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Polymer lasers:
recent advances
Ifor D. W. Samuel† and Graham A. Turnbull*
Semiconducting polymers combine attractive
optoelectronic properties with the scope for simple
fabrication. Now well established as materials for
light-emitting diodes (LEDs), they also offer
considerable potential as cheap visible lasers. In this
review, we describe the distinctive features of
semiconducting polymers that have motivated their
study as laser gain media. We discuss the latest
advances made in the physics and fabrication of
novel plastic laser resonators. Finally, we outline the
progress toward and future challenges for making
inexpensive plastic diode lasers that could operate
throughout the visible spectrum.
Plastics, or polymers, are found throughout everyday
life in applications ranging from food packaging to
videotape. There are two main reasons why they are
so widespread. One is that there is an enormous
range of possible structures, giving a huge variety of
properties. The other is that they are generally easy
to process and shape, leading to simple manufacture.
Another extremely important class of materials in
everyday life are semiconductors, which are usually
inorganic materials such as Si and GaAs. They are
used to make transistors, most lasers, and solar cells,
underpinning modern electronics and optoelectronics.
The discovery of semiconducting polymers is,
therefore, very exciting as it provides new materials
that combine the potential of polymers and
semiconductors to open new directions in electronics
and optoelectronics1. In particular, semiconducting
polymers combine novel semiconducting properties
with the simple processing of polymers, so we now
have semiconductors that can be dissolved in a
solvent and printed to make electronic devices1-3.
Ultrafast Photonics Collaboration and Organic
Semiconductor Centre,
School of Physics and Astronomy,
University of St Andrews
St Andrews,
Fife KY16 9NA UK
†E-mail: [email protected]
*E-mail: [email protected]
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September 2004
Polymers are usually long, chain-like molecules with a
regular repeat unit. A simple example would be polyethylene,
which is widely used in packaging. Semiconducting properties
arise from using conjugated repeat units, e.g. a repeat unit in
which there is a linear pattern of alternating single and
double bonds. Some examples of conjugated polymers are
shown in Fig. 1. The alternating single and double bonds lead
to one electron in a p orbital on each carbon atom. The p
ISSN:1369 7021 © Elsevier Ltd 2004
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orbitals overlap, leading to electron delocalization and
semiconducting behavior. There are three main families of
polymers that have been studied in the context of lasers4-7.
They are poly(phenylenevinylene)s (Fig. 1a)8-24, ladder-type
poly(paraphenylene) (Fig. 1b)25-30, and polyfluorenes
(Fig. 1c)31-35.
Motivation for polymer lasers
Many conjugated polymers are capable of light emission.
This can be generated either optically, by shining light onto
a sample to give fluorescence, or electrically, by applying a
voltage to a polymer light-emitting diode. As explained
elsewhere in this issue, the latter process is very promising as
a flat panel display technology in which color displays could
be made by ink-jet printing. The vigorous worldwide activity
relating to the development of light-emitting polymers has
led to remarkable advances in materials over the past decade,
facilitating the development of other application areas such
as electronics, solar cells, and lasers.
There are several reasons why semiconducting polymers
could be attractive laser materials4-7. The first is that there
exists a range of polymers that can emit light across the
visible spectrum, as illustrated in Fig. 2a. The polymers have
broad spectra (Fig. 2b), providing scope for making tunable
lasers. They have very strong absorption coefficients
(~105 cm-1), which implies that there is the potential for
extremely strong amplification of light. The absorption and
fluorescence spectra are well separated, so that absorption of
emitted light is weak. In many fluorescent organic molecules
(including laser dyes), light emission is severely quenched at
high concentrations – such as typically found in solid films. In
contrast, conjugated polymers can emit light as neat solid
Fig. 1 Generic structures of conjugated polymers commonly used for lasers:
(a) poly(phenylenevinylene)s; (b) ladder-type poly(paraphenylene); and
(c) polyfluorenes, where R1, R2, R3, and R4 represent alkyl or alkoxy groups.
films and, in addition, are capable of charge transport,
thereby providing the potential to make electrically pumped
lasers in the future. Semiconducting polymers combine the
specific advantages for lasing outlined above with the general
advantages of polymers relevant to all the applications in this
issue, namely the scope for tuning properties by changing the
structure, simple fabrication, and the possibility of working
with flexible substrates.
