Encounter with the future

The newly-discovered method of lasing in conjugated polymer microcavities may herald bright possibilities for the electronics industry

Published: Saturday 30 November 1996

lasers, like integrated circuits, are now an important part of many devices and techniques. Their unique properties like coherence and their non-dispersive nature help them play a crucial role in research in areas like quantum mechanics and communications. N Tessler and his colleagues at the Cavendish Laboratory in Cambridge, uk , have reported experiments that indicate lasing (the process of inducing laser) in a new medium, conjugated-polymer microcavities ( Nature , Vol 382, No 6593).

Conjugated polymers are a subclass of polymers (materials which have chain-like organic molecules linked in a well-defined sequence) that allow charge mobility and excitation in the optical frequency range. These polymers are finding many applications in the semiconductor industry. For instance, they are used in flat display units and optoelectronics. The basic phenomena occurring in these materials is electroluminescence: the emission of light when charges are injected into the material. Electroluminescent diodes, devices which allow injection of charges, can also be sandwiched between mirrors. This creates a microcavity which can be used to create a laser. Several inorganic materials have already demonstrated lasing in the microcavity. Tessler and his team have now shown that lasing is also possible with polymers.

The team used conjugated polymer microcavities, a material which was shown to have electroluminescence in 1990. A 100-nanometre (nm) (a nanometre is one thousand-millionth of a metre)-thick layer of the conjugated polymer p-phylenevinylene ( ppv ) was sandwiched between a 60-nm-thick upper mirror and a 100-nm-thick bottom mirror. The device was excited by using a laser with a central wavelength of 355 nm. Three phenomena which indicated lasing were observed. Firstly, the device coupled to a specific mode of the driving laser. Secondly, the emission spectrum was very narrow and finally, the directionality increased with increase in intensity of the pumping laser.

This demonstration of lasing in conjugated polymer microcavities holds tremendous potential for finding certain applications. The conversion of this optically (laser)-driven device to an electrically-driven one is considered fairly simple. The development of low-cost electrically-driven lasers with wavelengths across the visible spectrum and extending to near ultraviolet frequencies will be an important step. The resultant lasers are miniature and tunable. Though commercial developments seem a long way off, improvements in the performance of such devices may force industry to take a closer look at these lasers.

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