701 research outputs found
Control of coherent backscattering by breaking optical reciprocity
Reciprocity is a universal principle that has a profound impact on many areas
of physics. A fundamental phenomenon in condensed-matter physics, optical
physics and acoustics, arising from reciprocity, is the constructive
interference of quantum or classical waves which propagate along time-reversed
paths in disordered media, leading to, for example, weak localization and
metal-insulator transition. Previous studies have shown that such coherent
effects are suppressed when reciprocity is broken. Here we show that by
breaking reciprocity in a controlled manner, we can tune, rather than simply
suppress, these phenomena. In particular, we manipulate coherent backscattering
of light, also known as weak localization. By utilizing a non-reciprocal
magneto-optical effect, we control the interference between time-reversed paths
inside a multimode fiber with strong mode mixing, and realize a continuous
transition from the well-known peak to a dip in the backscattered intensity.
Our results may open new possibilities for coherent control of classical and
quantum waves in complex systemsComment: Comments are welcom
Binary holograms for shaping light with digital micromirror devices
Digital micromirror devices are a popular type of spatial light modulators
for wavefront shaping applications. While they offer several advantages when
compared to liquid crystal modulators, such as polarization insensitivity and
rapid-switching, they only provide a binary amplitude modulation. Despite this
restriction, it is possible to use binary holograms to modulate both the
amplitude and phase of the incoming light, thus allowing the creation of
complex light fields. Here, a didactic exploration of various types of binary
holograms is presented. A particular emphasis is placed on the fact that the
finite number of pixels coupled with the binary modulation limits the number of
complex values that can be encoded into the holograms. This entails an
inevitable trade-off between the number of complex values that can be modulated
with the hologram and the number of independent degrees of freedom available to
shape light, both of which impact the quality of the shaped field. Nonetheless,
it is shown that by appropriately choosing the type of hologram and its
parameters, it is possible to find a suitable compromise that allows shaping a
wide range of complex fields with high accuracy. In particular, it is shown
that choosing the appropriate alignment between the hologram and the
micromirror array allows for maximizing the number of complex values. Likewise,
the implications of the type of hologram and its parameters on the diffraction
efficiency are also considered
The Schr\"odinger operator on an infinite wedge with a tangent magnetic field
We study a model Schr\"odinger operator with constant magnetic field on an
infinite wedge with Neumann boundary condition. The magnetic field is assumed
to be tangent to a face. We compare the bottom of the spectrum to the model
spectral quantities coming from the regular case. We are particularly motivated
by the influence of the magnetic field and the opening angle of the wedge on
the spectrum of the model operator and we exhibit cases where the bottom of the
spectrum is smaller than in the regular case. Numerical computations enlighten
the theoretical approach
Controlling Light Through Optical Disordered Media : Transmission Matrix Approach
We experimentally measure the monochromatic transmission matrix (TM) of an
optical multiple scattering medium using a spatial light modulator together
with a phase-shifting interferometry measurement method. The TM contains all
information needed to shape the scattered output field at will or to detect an
image through the medium. We confront theory and experiment for these
applications and we study the effect of noise on the reconstruction method. We
also extracted from the TM informations about the statistical properties of the
medium and the light transport whitin it. In particular, we are able to isolate
the contributions of the Memory Effect (ME) and measure its attenuation length
Focusing and Compression of Ultrashort Pulses through Scattering Media
Light scattering in inhomogeneous media induces wavefront distortions which
pose an inherent limitation in many optical applications. Examples range from
microscopy and nanosurgery to astronomy. In recent years, ongoing efforts have
made the correction of spatial distortions possible by wavefront shaping
techniques. However, when ultrashort pulses are employed scattering induces
temporal distortions which hinder their use in nonlinear processes such as in
multiphoton microscopy and quantum control experiments. Here we show that
correction of both spatial and temporal distortions can be attained by
manipulating only the spatial degrees of freedom of the incident wavefront.
Moreover, by optimizing a nonlinear signal the refocused pulse can be shorter
than the input pulse. We demonstrate focusing of 100fs pulses through a 1mm
thick brain tissue, and 1000-fold enhancement of a localized two-photon
fluorescence signal. Our results open up new possibilities for optical
manipulation and nonlinear imaging in scattering media
High-fidelity multimode fibre-based endoscopy for deep brain in vivo imaging
Progress in neuroscience constantly relies on the development of new
techniques to investigate the complex dynamics of neuronal networks. An ongoing
challenge is to achieve minimally-invasive and high-resolution observations of
neuronal activity in vivo inside deep brain areas. A perspective strategy is to
utilise holographic control of light propagation in complex media, which allows
converting a hair-thin multimode optical fibre into an ultra-narrow imaging
tool. Compared to current endoscopes based on GRIN lenses or fibre bundles,
this concept offers a footprint reduction exceeding an order of magnitude,
together with a significant enhancement in resolution. We designed a compact
and high-speed system for fluorescent imaging at the tip of a fibre, achieving
micron-scale resolution across a 50 um field of view, and yielding 7-kilopixel
images at a rate of 3.5 frames/s. Furthermore, we demonstrate in vivo
observations of cell bodies and processes of inhibitory neurons within deep
layers of the visual cortex and hippocampus of anesthetised mice. This study
forms the basis for several perspective techniques of modern microscopy to be
delivered deep inside the tissue of living animal models while causing minimal
impact on its structural and functional properties.Comment: 10 pages, 2 figures, Supplementary movie:
https://drive.google.com/file/d/1Fm0G3TAIC49LVX6FaEiAtlefkWx1T2a5/vie
From music to mathematics and backwards: introducing algebra, topology and category theory into computational musicology
International audienceDespite a long historical relationship between mathematics and music, the interest of mathematicians is a recent phenomenon. In contrast to statistical methods and signal-based approaches currently employed in MIR (Music Information Research), the research project described in this paper stresses the necessity of introducing a structural multidisciplinary approach into computational musicology making use of advanced mathematics. It is based on the interplay between three main mathematical disciplines: algebra, topology and category theory. It therefore opens promising perspectives on important prevailing challenges, such as the automatic classification of musical styles or the solution of open mathematical conjectures, asking for new collaborations between mathematicians, computer scientists, musicologists, and composers. Music can in fact occupy a strategic place in the development of mathematics since music-theoretical constructions can be used to solve open mathematical problems. The SMIR project also differs from traditional applications of mathematics to music in aiming to build bridges between different musical genres, ranging from contemporary art music to popular music, including rock, pop, jazz and chanson. Beyond its academic ambition, the project carries an important societal dimension stressing the cultural component of 'mathemusical' research, that naturally resonates with the underlying philosophy of the âImagine Mathsâconference series. The article describes for a general public some of the most promising interdisciplinary research lines of this project
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