Slow light based optical frequency shifter
  We demonstrate experimentally and theoretically a controllable way of shifting the frequency of an optical pulse by using a combination of spectral hole burning, slow light effect, and linear Stark effect in a rare-earth-ion doped crystal. We claim that the solid angle of acceptance of a frequency shift structure can be close to $2\pi$, which means that the frequency shifter could work not only for optical pulses propagating in a specific spatial mode but also for randomly scattered light. As the frequency shift is controlled solely by an external electric field, it works also for weak coherent light fields, and can e.g. be used as a frequency shifter for quantum memory devices in quantum communication. 
Qian Li, Yupan Bao, Axel Thuresson, Adam N. Nilsson, Lars Rippe and   Stefan Kr\"oll
quant-ph physics.optics