A lot of effort is put into perfecting optical chips, which, among other applications, are used within quantum technology. However, a group at DUT Fotonik is saying that imperfection in the form of disordered structures on optical chips actually may be an advantage. This is an artist's impression of light emission in a disordered photonic crystal waveguide. Anderson localized modes appear as strongly confined "hot spots" where light emission is enhanced. (Image: Søren Stobbe) 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 millimeter. Up until now, a major problem has, however, been the fact that nanometer-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 Physicists at DTU Fotonik Department of Photonics Engineering, a department at the Technical University of Denmark, have now seemingly turned everything totally upside down and demonstrated that disordered structures on an optical chip may be used to capture light waves, for example. 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 approximately 15 times. The discovery allows the production of a brand-new type of optical chips, where disorder is used 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. Optical chips with ordered structures This electron microscope image of a photonic crystal membrane was 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. 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. 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, 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. Optical chips with disordered structures The researchers at DTU Fotonik have fabricated an optical chip where disorder has deliberately been introduced in the structure. Without disorder, the light will propagate along the waveguide, whereas the presence of disorder alters this picture completely. The light will 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 localizing light has proved surprisingly efficient, and in the experiment carried out at DTU Fotonik, the researchers succeeded in localizing the light in the waveguide within a region smaller than 25 µm (1 µm = one-thousandth of a millimeter). The figure shows how imperfections in a photonic crystal result in the localization 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 with the actual holes (black) in the structure), which results in the localization of light (orange areas). 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," in essence capturing and retaining the elementary constituent of the light: the photon. Unbreakable messages and quantum computers The figure shows measurements of the timing of the emission of a photon from quantum dots that are resonantly (red curve) or nonresonantly (black curve) coupled to an Anderson-localizing cavity. The measurements are performed by exciting the quantum dot with a short laser pulse and 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 analyzing 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 nonresonant conditions, which shows the increased light-matter coupling. The ability to localize 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, such as 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 percent unbreakable messages or, ultimately, for a quantum computer that 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 light waves was predicted in theory by the US researcher Philip W. Anderson, who was awarded the Nobel Prize in physics in 1977. This Illustration shows a photonic crystal waveguide (in gray) and an embedded quantum dot (red dot) emitting light (red arrows). The photonic crystals are realized by periodically etching holes in a nanometer-thick semiconductor membrane and the waveguide is obtained by leaving out a row of holes in the structure. (Image: Søren Stobbe) In the 1950s, Anderson predicted that the transport of electrons may be suppressed in a highly disordered lattice. This phenomenon, called Anderson localization, results from the fact that electrons in the world of quantum mechanics have wave properties, and that these waves can interfere – just as other types of waves can be mixed – a phenomenon well-known by everyone who has been swimming in the breakers. Anderson’s discovery has proved to be a universal phenomenon that not only applies to electrons but to all other types of waves. Disorder can thus be used to 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 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-localized 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. 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 nonresonant conditions. The research group behind the discovery The research has been conducted at the department of photonics engineering at Technical University of Denmark by a group consisting of postdoctoral students Luca Sapienza, Søren Stobbe and David Garcia, doctoral students Henri Thyrrestrup and Stephan Smolka, and Peter Lodahl, associate professor and group leader. These findings were published in the journal Science. For more information, visit: www.fotonik.dtu.dk