Controlling the Propagation of Light in Disordered Scatteringmedia

We report focusing of coherent light through opaque scattering materials, by control of the incident wavefront. The multiply scattered light forms a focus that is up to a factor 1000 brighter than the normal diffuse transmission. [This chapter has been published as: I. M. Vellekoop and A. P. Mosk, Opt. Lett. 32, 2309–2311 (2007)] Random scattering of light is what makes materials such as white paint, milk, or human tissue, opaque. In these materials, repeated scattering and interference distort the incident wavefront so strongly that all spatial coherence is lost.[1] Incident coherent light diffuses through the medium and forms a volume speckle field which has no correlations on a distance larger than the wavelength of light. The complete scrambling of the fieldmakes it impossible to control light propagation using the well established wavefront correction methods of adaptive optics (see e.g. [2]). We demonstrate focusing of coherent light through disordered scattering media by the construction of wavefronts that invert diffusion of light. Our method relies on interference and is universally applicable to scattering objects regardless of their constitution and scattering strength. We envision that, with such active control, random scattering will become beneficial, rather than detrimental, to imaging[1] and communication[3–5]. Figure 3.1 shows the principle of the experiment. Normally, incident light is scattered by the sample and forms a random speckle pattern (Fig. 3.1a). The goal is to Figure 3.1: Design of the experiment. a) A plane wave is focused on a disordered medium, a speckle pattern is transmitted. b) The wavefront of the incident light is shaped so that scattering makes the light focus at a predefined target. 44 Focusing coherent light through opaque strongly scattering media Figure 3.2: Schematic of the apparatus. A 632.8 nm HeNe laser beam is expanded and reflected off a Holoeye LC-R 2500 liquid crystal spatial light modulator (SLM). Polarization optics select a phase-mostly modulation mode. The SLM is imaged onto the entrance pupil of the objective with a 1:3 demagnifying lens system (not shown). The objective is overfilled; we only use segments that fall inside the pupil. The shaped wavefront is focused on the strongly scattering sample (S) and a CCD-camera images the transmitted intensity pattern. λ/4, quarter wave plate. λ/2, half wave plate. M, mirror. BS, 50% non-polarizing beam splitter. P, polarizer. match the incident wavefront to the sample, so that the scattered light is focused in a specified target area (Fig. 3.1b).