193 research outputs found
Adaptive pumping for spectral control of random lasers
A laser is not necessarily a sophisticated device: Pumping energy into an
amplifying medium randomly filled with scatterers, a powder for instance, makes
a perfect "random laser." In such a laser, the absence of mirrors greatly
simplifies laser design, but control over emission directionality or frequency
tunability is lost, seriously hindering prospects for this otherwise simple
laser. Lately, we proposed a novel approach to harness random lasers, inspired
by spatial shaping methods recently employed for coherent light control in
complex media. Here, we experimentally implement this method in an optofluidic
random laser where scattering is weak and modes extend spatially and strongly
overlap, making individual selection a priori impossible. We show that control
over laser emission can indeed be regained even in this extreme case by
actively shaping the spatial profile of the optical pump. This unique degree of
freedom, which has never been exploited, allows selection of any desired
wavelength and shaping of lasing modes, without prior knowledge of their
spatial distribution. Mode selection is achieved with spectral selectivity down
to 0.06nm and more than 10dB side-lobe rejection. This experimental method
paves the way towards fully tunable and controlled random lasers and can be
transferred to other class of lasers.Comment: 23 pages, 7 figure
Simulation of the active Brownian motion of a microswimmer
Cataloged from PDF version of article.Unlike passive Brownian particles, active Brownian particles, also known as microswimmers, propel themselves with directed motion and thus drive themselves out of equilibrium. Understanding their motion can provide insight into out-of-equilibrium phenomena associated with biological examples such as bacteria, as well as with artificial microswimmers. We discuss how to mathematically model their motion using a set of stochastic differential equations and how to numerically simulate it using the corresponding set of finite difference equations both in homogenous and complex environments. In particular, we show how active Brownian particles do not follow the Maxwell-Boltzmann distribution-a clear signature of their out-of-equilibrium nature- and how, unlike passive Brownian particles, microswimmers can be funneled, trapped, and sorted. (C) 2014 American Association of Physics Teachers
Continuous-wave phase-sensitive parametric image amplification
We study experimentally parametric amplification in the continuous regime
using a transverse-degenerate type-II Optical Parametric Oscillator operated
below threshold. We demonstrate that this device is able to amplify either in
the phase insensitive or phase sensitive way first a single mode beam, then a
multimode image. Furthermore the total intensities of the amplified image
projected on the signal and idler polarizations are shown to be correlated at
the quantum level.Comment: 14 pages, 7 figures, submitted to Journal of Modern Optics, Special
Issue on Quantum Imagin
Creating and probing macroscoping entanglement with light
We describe a scheme showing signatures of macroscopic optomechanical
entanglement generated by radiation pressure in a cavity system with a massive
movable mirror. The system we consider reveals genuine multipartite
entanglement. We highlight the way the entanglement involving the inaccessible
massive object is unravelled, in our scheme, by means of field-field quantum
correlations.Comment: 4 pages, 5 figure, RevTeX
Radiation-pressure self-cooling of a micromirror in a cryogenic environment
We demonstrate radiation-pressure cavity-cooling of a mechanical mode of a
micromirror starting from cryogenic temperatures. To achieve that, a
high-finesse Fabry-Perot cavity (F\approx 2200) was actively stabilized inside
a continuous-flow 4He cryostat. We observed optical cooling of the fundamental
mode of a 50mu x 50 mu x 5.4 mu singly-clamped micromirror at \omega_m=3.5 MHz
from 35 K to approx. 290 mK. This corresponds to a thermal occupation factor of
\approx 1x10^4. The cooling performance is only limited by the mechanical
quality and by the optical finesse of the system. Heating effects, e.g. due to
absorption of photons in the micromirror, could not be observed. These results
represent a next step towards cavity-cooling a mechanical oscillator into its
quantum ground state
Defocus test and defocus correction in full-field optical coherence tomography
We report experimental evidence and correction of defocus in full-field OCT
of biological samples due to mismatch of the refractive index of biological
tissues and water. Via a metric based on the image quality, we demonstrate that
we are able to compensate this index-induced defocus and to recover a sharp
image in depth.Comment: 7 pages, 3 figures, minor changes, 1 figure adde
Characterization of the angular memory effect of scattered light in biological tissues.
This is the final version of the article. Available via open access from Optical Society of America via the DOI in this record.High resolution optical microscopy is essential in neuroscience but suffers from scattering in biological tissues and therefore grants access to superficial brain layers only. Recently developed techniques use scattered photons for imaging by exploiting angular correlations in transmitted light and could potentially increase imaging depths. But those correlations ('angular memory effect') are of a very short range and should theoretically be only present behind and not inside scattering media. From measurements on neural tissues and complementary simulations, we find that strong forward scattering in biological tissues can enhance the memory effect range and thus the possible field-of-view by more than an order of magnitude compared to isotropic scattering for âŒ1 mm thick tissue layers.This work was funded by European Research Council Grant 278025 and the Agence Nationale
de la Recherche (Investissements dâAvenir ANR-10-LABX-54 MEMO LIFE, ANR-11-IDEX-
0001-02 PSL* Research University). We thank Prof. Georg Maret for enabling Sam Schottâs
stay at institut Langevin and his support of the project and David Martina for technical help in
the development of the experimental setup
Far-Field Wavefront Control of Nonlinear Luminescence in Disordered Gold Metasurfaces
We demonstrate the local optimization of nonlinear luminescence from disordered gold metasurfaces by shaping the phase of femtosecond excitation. This process is enabled by the far-field wavefront control of plasmonic modes delocalized over the sample surface, leading to a coherent enhancement of subwavelength electric fields. In practice, the increase in nonlinear luminescence is strongly sensitive to both the nanometer-scale morphology and the level of structural complexity of the gold metasurface. We typically observe a 2 orders of magnitude enhancement of the luminescence signal for an optimized excitation wavefront compared to a random one. These results demonstrate how disordered metasurfaces made of randomly coupled plasmonic resonators, together with wavefront shaping, provide numerous degrees of freedom to program locally optimized nonlinear responses and optical hotspots
A high-reflectivity high-Q micromechanical Bragg-mirror
We report on the fabrication and characterization of a micromechanical
oscillator consisting only of a free-standing dielectric Bragg mirror with high
optical reflectivity and high mechanical quality. The fabrication technique is
a hybrid approach involving laser ablation and dry etching. The mirror has a
reflectivity of 99.6%, a mass of 400ng, and a mechanical quality factor Q of
approximately 10^4. Using this micromirror in a Fabry Perot cavity, a finesse
of 500 has been achieved. This is an important step towards designing tunable
high-Q high-finesse cavities on chip.Comment: 3 pages, 2 figure
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