Opaque Lens

Physicists at the University of Twente in the Netherlands are able to focus a beam of light by sending it through an opaque medium. Normally an opaque substance, such as milk or paint, will only serve to scatter light waves. But by carefully sculpting the incoming laser beam-processing tiny portions of the forward-moving wavefront---the Dutch researchers were able to focus the beam to an intensity 1000 times brighter than for the normal diffuse transmission at that same point previously.

They do this by first sending the light through the medium and, with a CCD camera, recording the attenuation of the beam at various points behind the sample. From this one can calculate scattering coefficients corresponding to the degree of statistical scattering across the face of the sample. An optical device, called a phase modulator, is used to anticipate and correct for the scattering yet to occur on a point-by-point basis.

To see a movie of the before- and after-modulator light scattering, go to cops.tnw.utwente.nl/research/animation_focus/speckle.html. The titanium-oxide sample normally admits light at the 10% level (through the entire sample), but with the modulator in place, the transmission goes way up.

One of the Twente researchers, Ivo Vellekoop (i.m.vellekoop@tnw.utwente.nl, 31-53489-5390), says that the diffusion of light in a dispersive medium is, after this, not an inevitability, and that spectroscopy and microscopy (even of single cells embedded in human tissue) should be greatly improvable. (Vellekoop and Mosk, Optics Letters, 15 August 2007)