Multiband Laser Action from Organic-Organic Heteroepitaxial Nanofibers
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Multiband Laser Action from Organic-Organic Heteroepitaxial Nanofibers Francesco Quochi1, Francesco Floris2, Clemens Simbrunner3, Guenther Schwabegger3, Michele Saba1, Andrea Mura1, Helmut Sitter3, and Giovanni Bongiovanni1 1
Department of Physics – University of Cagliari, S.P. Monserrato-Sestu Km 0.7, I-09042 Monserrato (CA), Italy. 2 Department of Physics – University of Pavia, Via Agostino Bassi 6, I-27100 Pavia (PV), Italy. 3
Institute of Semiconductor and Solid State Physics, University Johannes Kepler of Linz, Austria
ABSTRACT We report successful tuning of laser wavelength from ~420 nm to ~600 nm in epitaxially aligned nanofibers grown by periodic deposition of para-sexiphenyl (p6P) and sexithiophene (6T) on p-6P/muscovite mica templates. The nanofibers were photoexcited by subpicosecond pulses tuned to the lowest p6P absorption band, and the emission of 6T, whose coverage was kept in the submonolayer regime, was efficiently sensitized through resonance energy transfer (RET). The 6T lasing was achieved at room temperature with threshold fluences as low as 10 2 J/cm per pulse. Transient photoluminescence measurements, with picosecond resolution, showed that at these pump fluences the decay dynamics of 6T emission is independent of the excitation density, thereby demonstrating the attainment of room-temperature monomolecular lasing from epitaxially oriented 6T submonolayer aggregates. Main lasing properties remained unaltered upon direct photoexcitation of 6T below the p6P absorption edge. INTRODUCTION Vacuum deposition of para-sexiphenyl (p6P) on muscovite is known to yield linear aggregates of co-oriented nanocrystals, usually referred to as nanofibers [1-3], having precise epitaxial relationships to the muscovite substrate [4]. These epitaxial nanofibers exhibit important optical properties, such as highly polarized blue luminescence and laser action [5-9], which could be exploited for applications in photonic and sensing technologies. Recent advances in the field include the achievement of highly polarized, red-green-blue (RGB) emission from epitaxially aligned bilayer nanofibers obtained upon deposition of sexithiophene (6T) on p6P/muscovite templates, and the development of a roll-printing technique for high-yield and massive transfer of nanofibers from the native substrate to a receiving substrate for device applications [10]. EXPERIMENT All nanofiber samples were grown on muscovite mica substrates (SPI, Structure Probe, Inc.) by Hot-Wall Beam Epitaxy (HWBE). The growth chamber was equipped with two HWE reactors for p6P (from TCI) and 6T (from Sigma-Aldrich) deposition. Optimized evaporation temperature was 240°C for p6P and 190 °C for 6T. The substrate was pre-heated for 30 min before deposition of the p6P fiber template. Samples were fabricated with different substrate temperatures ranging from 90 to 170 °C.
All the samples were optically pumped by 150-fs pulses delivered by a Ti:Sapphire regenerative amplifier (Quantronix Integra C) working at a repetition rate of 1 kHz, focused to a circular sp
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