Wavefront shaping techniques in complex media
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- Category: Spatial Lights Modulators (SLMs)
- Published on Saturday, 20 July 2013 03:18
[tutorial] How to use a binary amplitude Deformable Miror Device (DMD) as a phase modulator: The Lee hologram method
The optical field measured at the output of a complex medium (multimode waveguide, multimode cavity, scattering medium...) is the result of the interference of the many paths taken by the light. Most of the applications of wavefront shaping techniques in these media rely on taking advantage of these interference effects to force the medium to perform a desired function. For this reason, the phases of the controlled segments of the input wavefront are usually the most import degrees of freedom to control. Common phase-only Spatial Light Modulators (SLMs) have a limited refresh rate (~100 Hz) due to the liquid crystal technology. This limits the applications in media with a low decorrelation time (like biological tissues) or for experiments for which a long optimization process is needed. Although a new class of phase-only SLMs based on deformable mirrors allows high speed modulations (>30 kHz), these devices are currently very expensive compared to liquid crystal technology and have a limited resolution (~1000 pixels maximum). On the other hand, fast binary-amplitude modulators with high resolutions are wide spread and affordable since the technology is the same one as used in commercial digital projectors. In a recent publication, [D.B. Conkey et al., Opt. Express (2012)] introduced a method to use such a binary-amplitude Deformable Mirror Device (DMD) for phase modulation. This technique allows a fast phase modulation (up to 23 kHz) at the cost of a loss of resolution.
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- Category: Highlights
- Published on Friday, 14 June 2013 02:02
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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- 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.
- Details
- 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