Laser structures
The key features of a laser are a material that amplifies light
(known as the gain medium) and a resonator that applies
feedback36. As light passes through the gain medium, it
stimulates the emission of more light, thereby gaining
intensity. The resonator reflects the light backward and
forward through the gain medium to build up a very intense
light field. A very important point is that the additional light
stimulated has the same wavelength, direction, and phase as
the light passing through the gain medium, which leads to
the distinctive coherence properties of laser light, e.g. being
collimated in a narrow beam and (usually) monochromatic.
Fig.2 (a) Emission spectra of three commonly used conjugated polymers; the polymer
structures are inset. (b) Emission and absorption spectra of the polymer OC1C10-PPV; the
polymer structure is inset.
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Energy must be supplied to a laser in order for it to
operate, and there are two main ways of doing this. The first,
known as optical pumping, is to excite the gain medium with
a powerful light source. This would normally be either
another laser or a flash lamp. The other possibility, electrical
pumping, involves exciting the gain medium electrically by
passing a current through it. In all lasers there is a minimum
pumping rate or energy, known as the threshold, which must
be supplied in order for lasing to occur. Electrical pumping is
clearly very convenient, and is the approach used in inorganic
semiconductor diode lasers, as found in, for example, CD
players. So far, all polymer lasers have been optically pumped
by another laser. Prospects for electrical pumping will be
discussed later.
The most common laser configuration is to have a gain
medium in a resonator consisting of two (or more) mirrors as
depicted in Fig. 3a. Light passes repeatedly through the gain
medium as it bounces backward and forward between the
mirrors. One of the mirrors (the output coupler) reflects most
of the light, but transmits a small portion, giving the output
beam of the laser. The first semiconducting polymer laser
was reported by Moses8 in 1992. It used this resonator design
and a gain medium of a dilute solution of the polymer
poly(2-methoxy-5-(2’-ethylhexyloxy)-1,4phenylenevinylene), MEH-PPV, which has a structure very
similar to the material in the lower panel of Fig. 2. The use of
the polymer in solution was a good starting point as its
Fig. 3 Schematics of various polymer laser resonators; blue arrows show the resonant path
and emission direction of the laser light. (a) Cuvette of polymer solution inside a
conventional two-mirror resonator. (b) Planar microcavity with polymer film sandwiched
between two mirrors. (c) Microring resonator consisting of a silica fiber coated with a
polymer film. (d) Polymer distributed feedback (DFB) laser consisting of a corrugated
polymer film. (e) Polymer DFB laser with a two-dimensional grating.
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fluorescence efficiency is higher than in the solid state, and a
simple resonator structure can be used. The results obtained
were an important proof of principle.
A solid-state polymer laser, though, would be attractive
because it could be more compact and robust, and would be
better suited for electrical pumping in the future. However,
this proved to be a more challenging problem. In the same
year as Moses reported a solution laser, one of the present
authors (IDWS) was exploring the feasibility of a solid-state
laser by looking for gain in thin films of conjugated polymers.
