The Internet owes its power to countless light pulses with which enormous amounts of data are sent around the globe via optical fibers. To steer and control these light pulses, various technologies are employed. One of the oldest and most important is the diffraction grating, which deflects light of different colors in precisely determined directions. For decades, scientists have been trying to improve the design and production of diffraction gratings to make them suitable for today's demanding applications. Researchers now have developed a new method by which more efficient and more precise diffraction gratings can be produced.
Diffraction gratings are based on the principle of interference. When a light wave hits a grooved surface, it is divided into many smaller waves, each emanating from an individual groove. When these waves leave the surface, they can either add together or cancel each other, depending on the direction in which they travel and on their wavelength (which is related to their color). This explains why the surface of a CD, on which data is stored in tiny grooves, generates a rainbow of reflected colors when it is illuminated by white light.
For a diffraction grating to work properly, its grooves need to have a separation similar to the wavelength of the light, which is around one micrometer — a hundred times smaller than the width of a human hair. Traditionally, these grooves are etched into the surface of a material using manufacturing techniques from the microelectronics industry; however, this means that the grooves of the grating are square in shape. Physics indicates that grooves should have a smooth and wavy pattern, like ripples on water. Grooves made with traditional methods can, therefore, only ever be rough approximations, which in turn means that the diffraction grating will steer light less efficiently.
The new approach is based on the scanning tunneling microscope in which material surfaces are scanned by the sharp tip of a probe with high resolution. The images resulting from such a scan can even show the individual atoms of a material. One can also use the sharp tip to pattern a material and thus produce wavy surfaces. To do so, the researchers heat the tip of a scanning probe to almost 1000 °C and pressed it into a polymer surface at certain locations. This causes the molecules of the polymer to break up and evaporate at those locations, allowing the surface to be precisely sculpted. In this way, the scientists can write almost arbitrary surface profiles point-by-point into the polymer layer with a resolution of a few nanometers. Finally, the pattern is transferred to an optical material by depositing a silver layer onto the polymer. The silver layer can then be detached from the polymer and used as a reflective diffraction grating.
This allows the researchers to produce arbitrarily shaped diffraction gratings with a precision of just a few atomic distances in the silver layer. Unlike traditional square-shaped grooves, such gratings are no longer approximations but practically perfect and can be shaped in such a way that the interference of the reflected light waves creates precisely controllable patterns.
The perfect gratings enable new possibilities for controlling light; for example, to build tiny diffraction gratings into integrated circuits with which optical signals for the Internet can be sent, received, and routed more efficiently.