Created by sebastien.popoff on 01/07/2014

Highlights

From diffusive to ballistic-like transport in absorbing media

[S.F. Liew et al., Phys. Rev. B, 89, (2013)]

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 a known obstacle for imaging as it mixes light sending it in all the directions. 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.

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Created by sebastien.popoff on 09/09/2013

Highlights

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

[A. Liutkus et al., Sci. Rep. 5, (2014)]

The idea of compressive sensing is to acquire an image with fewer measurements than dictated by the Shannon-Nyquist theorem. In other words, an image divided into "pixels" can usually be reconstructed using fewer measurements than the total number of pixels. To do so, one needs a way to mix the information, so that any measurement contains at least a bit of information on any input element. Previous implementations of compressive sensing consisted of artificially designing hardware and a sampling procedure to generate randomness. In the present paper, the authors show 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 show that sparse images can be successfully reconstructed using compressed sensing techniques taking advantage of the randomness generated by multiple scattering.

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Created by sebastien.popoff on 15/07/2013

Highlights

Control of random lasing by wavefront shaping of the pump

[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 simplifies their design. They are potentially cheaper and more robust in the 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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Created by sebastien.popoff on 14/06/2013

Highlights

A noninvasive measure of the transmission matrix in scattering media using the photo-acoustic effect

[T. Chaigne et al., Nat. Photon., 8, (2013)]

Optical wavefront shaping allows imaging or focusing of light in strongly scattering media at a depth where usual microscopy techniques fail. 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 method uses an iterative optimization scheme 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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