Ultrafast all-optical switching and laser action in rotaxane
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1230-MM04-09
Ultrafast all-optical switching and laser action in rotaxane Marta M. Mroz1, Stefano Perissinotto1,2, Tersilla Virgili1, Marco Salerno2, Giuseppe Sforazzini3, Harry L. Anderson3 and Guglielmo Lanzani1,2 1 IFN-CNR, Dipartimento di Fisica, Politecnico di Milano, P.zza L. Da Vinci 32, 20133 Milan, IT 2 IIT, Italian Institute of Technology, via Morego 30, Genova 16163 (IT) 3 Chemistry Research Laboratory, Department of Chemistry, University of Oxford, Mansfield Road, Oxford OX1 3TA (UK) 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 βcyclodextrin (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. INTRODUCTION Conjugated polymers (CPs) have found widespread interest for application in organic light emitting devices (OLED), since they promise several advantages compared to their inorganic counterparts. Moreover the organic semiconductors have attracted much interest as an gain media in devices such as lasers, optical switches, and broadband amplifiers thanks to their borad emission range, high efficiency etc. CP combine the scope of simple shaping and manufacturing of plastics with good optoelectronics properties. Optical gain in CPs has been observed in a number of experiments such as pump-probe (PP), amplified spontaneous emission (ASE), and lasing. Like other lasers, organics semiconductor laser consists of gain medium and resonator. Out of a large variety of configurations distributed feedback (DFB) grating is widely used for testing and demonstrating lasing properties. Almost all rep
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