We propose and theoretically study a parity-time ($\mathcal{P}\mathcal{T}$)-symmetric photonic-crystal coupled-resonator optical waveguide (CROW) based on buried heterostructure nanocavities which has potential scalability and controllability. We analytically reveal its spectral transport properties with a tight-binding model and show the possibility of the wide-range control of its group velocity using the $\mathcal{P}\mathcal{T}$ phase transition. While the group velocity at the $\mathcal{P}\mathcal{T}$ phase-transition point diverges, the group-velocity dispersion converges. A numerical estimation of the system response to temporal pulse inputs shows that the pulse broadening is not severe in a device of hundreds of micrometers in size. Furthermore, a longer pulse duration results in a higher upper limit of the pulse peak velocity, which can be, in principle, superluminal. We next perform numerical simulations on the considered photonic-crystal slab structures with the finite-element method, and we successfully observe $\mathcal{P}\mathcal{T}$ phase transitions. In the simulated parameter range, gain and loss coefficients of the order of $100{\mathrm{cm}}^{-1}$ meet the condition for the maximum group-velocity coefficient in the context of the tight-binding approach. A 9.3-fold increase in the group velocity at 1502 nm is obtained in a three-dimensional device by switching between the conventional and $\mathcal{P}\mathcal{T}$-symmetric CROWs. Meanwhile, we also encounter band smoothing around the phase transition, which hampers the group-velocity divergence. Our simulation result indicates that it arises from interfering evanescent waves decaying out of the device structure, and we discuss ways to suppress this effect.

• Received 15 February 2017

DOI:https://doi.org/10.1103/PhysRevApplied.7.054023