Molecular light sources sensitive for environment
A Dutch-French team of scientists led by FOM researcher Dr. Danang Birowosuto and FOM workgroup leader Dr Alard Mosk have performed experiments which demonstrate for the first time that fluorescing molecules in opaque, scattering materials behave differently than in transparent materials. This had already been predicted theoretically twenty years ago but had never been observed to date. Understanding this process is important for the design of nanomaterials for energy-efficient lighting, powerful microscopes and efficient solar cells. The scientists published their results online on 2 July 2010 in the prestigious journal Physical Review Letters.
Fluorescing molecules behave as highly-efficient light sources at the nanoscale. That is why they are frequently used in energy-efficient lighting, computer screens and imaging techniques in the biomedical sciences. In transparent materials identical molecules emit the same number of light particles (photons) per second. Maze for photons
Yet in many applications the molecules are not in a transparent material. The white scattering phosphorous in energy-saving lamps and white LED lamps is a milky opaque colour, for example. That is because the material forms a maze for photons; the light particles are scattered and the direction they move in regularly changes. In the 1990s it was predicted that in such materials the emission of light by the molecules would be variable. Dependent on how they are surrounded by scattering nanoparticles, some molecules would emit more photons per second, whereas others would emit less.
This variability arises because the process in which a molecule emits a photon (spontaneous emission) is sensitive for the nano environment. Dependent on its size and position, a scattering particle at a distance of several nanometres can make it easier or more difficult for a molecule to emit a photon.
Nanobeads
The scientists from the MESA+ Institute for Nanotechnology at the University of Twente and from the University Joseph Fourier in France carried out experiments that clearly demonstrated this variability of the light sources for the first time. The experiments were performed using nanobeads filled with fluorescent molecules. Each nanobead had a diameter of no more than 25 nanometres – more than a million times smaller than a human cell. With the aid of sensitive measurements, the beads could even be detected in the middle of a myriad of scattering particles.
The nanobeads were mixed with strongly scattering materials made of polystyrene and zinc oxide (a paint pigment). The researchers subsequently measured the amount of light emitted per second. In a transparent medium that was the same for each nanobead. But in the scattering materials the amount of light emitted varied considerably. The stronger the medium scattered, the greater the variability. Based on the measurements, the researchers developed a new model to explain the strength of this variability.
This result has increased our understanding of how light is emitted in scattering materials. This knowledge can be used to improve existing light sources but also, for example, to develop new imaging techniques to study the biochemical processes in cells.
Reference
'Observation of spatial fluctuations of the local density of states in random photonic media' published online by Physical Review Letters: http://link.aps.org/doi/10.1103/PhysRevLett.105.013904
The research was performed by junior scientist Dr Danang Birowosuto, supported by Prof. Willem Vos, and workgroup leader Dr Allard Mosk from the group Complex Photonic Systems (COPS, www.photonicbandgaps.com, MESA+ Institute for Nanotechnology, University of Twente, Enschede, and researcher Dr Sergey Skipetrov from the University Joseph Fourier and CNRS in Grenoble, France.
The research was funded by the Foundation for Fundamental Research on Matter (FOM), the Netherlands Organisation for Scientific Research (NWO) and CNRS.
Information
Dr. Allard Mosk, University of Twente, Enschede, the Netherlands, phone +31 (0)53 489 53 90.