234 research outputs found
Resonant infiltration of an opal: reflection lineshape and contribution from in-depth regions
We analyze the resonant variation of the optical reflection on an infiltrated
artificial opal made of transparent nanospheres. The resonant infiltration is
considered as a perturbation in the frame of a previously described
one-dimensional model based upon a stratified effective index. We show that for
a thin slice of resonant medium, the resonant response oscillates with the
position of this slice. We derive that for adequate conditions of incidence
angle, this spatially oscillating behavior matches the geometrical periodicity
of the opal, and hence the related density of resonant infiltration. Close to
these matching conditions, the resonant response of the global infiltration
varies sharply in amplitude and shape with the incidence angle and
polarization. The corresponding resonant reflection originates from a rather
deep infiltration, up to several wavelengths or layers of spheres. Finally, we
discuss the relationship between the present predictions and our previous
observations on an opal infiltrated with a resonant vapor.Comment: to appear in J Chem Phy
A 2D nanosphere array for atomic spectroscopy
We are interested in the spectroscopic behaviour of a gas confined in a
micrometric or even nanometric volume. Such a situation could be encountered by
the filling-up of a porous medium, such as a photonic crystal, with an atomic
gas. Here, we discuss the first step of this program, with the generation and
characterization of a self-organized 2D film of nanospheres of silica. We show
that an optical characterization by laser light diffraction permits to extract
some information on the array structure and represents an interesting
complement to electron microscopy.Comment: accept\'e pour publication \`a Annales de Physique- proceedings of
COLOQ1
Optics of an opal modeled with a stratified effective index and the effect of the interface
Reflection and transmission for an artificial opal are described through a
model of stratified medium based upon a one-dimensional variation of an
effective index. The model is notably applicable to a Langmuir-Blodgett type
disordered opal. Light scattering is accounted for by a phenomenological
absorption. The interface region between the opal and the substrate -or the
vacuum- induces a periodicity break in the photonic crystal arrangement, which
exhibits a prominent influence on the reflection, notably away from the Bragg
reflection peak. Experimental results are compared to our model. The model is
extendable to inverse opals, stacked cylinders, or irradiation by evanescent
wavesComment: arXiv admin note: substantial text overlap with arXiv:1407.577
Detection of slow atoms confined in a Cesium vapor cell by spatially separated pump and probe laser beams
proceedings of 17th International School on Quantum Electronics: Laser Physics and Applications, neesebar, bulgaria Sept 2012 edited by Tanja Dreischuh, Albena DaskalovaInternational audienceThe velocity distribution of atoms in a thermal gas is usually described through a Maxwell-Boltzman distribution of energy, and assumes isotropy. As a consequence, the probability for an atom to leave the surface under an azimuth angle θ should evolve as cos θ, in spite of the fact that there is no microscopic basis to justify such a law. The contribution of atoms moving at a grazing incidence towards or from the surface, i.e. atoms with a small normal velocity, here called "slow" atoms, reveals essential in the development of spectroscopic methods probing a dilute atomic vapor in the vicinity of a surface, enabling a sub-Doppler resolution under a normal incidence irradiation. The probability for such "slow" atoms may be reduced by surface roughness and atom-surface interaction. Here, we describe a method to observe and to count these slow atoms relying on a mechanical discrimination, through spatially separated pump and probe beams. We also report on our experimental progresses toward such a goal
Sub-Doppler optical resolution by confining a vapour in a nanostructure
We show that a thermal vapor confined in a nanostructure is of spectroscopic
interest. We perform reflection spectroscopy on a Cs vapour cell whose window
is covered with a thin opal film (typically, 10 or 20 layers of ~ 1{\mu}m
diameter spheres). Sub-Doppler structures appear in the optical spectrum in a
purely linear regime of optical excitation and the signal is shown to originate
from the interstitial regions of the opal. These narrow spectral structures,
