89 research outputs found
Superpixel-based spatial amplitude and phase modulation using a digital micromirror device.
This is the final version of the article. Available via open access from Optical Society of America via the DOI in this record.We present a superpixel method for full spatial phase and amplitude control of a light beam using a digital micromirror device (DMD) combined with a spatial filter. We combine square regions of nearby micromirrors into superpixels by low pass filtering in a Fourier plane of the DMD. At each superpixel we are able to independently modulate the phase and the amplitude of light, while retaining a high resolution and the very high speed of a DMD. The method achieves a measured fidelity F = 0.98 for a target field with fully independent phase and amplitude at a resolution of 8 × 8 pixels per diffraction limited spot. For the LG10 orbital angular momentum mode the calculated fidelity is F = 0.99993, using 768 × 768 DMD pixels. The superpixel method reduces the errors when compared to the state of the art Lee holography method for these test fields by 50% and 18%, with a comparable light efficiency of around 5%. Our control software is publicly available.We thank Duygu Akbulut, Hasan Yılmaz, Henri Thyrrestrup, Michael J. Van De Graaff, Pepijn W.H. Pinkse, Ad Lagendijk and Willem L. Vos for discussions. This work is part of the research program of the Stichting voor Fundamenteel Onderzoek der Materie (FOM). A.P.M. acknowledges European Research Council grant no. 279248
Optical transmission matrix as a probe of the photonic strength
We demonstrate that optical transmission matrices (TM) of disordered complex media provide a powerful tool to extract the photonic interaction strength, independent of surface effects. We measure TM of strongly scattering GaP nanowires and plot the singular value density of the measured matrices and a random matrix model. By varying the free parameters of the model, the transport mean free path and effective refractive index, we retrieve the photonic interaction strength. From numerical simulations we conclude that TM statistics is hardly sensitive to surface effects, in contrast to enhanced backscattering or total transmission based methods.We acknowledge support from ERC grant 27948, NWOVici, STW, the Royal Society, and EPSRC through fellowship EP/J016918/1
Designing disorder
This is the author accepted manuscript. The final version is available from Springer Nature via the DOI in this recordMetasurfaces can in principle provide a versatile platform for optical functionalities, but in practice designing and fabricating them to specifications can be difficult. Now, the realization of metasurfaces with engineered disorder allows for versatile optical components that combine the best features of periodic and random systems
Scattering invariant modes of light in complex media
Random scattering of light in disordered media is an intriguing phenomenon of
fundamental relevance to various applications. While techniques such as
wavefront shaping and transmission matrix measurements have enabled remarkable
progress for advanced imaging concepts, the most successful strategy to obtain
clear images through a disordered medium remains the filtering of ballistic
light. Ballistic photons with a scattering-free propagation are, however,
exponentially rare and no method so far can increase their proportion. To
address these limitations, we introduce and experimentally implement here a new
set of optical states that we term Scattering Invariant Modes (SIMs), whose
transmitted field pattern is the same, irrespective of whether they scatter
through a disordered sample or propagate ballistically through a homogeneous
medium. We observe SIMs that are only weakly attenuated in dense scattering
media, and show in simulations that their correlations with the ballistic light
can be used to improve imaging inside scattering materials
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
Speckle-scale focusing in the diffusive regime with time reversal of variance-encoded light (TROVE)
Focusing of light in the diffusive regime inside scattering media has long been considered impossible. Recently, this limitation has been overcome with time reversal of ultrasound-encoded light (TRUE), but the resolution of this approach is fundamentally limited by the large number of optical modes within the ultrasound focus. Here, we introduce a new approach, time reversal of variance-encoded light (TROVE), which demixes these spatial modes by variance encoding to break the resolution barrier imposed by the ultrasound. By encoding individual spatial modes inside the scattering sample with unique variances, we effectively uncouple the system resolution from the size of the ultrasound focus. This enables us to demonstrate optical focusing and imaging with diffuse light at an unprecedented, speckle-scale lateral resolution of ~5 µm
