Ultrafast All-optical Switching and Laser Action in Rotaxane

Abstract

Linear and non-linear optical properties of conjugated polymers are often masked by the inter-chain network in solid state. The formation of aggregates may trap excitons, reduce oscillator strength and modify relaxation processes. The control of the inter-chain interaction is the main reason for developing “threaded” polymers, where supra-molecular encapsulation should reduce aggregation. Here, we investigate the influence of the encapsulation with β-cyclodextrin (β-CD) macrocycles on the photophysics of the polyfluorene-alt-biphenylene (PFBP) using femtosecond non-linear spectroscopy. Upon threading we observe enhancement of the stimulated emission (SE) in the visible range and reduction of the charge absorption. These phenomena are ascribed to the reduced inter-chain interaction. In more isolated chains the dynamics of intra and inter-chain charge states are distinguished. In addition, we performed three-beam experiments in which a first pulse (pump) creates singlet excited states; a second (push) pulse re-excite the singlet state and a broadband probe pulse detects the induced changes in transmission. This technique shows: (i) charges are generated from higher lying singlet states also in isolated chains (ii) ultrafast optical gain switching is possible in threaded chain. Finally, we demonstrate that ASE occurs in films of threaded polymers and lasing can be achieved with much lower threshold than the neat polymer chain in the DFB configuration. All our findings point out the potential role of rotaxanes in photonics, as amplifiers and reopen the route to the electrically pumped organic lasers and all-optical logic devices.