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Created by sebastien.popoff on 13/05/2013
Subwavelength light focusing through a scattering medium
[J. H. Park et al., Nat. Photon., (2013)] After the first experiment of light focusing through a scattering medium using wavefront shaping (see A pioneer experiment), the same group demonstrated in [I. M. Vellekoop et al., Nat. Photon., 4, 320, (2010)] that a random medium can improve the sharpness of the focus. The scattering in a medium behind a lens randomizes the direction of the light. The speckle pattern shows high spatial frequencies not allowed by the lens alone because of its finite numerical aperture. After optimization of the input wavefront, the focus spot obtained is sharper than the resolution limit of the lens. In these experiments, the intensity profile was always measured in the far field, i.e. at least several wavelengths away from the surface, where only the propagating waves contribute to the optical field. In the present paper, J. H. Park and his colleagues optimize the input wavefront impinging on turbid media to increase the intensity measured in the near field at a given position. Subwavelength focusing is achieved thanks to the contributions of the evanescent waves. |
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Created by sebastien.popoff on 28/04/2013
From the bimodal distribution to the quarter circle law
[A. Goetschy and A. D. Stone, Phys. Rev. Lett., 1304.5562, (2013)] Almost thirty years ago, theoreticians predicted that the distribution of the transmission values of a multiple scattering sample should follow a 'bimodal distribution'. Physically, that means that, in the diffusive regime, there is a large number of strongly reflected channels - the closed channels - and a small number of channels of transmission close to one - the open channels. The existence of these open channels regardless of the thickness of the medium is of big interest for researchers, especially for imaging or communication applications. Nevertheless, such channels have not yet been directly observed. A investigation on those channels requires a measurement of the entire transmission matrix of a lossless scattering medium. For practical reasons (open geometry, limited numerical aperture, noise...) one usually has access to a subpart of the total transmission matrix. In recent experimental measures of the transmission matrix in optics [S.M. Popoff et al., Phys. Rev. Lett., 104, 100601, (2010)] the distribution of the transmission values follows a 'quarter circle law', characteristic of totally uncorrelated systems. This means that the fraction of the transmission matrix measured shows no effect of the correlations at the origin of the bimodal distribution due to the loss of information. In this paper, A. Goetschy and D. Stone theoretically study the effect of the loss of information or the imperfect control on the statistics of the transmission matrix of a scattering system. |
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Created by sebastien.popoff on 26/04/2013
Retrieving an optical scale resolution with light focusing guided by ultrasound
[B. Judkewitz et al., Nat. Photon., 7, 300, (2013)] To focus light in or through a scattering medium using wavefront shaping techniques, one needs a way to probe the intensity or the field at the target position. To avoid having to insert a probe in the medium, Xu et al. proposed in 2011 the use of an ultrasonic focused beam to select a target area by photo-acoustic effect [X. Xu, H. Liu and L.V. Wang, Nat. Photon., 5, 154, (2011)]. This technique allows focusing light on a spot of the size of the ultrasound focused beam, which is typically at least one order of magnitude larger than the optical wavelength. In this new study, B. Judkewitz and co-authors used an innovative method to be able to focus light on a much smaller scale. |
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Created by sebastien.popoff on 24/04/2013
A pioneering experiment: Focusing through scattering media using wavefront shaping
[I.M Vellekoop and A.P. Mosk, Opt. Lett., 15, 2309, 2007] In 2007 I.M. Vellekoop and A.P. Mosk published their work on the first demonstration of focusing light through a highly scattering medium. Most techniques to image or focus through scattering media relied on selecting only the part of the light that has not been scattered - the ballistic light. The ballistic signals decay exponentially with the thickness of the medium, limiting drastically the depth at which light can be focused. The idea developed by the authors is to use the scattered waves, that are randomly mixed, to focus light through the medium. A scattering sample illuminated by a coherent wave gives rise to a so-called speckle pattern, that results from the interference of the scattered waves. Using a spatial light modulator (SLM), the authors are able to control independently the phase of the different parts of the incident beam. Each segment gives an output seemingly random complex field. By testing different values of the phase for each segment, they are able to put in phase all the contributions, giving rise to a very bright focus spot. |
