Fluorene-Based Rib Waveguides with Optimized Geometry for Long-Term
Amplified Spontaneous Emission Stability
Gonzalo Del Pozo,
1
Noureddine Bennis,
2
Xabier Quintana,
3
Jose Manuel Ot
on,
3
Jinyi Lin,
4
Linghai H. Xie,
4
Qi Wei,
4
Ruidong Xia,
4
Ramon Bernardo-Gavito,
1,5
Daniel Granados,
1
Juan Cabanillas-Gonzalez
1
1
Madrid Institute for Advanced Studies-IMDEA Nanociencia, Calle Faraday 9, Ciudad Universitaria de Cantoblanco 28049, Spain
2
Institute of Applied Physics, Military University of Technology, Kaliskiego 2, 00–908 Warsaw, Poland
3
CEMDATIC, Escuela T
ecnica Superior de Ingenieros de Telecomunicaci
on, Universidad Polit
ecnica de Madrid, 28040 Madrid,
Spain
4
Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials, National Jiangsu Synergistic
Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts and Telecommunications, Nanjing 210046,
People’s Republic of China
5
Departamento de F
ısica de la Materia Condensada, U. Autonoma de Madrid, 28049 Madrid, Spain
Correspondence to: J. Cabanillas-Gonzalez (E -mail: juan.cabanillas@imdea.org) or R. Xia (E-mail: ruidong.xia@gmail.com)
Received 5 February 2015; accepted 13 March 2015; published online 11 April 2015
DOI: 10.1002/polb.23730
ABSTRACT: Amongst the different optoelectronic applications of
conjugated polymers, waveguide amplifiers and optically
pumped lasers are those requiring larger photochemical stabil-
ity, owing to the large irradiation conditions under operation.
In this context, suitable waveguide optimization enabling the
reduction of amplified spontaneous emission (ASE) threshold
values appears as important as synthetic chemistry protocols
to promote polymer robustness against photo-oxidation. In
this work, we develop rib waveguides with different geome-
tries based on four different fluorene-based compounds and
assess the influence of rib confinement on ASE properties. We
observe ASE threshold values as low as 8.9 3 10
24
mJ cm
22
,
being among the lowest threshold values reported so far on
blue emitting polymer/oligomer waveguides. We demonstrate
that the enhanced ASE efficiency on some of these rib wave-
guides leads to a fivefold increase in operation lifetime respect
to spin-coated slab waveguides, thus confirming the impact of
waveguide geometry on ASE operation stability.
V
C
2015 Wiley
Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53,
1040–1045
KEYWORDS: conjugated polymers; fluorescence; photolithogra-
phy; photooxidation; photophysics
INTRODUCTION
Conjugated polymers are materials which
exhibit outstanding optical gain properties.
1–3
Processed as
thin films they behave as slab waveguides. Their large stimu-
lated emission cross-section values and notable waveguiding
properties enable amplified spontaneous emission (ASE) at
very low pump thresholds compared with organic dyes.
4,5
These assets together with their semiconducting charge
transport behavior open up prospects for the future develop-
ment of polymer-based electrically pumped lasers.
6,7
Among
conjugated polymers, poly(9,9-di-octyl-fluorene) (PFO) is one
of the most representative optical amplifying polymers.
8–10
Its notable properties as optical gain medium arise from a
combination of efficient solid-state blue emission
11
(ca. 40%)
and excellent thin film optical quality which manifests into
reduced scattering-induced losses and excellent waveguide
properties.
12
Hitherto, its operation lifetime is compromised by its low
emission stability associated to inter-chain aggregation-
induced energy transfer to fluorenone defects formed upon
photo-oxidation.
13
This behavior is particularly pronounced
under intense UV irradiation. It has already been shown that
waveguide encapsulation of the polymer gain leads to signifi-
cant improvement in emission stability.
14
Other approaches
based on synthetic chemistry consist of promoting polymer
backbone isolation to prevent aggregate-induced energy
transfer to fluorenone. Examples are the synthesis of rotaxi-
nated polyfluorenes with cyclodextrin rings around the poly-
mer backbone,
15,16
or the implementation of lateral side-
chain substituents which interfere with backbone p–p stack-
ing.
17–19
In line with this second strategy, some of us have
already reported on H-shape fluorene-based oligomers bear-
ing one terfluorene arm with flexible alkyl chains and a
V
C
2015 Wiley Periodicals, Inc.
1040 JOURNAL OF POLYMER SCIENCE, PART B: POLYMER PHYSICS 2015, 53, 1040–1045
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