observable for a large range of oblique incidence angles (~ 30-50°), are an
original feature associated to the 3-D vapor confinement. It remembers a Dicke
narrowing, i.e. a Doppler broadening suppression when the atomic motion is
sub-wavelength confined. This narrowing, commonly observed in the r.f. domain
when a buffer gas ensures a collision confinement effect, had remained elusive
in the optical frequency. Also, we describe preliminary experiments performed
in a pump-probe technique, intended to elucidate the spatial origin of the
narrow contribution. We finally discuss how our results allow envisioning
micron-size references for optical frequency clocks, and high resolution
spectroscopy of weak and hard-to-saturate molecular lines
Dicke Coherent Narrowing in Two-Photon and Raman Spectroscopy of Thin Vapour Cells
The principle of coherent Dicke narrowing in a thin vapour cell, in which
sub-Doppler spectral lineshapes are observed under a normal irradiation for a
l/2 thickness, is generalized to two-photon spectroscopy. Only the sum of the
two wave vectors must be normal to the cell, making the two-photon scheme
highly versatile. A comparison is provided between the Dicke narrowing with
copropagating fields, and the residual Doppler-broadening occurring with
counterpropagating geometries. The experimental feasibility is discussed on the
basis of a first observation of a two-photon resonance in a 300 nm-thick Cs
cell. Extension to the Raman situation is finally considered
Infiltrating a thin or single layer opal with an atomic vapour: sub-doppler signals and crystal optics
Artificial thin glass opals can be infiltrated with a resonant alkali-metal
vapour, providing novel types of hybrid systems. The reflection at the
interface between the substrate and the opal yields a resonant signal, which
exhibits sub-Doppler structures in linear spectroscopy for a range of oblique
incidences. This result is suspected to originate in an effect of the
three-dimensional confinement of the vapour in the opal interstices. It is here
extended to a situation where the opal is limited to a few or even a single
layer opal film, which is a kind of bidimensional grating. We have developed a
flexible one-dimensional layered optical model, well suited for a
Langmuir-Blodgett opal. Once extended to the case of a resonant infiltration,
the model reproduces quick variations of the lineshape with incidence angle or
polarization. Alternately, for an opal limited to a single layer of identical
spheres, a three-dimensional numerical calculation was developed. It predicts
crystalline anisotropy, which is demonstrated through diffraction on an empty
opal made of a single-layer of polystyrene spheres.Comment: to appear in Europhysics Letters, Special Issue (Proceedings of META
14-Singapore May 2014
EXTRA SUB-DOPPLER LINES IN THE VICINITY OF THE THIRD RESONANCE 6S-8P OF ATOMIC Cs ATTRIBUTED TO OPTICALLY INDUCED Cs DIMERS
International audienceWe report on the observation of extra sub-Doppler lines in a saturated absorption experiment when exploring the vicinity of the 6S1/2 8P3/2 transition of Cs ( = 388 nm). These extra lines are observed only under a relatively strong irradiation of both the pump and the probe beams. Extra narrow lines are also observed in co-propagating nonlinear spectroscopy, and around the lines of the V-type three-level system 8P3/2 - 6S1/2 - 8P1/2 (1 = 388 nm, 2 = 389 nm). We attribute theses extra-lines to a probing of high-lying molecular caesium, produced as a result of the optical excitation of Cs atoms, as the low Cs atom density ( 1012cm-3) is unable to populate significantly the dimer states in the condition of thermal equilibrium
Laser spectroscopy with nanometric gas cells : distance dependence of atom-surface interaction and collisions under confinement
The high sensitivity of Laser Spectroscopy has made possible the exploration
of atomic resonances in newly designed "nanometric" gas cells, whose local
thickness varies from 20nm to more than 1000 nm. Following the initial
observation of the optical analogous of the coherent Dicke microwave narrowing,
the newest prospects include the exploration of long-range atom surface van der
Waals interaction with spatial resolution in an unprecedented range of
distances, modification of atom dielectric resonant coupling under the
influence of the coupling between the two neighbouring dielectric media, and
even the possible modification of interatomic collisions processes under the
effect of confinement
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