Photonic crystals in the optical regime — past, present and future
Introduction
From the beginning of civilisation, man has sought to illuminate the darkness. Fires, gas lights and Edison's light bulb are but a few in a long chain of light sources that have been employed. Considering the high level of technical sophistication that we have achieved, it is surprising that we still rely mainly on such archaic light sources as the incandescent light bulb or on fluorescent strip-lights, particularly since these light sources typically waste 80–95% of the electrical energy input. This is a state of affairs that needs to be improved, so better light sources are required. This need has already been recognised in a recent report compiled by the U.S. Academy of Sciences [1], who cite lighting by LEDs as having a major impact on the economy in the next century, both in terms of lighting and energy saving. In order to make more efficient LEDs, we need to use the concepts of photonic crystals, which brings us to the main topic of this paper.
As far as the material for LEDs is concerned, semiconductor-based light emitters are the obvious choice. Mainly driven by the needs of the information technology industry, semiconductor light-emitting materials have reached a level of sophistication that surpasses that of any other light-emitting material known to man, with internal quantum efficiencies above 90% being routinely obtained. Unfortunately, most of this light is trapped inside the material, and only 3–20% can escape, the remainder being eventually re-absorbed and lost as heat (Fig. 1a).
Laser diodes offer one way out. They rely on stimulated emission and are therefore more efficient than LEDs, but are not applicable to every problem. Some applications, such as displays, demand the simpler and cheaper LED, since the high level of coherence of laser light is not desirable, or the generally higher cost and temperature dependence of laser diodes is prohibitive. Therefore, a concept that offers the ability to improve the efficiency of LEDs is a welcome addition to the toolbox.
Photonic crystals represent this concept, and their advantage lies in addressing the extraction problem mentioned above. Moreover, photonic crystals not only offer a simple improvement of the extraction of light from LEDs, they can also alter the fundamental process of light generation in the material itself (Fig. 1b). Perhaps more significantly, they can improve the operating characteristics of laser diodes. Laser diodes have experienced a tremendous development in recent years, and specific devices have already shown total external (“wallplug”) efficiencies of 50% and above [2], [3], [4]. Most types of laser, however, still waste too much light, light that could be re-used, or not be emitted in the first place, if photonic band gap principles were applied.
So, be it a laser or an LED, photonic crystals promise improvement. Their ability to control the flow of light and their capacity to concentrate light and enhance light–matter interaction is certainly inspiring. Examining whether this promise is realistic and what else they have to offer, e.g. in the realm of passive devices, such as filters and waveguides, is the purpose of this paper. In Section 2, we introduce photonic crystals and show how they have emerged. A review of the state of the art is given in Section 3 and a speculative forward-look at what they may be used for in the short and longer term is attempted in Section 4.
Section snippets
The past — how did it all start?
In 1946, Purcell [5] was the first to discuss the fact that the presence of a mirror can substantially alter the radiation properties of an electromagnetic dipole. This insight was further developed over the years, and in 1987 led to the concept of a photonic band gap [6], [7], a concept that was borrowed from semiconducting crystals in analogy to their electronic band gap.
How to make a photonic crystal?
There is a variety of ways of making photonic crystals, many of which have been borrowed from the silicon microelectronics industry.
Light emitting diodes (LEDs)
As indicated in the introduction, we see some of the major applications for photonic crystals in novel light sources. Consider, for example, the light emitting diode (LED). The active material of LEDs is still invariably semiconductor, which is the most efficient light-generating material known, with up to 99.7% internal quantum efficiency reported [83]. The problem then lies in the extraction of this light: as a consequence of Snell's law, only the radiation within a small cone (around 16° for
Conclusions
We have reviewed the current range of activity in the field, emphasising semiconductor-based photonic crystals because we feel that they are particularly promising due to their high refractive index and light emitting properties. We have also highlighted some of the material problems that occur with semiconductor-based photonic crystals, such as broad emission bandwidth and surface recombination, which hamper cavity enhancement and compromise luminescence efficiency, respectively. Some of these
Acknowledgements
The authors would like to thank C.J.M. Smith, C.N. Ironside and S.G. Romanov at Glasgow University, D. Labilloy, H. Benisty and C. Weisbuch at Ecole Polytechnique, Palaiseau, as well as M. Boroditsky and E. Yablonovitch at UCLA for fruitful discussions. We would also like to express our gratitude to the Royal Society and the EPSRC of the UK, and the ESPRIT-programme of the European Union for financial support.
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