Final Activity Report Summary - SLIPPRY (Slow LIght Propagation in Photonic cRYstals (SLIPPRY)) Slowing light down is not the most obvious way to speed up optical networks and reduce their power consumption, but this was the goal of the project SLIPPRY (Slow LIght Propagation in Photonic cRYstals), undertaken by Dr Thomas White in the Microphotonics Group led by Prof. Thomas Krauss at the University of St Andrews. During the project, Dr White and colleagues collaborated with researchers from around the world to study the most efficient ways to slow down light, and to use it to switch, regenerate and monitor optical data signals. Controlling the transmission of light is crucial to the optical communication networks that form the backbone of today's connected society. Light provides an efficient way to transmit large amounts of data, but whenever that data needs to be processed the light signal is converted back to an electrical signal so it can be dealt with using electronics. This is a relatively slow and inefficient step that limits the speed of the network and consumes a lot of power. One solution is to develop optical devices that process the light signal directly, avoiding the need to convert it to an electrical signal. The ability to slow the light down inside such devices means they can be made more efficient and smaller than would otherwise be possible. The devices studied in SLIPPRY are based on planar photonic crystals: thin membranes of silicon patterned with thousands of air holes about 250nm in diameter and 450nm apart. By carefully arranging the holes it is possible to precisely control the speed of light as it moves through the silicon. The first major result of SLIPPRY was the demonstration of an optimized photonic crystal design for slowing light to anywhere between 8 and 30 times less than its normal speed in silicon. These designs provided a basis for more advanced experiments to demonstrate optical switching and other functions. The first practical device demonstrated during SLIPPRY was a slow light optical switch less than one tenth the size of a human hair - about thirty-six times smaller than a conventional optical switch. At this size, hundreds of switches could be placed onto a single optical chip and used to redirect light signals around a network. Another highlight of SLIPPRY was the demonstration of enhanced optical nonlinearity by slow light. Optical nonlinearities occur when a very intense light pulse passes through a material and instantaneously changes its properties. This is the key to many interesting and useful processes in optical communications, and these nonlinearities are strengthened when light is slowed down. Several nonlinear processes were studied during SLIPPRY in collaboration with researchers at the University of Sydney. The most exciting result was also the most unexpected. While performing another experiment, green light was observed from the photonic crystal under test. This was surprising since the only light being used in the experiment was invisible infrared radiation. Further investigation revealed that the green light resulted from a nonlinear process called third harmonic generation, which was converting the infrared light into visible green light with a wavelength exactly one third of the original. Measurements also showed that this was occurring for light powers about one million times lower than previous measurements in silicon. This huge improvement was due to the combined effects of the slow light and the photonic crystal. In an extension of these results, researchers at the University of Sydney used the green light to measure the quality of an ultrafast data signal - a procedure known as ultrafast performance monitoring. This was the first demonstration of high-speed signal processing on a chip using slow light and as such is a very significant result. The SLIPPRY project has made a significant contribution to the understanding of slow light effects in photonic crystals and the potential for using these effects in real applications. The work has helped to establish the microphotonics group at St Andrews University as one of the leaders in slow-light photonic crystal research.