Download Diode pumped distributed Bragg reflector lasers

Diode pumped distributed Bragg reflector lasers
based on a dye-to-polymer energy transfer blend
A. E. Vasdekis, G. Tsiminis, J.-C. Ribierre, Liam O’ Faolain, T. F. Krauss,
G. A. Turnbull and I. D. W. Samuel
Organic Semiconductor Centre & Ultrafast Photonics Collaboration, SUPA, School of Physics and Astronomy,
University of St Andrews, St Andrews, Fife, KY16 9SS, UK
[email protected], [email protected]
http://www.st-andrews.ac.uk/~OSC
Abstract: We report the demonstration of a compact, all-solid-state
polymer laser system comprising of a Gallium Nitride (GaN) semiconductor
diode laser as the pump source. The polymer laser was configured as a
surface emitting, distributed Bragg reflector laser (DBR), based on a novel
energy transfer blend of Coumarin 102 and the conjugated polymer poly(2methoxy-5-(2’-ethylhexyloxy)-1,4-phenylene
vinylene).
In
this
configuration, diode pumping was possible both due to the improved quality
of the resonators and the improved harvesting of the diode laser light.
©2006 Optical Society of America
OCIS codes: (250.3680) Light-emitting polymers; (140.3380) Laser Materials;
(140.5960) Semiconductor
lasers;
(250.7270)
Vertical
emitting
lasers;
(230.3990) Microstructure devices;
References and links
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semiconducting polymer laser pumped by a microchip laser,” Appl. Phys. Lett. 82, 313-315 (2003).
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Received 10 August 2006; revised 15 September 2006; accepted 18 September 2006
2 October 2006 / Vol. 14, No. 20 / OPTICS EXPRESS 9211
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1. Introduction
In recent years, semiconducting (conjugated) polymers have emerged as an attractive new
gain medium for widely tuneable visible lasers [1-6]. Several families of materials have been
studied, though much of the work has concentrated on derivatives of poly(paraphenylenevinylene) and poly(fluorene) [4-6]. These are highly fluorescent and exhibit little
concentration quenching, allowing them to be used in the undiluted solid state. In such a
configuration, they are electroluminescent, suggesting the possibility of electrically excited
plastic lasers. The polymers can also be readily processed from solution to form low-loss
optical waveguides and compact diffractive resonators. Such lasers offer new possibilities for
spectroscopic applications, including ultrasensitive chemical sensing [7]. While direct
electrical pumping remains a major outstanding challenge optically pumped polymer lasers
have developed to become very attractive visible sources [1-6]. Lasing thresholds of polymer
distributed feedback (DFB) lasers are now commonly low enough to be pumped by pulsed
microchip lasers [8]. GaN diode-laser-pumped organic lasers based on polyfluorene
derivatives have recently been reported [9, 10].
In this paper we demonstrate diode-pumped organic lasers based on the
poly(paraphenylene-vinylene) derivative MEH-PPV. To achieve the very low oscillation
thresholds required for direct diode pumping, we combine a novel surface-emitting resonator
structure with an energy-transfer gain medium. The gain medium is to the best of our
knowledge, the first example of a concentrated laser dye host doped with a luminescent
polymer for lasing applications. We show that efficient energy transfer is possible from the
dye to the polymer and describe the photo-physical performance of the gain medium for diode
laser excitation. We then describe the operation principle of the DBR polymer cavities and
finally the operating characteristics of the resulting devices.
2. Dye-to-polymer energy transfer
MEH-PPV is a well-known prototypical polymer, readily available from a variety of
commercial sources. However, its strongest absorption is at 500 nm (Fig. 1) hindering thus the
efficient harvesting of the GaN diode violet emission. To overcome this, our strategy was to
employ non-radiative energy transfer by blending the emissive species in an appropriate host
[9, 11-14]. A potential host would be another conjugated polymer, but polymer blends tend to
phase separate, particularly when mixed in similar quantities.
We explore instead the blending of the MEH-PPV with the dye molecule Coumarin 102.
The dye fluoresces at the absorption maximum of the MEH-PPV and also exhibits a strong
absorption band at the GaN laser emission wavelength. We study the energy transfer process
from a concentrated dye film [14] and its viability for lasing in the polymer guest. The study
involved the measurement of the absorption and photoluminescence spectra, the absolute
photoluminescence quantum yield (PLQY) and the amplified spontaneous emission
thresholds of thin films.
