Making organics shine
By coupling light to matter, physicists of the Surface Photonics group at Photonics and Semiconductor Nanophysics are unlocking new material properties. Recently, they published a paper in Nano Letters describing a technical breakthrough that brings the idea of electrically driven laser-like light emission from organic devices an important step closer.
Before diving into the details of their paper, postdoc researcher Matthijs Berghuis and his supervisor Jaime Gómez Rivas first put their research into context. Demonstrating two set-ups of pendulums, Gómez Rivas shows what happens when two systems are coupled: ‘When I have two individual pendulums moving independently from each other, if I add energy to one of the pendulums by pushing it, only that pendulum will start to move. But when I do the same in a system where both pendulums are coupled, not only the second pendulum will also start to move, but its movement will in turn influence the behavior of the first pendulum: energy is exchanged between the two at a certain rate. The same thing happens when you couple light to matter: the resulting system will have a mix of the properties of both the light and the matter.’
For excitons – bound states of negatively charged electrons and positively charged ‘holes’ in a semiconductor – coupling them to light has some major advantages, Gómez Rivas lectures.
Jaime Gómez Rivas | Full Professor
‘Excitons carry electrical charges, but they cannot travel very far in material. Photons can travel easily, but do not carry any charge. When you combine the two you get a so-called exciton-polariton that is able to transport electrical charges over relatively large distances. This is for example of interest for applications in solar cells.’
Moving to organics
It is in this context that Berghuis and Gómez Rivas performed the experiments they reported about in their recent paper. ‘We wanted to explore the possibilities of light-matter coupling in organic materials,’ explains Berghuis, ‘since organics are a more sustainable and easier to produce alternative to the inorganic semiconductors that are currently used for electronics and photonics.’ What’s more, in organic materials, excitons are stable at room temperature, which makes experimenting with them relatively easy since no extensive cooling systems are needed. And any application will also be only possible at room temperature.
The physicists developed and fabricated a specially designed silicon nanoparticle array and combined it with a layer of polystyrene doped with light emitting organic molecules. The nanoparticles effectively construct an optical cavity, where the light is confined. The better the quality of this cavity, the more of the excitons will condense to the same quantum state before recombining and emitting light.
‘In our experiments, we managed to fabricate virtually perfect optical cavities,’ Berghuis explains. ‘The photons in the cavity are coupled to the excitons, which in turn decay. Since the excitons are in the same quantum state, the light that they emit is coherent, and thus resembles the light emitted by a laser, though the physical principle behind the light emission is entirely different from that of a laser.’
“We wanted to explore the possibilities of light-matter coupling in organic materials, since organics are a more sustainable and easier to produce alternative to the inorganic semiconductors that are currently used for electronics and photonics.”
Matthijs Berghuis | Postdoc Researcher
Lowering the threshold
Whereas such so-called polariton lasers already exist in inorganic materials, so far in organic materials it has not been possible to demonstrate electrically driven versions of these light sources. This is a prerequisite for organic polariton lasers to be integrated on a chip. The problem is that until now, the threshold for non-linear amplification of the light was so high, that the amount of energy you would have to put into the organic molecules would effectively break them. ‘With our nearly perfect optical cavities, we have significantly brought down this threshold,’ Gómez Rivas explains the significance of the findings. Though in their current system, it is not yet possible to electrically pump the laser, both researchers are convinced that they are on the right track to get there. ‘With this system we have provided a first proof of principle that by designing a dielectric metasurface of nanoparticles, it is possible to construct perfect optical cavities. These enable room temperature condensation of the excitons, leading to lower lasing thresholds. By playing around with other materials and nanopatterns, we are convinced there is still a lot to gain.’
And that is exactly what the next steps are going to be, they explain. ‘Together with the Institute for Complex Molecular Systems, we are looking into other organic molecules, since at the moment, it is those molecules that are the limiting factor in terms of the wavelengths we can emit,’ says Gómez Rivas. ‘We are also looking into possibilities of fabricating more complex nanostructures to confine the light waves in, such as chiral cavities that can result in polarized cavity modes,’ Berghuis adds.
The image above is showing the density distribution of exciton-polaritons when the total number of polaritons is increased from the left to the right panel. The left panel shows the distribution below the condensation threshold, the middle panel the distribution at the condensation threshold and the right panel above condensation threshold. From the image it is clear that above threshold all exciton polaritons condense to two points in the (energy-momentum) space which results in laser-like emission. The left panel is multiplied by a factor 105 and the middle panel by a factor 103 to increase the visibility.
Potential of cavities
Though this research is of a rather fundamental nature, there are plenty links with future applications, Gómez Rivas points out. ‘What we have now are high quality optical cavities based on an array of nanoparticles. Those can be used for a myriad of things. These types of particle arrays are for example also used in solid state lighting, which is an application that we are investigating with Lumileds. And perfect optical cavities are very relevant to enhance the sensitivity in optical sensors. The field of terahertz sensing is another topic our research group is actively investigating.’