467 research outputs found
Spatially-entangled Photon-pairs Generation Using Partial Spatially Coherent Pump Beam
We demonstrate experimental generation of spatially-entangled photon-pairs by
spontaneous parametric down conversion (SPDC) using a partial spatially
coherent pump beam. By varying the spatial coherence of the pump, we show its
influence on the downconverted photon's spatial correlations and on their
degree of entanglement, in excellent agreement with theory. We then exploit
this property to produce pairs of photons with a specific degree of
entanglement by tailoring of the pump coherence length. This work thus unravels
the fundamental transfer of coherence occuring in SPDC processes, and provides
a simple experimental scheme to generate photon-pairs with a well-defined
degree of spatial entanglement, which may be useful for quantum communication
and information processingComment: Main: 5 pages and 3 Figures ; Supplementary: 5 pages and 3 Figure
Spectral Method for Multiplexed Phase Retrieval and Application in Optical Imaging in Complex Media
We introduce a generalized version of phase retrieval called multiplexed
phase retrieval. We want to recover the phase of amplitude-only measurements
from linear combinations of them. This corresponds to the case in which
multiple incoherent sources are sampled jointly, and one would like to recover
their individual contributions. We show that a recent spectral method developed
for phase retrieval can be generalized to this setting, and that its
performance follows a phase transition behavior. We apply this new technique to
light focusing at depth in a complex medium. Experimentally, although we only
have access to the sum of the intensities on multiple targets, we are able to
separately focus on each ones, thus opening potential applications in deep
fluorescence imaging and light deliver
Brownian Motion in a Speckle Light Field: Tunable Anomalous Diffusion and Deterministic Optical Manipulation
The motion of particles in random potentials occurs in several natural
phenomena ranging from the mobility of organelles within a biological cell to
the diffusion of stars within a galaxy. A Brownian particle moving in the
random optical potential associated to a speckle, i.e., a complex interference
pattern generated by the scattering of coherent light by a random medium,
provides an ideal mesoscopic model system to study such phenomena. Here, we
derive a theory for the motion of a Brownian particle in a speckle and, in
particular, we identify its universal characteristic timescale levering on the
universal properties of speckles. This theoretical insight permits us to
identify several interesting unexplored phenomena and applications. As an
example of the former, we show the possibility of tuning anomalous diffusion
continuously from subdiffusion to superdiffusion. As an example of the latter,
we show the possibility of harnessing the speckle memory effect to perform some
basic deterministic optical manipulation tasks such as guiding and sorting by
employing random speckles, which might broaden the perspectives of optical
manipulation for real-life applications by providing a simple and
cost-effective technique
Robust phase retrieval with the swept approximate message passing (prSAMP) algorithm
In phase retrieval, the goal is to recover a complex signal from the
magnitude of its linear measurements. While many well-known algorithms
guarantee deterministic recovery of the unknown signal using i.i.d. random
measurement matrices, they suffer serious convergence issues some
ill-conditioned matrices. As an example, this happens in optical imagers using
binary intensity-only spatial light modulators to shape the input wavefront.
The problem of ill-conditioned measurement matrices has also been a topic of
interest for compressed sensing researchers during the past decade. In this
paper, using recent advances in generic compressed sensing, we propose a new
phase retrieval algorithm that well-adopts for both Gaussian i.i.d. and binary
matrices using both sparse and dense input signals. This algorithm is also
robust to the strong noise levels found in some imaging applications
Temporal recompression through a scattering medium via a broadband transmission matrix
The transmission matrix is a unique tool to control light through a
scattering medium. A monochromatic transmission matrix does not allow temporal
control of broadband light. Conversely, measuring multiple transmission
matrices with spectral resolution allows fine temporal control when a pulse is
temporally broadened upon multiple scattering, but requires very long
measurement time. Here, we show that a single linear operator, measured for a
broadband pulse with a co-propagating reference, naturally allows for spatial
focusing, and interestingly generates a two-fold temporal recompression at the
focus, compared with the natural temporal broadening. This is particularly
relevant for non-linear imaging techniques in biological tissues.Comment: 4 pages, 3 figure
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
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