Stopping Light All Optically

We show that light pulses can be stopped and stored coherently, with an all-optical adiabatic and reversible pulse bandwidth compression process. Such a process overcomes the fundamental bandwidth-delay constraint in optics and can generate arbitrarily small group velocities for any light pulse with a given bandwidth, without any coherent or resonant light-matter interactions. We exhibit this process in optical resonators, where the bandwidth compression is accomplished only by small refractive-index modulations performed at moderate speeds.

Figure 1

Schematic of a tunable microcavity system used to stop light. The disks represent cavities, and the arrows indicate available evanescent coupling pathways between the cavities. The system consists of a periodic array of coupled cavities. Each unit cell of the array contains a waveguide cavity $A$, which couples to nearest neighbor cells via evanescent coupling with a coupling strength $\alpha$. Each waveguide cavity $A$ is also coupled to either one or more side cavities (with coupling strength ${\beta }_i$). The figure shows the case with two side cavities, labeled $B_1$ and $B_2$.

Figure 2

Schematic of the frequency bands ${\omega }_{+}$ and ${\omega }_{-}$ for the system shown in Fig. 1 with a single side cavity in each unit cell. ${\omega }_A$ and ${\omega }_B$ are the resonance frequencies for the waveguide cavities $A$ and the side cavities $B$, and $\beta$ is the coupling constant between them. The widths of the lines represent the widths of the frequency bands. (a) ${\omega }_A-{\omega }_B\ll -|\beta |$. The frequency band ${\omega }_{-}$ exhibits a large bandwidth centered at the pulse frequency ${\omega }_0$. (b) ${\omega }_A-{\omega }_B\gg |\beta |$. The frequency band ${\omega }_{-}$ exhibits a small bandwidth.

Figure 3

Propagation of an optical pulse through a coupled microcavity complex in a PC as the resonant frequencies of the cavities are varied. The PC consists of 100 cavity pairs. The pulse is generated by exciting the first cavity, and the excitation reaches its peak at $t=0.8t_{\mathrm{p}\mathrm{a}\mathrm{s}\mathrm{s}}$, where $t_{\mathrm{p}\mathrm{a}\mathrm{s}\mathrm{s}}$ is the traversal time of the pulse through the waveguide by itself. Fragments of the PC are shown in (b). The three fragments correspond to cavity pairs 3–6, 56–60, and 97–100. The dots indicate the positions of the dielectric rods. The black dots represent the cavities. (a) The dashed green and black lines represent the variation of ${\omega }_A$ and ${\omega }_B$ as a function of time, respectively. The blue solid line is the intensity of the incident pulse as recorded in the first waveguide cavity. The red dashed and solid lines represent the intensity in the last-waveguide cavity, in the absence and in the presence of group velocity reduction, respectively. The group velocity reduction occurs from $1.0t_{\mathrm{p}\mathrm{a}\mathrm{s}\mathrm{s}}$ to $1.5t_{\mathrm{p}\mathrm{a}\mathrm{s}\mathrm{s}}$. The pulse is then held near stationary in the system until $6.0t_{\mathrm{p}\mathrm{a}\mathrm{s}\mathrm{s}}$. Afterwards, the pulse is completely released at $6.5t_{\mathrm{p}\mathrm{a}\mathrm{s}\mathrm{s}}$. Open circles are FDTD results, and the red and blue lines are from coupled mode theory. (b) Snapshots of the electric field in the PC at the indicated times. Red and blue represent large positive and negative electric fields, respectively. The same color scale is used for all the panels.