Tunable Optical Gain from Soluble Thiophene-Based Oligomers
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Tunable Optical Gain from Soluble Thiophene-Based Oligomers M. Anni1, G. Gigli1, M. Zavelani-Rossi2, C. Gadermaier3, G. Lanzani2, G. Barbarella4, L. Favaretto4and R.Cingolani1 1 Istituto Nazionale Fisica della Materia (INFM), Dip. Ingegneria dell’Innovazione, Università di Lecce, Via per Arnesano, 73100 Lecce, ITALY 2 Dip. Fisica, Politecnico di Milano, Piazza L. Da Vinci 32, 20133 Milano, ITALY 3 Dip. Matematica e Fisica, Università di Sassari, Via Vienna 2, 07100 Sassari, ITALY 4 Consiglio Nazionale delle Ricerche (CNR), ICOCEA, Area della Ricerca di Bologna, Via Gobetti 101, 40129 Bologna ITALY
ABSTRACT Tunable optical gain is demonstrated, through pump and probe measurements, in spin coated neat films of soluble S,S-dioxide oligothiophenes with different functionalization pattern. The molecular functionalization with alkyl and aryl groups allows us to obtain photoluminescence efficiency up to 70%, good chemical stability, high solubility and tunable gain along the visible range from 470 nm to 660 nm. The gain cross section is up to 9*10-18 cm2. The gain relaxation dynamic is studied through time resolved pump and probe measurements. A comparison with some of the best organic materials for laser application is presented. From these results we conclude that thienyl-S,S-dioxide oligothiophenes are interesting materials for applications to organic lasers and amplifiers. INTRODUCTION Organic conjugated molecules have received an increasing attention in the last decade due to their potential applications in electronic and optoelectronic devices, such as Field Effect Transistors (FETs) and Light Emitting Diodes (LEDs). After the first demonstration of stimulated emission in neat films of poly(para-phenylene-vinylene) (PPV) and PPV derivatives [1-5] great attention has been devoted to the research of good active material for solid state organic lasers. This has led to the demonstration of optical gain in many different conjugated materials [6-11] and of lasing in optically pumped devices with different cavity geometry [1219]. Recently the first evidence of lasing in an electrically injected device [20] based on ultrapure organic single crystals has been reported. Compared with conventional laser materials, soluble conjugated molecules offer important advantages. One of the main advantages stands in low cost deposition techniques such as spin coating or direct printing, eventually on flexible large area plastic substrates. Moreover the usually broad density of states and the strong electronphonon coupling give rise to broad gain spectra, which can be exploited to obtain spectral tuning of the laser emission [21]. To date, despite their high chemical stability and their emission tunability, thiophene oligomers have not received great attention, due to their poor luminescence in the solid phase and their lack of solubility. The study of optical amplification on thiophenes is in fact limited to single crystals of short insoluble molecules, not useful for device applications [22]. However, we recently reported on a new clas
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