No gain was observed – only photo-induced absorption9, so a
solid-state laser could not be made from the materials being
studied at that time. Fortunately, materials and their
synthesis were rapidly advancing, driven by the development
of organic LEDs, and a few years later a range of experiments
indicated the presence of gain and lasing10-13. An example of
one of these early lasers is the work of Tessler, who studied
thin films of the polymer poly(phenylene vinylene) (PPV)
sandwiched in between two mirrors12. This resonator design
is shown in Fig. 3b. When excited by an extremely intense
laser, lasing was observed at a wavelength of 545 nm. An
interesting feature of this result is that lasing was achieved in
a gain medium only 100 nm thick. In contrast, the gain
medium in many lasers is of centimeter dimensions. The fact
that such a short length of gain medium can be used shows
that extraordinarily high gains can be achieved from
semiconducting polymers. This is something that would be
expected from their very strong optical transitions, as
measured by absorption.
Many laser gain media are crystals that must be carefully
cleaved and then used in resonators such as that shown in
Fig. 3a. However, the processing flexibility of polymers allows
a much wider range of resonators to be made in simple ways.
For example, by dipping an optical fiber into a
semiconducting polymer solution one may form a polymer
‘microring’ surrounding a glass core, as shown in Fig. 3c. Light
can travel round and round this structure, being amplified as
it goes, and giving lasing14,15. An advantage of this structure
is that the gain medium is long and losses are very low,
making it easier to reach threshold and achieve lasing.
However, light is emitted in all directions from the structure,
and the ill-defined output is a disadvantage.
Instead of using mirrors, feedback can be applied by
making suitable periodic structures in a polymer film
(Fig. 3d). The simplest example is a structure shaped like a
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corrugated iron sheet, but on a very much finer length scale
with the spacing between peaks typically 400 nm. This length
is similar to the wavelength of light, so that the structure can
act as a diffraction grating. Light traveling in one direction in
the film will be diffracted in a new direction. If the period of
the corrugation is chosen carefully, then one may arrange it
such that the light is diffracted through 180° into the
counter-propagating direction; the effect is similar to light
being reflected by a mirror. A corrugated semiconducting
polymer film can, therefore, be used to make a laser: the
polymer is the gain medium and the corrugation provides
feedback. Lasers of this type are known as distributed
feedback (DFB) lasers. There are many examples of polymer
DFB lasers16-30,33, including a flexible polymer laser25. It is
important to note that the DFB geometry has also proved
very useful for lasers made from evaporated organic
semiconductors37-40 and solid-state dye lasers41,42. The DFB
geometry is very attractive for semiconducting polymer
lasers for several reasons. Polymer thin films can be readily
made by spin-coating and the laser structure allows light to
propagate through significant distances in the gain medium,
thereby giving low threshold operation. As the feedback is
provided by corrugation rather than mirrors, no alignment of
the laser is required, and the corrugation can also be used to
give a well-defined output beam, by diffracting some light
out of the face of the film.
In addition to the simple diffraction grating described
above, polymer DFB lasers have been fabricated with more
complicated two-dimensional periodic structures19-22,26-29,35
(Fig. 3e). In these lasers, the feedback is more effective,
thresholds lower, and the direction of light emission better
defined, giving good quality collimated beams. Fig. 4 shows
typical lasing characteristics of a polymer DFB laser based on
a structure consisting of two perpendicular gratings coated
with an MEH-PPV film. Fig. 4a shows the output pulse energy
from the device, while Fig. 4b shows the spectrum of the
emitted light. Above a threshold pump energy of 4 nJ, the
laser output increases linearly with pump energy, while the
spectrum of surface-emitted light abruptly narrows to a
linewidth of <1 nm.
some of the more recent developments in polymer lasers.
These include understanding the detailed operation of DFB
lasers, reducing the size of pump lasers, simple fabrication of
polymer lasers, and polymers in ultrafast photonics. Finally,
we will assess the progress that has been made toward
electrical pumping.
