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Rapid change of colour in nano-sized hall of mirrors

A team of researchers from the FOM Institute for Atomic and Molecular Physics (AMOLF) in Amsterdam and the University of Minnesota (USA) have succeeded in rapidly changing the selective colour of a nano-sized hall of mirrors – a so-called photonic band gap crystal. The successful experiment of Dr Tijmen Euser and his colleagues has been published in the printed version of the Journal of Applied Physics and is also available online.

Managing the emission and propagation of light are important objectives in physics, nanotechnology and life sciences because light, as a carrier of energy and information, propagates itself at an incredible speed. The new discipline of nanophotonics pursues these objectives. In 1987, the American researcher Eli Yablonovitch predicted that, theoretically, light can be completely controlled in special nanostructures known as photonic crystals. This prediction initiated the worldwide research into such structures. Researchers expect that these will form the building blocks for optical 'chips' in which information can be manipulated in the form of light. Experimentally, this may lead to questions such as: "How can you manipulate atoms in such a way that they emit light if you want them to?" and "How can you control the propagation and even halting of photons?".

Hall of mirrors for photons
A photonic crystal is a highly ordered nanostructure containing many air bubbles (see Figure 1). The typical diameter of the air bubbles is the wavelength of light. Light of a given colour that enters the photonic crystal is scattered in all directions by the structure. The crystal works like a hall of mirrors for photons. Interference¹ effects lead to certain colours being completely excluded by the crystal and therefore not being scattered. The forbidden colour range is termed the photonic band gap. The exclusion of certain colours leads to the special phenomenon of a light source that has entered the crystal (for example an atom or quantum dot) being unable to emit light (see Figure 2, left). The size of the air bubbles and the refractive index² determine the colour of light that can and cannot penetrate the crystal. Once a photonic crystal has been made, these properties cannot be altered.
Or can they…?

If you could briefly change the band gap of a photonic crystal, the options for using the crystals increase enormously, for example being able to switch light on and off. For future 'photonic computer chips' it is important that you can switch the band gap rapidly enough to instantaneously stop the light. Euser and his colleagues are the first to experimentally demonstrate an ultrafast change of colour for a photonic band gap crystal. Figure 2 illustrates a possible application of photonic switching. By rapidly switching the photonic crystal, the light source in the crystal can suddenly emit light when you want it to.

The crystal is switched by 'tapping' it with a short laser pulse that briefly changes the optical properties of the silicon. This results in a change in optical characteristics of the photonic crystal. The colour of the switched crystal is measured with a brief second pulse.

Colour change
The researchers observed the colour of the photonic band gap becoming bluer within 1 picosecond³ (see Figure 3, left). In principle this is fast enough to capture light in the cavity resonators within the crystal. After the crystal has been changed by the first laser pulse, it returns to the original state within 20 picoseconds (see Figure 3, right). The entire switching cycle proceeds so rapidly that the researchers conclude that it may be able to switch 10 times faster than the fastest computers currently available.

The photonic crystals used by Euser and his colleagues have properties similar to the colours used in modern telecommunications media such as glass fibres. The results of this research are therefore important for potential applications. This is partly because the crystals are produced from silicon, a material widely used by the computer industry and one with which it is therefore familiar.

The ultrafast switching of optical crystals lays the foundation for possible binary switches of photons instead of the current electronic switches on silicon chips. However a lot more research is required before such applications can be realised. Current research is focussed on perfecting the structures and the effect of unavoidable imperfections of the photonic crystals. The researchers are also investigating the 'capture and release' of light pulses in minuscule cavity resonators. It should be noted that recently Tijmen Euser, Philip Harding, Willem Vos (AMOLF), together with Yoanna-Reine Nowicky-Bringuier and Jean-Michel Gérard of the CEA in Grenoble (France), were also able to switch photonic cavity resonators. These results were published on 10 September 2007 in the American journal Applied Physics Letters⁴.

