Molecular light sources sensitive to environment

July 30, 2010
(a) In a transparent material, identical molecules all emit the same number of photons per second, which means that they produce the same amount of light (b) Opaque materials, such as paint and organic tissue, are a maze for photons. The molecules of these materials emit variable numbers of photons per second.

A Dutch-French team of scientists led by FOM (Foundation for Fundamental Research on Matter) researcher Dr Danang Birowosuto and University of Twente researcher Dr Allard Mosk has obtained the first experimental evidence that fluorescing molecules behave differently in opaque, light-scattering materials than they do in transparent materials.

Although this effect was predicted on a theoretical basis twenty years ago, it has never before been observed. An understanding of this process is important for the design of for energy-efficient lighting, powerful microscopes and efficient . These researchers published their findings in the prestigious journal .

At the nanoscale, fluorescing molecules behave as highly-efficient light sources. 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 (photons) per second.

Maze for photons

In many applications, however, these molecules are not contained in a transparent material. The white, light-scattering phosphorous layer in energy-saving lamps and white LED lamps is an opaque milky colour, for example. This is because the material forms a light-scattering maze, in which photons regularly switch direction. In the 1990s, it was predicted that the molecules of such materials would emit variable amounts of light. Dependent on the way in which the surrounding, light-scattering nanoparticles are arranged, some molecules emit more photons per second than others.

The process by which a molecule emits a photon (spontaneous emission) is influenced by the nano-environment, and it is this effect that gives rise to the variation in . Dependent on its size and position, a light-scattering particle at a distance of several nanometres can either make it easier or more difficult for a molecule to emit a photon.


These scientists from the MESA+ Institute for Nanotechnology at the University of Twente and from Grenoble University in France were the first to obtain clear experimental evidence that these light sources varied in this way. The experiments were performed using nanobeads filled with fluorescent molecules. These nanobeads were just 25 nanometres in diameter - more than a million times smaller than a human cell. Sensitive measurements were able to detect the beads, even when they were surrounded by a myriad of light-scattering particles.

After mixing the nanobeads with strongly scattering materials made of polystyrene and zinc oxide (a paint pigment), the researchers measured the amount of light emitted per second. In a transparent medium, this quantity was the same for each nanobead. In light-scattering media, however, the amount of light emitted varied considerably. The greater the light-scattering effect of the medium, the greater the variability. On the basis of these measurements, the researchers developed a new model to explain the range of this variability.

This result has increased our understanding of how light is emitted in light-scattering materials. In addition to improving existing light sources, this knowledge can be used in the development of new imaging techniques for studying biochemical processes in cells.

Explore further: Emission of light under control: "lifetime-warp" in a photonic crystal

More information: 'Observation of spatial fluctuations of the local density of states in random photonic media' published online by Physical Review Letters:

Related Stories

Light Scattering Method Reveals Details under Skin

April 12, 2005

A new optical method that can image subsurface structures under skin has been demonstrated by scientists at the National Institute of Standards and Technology (NIST) and the Johns Hopkins University Applied Physics Laboratory. The ...

Mass weddings -- NIST's new efficient 2-photon source

April 12, 2007

For a variety of applications in physics and technology, ranging from quantum information theory to telecommunications, it’s handy to have access to pairs of photons created simultaneously, with a chosen energy. In a significant ...

How to see through opaque materials

March 8, 2010

New experiments show that it's possible to focus light through opaque materials and detect objects hidden behind them, provided you know enough about the material.

'Seeing' through paint

March 18, 2010

( -- When light passes through materials that we consider opaque, such as paint, biological tissue, fabric and paper, it is scattered in such a complex way that an image does not come through. "It is possible ...

Recommended for you

Just a touch of skyrmions

October 13, 2015

Ancient memory devices such as handwriting were based on mechanical energy—but in the modern world they have given way to devices based generally on electrical manipulation.

Perfectly accurate clocks turn out to be impossible

October 7, 2015

Can the passage of time be measured precisely, always and everywhere? The answer will upset many watchmakers. A team of physicists from the universities of Warsaw and Nottingham have just shown that when we are dealing with ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.