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Coumarin 102 in
Chlorobenzene solution (x5)
Absorbance
(b)
(a)
2
N
O
O
Coumarin 102 : MEH-PPV
weight ratio:
0%
33 %
50 %
66 %
1
200
150
100
50
40
ASE threshold (nJ)
incident energy
absorbed energy
PLQY (%)
514 nm
35
O
407 nm
30
n
0
350
O
400
450
500
wavelength (nm)
550
600
25
0
10
20
30
40
50
60
dye - polymer weight ratio (%)
70
Fig. 1. (a). The absorbance for a neat MEH-PPV film, Coumarin 102 in chlorobenzene
solution (x5) and their blends in solid state with dye concentrations of 33, 50 and 60
wt.%. The absorbance of each film is normalized at 496 nm to compensate for the
different red chromophore densities in each blend. The emission wavelength of the GaN
diode is also shown (black arrow). (b): The PLQY values (lower) and ASE thresholds
(upper) are plotted as a function of the doping concentration. The molecular structures of
MEH-PPV (red) and the Coumarin 102 dye (blue) are also shown.
The MEH-PPV (American Dye Source Inc.) and the Coumarin 102 (Lambda Physik) were
first co-dissolved in chlorobenzene in a range of blend ratios, and spin-cast onto fused silica
substrates to form films ~100 nm thick. We find that we can form good quality films for a
wide range of blend ratios, even at near equal weights. Figure 1(a) shows the absorption of
solid-state blends of various weight fractions of Coumarin 102, plus the absorption of the dye
in chlorobenzene solution and the emission wavelength of the GaN laser (black arrow). We
find we can significantly increase the absorption at the diode laser wavelength relative to that
of the MEH-PPV. In addition, for all these blends the emission is dominated by the MEHPPV photoluminescence spectrum. This result is characteristic of efficient energy transfer, but
does not by itself confirm its presence, as the concentrated dye film is also likely to exhibit a
rapid non-radiative decay path that will quench its emission, competing with the energy
transfer.
The PLQY is the ratio of photons emitted to photons absorbed, and is measured for films
mounted inside an integrating sphere in order to collect light emitted in all directions [15]. In
Fig. 1(b), the PLQY is plotted as a function of the doping concentration for both the direct
excitation of the polymer at 514 nm, and excitation into the dye absorption band at 407 nm. A
substantial increase in the blend PLQY is achieved as compared to neat MEH-PPV films for
increasing dye concentration. This increase is indicative that non-radiative decay in the
polymer is lowered due to reduced intra-molecular interactions and self absorption in the
blends. The similarity in PLQY values between direct (514 nm) and indirect (407 nm)
excitation of the MEH-PPV confirms that we do have an efficient energy transfer (> 80% for
the 50:50 wt. % blend) from the dye to the polymer.
The ASE experiments were performed by exciting the films with the 407 nm emission
from an OPO (Continuum Panther EX) focused to a stripe with dimensions 170 μm by 3.8
mm. The spectrum of the blend emission was detected from the edge of the film using a CCD
spectrograph for a range of excitation densities. For excitation above a critical pumping
density, we observe the onset of ASE in each of the films, characterized by a gain narrowing
of the edge-emitted spectrum. The ASE was centered at 632 nm for the blends and at 621 nm
for the neat MEH-PPV, with linewidths of ~ 9 nm, confirming thus the efficient energy
transfer from the dye molecule to the MEH-PPV. The 11 nm difference follows the red-shift
in the MEH-PPV absorption in the blends. From Fig. 1(b) (blue line) it can be observed that in
terms of incident pump energies a clear threshold minimum occurs at the concentration of
50:50 wt.%. When correcting for the amount of light absorbed in each blend, the ASE
thresholds become comparable indicating the successful light harvesting of the blend, while
maintaining the amplifying properties of the conjugated polymer. At high concentrations
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(>50% per weight), the ASE threshold increases and a possible explanation for this effect may
be the increased scattering losses due to the formation of aggregates. In the following
experiments we used the optimal blending concentration of 50:50 wt. %.
