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Laser physics

Fantastic plastic

Plastics are ubiquitous, thanks to the cheapness and versatility of these materials. Now plastic lasers are in prospect, battery-operated for low-cost communication and display applications.

Nearly all plastics are electrical insulators, but one class of plastics is different. These are conjugated polymers, whose discovery was celebrated with the award of the 2000 Nobel Prize in Chemistry. They differ from other polymers in having a backbone of alternating single and double bonds, and this difference in structure makes them semiconductors. Conjugated polymers are therefore interesting materials, with their unique combination of semiconducting properties and scope for simple fabrication and shaping. Lasers are a promising application, and now Reufer et al.1, writing in Applied Physics Letters, report fresh progress towards the creation of low-cost polymer lasers.

A major breakthrough in the development of semiconducting polymers was the discovery that when a voltage is applied to a thin layer of one of these materials, light can be emitted2. Light-emitting diodes using this effect are now the basis of a modern flat-panel display technology. Other polymer optoelectronic devices have followed, including polymer solar cells, optical amplifiers and lasers, although further development is necessary before they reach the market. Polymer lasers are attractive as light sources for several reasons. Polymers can emit light across the visible spectrum, and so wavelengths can be generated that are not readily available from other lasers. The polymers have broad spectra, meaning that a polymer laser can be tuned over a range of wavelengths. Polymer lasers should also be simple to make, can be flexible, and should be suitable for a wide range of applications, from plastic optical circuits to biological screening and assays.

Lasers consist of two key components: the gain medium and a resonator. The gain medium is a material in which light is amplified. In most lasers, this would be a crystalline inorganic material, such as ruby or gallium arsenide (the latter is commonly used in CD players and laser pointers). Light passes backwards and forwards through the gain medium, thanks to the feedback generated by the resonator. In plastic lasers, a semiconducting polymer is used as the gain medium3,4,5. Light can be passed backwards and forwards through the polymer with mirrors3,4, but in most polymer-laser work, including that of Reufer et al.1, a corrugated gain medium is used instead (Fig. 1a). The corrugation acts as a diffraction grating that diffracts light travelling in one direction back in the opposite direction, thereby creating feedback and enabling lasing to occur. As there are no mirrors in these lasers, they are compact, robust and exceptionally easy to align.

Figure 1: Low-cost lasers from plastic.

a, A laser consists of a light-amplifying material and a resonator. Polymer (plastic) semiconductors can be used to amplify light, with a corrugated structure acting as a resonator. The corrugation diffracts light travelling to the left back towards the right, and vice versa. Hence, light is sent backwards and forwards through the amplifying medium. Typically, the excitation is provided by another laser. It would be more convenient, however, to make polymer lasers that operate using electrical excitation — say, from a battery. b, Reufer et al.1 have shown how to apply electrical contacts to a semiconducting polymer laser without spoiling its optical properties. With a thick semiconducting polymer layer in the laser, the electric field of light in the polymer is weak at the contacts. So absorption (hence, loss) of light at the contacts is low and the laser works effectively (although still, for now, with laser excitation).

A laser needs energy in order to operate. This can be supplied in two ways: either optically (from flash-lamps or another laser) or electrically. There is a minimum energy, the threshold, required for operation. Above threshold, the gain (or amplification) exceeds all the losses in the device, and lasing begins. At present, all polymer lasers are optically excited, or ‘pumped’. However, as mentioned above, light emission from semiconducting polymers can be induced electrically, and an electrically pumped laser would be much more convenient.

There are three main reasons why it has so far been possible to make only optically pumped polymer lasers. The first is that, in order to reach the threshold typical of existing lasers, a higher current density would be required in a polymer laser than most semiconducting polymers can readily withstand. The second is that an electrically pumped polymer laser would require electrical contacts. These would normally be metal and would introduce substantial additional losses, raising the threshold even further. Finally, the electrical charges injected would also absorb light, leading to increased losses and higher threshold.

The first problem can be solved by using pulsed excitation. Reufer et al.1 have addressed the second problem — the losses associated with electrical contacts. A few groups have suggested possible solutions5,6,7,8, and lasing has been achieved in the presence of a metal contact but with a considerably increased threshold9. Now Reufer et al. have demonstrated a way in which a metal contact can be applied without a threshold increase (Fig. 1b). The losses depend on the electric field distribution through the polymer layer. If the polymer layer is made thicker, this reduces the strength of the electric field at the metal contact, thereby reducing the associated losses. The metal contact used was silver, but the approach should be more generally applicable. To allow the light out of the laser, Reufer et al. used a very thin (20 nm) layer of indium–tin oxide, a transparent conductor, as the other electrical contact. This additional layer only slightly increased the threshold.

It should be noted that the polymer laser was optically pumped (that is, energy was supplied by another laser). But this latest work does show a way of dealing with the losses associated with electrical contacts, and hence is a significant step towards an electrically pumped polymer laser. There are limitations to Reufer and colleagues' laser, however. In particular, the amount of current that could be passed through the device is limited for several reasons: silver contacts are not ideal for injecting charge effectively into semiconducting polymers; the thicker polymer film used in this set-up makes charge injection and transport more difficult; and the electrical conductivity of the thin indium–tin oxide film is relatively poor, limiting the area of such lasers and increasing their operating voltage. Overall, however, Reufer and colleagues' results1 bring low-cost, battery-operated, visible lasers a step closer, and will stimulate renewed interest in plastic lasers.


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