Reversible oxygen induced emission quenching of both the Spontaneous Emission (SE) and the Amplified Spontaneous Emission (ASE) of poly(9,9-dioctylfluorene) waveguides is demonstrated. We show that ASE shows a stronger quenching than SE, up to about 6.2 times, but also a stronger decrease when the excitation density increases. We conclude that the fast increase of the ASE decay rate is the main process in determining the ASE detection sensitivity, limiting the potentiality of sensitivity improvement of ASE with respect to SE. 1. Introduction The development of novel active systems for gas sensing is receiving increasing attention due to the very wide range of possible applications. In particular the detection of oxygen is particularly interesting in the medical field for the monitoring of oxygen content in both air and blood, as well as in the environmental monitoring field [1]. Among the different proposed techniques, exploiting the oxygen effects on the electric conductivity [2], material color [3], and chemiluminescence [4], optical sensors, based on the oxygen induced photoluminescence quenching, are characterized by fast response time and high sensitivity. The typical active systems are blends between an inert matrix and a phosphorescent molecule [5], while reversible oxygen induced fluorescent quenching in poly(9,9-dioctylfluorene) (PF8) neat films was recently demonstrated [6]. A recent breakthrough in the sensitivity enhancement was represented by the demonstration that in neat active films showing optical gain, the Amplified Spontaneous Emission (ASE) and the laser intensity show a stronger relative quenching with respect to the Spontaneous Emission (SE) [7], exploited in efficient explosive vapours detection. This first result was followed by similar demonstration [8] of explosive detection in PF8 and in few evidences of oxygen detection [9, 10]. In this letter we demonstrate reversible oxygen induced emission quenching of both the Spontaneous Emission (SE) and the Amplified Spontaneous Emission (ASE) in a PF8 waveguide. We demonstrate that the ASE shows a detection sensitivity up to 6.2 times larger than the SE and a stronger sensitivity decrease as the excitation density increases. The relaxation processes affecting the intensity quenching of both SE and ASE are discussed concluding that the fast ASE rate increase with the excitation density is the process limiting the sensitivity improvement of ASE with respect to SE. 2. Materials and Methods The PF8 film, with a thickness of about 650？nm, was realized by spin coating on a glass substrate
References
[1]
Y. Amao, “Probes and polymers for optical sensing of oxygen,” Microchimica Acta, vol. 143, no. 1, pp. 1–12, 2003.
[2]
K. R. Sridha and J. A. Blanchard, “Electronic conduction in low oxygen partial pressure measurements using an amperometric zirconia oxygen sensor,” Sensors and Actuators B, vol. 59, no. 1, pp. 60–67, 1999.
[3]
Y. Suzuki, H. Nishide, and E. Tsuchida, “Membranes of the picket fence cobalt porphyrin complexed with poly(vinylimidazole and -pyridine)s: selective optical response to oxygen,” Macromolecules, vol. 33, no. 7, pp. 2530–2534, 2000.
[4]
G. E. Collins and S. L. Rose-Pehrsson, “Chemiluminescent chemical sensors for oxygen and nitrogen dioxide,” Analytical Chemistry, vol. 67, no. 13, pp. 2224–2230, 1995.
[5]
P. Douglas and K. Eaton, “Response characteristics of thin film oxygen sensors, Pt and Pd octaethylporphyrins in polymer films,” Sensors and Actuators B, vol. 82, no. 2-3, pp. 200–208, 2002.
[6]
M. Anni and R. Rella, “Interplay between amplified spontaneous emission, F？rster resonant energy transfer, and self-absorption in hybrid poly(9,9-dioctylfluorene)-CdSe/ZnS nanocrystal thin films,” Journal of Physical Chemistry B, vol. 114, no. 5, pp. 2086–2090, 2010.
[7]
A. Rose, Z. Zhu, C. F. Madigan, T. M. Swager, and V. Bulovi？, “Sensitivity gains in chemosensing by lasing action in organic polymers,” Nature, vol. 434, no. 7035, pp. 876–879, 2005.
[8]
Y. Yang, G. A. Turnbull, and I. D. W. Samuel, “Sensitive explosive vapor detection with polyfluorene lasers,” Advanced Functional Materials, vol. 20, no. 13, pp. 2093–2097, 2010.
[9]
M. Gaal, S. Sax, H. Plank et al., “Directly imprinted surface-emitting distributed feedback structure polymer sensor laser devices for enhanced oxygen sensitivity,” Japanese Journal of Applied Physics, vol. 47, pp. 304–306, 2008.
[10]
H.-W. Lin, M.-H. Huang, Y.-H. Chen et al., “Novel oxygen sensor based on terfluorene thin-film and its enhanced sensitivity by stimulated emission,” Journal of Materials Chemistry, vol. 22, pp. 13446–13450, 2012.
[11]
M. Ariu, M. Sims, M. D. Rahn et al., “Exciton migration in β-phase poly(9,9-dioctylfluorene),” Physical Review B, vol. 67, no. 19, Article ID 195333, 11 pages, 2003.
[12]
M. Anni, “Photodegradation effects on the emission properties of an amplifying poly(9,9-dioctylfluorene) active waveguide operating in air,” Journal of Physical Chemistry B, vol. 116, no. 15, pp. 4655–4660, 2012.
[13]
O. Svelto, S. Taccheo, and C. Svelto, “Analysis of amplified spontaneous emission: some corrections to the Linford formula,” Optics Communications, vol. 149, no. 4–6, pp. 277–282, 1998.
[14]
C. Kallinger, S. Riechel, O. Holderer et al., “Picosecond amplified spontaneous emission bursts from a molecularly doped organic semiconductor,” Journal of Applied Physics, vol. 91, no. 10, p. 6367, 2002.