3. Distributed Bragg reflector polymer lasers
DFB polymer lasers have been widely studied in the past and have been shown to exhibit
enhanced performance in terms of low thresholds and collimated emission [8]. A thin
polymer film is spun on top of a grating and a standing wave is created due to interference
effects originating from the periodic modulation of the refractive index. This modulation
however can act as a source of loss due to mainly two possible mechanisms. The first one is
the diffractive output coupling that provides the advantageous surface emission in 2nd order
DFB lasers, but has been identified as a major source of loss in these devices [16]. A further
probable source of loss is the incoherent scattering associated with the inevitable surface nonuniformity of a polymer film spun on a corrugated substrate. In both cases lower net-gain is
expected.
pump
emission
emission
Reflector
Air
Amplification
Region
Polymer
(120nm)
Reflector
Silica
Reflector
Amplification
region
Reflector
2 μm
Fig. 2. Left: a cross-sectional schematic of a polymer DBR laser. The planar polymer film
is the amplifying region and the corrugated surfaces on the right and left are the Bragg
reflectors. Right: An SEM image of a representative grating structure fabricated on fused
silica using electron-beam lithography. The area between the gratings is also etched to
form a channel waveguide for lateral confinement.
In order to further improve the performance of polymer solid-state lasers, we investigate DBR
resonators. In DBR lasers, the amplifying medium is placed between two Bragg mirrors that
provide the necessary feedback for lasing. By this means, the areas of population inversion
are separated from the Bragg gratings and hence both of the aforementioned possible losses
are addressed. In Fig. 2 a general schematic of such a structure is shown: the planar polymer
film acts as an amplifier and the adjacent gratings form the resonator. The optical excitation is
centered in the planar polymer film and the emitted photons can form a standing wave due to
the reflection from the gratings. If a 2nd order grating is used, the amplified light will be both
reflected in the plane of the guide and scattered from the surface, thus forming a compact,
surface-emitting laser.
Such polymer lasers have been previously investigated, where the mirror separation was
of the order of 1 mm [17, 18]. In addition, a lot of attention has been focused on similar
structures, namely the organic VCSEL type of lasers (or microcavities) that nonetheless entail
certain drawbacks, such as the additional step of the mirrors’ evaporation but also the short
gain length limited by the thickness of the organic film [11, 19]. In contrast to these reports,
the mirror separation in our experiments was kept within the range of 10 to 110 μm in order to
achieve increased gain length but also to avoid severe multi-mode operation and the
associated gain competition. In addition, for these cavity lengths we could achieve high
excitation densities even for the low pulse energies that are currently available from GaN
diode lasers.
In the current experiments, the resonators were defined on a 140 nm film of poly(methyl
methacrylate) (PMMA) using electron-beam lithography (Hybrid LEO Gemini 1530
SEM/RAITH ELPHY lithography system) at 30 kV. In order to improve the resolution of the
lithography, a 15 nm gold and a 20 nm PEDOT film were added on top of the resist. The
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2 October 2006 / Vol. 14, No. 20 / OPTICS EXPRESS 9214
pattern was subsequently transferred onto a fused silica substrate using reactive ion plasma
chemistry. In each resonator, both mirror gratings had a period of 410 nm, thus providing inplane reflectivity and surface emission of the laser light via second and first order diffraction
respectively. The length of each mirror was 41 μm (100 layers) providing high reflectivity for
the laser field [20]. In addition, the substrate area between the mirrors was etched to form a
channel waveguide and hence confine the light in the lateral direction to the feedback. A
typical grating structure for a mirror separation of 10 μm is shown in the SEM image of Fig.
2. Additionally, the mirror separation was varied in order to confirm the principle of
operation by studying the resonant behaviour for a range of lengths and also to identify the
optimal length.
4. Diode pumping
The pump source was a GaN diode laser (Jobin Yvon Horiba) with maximum pulse energy of
0.67 nJ, pulse duration of 1 nsec and a repetition rate of 10 kHz. The astigmatic and highly
divergent output beam was focused using a spherical lens to an elliptical spot size with a
diameter along the major and minor axis of 76 μm and 66 μm respectively. In the optical
experiments care was taken so that the major axis of the excitation area was parallel to the
resonant direction of the cavity and the polarisation of the pump light parallel to the grating
grooves. The excitation wavelength was 409 nm, matching the absorption of the host
molecule. The emission was collected normal to the surface of the polymer film using a fibre
coupled CCD spectrometer.
