Optical gain characteristics of β-phase poly(9,9-dioctylfluorene)

We report an investigation of the effect of morphology on the gain properties of poly(9,9-dioctylfluorene) (PFO). The PFO β-phase morphology has previously been reported to be detrimental to lasing threshold, a conclusion at odds, however, with pump–probe measurements on PFO/poly(methylmethacrylate) blend films that show enhanced stimulated emission characteristics for β-phase chains. In order to understand these conflicting indications, we have undertaken a detailed study of the gain properties for spin-coated PFO films, including samples in which the fraction of β-phase chains is deliberately enhanced by post-deposition exposure to toluene vapour. We find that the amplified spontaneous emission (ASE) threshold (390 nm pump, 10 ns pulses, 10 Hz repetition rate) is of order 80 nJ/pulse, independent of the presence of a significant β-phase component. Surface emitting distributed feedback lasers comprising polymer-coated second-order gratings etched into silica substrates are also insensitive to the β-phase morphology: lasing threshold energies are equivalent so long as the structures are tuned to the correct peak gain wavelength for each film morphology. This occurs at the 0–1 vibronic peak in the corresponding photoluminescence emission spectra, namely 465 nm for films with and 450 nm for films without a significant β-phase component. We can thus conclude that whilst the introduction of β-phase chains leads to new lasing wavelengths (some 15 nm red shifted from those for films without β-phase chains) it is not obviously detrimental to lasing performance. An additional effect does occur, however, when the pump beam energy is increased by one to two orders of magnitude above the ASE threshold energy: the ASE peak position for the β-phase films then migrates from 465 to 450 nm. This phenomenon is irreversible and appears to be the result of exciton quenching on β-phase chains due to the photo-oxidative formation of fluorenone moieties.

[1]  D. Bradley,et al.  On the optical anisotropy of conjugated polymer thin films , 2005 .

[2]  D. Moses,et al.  Excitation energy transfer from polyfluorene to fluorenone defects , 2004 .

[3]  Guglielmo Lanzani,et al.  Ultrafast resonant optical switching in isolated polyfluorenes chains , 2005 .

[4]  G. Bazan,et al.  Light Amplification by Optical Excitation of a Chemical Defect in a Conjugated Polymer , 2004 .

[5]  A. Köhler,et al.  Morphology-dependent energy transfer within polyfluorene thin films , 2004 .

[6]  M. Grell,et al.  Film morphology and photophysics of polyfluorene , 2000 .

[7]  David G Lidzey,et al.  A study of the different structural phases of the polymer poly(9,9′-dioctyl fluorene) using Raman spectroscopy , 2001 .

[8]  Piers Andrew,et al.  Blue, surface-emitting, distributed feedback polyfluorene lasers , 2003 .

[9]  Donal D. C. Bradley,et al.  Interplay of Physical Structure and Photophysics for a Liquid Crystalline Polyfluorene , 1999 .

[10]  Z. Vardeny,et al.  Film morphology and ultrafast photoexcitation dynamics in polyfluorene , 2002 .

[11]  Donal D. C. Bradley,et al.  Fluorene-based polymer gain media for solid-state laser emission across the full visible spectrum , 2003 .

[12]  D. Bradley,et al.  Thickness-dependent thermal transition temperatures in thin conjugated polymer films , 2006 .

[13]  Donal D. C. Bradley,et al.  Chain geometry, solution aggregation and enhanced dichroism in the liquidcrystalline conjugated polymer poly(9,9-dioctylfluorene) , 1998 .

[14]  R. Friend,et al.  Morphology dependence of the triplet excited state formation and absorption in polyfluorene , 2005 .

[15]  Donal D. C. Bradley,et al.  Nondispersive hole transport in an electroluminescent polyfluorene , 1998 .

[16]  Donal D. C. Bradley,et al.  Exciton migration in β -phase poly(9,9-dioctylfluorene) , 2003 .

[17]  D. Bradley,et al.  Temperature and field dependence of hole mobility in poly(9,9-dioctylfluorene) , 2006 .

[18]  G. Lanzani,et al.  Ultrafast intrachain photoexcitation of polymeric semiconductors. , 2005, Physical review letters.

[19]  P. Etchegoin,et al.  Ellipsometric Characterization of the Optical Constants of Polyfluorene Gain Media , 2005 .

