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Opaque materials let light through

15 Aug 2007

How do you focus coherent light through daisy petals and other opaque scattering materials? optics.org speaks to researchers in the Netherlands who have an answer.

By constructing wavefronts that invert the diffusion of light, researchers in the Netherlands say that they can focus coherent light through opaque scattering materials. The result could aid a number of lines of research including spectroscopy in scattering media and metamaterials. (Optics Letters 32 2309)

"Light propagation in opaque materials such as biological tissue is dominated by diffusion," Ivo Vellekoop of the MESA+ Research Institute at the University of Twente told optics.org. "Since diffuse light has no preferential direction, it seems impossible to directionally transmit light through a multiple scattering material."

As Vellekoop and his colleague Allard Mosk discovered, the answer was to modify the incident wavefront of the laser beam. The method relies on interference: the incident light is manipulated in such a way that the scattered light interferes constructively to form a sharp focus after it has passed through the opaque material.

Vellekoop and Mosk start by illuminating the sample with a focused plane wave. Behind the sample, there is a random mixture of constructive and destructive interference leading to a small signal in the form of a speckle pattern.

"We divide the incident wavefront into segments and cycle the phase of each segment," explained Vellekoop. "This allows us to find the phase at which the light from this segment interferes constructively with the small initial background signal. Once we know the optimal phase for each segment, we program a phase modulator with the optimal incident wavefront and a single bright focus appears behind the sample."

The researchers have successfully tested their phase-altering algorithm on titanium-dioxide pigment, daisy petals, eggshell and a human baby tooth where the thicknesses varied from 10 microns to 1.5 mm. According to Vellekoop, the algorithm creates a spot that is up to 1000 times brighter that a normal diffuse transmission spot.

"The algorithm is fully automatic and performs a measurement for each segment and constructs the optimal wavefront," said Vellekoop. "The time required to construct the wavefront depends on the number of segments. A contrast of 10 can be achieved in a matter of seconds, while a contrast of 1000 takes over 15 minutes."

The team is now experimenting with other materials such as porous gallium phoshide. "We are starting work on wavefront optimization using local nanoscale probes which can be put inside tissue," concluded Vellekoop. "We are also investigating real time wavefront construction."

Jacqueline Hewett is editor of Optics & Laser Europe magazine.

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