Research activities Photon Scattering
Research highlights 2005-2010
Exploiting wave front shaping (in collaboration with A. P. Mosk at Univ. of Twente and L. Kuipers at AMOLF)
It is our strong conviction that wave front shaping will revolutionize optics. Scattering makes it impossible to focus light within or through an optically diffuse medium by conventional means. We have demonstrated that for a stationary object, fine control over the phase of an array of thousands of channels of incident light makes it possible to compensate for any form of diffraction and of multiple scattering. Our group has been able to excite in this way the fluorescence of individual nanospheres in a fully opaque sample. In many media relevant for nanotechnology scattering occurs and this continuous redirection of light is thought to fundamentally limit the resolution and penetration depth of optical methods. We have demonstrated that scattering can be used to improve, rather than deteriorate, the sharpness of the focus. The resulting focus is even sharper than that in a transparent medium. very recently we have manipulated the wavefronts of surface plasmon polaritons connected to nanohole arrays using wave front shaping. Using phase optimization via feedback we focus SPPs at a pre-chosen point on the surface of the array with a high SPP numerical aperture. High resolution addressing of arbitrary spots on a metal surface offers the potential for optical data storage, multiplexing, sensing and other, without hardware translation.
Modes in open systems
The concept of a mode is at the heart of understanding propagation of light in open, complex systems. By introducing randomly distributed gain in a random system, the resulting random lasing act as a perfect monitor of the (cold) modes of the system. We have experimentally studied the distribution of the spatial extent of modes and the crossover from essentially single-mode to distinctly multimode behavior inside a porous semi-conductor random laser. This system serves as a paragon for random lasers due to its exemplary high index contrast. In the multimode regime, we observed mode competition. The experimental parameters that influence the type of modes that lase are observed, are identified. The spatial structure of random laser modes is accessed experimentally by a spatially (confocal) and spectrally selective detection scheme. We show that the modes that lase are distinct from laser speckle.
The speed of light under pressure
Measuring how fast light travels is not so easy, but inside opaque materials, like clouds, bone, skin, or paint the problem is particularly daunting. We have teased out this rate of transport using a simple, yet novel, trick: changing the ambient pressure. As the pressure is slowly tuned so is the so-called effective refractive index, which determines the speed of light. even though the light is following an extremely complex path as it bounces through the material, its speed can be characterized by tracking the influence of pressure on the outgoing light intensity pattern. So simple and direct is the technique that it offers an entirely new way for probing inside important biological
materials such as bone or wood as well as complex photonic materials such as photonic crystals or metamaterials. We have developed a rigorous theory, valid for any scattering strength, that maps the dependence on effective index on the dependence of the optical frequency.
Future directions
Wave front shaping (in collaboration with A. P. Mosk at Univ. of Twente, M. Fink and A. Tourin of LOA in Paris, and L. Kuipers at AMOLF)
We will generalize the technique of wave front shaping to include the creation of virtual light sources and the development of time reversal in various types of complex nanophotonic environments, from fully random - including random lasers - to structures supporting plasmon polaritons. We were already successful in applying wave front shaping to selectively optimize the transmission through light paths of a chosen length through the scattering medium. By that, we are able create a short pulse on the transmission side, much more intense than the randomly scattered light.
Sources and sinks (in collaboration with B.A. van Tiggelen, Grenoble and with D. S. Wiersma, Florence, Italy)
In our opinion the importance of characterizing embedded light sources and sinks is grossly underestimated in nanophotonics. The surroundings of sources and sinks determine the power balance and the feedback into the source. We will create and study sources of (quantum) light inside a complex nanophotonic host, from fully random to a fully designed structure with direct laser writing, using nonlinear optics, either through second harmonic generation (SHG), through two-photon luminescence (TPL), and by using (approximately) linear sources as quantum dots, dye molecules and cold atoms. We seek to observe the influence of the complexity of the nanophotonic environment on sinks (absorption processes), including Förster energy transfer, which is just absorption in disguise. Absorption is a highly disliked phenomenon in the community of nanophotonics, and for that reason its infl uence is either fully neglected or handled in an oversimplified way.
Knowledge transfer to industry and society
We collaborate with AKZO-Nobel and with Netherlands Institute for Cultural Heritage on light scattering in paint. We collaborate with Amsterdam University Press in the development of a combined web site for scientists. We collaborate with Shell in a Shell-FOM IPP project Innovative physics for oil and gas in which we investigate analogies between light and soud in complex media.
