Disorder is OK, is the surprising message from a research group at DTU Fotonik, thus overturning the common notion that optical chips must be perfect. This discovery was published on 12 March 2010 in the prestigious international journal Science
Messy accounts, noise on the line or production errors: Disorder is, in many respects, considered an evil. This also applies within photonics, and researchers all over the world put a lot of effort into perfecting the optical chips which, among other applications, are used within quantum technology, e.g. in quantum computers.
An optical chip can be used to manipulate information in the form of light, and the functionalities are integrated in a few thousandths of a millimetre. Up until now, a major problem has, however, been the fact that nanometre-scale imperfections are inevitable during optical chip production. So far, it has been the general conviction that this reduces or simply destroys functionality, and that this has hampered the possibility of upscaling optical chips to larger and more complex circuits.
Disorder as a valuable resource
A group of physicists from DTU Fotonik have now turned everything totally upside down and demonstrated that imperfection in the form of disordered structures on optical chips may actually be an advantage: The disordered structures on an optical chip may be used to capture, e.g., light waves.
The research group has demonstrated that when the light is captured on the imperfect optical chip, the interaction of light with matter (an atom) is increased approx. 15 times. The discovery allows the production of a brand new type of optical chips where disorder is utilised as a valuable resource instead of being considered a limitation. It may potentially be used to develop efficient miniature lasers, solar cells and sensors and to pave the way for a completely new quantum information technology, including quantum computers.
This finding is a major basic scientific breakthrough, which will be published in the international journal Science on 12 March 2010.
Optical chips with ordered structures
On optical chips based on photonic crystals, a structure of holes is normally etched, and so far the aim has been to achieve a regular and ordered structure (see Figure 1). Even though modern nanotechnological techniques make it possible to fabricate very precise structures, a certain element of disorder is inevitable in any real system. There will thus be roughness and variations in the positioning of the holes of which a photonic crystal is made up. By changing the distance between the holes in the photonic crystal and omitting a row of holes, a waveguide is created, which can guide light in desired directions, thus providing new possibilities for taming light. A properly designed photonic crystal thus makes it possible to stop or capture light – and even control the emission of light.
Figure 1. Electron microscope image of a photonic crystal membrane made by etching holes in a gallium arsenide (GaAs) substrate. By omitting a row of holes, a waveguide is created, along which the light will propagate. Nanoscopic light sources (so-called quantum dots) are placed in the middle of the membrane, indicated by the yellow triangles on the image.
Optical chips with disordered structures
The researchers at DTU Fotonik have fabricated an optical chip where disorder has deliberately been introduced in the structure (see Figure 2). Without disorder, the light will propagate along the waveguide, whereas the presence of disorder alters this picture completely. The light will thus be captured in the waveguide as it is scattered on the imperfections and subsequently interferes with other parts of the light wave. This way of localising light has proved surprisingly efficient, and in the experiment carried out at DTU Fotonik, the researchers succeeded in localising the light in the waveguide within a region smaller than 25 microns (one micron = one thousandth of a millimetre). In their experiment, the researchers used nanoscopic light sources inside the photonic crystal (the so-called quantum dots). A quantum dot can be seen as an artificial atom emitting exactly one photon at a time. The researchers have thus succeeded in making a ‘box for photons’, i.e. capturing and retaining the elementary constituent of the light: the photon.
Figure 2. The figure shows how imperfections in a photonic crystal result in the localisation of light in a very small area. The red circles show the position of holes in an ideal structure. Random disorder has been introduced in this structure (compare the position of red circles to the actual holes (black) in the structure), which results in the localisation of light (orange areas).
Unbreakable messages and quantum computers
The ability to localise light is crucial for many applications, as light in many contexts is intractable: It propagates at a speed of almost 300,000 km/s, making it very useful for transmitting information for use in optical communication. Unfortunately, it also means that the interaction with matter is generally inefficient, which is a problem for a number of applications, e.g. in solar cells and optical sensors or within quantum information technology. The dawning quantum information technology promises fundamentally new ways of coding and processing information, using the laws of quantum mechanics. This can, among other things, be used to exchange 100% unbreakable messages or, ultimately, for a quantum computer which can perform a number of calculation tasks far more efficiently than even the supercomputers of today.
Research based on Nobel Prize winner’s theory
The use of very disordered structures to capture, e.g., light waves was predicted in theory by the US researcher Philip W. Anderson, who was awarded the Nobel Prize in physics back in 1977.
In the 1950s, Philip W. Anderson predicted that the transport of electrons may be suppressed in a highly disordered lattice. This phenomenon is called Anderson localisation. This is due to the fact that electrons in the world of quantum mechanics have wave properties, and that these waves can interfere, like other types of waves can be mixed, which is a well-known phenomenon by everyone who has been swimming in the breakers. Anderson’s discovery has proved to be a universal phenomenon which not only applies to electrons, but to all other types of waves. Disorder can thus also be used to localise light waves, i.e. capture light in a very small area.
In quantum information technology, it is crucial to have a very strong light-matter coupling at the most elementary level, i.e. so that one photon interacts efficiently with one atom. Such an increased coupling is exactly what the researchers at DTU Fotonik have demonstrated, where a photon in an Anderson-localised cavity interacts with a quantum dot. The increased coupling results in the quantum dot emitting a photon more rapidly when its wave length matches that of the cavity (i.e. is in resonance). This is exactly what the researchers have observed, as shown in Figure 3, which shows that the quantum dot emits photons up to 15 times more rapidly under resonant conditions than under non-resonant conditions.
Figure 3. The figure shows measurements of the timing of the emission of a photon from quantum dots which are resonantly (red curve) or non-resonantly (black curve) coupled to an Anderson-localising cavity. The measurements are performed by exciting the quantum dot with a short laser pulse and subsequently recording when a photon is emitted. If the experiment is repeated several times, the above decay curves are obtained, showing the emitted intensity of photons vs. time. By analysing the curves, the mean decay time for the quantum dot in the two situations is found. Under resonant conditions, the quantum dot decays 15 times faster than under non-resonant conditions, which shows the increased light-matter coupling.
The research group behind the discovery
The research has been conducted at the Department of Photonics Engineering at the Technical University of Denmark by a research group consisting of postdocs Luca Sapienza, Søren Stobbe and David Garcia, PhD students Henri Thyrrestrup and Stephan Smolka as well as Associate Professor and group leader Peter Lodahl.
For further information, please contact:
Associate Professor Peter Lodahl, DTU Fotonik, Quantum Photonics group, tel. (mobile): +45 51 64 74 83, email: .
See also: www.fotonik.dtu.dk/quantumphotonics
Artist's impression of localization of light
in a disordered photonic crystal waveguide