Reducing threshold
Most existing polymer lasers are excited by very large,
powerful, and expensive lasers. This is a result in part of the
threshold energy required to reach lasing. The thresholds are
also higher than can be achieved comfortably by electrical
pumping. It is, therefore, desirable to understand and improve
the optical design of polymer lasers in order to reduce
threshold. This would enable more compact and economical
pump lasers to be used, and take us closer to being able to
use electrical pumping. One important issue is to understand
how light emission from a laser relates to the corrugated
structure used. The lasing behavior can be characterized by
measuring the spectrum of the light emitted as a function of
Recent developments
There are a number of excellent reviews giving further
information about the development of polymer lasers up
until 20014-7. In the remainder of this article we will look at
Fig. 4 (a) Output versus pump energy for a polymer DFB laser. (b) Emission spectrum
from a polymer laser operating just above and just below the lasing threshold.
(Reprinted with permission from20. © 2003 American Institute of Physics.)
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(b)
(a)
Fig. 5 (a) Light emission from a polymer DFB laser as a function of wavelength and observation angle, red corresponds to the strongest emission, deep blue the weakest emission; A is the
intense laser mode, B is the diffracted spontaneous emission from the film, and C is the spectral region within which the grating blocks the propagation of light. (b) Transmission of light
through the same laser structure, blue corresponds to high transmission, red to weak transmission; inset shows the polymer laser structure and geometry of observation. (Reproduced with
permission from43. © 2004 Institute of Physics Publishing.)
observation angle. This is shown as a color contour plot in
Fig. 5a. There is a very strong feature at 612 nm (marked A)
at zero degrees, corresponding to light emitted perpendicular
to the substrate. This is the lasing mode. There is a weaker
(though larger) ‘X’ shaped feature (marked B) produced by
diffraction of waveguided light in the corrugated structure.
There is also a region of low intensity (marked C) at 610 nm,
which is a wavelength whose propagation is blocked by the
grating structure. In collaboration with Bill Barnes at the
University of Exeter, we have shown that all the features of
this diagram can be explained by measuring the photonic
properties of the structure17,19. This is done by measuring the
transmission of light through the laser structure as a function
of angle, the results of which are shown in Fig. 5b. At some
angles, light will couple to waveguide modes, leading to a
reduction in the light transmitted. This is shown by ‘hot’
regions in the diagram. There is a remarkable similarity
between this figure, which measures the coupling to
waveguide modes, and the laser behavior in Fig. 5a. Every
feature of the light emission can be explained in terms of the
photonic band structure17,19. A complementary approach is
to calculate the effect of the grating profile44.
This kind of detailed understanding is helpful for
developing improved lasers. The pump lasers used for
polymer lasers include regenerative amplifier, Ti:sapphire,
large Q switched Nd:YAG, and nitrogen lasers, which are
typically the size of a large desk. We have recently shown
that a polymer DFB laser like that in Fig. 3e can have
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September 2004
sufficiently low threshold to be pumped by a much smaller
laser – a microchip laser the size of a large matchbox20. A
similar pumping scheme has also been applied to evaporated
organic semiconductor lasers39. The pump and polymer lasers
are shown together in Fig. 6. It is even possible to hold the
polymer laser in the pump beam by hand and for it to lase.
This more compact and lightweight pump source brings
practical polymer lasers much closer.
Fig. 6 Polymer laser (MEH-PPV) pumped by a matchbox-sized microchip laser. The
emission pattern is typical of a DFB structure consisting of two perpendicular gratings
when pumped far above threshold; at lower pumping powers only the bright central spot
is emitted.
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Simple fabrication of microstructure
One of the attractive features of semiconducting polymers is
their simple processing. However, regular 400 nm corrugated
features are required in DFB lasers. This is a very small length
scale – smaller than can be conveniently made by simple
photolithography – so holographic techniques are often used.
