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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] 28 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 REVIEW FEATURE 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. September 2004 29 REVIEW FEATURE 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. 30 September 2004 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 REVIEW FEATURE 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.) September 2004 31 REVIEW FEATURE (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 32 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. REVIEW FEATURE 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 33 REVIEW FEATURE 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 34 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. REVIEW FEATURE 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, REFERENCES 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. 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