In Fig. 3(a) a typical emission spectrum below and above threshold is shown for a mirror
separation of 20 μm. Below threshold, the spontaneous emission couples to four distinct
optical modes at 624 nm, 625.7 nm, 628.2 nm and 630.4 nm. Angular dispersion
measurements indicated that the modes at the shortest and longest wavelength correspond to
the Bragg scattered modes that originate from the periodic nature of the gratings [21]. These
modes appear at the edges of the stop-band at normal incidence and shift in wavelength for
different scattering angles. The presence of these modes is enhanced by the optical excitation
of the grating mirrors since the excitation spot in our experiments is larger than the mirror
separation.
The intermediate modes at wavelengths 625.7 nm and 628.2 nm are the resonant modes of
the DBR cavity. These modes appear within the stop-band and are excited only when the
pump light is centered between the Bragg mirrors. These modes do not exhibit such strong
angular dispersion and from their free spectral range we deduced a total cavity length of 49
μm corresponding to the actual mirror separation length plus a photon penetration length of
~14.5 μm at each mirror. In addition, respectable Q-factors ( Δλ λ ~103) were obtained
confirming the high quality of the cavity.
(b)
(a)
3
intensity (a.u.)
above threshold
below threshold
3
spontaneous emission
at 632.1 nm
2
(c)
3
laser peak at 625.6 nm
2
0.67 nJ
0.53 nJ
0.42 nJ
0.34 nJ
normalisation
1
1
0
615
620
625
630
wavelength (nm)
635
640
100
200
300
400
Epump (pJ)
500
600
700 610
615
620
625
630
635
640
wavelength (nm)
Fig. 3. (a). The emission spectra above (0.67 nJ) and below threshold (0.27 nJ), where both the
DBR and ‘mirror modes’ are shown. (b) The input-output relationship for the laser peak at 625.6
nm (blue) and the spontaneous emission at 632.1 nm (red). The lines are a guide to the eye. (c)
The normalized emission spectra for different excitation densities. The normalization level was
chosen at the wavelength of 632.1 nm corresponding to the spontaneous emission and is denoted
by the blue arrow.
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Above a pump energy of 0.42 nJ, we observe a change in the emitted light. This can be seen
in Fig. 3(b). There is an increase in growth of the DBR mode at 625.6 nm and a flattening of
the spontaneous emission at 632.1 nm. The faster growth of one of the modes and the pinning
of the others is a clear indication of lasing [1]. This behaviour can also be seen in Fig. 3(c),
where the emission spectra are normalized at the wavelength of 632.2 nm. Below threshold,
the spectra evolve linearly with the pump intensity and hence completely overlap when
normalized. Above threshold, the laser peak increases faster than the background indicating
the non-linearity associated with stimulated emission. The same behaviour was observed for
30 and 50 μm cavity lengths, but with more longitudinal modes within the stopband, and a
decreasing free spectral range, for increasing cavity length. Finally, when using a frequency
doubled Q-switched Nd:YVO4 microchip pump laser that allowed for higher excitation
densities, the same behaviour was observed with the lasing mode rapidly growing to
completely dominate the surface emission. In addition, with this pump laser we also found
that the DFB lasers have higher thresholds than the DBR.
5. Conclusion
In conclusion, we have demonstrated a solid-state polymer laser pumped with an inorganic
diode laser. We used a novel blend based on the dye Coumarin 102 and the conjugated
polymer MEH-PPV, both of which were commercially sourced. This allowed the efficient
transfer of the GaN pump excitation to the conjugated polymer. In regards to the cavity
optimization, we used a polymer DBR resonator that acted as a low threshold, surfaceemitting laser. In comparison to 2nd order DFB lasers, we attribute the optimised performance
to the separation of the Bragg gratings with the amplifier. The latter allowed the reduction of
coherent and incoherent scattering losses and cavity modes of high quality factors.
Acknowledgments
We are grateful to EPSRC for financial support.
#73974 - $15.00 USD
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Received 10 August 2006; revised 15 September 2006; accepted 18 September 2006
2 October 2006 / Vol. 14, No. 20 / OPTICS EXPRESS 9216