raman gain grating).
We show that it is possible to control both the probe pulse amplitude and the group velocity of
the pulse from subluminal to superluminal by varying the pump or control field. Such a grating is
of interest forall-optical switches and transistors
Parton Energy Loss in Heavy-Ion Collisions Via Direct-Photon and Charged-Particle Azimuthal Correlations
Charged-particle spectra associated with direct photon Ydir) and Ο0 are measured in p+p and Au+Au collisions at center-of-mass energy βsNN=200 GeV with the STAR detector at the Relativistic Heavy Ion Collider. A shower-shape analysis is used to partially discriminate between Ydirand Ο0. Assuming no associated charged particles in the Ξ³dir direction (near side) and small contribution from fragmentation photons (Yfrag), the associated charged-particle yields opposite to Ydir(away side) are extracted. In central Au+Au collisions, the charged-particle yields at midrapidity (|Ξ·|\u3c1) and high transverse momentum (3 \u3c PTassoc \u3c 16 GeV/c) associated with Ξ³dir and Ο0 (|Ξ·|\u3c0.9, 8 \u3c PTtrig \u3c16 GeV/c) are suppressed by a factor of 3β5 compared with p+p collisions. The observed suppression of the associated charged particles is similar for Ydir and Ο0 and independent of the Ξ³direnergy within uncertainties. These measurements indicate that, in the kinematic range covered and within our current experimental uncertainties, the parton energy loss shows no sensitivity to the parton initial energy, path length, or color charge
Multiple nonlinear Bragg diffraction of femtosecond laser pulses in a Ο2 photonic lattice with hexagonal domains
The frequency doubling of femtosecond laser pulses in a two-dimensional (2D) rectangular nonlinear photonic lattice with hexagonal domains is studied experimentally and theoretically. The broad fundamental spectrum enables frequency conversion under nonlinear Bragg diffraction for a series of transverse orders at a fixed longitudinal quasi-phase-matching order. The consistent nonstationary theory of the frequency doubling of femtosecond laser pulses is developed using the representation based on the reciprocal lattice of the structure. The calculated spatial distribution of the second-harmonic spectral intensity agrees well with the experimental data. The condition for multiple nonlinear Bragg diffraction in a 2D nonlinear photonic lattice is offered. The hexagonal shape of the domains contributes to multibeam second harmonic excitation. The maximum conversion efficiency for a series of transverse orders in the range 0.01%-0.03% is obtained. Β© 2018 Astro Ltd
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