Wavefront shaping techniques in complex media
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- Category: Highlights
- Published on Monday, 15 July 2013 23:01
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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- Category: Highlights
- Published on Monday, 13 May 2013 00:03
Subwavelength light focusing through a scattering medium [highlight]
[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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- Category: Spatial Lights Modulators (SLMs)
- Published on Saturday, 04 May 2013 01:42
How to characterize and calibrate a phase only SLM [tutorial]
For most applications in complex media, spatial light modulators are used for their ability to control the phase of a laser beam. Whereas deformable mirrors are insensitive to the input polarization, liquid crystal based SLMs need to work with a given input polarization or sometimes a precise combination of input and output polarizations. It is then necessary for LC SLMs to carefully characterize the modulation to find the setup conditions where amplitude variations are minimal and for which the phase range is at least 2π. In any case, for a given wavelength, it is necessary to know the relation between the value given to a pixel on the SLM and the relative phase shift associated. I present here a typical way to characterize the complex modulation of a SLM.
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- Category: Events
- Published on Thursday, 02 May 2013 15:58
Complex Nanophotonic - Science Camp
Aug 27 - Aug 30, 2013
Cumberland Lodge, Windsor Great Park, UK
Organized by:
Jacopo Bertolotti (Twente University, NL) / Otto Muskens (Southampton University, UK)
Sylvain Gigan (Langevin Instiute, ESPCI, FR) / Riccardo Sapienza (King’s College London, UK)
Link: here
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- Category: Highlights
- Published on Sunday, 28 April 2013 23:47
From the bimodal distribution to the quarter circle law [highlight]
[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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- Category: Highlights
- Published on Friday, 26 April 2013 16:49
Retrieving an optical scale resolution with light focusing guided by ultrasound [highlight]
[B. Judkewitz et al., Nat. Photon., 7, 300, (2013)]
To focus light in or through a scattering medium using wavefront shaping techniques, one need 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 to focus 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.
