Light emission from organic molecules is promising for applications ranging from fluorescence imaging to data storage. To overcome intrinsically weak emission efficiencies, Shin-ichiro Inoue and Shiyoshi Yokoyama from Kyushu University in Japan1 now demonstrate that photonic crystals can be used to significantly enhance non-linear optical emission from fluorescent molecules.

The advantage of non-linear optical processes compared to conventional light-emission techniques is that the intensity varies strongly with the excitation energy, which produces higher definition images. The high resolution that can be achieved is not only relevant for imaging, but can also be used in optical data storage where it allows for a higher data integration density.

One of the most widely used non-linear processes is two-photon excited fluorescence (TPEF), where the combined energy of two low-energy photons from a laser beam is used to excite an electron into an higher state. As the excited electrons relax back to the ground state, they emit the energy gained and this fluorescence is detected.

Fig. 1: The two-photon excited fluorescence process from organic molecules embedded within a polymer layer can be significantly enhanced by a photonic crystal at the surface.

Unfortunately, “despite the important technological potential of this method, the majority of known organic molecules have very small two-photon excitation efficiencies, hampering the widespread use of TPEF applications”, says Inoue. To solve the problem of low efficiencies, the researchers use a photonic crystal that is placed on top of a thin polymer layer containing the fluorescent molecules (Fig. 1).

The photonic crystal facilitates the coupling between the incident laser and the molecules. At certain laser incident angles this can lead to a strong coupling of light into the photonic crystal. This subsequently increases the light absorption by the molecules, leading to a much stronger TPEF process.

Indeed, in their experiments Inoue and Yokoyama observed a significant enhancement of TPEF at the predicted laser beam angles. These enhancements can be as high as two orders of magnitude. Therefore, much lower and less damaging incident laser powers than in conventional TPEF experiments can be used, making this technique particularly suitable for molecules with low TPEF efficiencies.

Inoue is very confident that “this dramatic improvement in performance will stimulate a number of applications, for example in optical data storage”.