Ellipsometric Characterization of the Optical Constants of Polyfluorene Gain Media** By Mariano Campoy-Quiles , George Heliotis , Ruidong Xia, Marilu Ariu, Martina Pintani, Pablo Etchegoin, and Donal D. C. Bradley* 1. Introduction Semiconducting (conjugated) polymers are now attracting considerable attention as a new class of materials for use in electronics and optoelectronics. Following encouraging early successes in the commercialization of electroluminescent poly- mer light-emitting diode (LED)-based displays, there is cur- rently increasing interest being paid to the development of these materials as novel gain media for devices such as lasers and optical amplifiers. [1±3] In particular, an important target is thefuturerealizationofanelectricallypumpedsolid-statepoly- mer laser diode. Even though issues of long-term stability still persist, efficient lasing has been achieved across a wide spectral range (400±800 nm) by optical pumping of various resonator structures. However, electrically driven devices still represent a major challenge for research and development. Fluorene-based polymers (polyfluorenes) are a particularly attractive class of semiconducting polymer for such applications because they show excellent charge-transport properties, have low stimulat- ed-emission thresholds, and their gain spectra can be tuned to covertheentirevisiblespectrum. [4±7] Spectroscopic ellipsometry has been recently employed to determine the optical constants of conjugated polymers and related materials used in organic devices. [8±16] These studies are often focused on morphological issues. For example Miller et al. [12] have characterized the in-plane ordering induced optical birefringenceofpoly[2-butyl-5-(e¢-ethyl)hexyl-1-1,4-phenylene vinylene], BuEH-PPV, a conjugated polymer that exhibits am- plifiedspontaneousemission(ASE).Fromtheordinaryandex- traordinary refractive indices in the transparency region they deduced the TE (transverse-electric) and TM (transverse-mag- netic) cut-off thicknesses for BuEH-PPV slab waveguides com- prised of the polymer coated on a fused silica substrate and with an air cladding. Tammer et al. [11] have also examined opti- cal birefringence in thin films and have addressed the issue of the influence that it has on the outcoupling efficiency from LEDs. This paper reports the results of a spectroscopic ellipsometry study of the optical properties of conjugated-polymer thin (» 100±200 nm) films: in all, five different polymers have been investigated, namely poly(9,9-dioctylfluorene) (PFO), poly(9,9- dihexylfluorene) (F6), poly[(9,9-dioctylfluorene)-co-(9,9-di(4- methoxy)phenylfluorene)] (F8DP), poly[(9,9-dioctylfluorene)- co-(benzothiadiazole)] (F8BT) (see Scheme 1 for the corre- sponding chemical structures), and a proprietary red-emission copolymer, known as Dow Red F. Specific points addressed in thepaperinclude: 1) The use of the standard critical point (SCP) model to ana- lyze the ellipsometric data from conjugated-polymer thin films. Adv. Funct. Mater. 2005, 15, 925±933 DOI: 10.1002/adfm.200400121  2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 925 ± [*] Prof. D. D. C. Bradley, M. Campoy-Quiles, Dr. G. Heliotis, Dr. R. Xia, Dr. M. Ariu, M. Pintani Ultrafast Photonics Collaboration and Experimental Solid State Physics Group Blackett Laboratory, Imperial College London Prince Consort Road, London, SW7 2BZ (UK) E-mail: d.bradley@imperial.ac.uk Dr. P. Etchegoin The McDiarmid Institute for Advanced Materials and Nanotechnology School of Chemical and Physical Sciences Victoria University of Wellington PO Box 600, Wellington (New Zealand) [**] We thank the UK Engineering and Physical Sciences Research Council for financial support through the Ultrafast Photonics Collaboration (GR/R55078) and grant GR/R26641 (MA). We also thank Mark Ber- nius, Rob Fletcher, Mike Inbasekaran, and Jim O'Brien of The Dow Chemical Company for providing the polyfluorene polymers that we have studied. Variable-angle spectroscopic ellipsometry (VASE) has been applied to five polyfluorene gain media. The ellipsometric data have been analyzed using an electronic model based on critical points of zero dimension, i.e., an exciton model. The optical constants of thin-film samples on spectrosil B substrates have been deduced and are used to characterize the waveguiding conditions in these asymmetric slab structures. The exciton model that we have used leads to a small correlation of the parame- ters and accurate fits. The good match with normal-incidence transmission spectrophotometry data and surface profilometry determinations of thickness gives further confidence in the suitability of this model. Based on our measurements, we calculate the cut-off thicknesses for the fundamental TE-guided modes in our silica±polymer±air structures to lie within the range of 40 nm to 70 nm (depending on the specific polymer), and demonstrate corresponding confinement factors (C) between 37% and 63%. The calculated thickness dependence of the cut-off wavelength agrees well with experimental data for the thickness dependence of the peak amplified spontaneous emission (ASE) wavelength and is, therefore, consistent with previous explana- tionsoftheASEspectralshifts. FULL PAPER