Metallic and dielectric microcavities can be used to boost light emitting diodes. Now, researchers in Italy have demonstrated an ultra strong coupling between molecular excitons and light in an electroluminescent organic microcavity. The phenomenon leads to a twelvefold narrowing of the electroluminescent emission compared with the corresponding photoluminescence. Such a discovery bodes well for developing organic microcavities for applications in polariton lasing, enhanced luminescence, all-optical circuits and photonic quantum devices.
Salvatore Gambino of the CNR NANOTEC - Istituto di Nanotecnologia and Dipartimento di Matematica e Fisica "Ennio De Giorgi", Università del Salento - and colleagues Armando Genco, Gianluca Accorsi, Omar Di Stefano, Salvatore Savasta, Salvatore Patanè, Giuseppe Gigli, and Marco Mazzeo, explain how many microcavities operate in the weak coupling regime, wherein energy is lost quickly and so efficiency is not as high as it might otherwise be. They point out that if coupling can be boosted then the energy exchange rate between light and matter becomes faster than any other energy dissipation process and new phenomena begin to emerge.
The team has focused on the squaraine dye, 2,4-bis[4- (N,N-diisobutylamino)-2,6-dihydroxyphenyl], which is a thermally evaporable material that can be embedded in a multi-layer metal-metal (two silver mirrors) microcavity in which it acts as an emitting/coupling organic material. Organic dopants and an inorganic dopant (cesium metal) complete the device. By varying the thickness of the squaraine dye film from 20 to 60 nanometers, the team was able to tune the light-matter coupling, with a change over that thickness range in "g" (the light-matter coupling factor) from 31 to 48 percent.
The researchers then used ellipsometer transmission measurements at different incidence angles of transverse-electric (TE) polarized white light to optically characterize their device. Their experimental data finely match the simulated upper and lower polariton branches, the team reports [Gambino et al, Appl Mater Today, 2015, 1, 33-36; DOI: 10.1016/j.apmt.2015.08.003]
"Our device structure ensures an efficient electron–hole recombination within the emissive/coupling layer," the team says. "Thus, we were able to obtain polariton devices displaying quite low turning on voltages and still being in ultra strong coupling (USC) regime despite the thin thickness of the active layer." They add that, "An analogous USC regime and line-width reduction has been shown theoretically in hybrid metal-squaraine nanoshells." This they suggest points to the viability of the USC regime for the future development of ultra-compact photonic devices.
Gambino told Materials Today that the next step will be "to study room temperature Bose-Einstein Condensation (BEC) in organic optoelectronic devices under electrical pumping as a first step towards organic polariton lasing."
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling popular science book "Deceived Wisdom".