Rainbow trapping effect in horizontal and vertical directions

It is known that light can be slowed down in the vicinity of resonances in dispersive materials.5 In order to reduce the group velocity (vg) of light coherently, there are primarily two major approaches employing either electronic or optical resonances. In the electronic scheme, drastic slowing down and complete stopping of light pulses can be accomplished by converting optical signals into electronic coherences.6-9 The use of electronic states to coherently store the optical information, however, imposes severe constraints in the scheme, including narrow bandwidth, limited working wavelengths and strong temperature dependence. While promising steps have been taken towards slowing light in solidstate media and semiconductor nanostructures operating at room temperature,10 it still remains a great challenge to implement such schemes on-a-chip incorporating optoelectronic devices.9 Besides, only a few special and delicate electronic resonances available in nature possess the required properties. As a result, there has been great interest in pursuing alternative approaches utilizing optical resonances in photonic structures, such as microcavities,11photonic crystals,12 semiconductor waveguide ring resonators,13etc. In 2007, researchers in UK proposed a tapered structure based on metamaterial with negative refractive index.14 Theoretically, this structure should be able to slow light to a standstill. The material would halt each frequency of light, and hence color, at slightly different places, to make so called “trapped rainbow” (see Fig. 9.1), which opened a new and attractive approach to stopping light.