`Slow’ and `fast’ light in resonator-coupled waveguides

We describe a device constructed of a sequence of microresonators coupled to an optical waveguide. The in ̄uence of these resonators is to enhance nonlinearities and to induce strong dispersive e€ ects, leading to exotic optical properties including slow and superluminal group velocities of propagation. In recent years there has been a ̄urry of activity aimed at the development of techniques that can lead to a signi®cant modi®cation of the group velocity of propagation of a light pulse through a material medium [1]. Proposed applications of these procedures include the development of optical delay lines [2] and the `storage` of light pulses [3, 4] with perhaps implications for the ®eld of quantum information. Most of this research has made use of the response of resonant media [5] and much of it has made use of the concept of electromagnetically induced transparency [3, 6]. In this contribution, we describe an alternative procedure for the propagation of slow light based on inducing large dispersive e€ ects in optical waveguides by coupling the waveguide to an array of optical resonators. A typical device of this sort, which we refer to [7, 8] as a side-coupled integrated spaced sequence of resonators (SCISSOR), is shown in ®gure 1. The resonators can be of arbitrary design, although in our experimental work we are concentrating on resonators in the form of ring waveguides or of a whispering gallery mode [9] of dielectric discs. Alternatively, the resonators could be dielectric spheres coupled to an optical ®bre; the excitation of the resonances of such spheres has been observed previously [10]. Since the light ®eld e€ ectively circulates many times in each resonator before passing to the next, the group velocity of propagation of a pulse of light through such a structure is signi®cantly reduced. In addition, the phase shift experienced by the light wave in interacting with each resonator depends sensitively on its detuning from the cavity resonance, and thus this structure produces large and controllable dispersion. Moreover, if the resonator is constructed of a material that displays a nonlinear optical response, the nonlinear phase shift acquired by a light wave in interacting with the resonator scales as the square of the ®nesse of the resonator [11]; thus such a structure displays an enhanced nonlinear optical response. For these reasons, devices of this sort may prove extremely useful for many applications in modern photonics. Related approaches o€ ering similar promise, but possessing photonic bandgaps (which are necessarily attenuating) Journal of Modern Optics ISSN 0950±0340 print/ISSN 1362±3044 online # 2002 Taylor & Francis Ltd http://www.tandf.co.uk/journals DOI: 10.1080/0950034021000011527 journal of modern optics, 2002, vol. 49, no. 14/15, 2629±2636