{jcomments on}

\(
\def\ket#1{{\left|{#1}\right\rangle}}
\def\bra#1{{\left\langle{#1}\right|}}
\def\braket#1#2{{\left\langle{#1}|{#2}\right\rangle}}
\)

Wavefront shaping in complex media for analog computation [highlight]

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[M. W. Matthès et al., Optica, 6 (2019)]

Performing linear operations using optical devices is a crucial building block in many fields ranging from telecommunications to optical analogue computation and machine learning. For many of these applications, key requirements are robustness to fabrication inaccuracies, reconfigurability and scalability. Traditionally, the conformation or the structure of the medium is optimized in order to perform a given desired operation. Since the advent of wavefront shaping, we know that the complexity of a given operation can be shifted toward the engineering of the wavefront, allowing, for example, to use any random medium as a lens. 

In [M. W. Matthès et al., Optica, 6 (2019)], we propose to use this approach to use complex optical media such as multimode fibers or scattering media as a computational platform driven by wavefront shaping to perform analogue linear operations. Given a large random transmission matrix representing the light propagation in such a medium, we can extract any desired smaller linear operator by finding suitable input and output projectors. We demonstrate this concept by finding input wavefronts using a Spatial Light Modulator that cause the complex medium to act as a desired complex-valued linear operator on the optical field.

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[highligth] A microwave spatial modulator to improve in-home WiFi

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[N. Kaina et al., Sci. Rep. (2014)]

Wavefront shaping is not limited to optical waves. Similar techniques can be used for any kind of wave for which one can control dynamically the phase over a large number of independent elements. In [N. Kaina et al., Sci. Rep. (2014)], the authors demonstrate the use of their Spatial Microwave Modulator (SMM) to control the propagation of radio frequency waves inside a room to improve the WiFi signal at any chosen position. The system is passive as there is no energy transfer from the modulator to the WiFi signal, it only controls the local phase of the waves reflected of the modulator. The device is thin and has the typical size of small poster, it can be conveniently placed on the wall of a typical room without any loss of space.

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[highlight] From diffusive to ballistic-like transport in absorbing media

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Intuitively, absorption of light is detrimental for imaging as it reduces the intensity of the image we see. On the other hand, scattering is also an known obstacle for imaging as it mixes light sending it in all the dirrections. In the present paper, S.F. Liew and his collaborators from Yale University (CT, USA) and the University of Twente (The Netherlands) show that, contrary to appearances, absorption can in fact help light to follow a direct path through disordered media.

Without absorption, spatial information of an object transmitted through an opaque material is totally mixed and difficult to recover. The reason is that the photons are multiply scattered, hence their propagation directions are randomized at every scattering event. In their recent numerical calculation study, the authors noticed that when absorption becomes strong, the transport of light occurs via much straighter paths.

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Imaging with nature: Using a scattering medium as a universal scrambler for imaging by compressed sensing [highlight]

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[A. Liutkus et al., arXiv, 1309.0425, (2013)]

The idea of compressive sensing is to acquire an image with fewer measurement that dictated by the Shannon-Nyquist theorem. In other words, an image divided in "pixels" can usually be reconstructed using less measurements than the total number of pixels. To do so, one need a way to mix the information, so that any measurement contain at least a bit of information on any input element. Previous implementation of compressive sensing consisted in artificially designing a hardware and a sampling procedure to generate randomness. In the present paper, the authors shows that one can use a random scattering medium as a universal image scrambler. The light reflected from an image propagates through a layer of white paint and the field is measured on different receptors on the other side of the sample. By previously measuring the transmission matrix, the authors shows that sparse images can be successfully reconstructed using compressed sensing techniques taking advantage of the randomness generated by multiple scattering.

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Control of random lasing by wavefront shaping of the pump [highlight]

[N. Bachelard et al., Phys. Rev. Lett., 109, (2012)]

[M. Leonetti et al., Appl. Phys. Lett., 102, (2013)]

[N. Bachelard et al., arXiv, 1303.1398, (2013)]

[T. Hirsch et al., Phys. Rev. Lett., 111, (2013)]

 

While conventional lasers use mirrors to confine light in a cavity with gain to achieve spontaneous emission, random lasers take advantage of multiple scattering to trap light in a disordered medium [1]. Such lasers do not require to carefully tune the geometry of the cavity, which greatly simplify their design. They are potentially cheaper and more robust in presence of perturbations (temperature, vibration). The resulting emission spectrums and radiation patterns are broad but mainly uncontrolled. In recent studies [2-5] different groups demonstrated numerically and experimentally the modulation of the spatial profile of the pump to control the spectrum [2-4] or the emission pattern [5].

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Noninvasive measure of the transmission matrix in scattering media using the photo-acoustic effect [highlight]

[T. Chaigne et al., arXiv, 1305.6246, (2013)]

Optical wavefront shaping allows imaging or focusing of light in strongly scattering media at depth where usual microscopy techniques fails. However, wavefront shaping techniques usually require captors (like a CCD array) or probes (like fluorescent entities) to guide the focusing of light or to characterize the system for imaging purposes. Recently, [X. Xu, H. Liu and L.V. Wang, Nat. Photon., 5, 154, (2011)] and [X. Xu, H. Liu and L.V. Wang, Nat. Photon., 7, 300, (2013)]  (see Retrieving an optical scale resolution with light focusing guided by ultrasound) have shown how to use ultrasound to noninvasively guide light focusing in a scattering medium. This methods use an iterative optimization schemes for focusing on each target. This limits the applications for imaging due to the time requirements. In this paper, the authors use the photo-acoustic effect to measure the transmission matrix that links the optical field on the surface of a spatial light modulator (SLM) modulating the input light to the optical field on different points inside a scattering medium. This knowledge of this matrix allows selective focusing on multiple points and detection of targets buried in the medium.

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