Another option is to use electron beam lithography, but this
is slow and requires expensive equipment. Several groups
have recently developed an alternative approach that takes
advantage of the simple processing properties of
polymers21,24,45,46. Soft lithography – a family of techniques
pioneered by Whitesides47 – enables simple patterning at
very high resolution. One approach we have used is to make
the grating by hot embossing, in which the semiconducting
polymer is softened by heating above its glass transition
temperature and pressed against a conventionally made
corrugated master to transfer the pattern. Remarkably, using
apparatus as simple as a hotplate and some weights, 400 nm
gratings can be readily and accurately embossed into a
semiconducting polymer45. An even simpler method21 of
patterning a semiconducting polymer is shown in Fig. 7a. An
elastomeric mold with the required grating structure is made
in polydimethylsiloxane (PDMS). It is then ‘inked’ by applying
a solvent using a cotton bud and pressed against a thin film
of the semiconducting polymer. In only two minutes the
400 nm grating is transferred into the semiconducting
polymer. The procedure is very simple – the mold and
polymer film can be hand-held. The resulting structure is a
working laser, as shown in Fig. 7b. A threshold for laser
operation is seen and the inset shows the narrowing of the
spectrum above the threshold, which is characteristic of
lasing. The significant features of this result are that the
semiconductor has been patterned directly on a tiny length
scale in a very simple way, and that the procedure has
resulted in a working photonic device.
Semiconducting polymers and ultrafast photonics
The broad spectra of conjugated polymers make them
suitable candidates for ultrafast photonics. The generation of
short laser pulses requires a gain medium that operates over
a wide range of energies. This is because of the uncertainty
principle, ∆E∆t > h/4π (where h is Planck’s constant). A short
laser pulse has a time duration ∆t and, consequently, consists
of a large range of photon energies ∆E (or, equivalently, a
large range of wavelengths). In addition, high-speed data
transmission will require broadband amplifiers. In our lab, we
have explored these issues as part of the Ultrafast Photonics
Collaboration, a major project involving six UK universities48.
We have explored whether conjugated polymers are
suitable for use as optical amplifiers. Although many
experiments demonstrating the existence of gain have been
performed in conjugated polymers, until recently optical
amplifiers, i.e. devices that take a weak light pulse and
amplify it into a much stronger one, had not been made from
these materials. In our experiments, a solution of the polymer
OC1C10-PPV (Fig. 2b) was excited by a nitrogen laser. Weak
light pulses were passed through the conjugated polymer and
amplified by a factor of 1000 to 30 000 after passing through
1 cm of conjugated polymer solution49. This demonstrates
strong amplification with, as shown in Fig. 8, substantial gain
over more than 60 nm across the visible spectrum,
Fig. 7 (a) Schematic of a simple process for patterning the feedback resonator in polymer DFB lasers. (b) Energy characteristics and emission spectra above and below the lasing threshold
of a polymer laser fabricated by soft lithography. (Reproduced with permission from21. © 2003 American Institute of Physics.)
September 2004
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Fig. 8 Spectrum of gain in decibels from a 1 cm long optical amplifier based on a solution
of the conjugated polymer OC1C10-PPV. The experimental configuration is shown as an
inset. (Reproduced with permission from49. © 2002 American Institute of Physics.)
corresponding to a bandwidth of more than 50 THz. Hence,
polymer optical amplifiers would be suitable for ultrafast
data communications in the red region of the spectrum, and
would also be compatible with polymer optical fibers.
We have explored the scope for short pulse generation in
polymer DFB lasers consisting of films of the polymer
OC1C10-PPV22. The polymer lasers were excited by 100 fs
(1 fs = 10-15 s) light pulses from a Ti:sapphire laser, and the
time dependence of the emission was measured using a
streak camera – an instrument with time resolution of
approximately 800 fs. The results in Fig. 9 show that the
measured pulse shape from the polymer laser is very close to
the instrument response function, implying that the polymer
laser is generating light pulses of less than 800 fs duration.
This shows that the broad spectra of conjugated polymers
can be used for short pulse generation.
