Pumped photonic crystal accelerates slow light
It might seem paradoxical, but slow light can speed up telecommunications. Slow light increases the effective interaction between light and matter. Consequently, the fastest way of switching light, light-light switching, is now within reach. Researchers from the Nano-Optics group of the FOM Institute AMOLF and the University of St Andrews (UK) have demonstrated how they can allow a slow light pulse to emerge faster from a photonic crystal by illuminating the crystal with a second laser pulse. This week they published their results about the acceleration of slow light in the renowned journal Physical Review Letters. Earlier this year they also published an article in Physical Review Letters about the active delay of slow light.
Photonic crystals are nanostructures in which two or more materials with strongly differing indices of refraction are arranged in a regular pattern. Due to this periodic arrangement of the materials, light in such crystals behaves in a similar fashion to electrons in a crystalline solid: the relationship between wavelength and frequency (colour) is described by a photonic band structure. One important consequence of this band structure is that the light becomes slow for certain frequencies.
Slow light exhibits an increased light-matter interaction and therefore responds more strongly to changes in the refractive index of the photonic crystal. For example, the change in the refractive index needed to switch slow light is smaller than that for faster light. The refractive index of a material can be altered using external parameters such as temperature, pressure, electric field, etc., but also by using light. However, using an external parameter is difficult because the changes are small. Powerful (and expensive) lasers are needed to realise even minimal changes. As slow light is more sensitive, switching light with light becomes easier.
Researchers from AMOLF and the University of St Andrews have demonstrated that they can switch slow light with light. They created a slow light pulse by sending this into a wave-guide in a photonic crystal. Once the entire light pulse was inside the crystal they used a second pulse - the pump - to change the refractive index of the crystal. As a consequence the slow light changed colour: it shifted more towards blue, but its speed did not change. However, by not pumping the entire photonic crystal, but just the part in which the slow light pulse was located, the researchers ensured that in effect two different photonic crystals arose: one where the blue-shifted light flash was slow and one - the 'unpumped' part - in which this blue light pulse moved quicker than an 'unpumped' flash. In effect, the researchers caused an indirect transition of the light: the light shifted in both frequency and wavelength (wavevector). By altering the intensity of the pump laser, researchers could control the time at which the light flash leaves the structure.
Earlier this year the same team also revealed the opposite effect, namely how slow light could be delayed with the help of an external light pulse. The ability to both accelerate and delay light makes it possible to control optical data traffic. Switching light with light instead of light with electronics would allow the bandwidth of telecommunication to be increased by a factor of 10 to 100.
Publications
Daryl M. Beggs, Isabella H. Rey, Tobias Kampfrath, Nir Rotenberg, L. Kuipers, and Thomas F. Krauss, Ultrafast Tunable Optical Delay Line Based on Indirect Photonic Transitions, Phys. Rev. Lett. 108, 213901 (2012),
Daryl M. Beggs, Thomas F. Krauss, L. Kuipers and Tobias Kampfrath, Ultrafast tilting of the dispersion of a photonic crystal and adiabatic spectral compression of light pulses, Phys. Rev. Lett. 108, 033902 (2012).
For further information please contact Prof.dr. L. (Kobus) Kuipers, +31 20 754 7100.