Observation of simultaneous fast and slow light

We present a microresonator-based system capable of simultaneously producing time-advanced and time-delayed pulses. The effect is based on the combination of a sharp spectral feature with two orthogonally-polarized propagating waveguide modes. We include an experimental proof-of-concept implementation using a silica microsphere coupled to a tapered optical fiber and use a time-domain picture to interpret the observed delays. We also discuss potential applications for future all-optical networks.

Figure 1

(Color online) (a) The waveguide supports two degenerate modes with orthogonal polarizations, coincident with the resonator natural polarization axes ($\widehat{x}$ and $\widehat{y}$). The incident field ${\vec{E}}_0$ polarization is oriented halfway between the two axes. A polarizer at the output selects either the polarization parallel to the incoming one $\left(\theta =\pi /4\right)$ or the perpendicular one $\left(\theta =-\pi /4\right)$. (b) Optical microscope picture showing a microsphere close to a tapered optical fiber.

Figure 2

(a) Calculated transmission spectrum for both detected polarizations in a slightly overcoupled regime $\left(a=0.999963,r=0.999960\right)$. At resonance, a fraction of the radiation has its polarization rotated by 90°. (b) Phase spectrum for both polarizations. The parallel polarization shows anomalous dispersion near the resonance, while the perpendicular one shows normal but steep dispersion.

Figure 3

(a) Calculated transmitted pulses in the resonator natural polarization axes, for the same conditions as in Fig. 2. The $\theta =0$ component is affected by the resonator, while the $\theta =\pi /2$ one is not. The input pulse is polarized along $\theta =\pi /4$. (b) Calculated transmitted pulses for the parallel and orthogonal polarizations. The peak of the parallel pulse is advanced in time, while that of the perpendicular pulse is delayed.

Figure 4

(a) Measured time traces showing negative and positive group delay polarization angles. The dotted lines represent numerical calculations with estimated attenuation and coupling coefficients $a=0.999998$ and $r=0.999947$. Inset: spectrum of the unloaded cavity resonance used in the experiments. (b) Observed group delay measured for different laser frequencies. (c) Experimental transmission spectra complementing the group delay data in panel (b).

Figure 5

Experimental polarization conversion for different coupling strengths. Significant conversion is observed as the coupling becomes stronger.