Toward electrical pumping of polymer lasers
Electrically pumped lasers would be much more attractive
than existing optically pumped devices as they could be
powered by a battery instead of another laser – a much
simpler, smaller, and cheaper source. The challenges in
making an electrically pumped laser are, however,
considerable. The low mobilities of semiconducting polymers
limit the current densities that can be passed through these
materials, so that the excitation densities achieved in typical
LED operation are much lower than those required for laser
operation. However, sufficient excitation densities have been
achieved in pulsed LEDs5,7. This leaves two further serious
obstacles. One is that an electrically pumped laser would
require contacts, which would absorb light in the laser cavity
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September 2004
and increase the threshold for lasing. The other is that an
electrically pumped laser will necessarily contain many
injected charges and nonemissive triplet excited states, which
will lead to additional absorption not present in the optically
pumped case.
There are a few encouraging reports relating to the
problem of contact absorption. One possible solution is to
use a thin transparent contact of indium tin oxide (ITO) on
both sides of a waveguide laser37. The ITO layer must be very
thin to avoid acting as a waveguide, although this tends to
make the contacts very resistive, which would limit the area
of the laser. Another disadvantage is that ITO is not the
preferred contact for electron injection; most polymer LEDs
use a low work function metal as the electron injecting
contact. Optically pumped lasing has been demonstrated in
the presence of a metal contact for a 100 nm thick polymer
film (which is the typical thickness preferred in polymer LEDs
for good charge injection and transport)18. However, the
threshold of the laser was substantially increased as a result
of the additional resonator loss due to the metal. The
increased threshold can be avoided by using a much thicker
(>300 nm) polymer layer15,30, although this would not be
ideal for electrical pumping. The above reports indicate
different points in a trade-off between optimizing optical and
electrical properties, and provide ideas for the reduction of
losses arising from contacts to allow lasing.
This then leaves the absorption of injected carriers and
triplets as a major problem for electrically pumped lasers. It is
conceivable that materials engineering might modify their
spectra so that they overlap less with the emission spectrum.
While the triplet absorption occurs mainly at longer
wavelengths than the emission5, the charge absorption is
Fig. 9 Temporal pulse shape from a polymer laser pumped by a 100 fs pulsed Ti:sapphire
laser.
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generally very broad so it will be difficult to achieve this. It
might be possible to reduce the charge carrier density by
increasing mobility, although this could have other adverse
consequences50. Alternatively, it may be feasible to separate
the charge transporting and active laser regions of a device.
The challenges of direct electrical pumping of a polymer
laser make the use of compact optical pump sources an
attractive alternative in the medium term. Microchip pump
lasers represent a first step in this direction, while either GaN
or polymer LEDs could provide an integrated, efficient
pumping scheme for polymer lasers in the future. Such an
approach would allow polymer lasers to be electrically
pumped indirectly, while avoiding the problems of losses
caused by charge absorption and contacts outlined above.
Conclusions
Semiconducting polymers combine novel semiconducting
properties with simple processing and can be tuned to give
desired properties. This makes them promising for a wide
range of optoelectronic applications including LEDs, lasers,
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and solar cells. In the context of lasers, their broad spectra,
strong absorption, efficient solid-state fluorescence, and
potential for electrical pumping make them attractive
materials. The development of polymer lasers is at a much
earlier stage than polymer LEDs, but enormous progress has
been made in understanding and improving the optical
design, reducing the threshold, and exploiting polymer
properties to enable simple patterning. Major challenges
remain as, at present, all polymer lasers are optically pumped
and pulsed. Nevertheless, optically pumped lasers can be very
useful and could find a range of applications as low cost
tunable sources for spectroscopy. Their stability needs further
testing, but the remarkable progress in polymer LED lifetimes
is extremely encouraging for polymer lasers. An electrically
pumped polymer laser will require considerable further
progress but, in the meantime, indirect electrical pumping is a
promising route to battery-powered polymer lasers. MT
Acknowledgments
We are grateful to the UK Engineering and Physical Sciences Research Council, the Scottish
Higher Education Funding Council, and the Royal Society for financial support.
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