Net Gain Dynamics in Actively Driven Organic Microdevices

• Applicant: Dr. Hartmut Fröb
• Subject Area: Experimental Condensed Matter Physics ; Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: from 2016 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 319139519

The effect of field enhancement and squeezing of the operating volume in organic microcavities may lead to a drastically reduced lasing threshold. At the same time, the balance between the gain due to population inversion and various loss mechanisms and the photon number within the whole microresonator can change substantially. While the photonic losses are essentially defined by the geometry of the microresonator and optical quality of the materials, the excitonic losses are solely defined by the active gain medium. Moreover, when the mode volume is artificially squeezed and start to approach (lambda/n)³, the organic gain medium is often not capable to respond linearly due to the limited number of invertible states and population inversion starts to saturate. Therefore, for optically- or electrically-pumped organic microdevices, it is important to balance optical gain and absorption losses directly within the device in order to reduce pumping power to a level which organic materials can withstand. Therefore, a detailed understanding and knowledge of absorption and gain dynamics (both as a function of pump power and time) is of crucial importance and could lead to new prototypes of high quality organic solid state light microdevices.In this proposal, we develop a novel technique to investigate net gain dynamics of actively driven organic solid state light microdevices. It is based on a spectral analysis of light emission (or transmission) from the resonant mode. Our preliminary results show that emission spectra strongly depend on the optical constants of the materials used and show pronounced spectral narrowing with the pump power. We can reliably translate this dependence into the net gain dynamics in the organic gain medium using transfer matrix analysis. Mixing the proposed gain measurement technique with the possibility to apply current allows direct measurement of the charge-induced (polaronic) losses inside the cavity which is known to be one of the most critical points preventing observation of lasing in electrically driven systems. Besides the net gain characterization in actively driven organic light-emitting diodes or microlasers, we envision this method as a perfect tool to investigate modern and complex systems such as PT synthetic photonic structures or, for example, coherent perfect absorption in organic media.

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr. Karl Leo