The research team
Dr Tijmen Euser carried out his optical experiments at the FOM Institute for Atomic and Molecular Physics in Amsterdam. He gained his doctorate in March 2007 under the supervision of Prof. Willem Vos at the University of Twente. He is now carrying out postdoctoral research at the Max Planck Research Group of the University of Erlangen-Nuremberg in Germany.
Dr Euser can be reached by e-mail: euser@amolf.nl, or by phone +49 9131 687 73 20.

Prof. Willem Vos is group leader at the Centre for Nanophotonics, FOM Institute for Atomic and Molecular Physics in Amsterdam. He is also part-time professor at the University of Twente (Complex Photonic Systems (COPS), MESA⁺ Institute for Nanotechnology). Vos has carried out research into photonic crystals since 1993. He proposed the rapid change of photonic crystals and supervised the key experiments.
Prof. Vos can be reached by e-mail: w.l.vos@amolf.nl, or by phone: +31 (0)20 608 12 34.

The photonic gap crystals were made by the group of Prof. David J. Norris of the Department of Chemical Engineering and Materials Science, at the University of Minnesota, in Minneapolis (USA), in close cooperation with the group of Prof. Albert Polman of the Centre for Nanophotonics, FOM Institute for Atomic and Molecular Physics in Amsterdam.

Article
Tijmen G. Euser, Hong Wei, Jeroen Kalkman, Yoonho Jun, Albert Polman, David J. Norris and Willem L. Vos, Ultrafast optical switching of three-dimensional Si inverse opal photonic band gap crystals
Journal of Applied Physics, volume 102 (14 September 2007).
A preprint of the publication is available at: www.arxiv.org/abs/0705.4250.

Further information
Further information about photonic crystals is available at: www.photonicbandgaps.com
Website of the AMOLF Institute: www.amolf.nl
Website of the Norris group: www.cems.umn.edu/research/norris
Website of the Journal of Applied Physics: http://jap.aip.org/jap

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¹ Interference is the simultaneous effect of two movements that hinder or strengthen each other, in this case light wave movements.

² The refractive index indicates the size of the delay in the speed of light in a material. Light has a refractive index (n) of 1. In silicon light is 3.5 times slower (n= 3.5). Due to this large difference in the speed of light strong reflections occur on the many surfaces within the crystal: the nano-sized hall of mirrors.

³ A picosecond is a millionth-millionth of a second. To gain some idea of the extremely short times it is useful to translate these into distances: Light travels at 300,000 km/s, that is 7.5 times around the Earth in one second. The researchers use laser pulses of just 0.15 picosends. In this time the light travels just half a hair width (0.05 millimeter).
⁴ Reference: Harding et al., Appl. Phys. Lett. 91, 111103 (2007).

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Figure 2. Photonic band gap crystal

On the left hand side, a diagram of a photonic band gap crystal is shown. In the middle of the image a light source has been drawn that will emit light in the same colour as the band gap (red). However, this colour is not permitted in the crystal.
On the right, a short laser pulse rapidly changes the colour of the crystal from red to blue. This allows the red light of the light source to leave the hall of mirrors. Shortly after this the crystal recovers and the light from the light source can no longer escape.
Figure 1. Nano-sized hall of mirrors

An image of a photonic crystal â€“ a nano-sized hall of mirrors â€“ prepared with an electron microscope. The white line indicates two micrometres. The crystals have a periodic structure of air bubbles with a 'skeleton' of silicon (the material that computer chips are manufactured from). The air bubbles for this experiment have a diameter of approximately 900 nanometres. These structures â€“ called inverse opals â€“ form small mirrors for the light within a nano-sized hall of mirrors.
Figure 3. Ultrafast colour change

Observation of the ultrafast change of colour of the band gap as a function of time, after the first laser pulse. The selective reflection colour of the crystal turns bluer within 1 picosecond (left, blue arrow). In the following 20 picoseconds, the crystal rapidly recovers to its original state (right, red arrow).