Translation correlations in anisotropically scattering media
Controlling light propagation across scattering media by wavefront shaping
holds great promise for a wide range of communications and imaging
applications. However, finding the right wavefront to shape is a challenge when
the mapping between input and output scattered wavefronts (i.e. the
transmission matrix) is not known. Correlations in transmission matrices,
especially the so-called memory-effect, have been exploited to address this
limitation. However, the traditional memory-effect applies to thin scattering
layers at a distance from the target, which precludes its use within thick
scattering media, such as fog and biological tissue. Here, we theoretically
predict and experimentally verify new transmission matrix correlations within
thick anisotropically scattering media, with important implications for
biomedical imaging and adaptive optics.Comment: main article (18 pages) and appendices (6 pages
Fluorescence imaging through dynamic scattering media with speckle-encoded ultrasound-modulated light correlation
Fluorescence imaging is indispensable to biomedical research, and yet it remains challenging to image through dynamic scattering samples. Techniques that combine ultrasound and light as exemplified by ultrasound-assisted wavefront shaping have enabled fluorescence imaging through scattering media. However, the translation of these techniques into in vivo applications has been hindered by the lack of high-speed solutions to counter the fast speckle decorrelation of dynamic tissue. Here, we report an ultrasound-enabled optical imaging method that instead leverages the dynamic nature to perform imaging. The method utilizes the correlation between the dynamic speckle-encoded fluorescence and ultrasound-modulated light signal that originate from the same location within a sample. We image fluorescent targets with an improved resolution of ≤75 µm (versus a resolution of 1.3 mm with direct optical imaging) within a scattering medium with 17 ms decorrelation time. This new imaging modality paves the way for fluorescence imaging in highly scattering tissue in vivo
Wavefront shaping with disorder-engineered metasurfaces
Recently, wavefront shaping with disordered media has demonstrated optical manipulation capabilities beyond those of conventional optics, including extended volume, aberration-free focusing and subwavelength focusing. However, translating these capabilities to useful applications has remained challenging as the input–output characteristics of the disordered media (P variables) need to be exhaustively determined via O(P) measurements. Here, we propose a paradigm shift where the disorder is specifically designed so its exact input–output characteristics are known a priori and can be used with only a few alignment steps. We implement this concept with a disorder-engineered metasurface, which exhibits additional unique features for wavefront shaping such as a large optical memory effect range in combination with a wide angular scattering range, excellent stability, and a tailorable angular scattering profile. Using this designed metasurface with wavefront shaping, we demonstrate high numerical aperture (NA > 0.5) focusing and fluorescence imaging with an estimated ~2.2 × 10^8 addressable points in an ~8 mm field of view
Conservative entropic forces
Entropic forces have recently attracted considerable attention as ways to
reformulate, retrodict, and perhaps even "explain'" classical Newtonian gravity
from a rather specific thermodynamic perspective. In this article I point out
that if one wishes to reformulate classical Newtonian gravity in terms of an
entropic force, then the fact that Newtonian gravity is described by a
conservative force places significant constraints on the form of the entropy
and temperature functions. (These constraints also apply to entropic
reinterpretations of electromagnetism, and indeed to any conservative force
derivable from a potential.)
The constraints I will establish are sufficient to present real and
significant problems for any reasonable variant of Verlinde's entropic gravity
proposal, though for technical reasons the constraints established herein do
not directly impact on either Jacobson's or Padmanabhan's versions of entropic
gravity. In an attempt to resolve these issues, I will extend the usual notion
of entropic force to multiple heat baths with multiple "temperatures'" and
multiple "entropies".Comment: V1: 21 pages; no figures. V2: now 24 pages. Two new sections (reduced
mass formulation, decoherence). Many small clarifying comments added
throughout the text. Several references added. V3: Three more references
added. V4: now 25 pages. Some extra discussion on the relation between
Verlinde's scenario and the Jacobson and Padmanabhan scenarios. This version
accepted for publication in JHE
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