Print this page 16 April 2007 2007/19

Ultrafast changing 'fingerprints' of light

Researchers at the FOM Institute for Atomic and Molecular Physics (AMOLF) in Amsterdam, together with fellow-researchers at the University of Bristol (Great Britain), National Institute of Advanced Industrial Science and Technology (AIST Japan) and the University of Tsukuba (Japan), have been succeeded in taking a photograph of a light pulse while it is moving through a photonic chip. A detailed plan of the measurements was made, so that the researchers were able to construct a ‘photonic fingerprint’ of the moving light. Such a fingerprint enables them to see exactly the ways the light is allowed to travel in a crystal with a complex periodic structure. The improved insight into fundamental physical processes will lead to innovations in the field of optical telecommunication. The researchers will soon have their investigation published in Nature Physics.

Objects, such as a snare or the skin of a drum, or, say, air in an organ pipe or a flute, preferably vibrate in one particular way that depends on its geometry, for example, the length and width of the organ pipe. The bulk of all musical instruments uses this characteristic. The pitch of the musical instrument closely connects the outline of the preferential vibration, you may think of the typical outline of the trembling snare of a guitar.
The outline of the preferential vibration can be described into a mathematical function, the eigenfunction, that conveys how, for example, the divergence of the snare of a guitar or the air pressure in the organ pipe depends on its position alongside the snare of the organ pipe.

Eigenfunctions are extremely useful for describing nature around us, because they can simply convey the complex movements: each random movement of a snare can be calculated out of a sum of eigenfuncties. The eigenfunctions of a snare can be easily described by some wavelengths: every pitch has a specific wavelength. You might say that the combination of one pitch and one wavelength is the fingerprint of the eigenfunction. It is also possible to describe an eigenfunction for two-dimensional waves. Thus, a drop falling into a pond, will cause a wave that moves in all directions at the same pace. We then see a circular wave pattern of concentric bands and its centre is the place where the drop touches the water. The fingerprint that describes this wave is a circle: the shape of the circle indicates that the wave moves in all directions and that it has the same wavelengths in all directions.

Waves in a crystal
When looking at waves that move on in a crystal that has a periodic structure, it becomes much more complicated to describe them. In a case like this the wave ‘sees’ another structure that is dependent on the direction of the wave in the crystal. The wavelength will be strongly dependent on its direction. This effect plays a more important part as the wavelength of the wave corresponds more and more to the periodic structure of the crystal. Photonic crystals can be made for light that may influence this light to a great extent. Thus, some colours of light are not allowed to travel in the photonic crystal and therefore, can be locked in the crystal. Scientists already proved before that other colours of light will travel very slowly through the crystal (see Microscope makes light waves visible).
As these characteristics are so revolutionary, scientists expect that all data traffic in future telecommunication will be controlled by this technology of light. In order to give a good description of the characteristics of these crystals, eigenfunctions are needed again. The eigenfunctions can be estimated reasonably well. However, measuring the eigenfunctions is terribly difficult, because inside the crystal a photograph has to be made of the light itself.

Photonic fingerprint
Nevertheless, Rob Engelen (FOM/AMOLF), Yoshimasa Suginoto (AIST), Henkjan Gersen (Bristol), Noaki Ikeda (AIST), Kyoshi Asakawa (Tsukuba) and Kobus Kuipers (FOM/AMOLF) have recently been succeeded in making this photonic fingerprint. By using a unique microscope that is able to measure light inside a crystal structure, they have visualized the light that travels through a two-dimensional photonic crystal. In this photonic crystal (see figure 1) light paths have been created, in which the light is allowed to travel, whereas the light is not allowed to travel in the remaining crystal structure. In this way the light is able to take a sharp bend and at the joint climbing of the paths (see centre of figure) it can leap over one path to another. This structure might be considered to be one of the first photonic chips based on photonic crystals. Each of the light paths has its own preferential vibration, or eigenfunctions, described by photonic eigenfunctions that have their own fingerprint. In order to pass through the chip, the light of the eigenfunction of one section has to leap over to the eigenfunction of the next (or preceding) section. Light may also leap over to eigenfunctions that are travelling in the opposite way, which is unwanted for applications. In order to find out how the light behaves in the chip and whether the chip exactly behaves the way you want, you have to find out how the eigenfunctions are coupled together.

By using a microscope the researchers are able to shoot pictures of the movement of a light pulse through a crystal structure (see figure 2 and the pictures). They were able to determine from these pictures which wavelengths of light were in the structure and which direction they took at which moment: the photonic fingerprint. Figure 2 shows these fingerprints: the axes indicate the way light travels, whereas the distance up to the centre indicates the wavelength of light.

Scientists are able to discover characteristics that, otherwise, would be very hard to find out, by investigating the photonic fingerprint. By considering the fingerprint as a function of time, scientists are able to look at the working of the photonic chip in much more detail than had been possible in the past, even at a time scale shorter than 10^-12 seconds. The expectation is that the recent insights will lead to new and improved optical chips for faster telecommunication.

For more information, please contact Prof.dr. Kobus Kuipers, FOM Institute for Atomic and Molecular Physics ; phone (020) 608 12 34, website www.amolf.nl

The electronic versions of the illustrations can be asked for via Annemarie Zegers, Public Information Office, FOM Foundation, phone (030) 600 12 18.

At the top of this page you may download two pictures. The pictures show the travelling of a pulse light through the crystal structure.

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Film 1

Film 1 bij 2007/19
Film 2

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Figure 1a. Two-dimensional photonic crystal

The photograph shows a view from above - made by an electron microscope - of a two-dimensional photonic crystal, in which light conductors have been constructed. Top left the light conductor is visible that transmits the light. Next to a sharp bend is a section in which two light conductors run parallel, the coupling section. To the right you see two ‘output’ light conductors, through which the light is being transmitted to the outside. The researchers have measured the photonic fingerprints of this structure.
Figure 1b. Two-dimensional crystal (side-view)

The photograph shows a side-view of the two-dimensional crystal, in which one light conductor becomes visible. Notice that the crystal is being constructed in a very thin layer (GaAs) only 250 nanometers thick. The holes have a radius of 102 nanometers and a centre to centre-distance of 339 nanometers.
Figure 2. Photonic fingerprint

To the left you see a shot of a light pulse at different periods of time. The light leaves the structure via the upper conductor as well as the lower outward conductor. To the right you see the 'photonic fingerprint' of the light at the periods of time that correspond to those in the left series. The pictures show that there are different eigenfunctions to the coupling of the pulse. The fingerprint is roughly shaped by two parallel straight lines for each section. The axes indicate to which direction the light travels, whereas the distance to the centre indicates the wavelength of light. Some shots create a hexagonal shape, because the different sections have different directions. The quantity of energy that is found in each eigenfunction can be defined by investigating the clearness of the fingerprints. The circular pattern in the centre is the fingerprint of light that travels in all directions at the same wavelength (just like the waves in the pond) and that does not care about the photonic crystal: this is light that is (unwantedly) leaking from the structure.