Demixing light paths inside disordered metamaterials

Abstract

We experimentally demonstrate the first method to focus light inside disordered photonic metamaterials. In such materials, scattering prevents light from forming a geometric focus. Instead of geometric optics, we used multi-path interference to make the scattering process itself concentrate light on a fluorescent nanoscale probe at the target position. Our method uses the fact that the disorder in a solid material is fixed in time. Therefore, even disordered light scattering is deterministic. Measurements of the probes fluorescence provided the information needed to construct a specific linear combination of hundreds of incident waves, which interfere constructively at the probe.

Controlling waves in space and time for imaging and focusing in complex media

“…The first demonstrations of using interference to control multiply scattered light were made by Vellekoop and co-workers 39 , initially by focusing light transmitted through a layer of strongly scattering material and soon after by focusing light onto a fluorescent probe deep inside a strongly scattering layer 40 . They optimized thousands of degrees of freedom of the incident wavefront using the measured intensity at one -or several -targets as a feedback signal.…”

“…The wavefront-shaping procedure used by Vellekoop and coworkers 39,40 is mathematically equivalent to phase conjugation 44 . In 1966, Leith and Upatnieks gave a dramatic demonstration of how phase conjugation by holographic methods can counteract phase scrambling due to single scattering 8 .…”

Wavefront corrected light sheet microscopy in turbid media

“…For probes embedded within a turbid medium, such as fluorescent bio-markers in biological tissue, this technique provides full correction for both system and specimeninduced aberrations. Probe based wavefront measurement and correction has been demonstrated utilizing non-linear harmonic-generating particles, 12 direct imaging of the beam itself, 10 and on embedded fluorescent probes using interferometric 13 and Shack-Hartmann sensing. 14 An advantage of our method is its applicability to any type of probe; all that is required is that the probe produces an intensity signal.…”

Optimal concentration of light in turbid materials

“…[3][4][5][6][7][8] Although light scattering is detrimental to imaging, it is recently shown that scattering can be exploited to increase the amount of light energy deep inside turbid materials. [9] By spatially shaping the wave front of the incident light, the emission of a small dyed probe sphere hidden inside the turbid layer was strongly enhanced. Despite the fact that this enhancement proves an increase of excitation intensity at the probe position, it remains unclear what the spatial distribution of the excitation light is.…”

“…[14,15] Using a wave front synthesizer, similar to the one discussed in Ref. [9], we spatially divide a monochromatic laser beam (λ = 532 nm) into up to 640 square segments of which we individually control the phase. The shaped beam is focussed onto our sample using a microscope objective (NA = 0.95).…”