[20]  D. Bradley,et al.  Raman Anisotropy Measurements: An Effective Probe of Molecular Orientation in Conjugated Polymer Thin Films , 2003 .

[21]  Daniel Moses,et al.  Stabilized Blue Emission from Polyfluorene‐Based Light‐Emitting Diodes: Elimination of Fluorenone Defects , 2003 .

[22]  A. Monkman,et al.  Triplet exciton state and related phenomena in the β-phase of poly(9,9-dioctyl)fluorene , 2004 .

[23]  Donal D. C. Bradley,et al.  Light amplification and gain in polyfluorene waveguides , 2002 .

[24]  Michael Inbasekaran,et al.  Influence of aggregation on the optical properties of a polyfluorene , 1997, Optics & Photonics.

[25]  Donal D. C. Bradley,et al.  Fluorene-based conjugated polymer optical gain media , 2003 .

[26]  David G Lidzey,et al.  Influence of film morphology on the vibrational spectra of dioctyl substituted polyfluorene (PFO) , 2000 .

[27]  Donal D. C. Bradley,et al.  A glass‐forming conjugated main‐chain liquid crystal polymer for polarized electroluminescence applications , 1997 .

[28]  B. Larson,et al.  Near-term aging and thermal behavior of polyfluorene in various aggregation states , 2004 .

[29]  D. Huber,et al.  Structure, photophysics, and the order-disorder transition to the β phase in poly(9,9-(di-n, n-octyl)fluorene) , 2002, cond-mat/0211610.

[30]  V. Sundström,et al.  Lasing in a Microcavity with an Oriented Liquid‐Crystalline Polyfluorene Copolymer as Active Layer , 2001 .

[31]  Donal D. C. Bradley,et al.  Thermodynamic constants for excimer formation and dissociation in oxidized poly(9,9-dioctylfluorene) (PFO) , 2004, SPIE Optics + Photonics.

[32]  Donal D. C. Bradley,et al.  The effect of morphology on the temperature-dependent photoluminescence quantum efficiency of the conjugated polymer poly(9, 9-dioctylfluorene) , 2002 .

[33]  Donal D. C. Bradley,et al.  Exploring the potential of ellipsometry for the characterisation of electronic, optical, morphologic and thermodynamic properties of polyfluorene thin films , 2005 .

[34]  Donal D. C. Bradley,et al.  Spectral narrowing phenomena in the emission from a conjugated polymer , 1998 .

[35]  R. Cingolani,et al.  Microscopic investigation of the poly(9,9-dioctylfluorene) photoluminescence dependence on the deposition conditions by confocal laser microscopy , 2006 .

[36]  Piers Andrew,et al.  Emission Characteristics and Performance Comparison of Polyfluorene Lasers with One‐ and Two‐Dimensional Distributed Feedback , 2004 .

[37]  D. Bradley,et al.  Mobility enhancement through homogeneous nematic alignment of a liquid-crystalline polyfluorene , 1999 .

[38]  Wolfgang Kowalsky,et al.  Threshold Reduction in Polymer Lasers Based on Poly(9,9‐dioctylfluorene) with Statistical Binaphthyl Units , 2005 .

[39]  M. Grell,et al.  Highly polarized blue electroluminescence from homogeneously aligned films of poly(9,9-dioctylfluorene) , 2000 .

[40]  D. Bradley,et al.  Significant improvements in the optical gain properties of oriented liquid crystalline conjugated polymer films , 2005 .

[41]  D. Huber,et al.  Chain conformations and photoluminescence of poly(di-n-octylfluorene). , 2004, Physical review letters.

[42]  M. Grell,et al.  Understanding the Origin of the 535 nm Emission Band in Oxidized Poly(9,9‐dioctylfluorene): The Essential Role of Inter‐Chain/Inter‐Segment Interactions , 2004 .

[43]  D. Bradley,et al.  On the use of optical probes to monitor the thermal transitions in spin-coated poly(9,9-dioctylfluorene) films , 2005 .

[44]  D. Bradley,et al.  Investigation of amplified spontaneous emission in oriented films of a liquid crystalline conjugated polymer , 2003 .

[45]  M. Grell,et al.  Completely polarized photoluminescence emission from a microcavity containing an aligned conjugated polymer , 2001 .