Observation of Zeroth-Order Band Gaps in Negative-Refraction Photonic Crystal Superlattices at Near-Infrared Frequencies

We present the first observations of zero-$\overline{n}$ band gaps in photonic crystal superlattices consisting of alternating stacks of negative-index photonic crystals and positive-index dielectric materials in the near-infrared range. Guided by ab initio three-dimensional numerical simulations, the fabricated nanostructured superlattices demonstrate the presence of zeroth-order gaps in remarkable agreement with theoretical predictions across a range of different superlattice periods and unit cell variations. These volume-averaged zero-index superlattice structures present a new type of photonic band gap, with the potential for complete wave front control for arbitrary phase delay lines and open cavity resonances.

Figure 1

(a) SEM of a fabricated sample with eight stacks, whose PhC slab layer has a length of $d_1=3.5\sqrt{3}a$. Scale bar: $5\mu \mathrm{m}$. (b) A time-averaged steady-state numerical calculation of the field intensity distribution, $|E{|}^2$, corresponding to a propagating mode ($\lambda =1550\mathrm{nm}$). Scale bar: $5\mu \mathrm{m}$. Left-hand metamaterial (LHM), right-hand metamaterial (RHM). (c) Calculated photonic band structure of the fabricated PhC slab waveguide with $r=0.290a$ and $t=0.762a$ ($a=420\mathrm{nm}$). The TM-like [TE-like] photonic bands are depicted in blue (darker) [red (lighter)]. Inset: SEM of the PhC region of the fabricated superlattice. Scale bar: 500 nm. (d) A zoom-in of the spectral domain corresponding to our experiments. Inset: The first Brillouin zones of the hexagonal PhC and the PhC superlattice.

Figure 2

(a) Transmission highlighting the invariant zero-$\overline{n}$ gaps for a superlattice with $d_2/d_1=0.746$ (design 1), containing 7, 11, and 15 unit cells in each PhC slab. Three superperiods in the PhC superlattice are used for the case of PhC layers with 11 and 15 unit cells, and 5 superperiods for the case of PhC layers with 7 unit cells. The shaded region illustrates the negative-index range, in order to observe the zero-$\overline{n}$ gap. (b) Same as in (a) for $d_2/d_1=0.794$ (design 2). The shifting gaps are the conventional Bragg gaps. The transmission plots are offset vertically for viewing clarity. (c) Transmission for a superlattice with $d_2/d_1=0.794$, containing 3, 5, and 8 superperiods. Each PhC layer contains 7 unit cells. (d) Transmission through a regular PhC slab (nonsuperlattice) containing 15, 30, and 45 unit cells, illustrating a regular Bragg gap. All designs are obtained through full 3D FDTD numerical simulations. In all plots, the shaded region illustrates the negative-index region.

Figure 3

(a) Measured transmission for a superlattice with $d_2/d_1=0.746$, with 7 unit cells in the PhC layers and 5 superperiods; for comparison, results of numerical simulations are also shown. (b) The same as in (a), but for a superlattice with 0.794. (c) Measured transmission for a superlattice with $d_2/d_1=0.746$, with 3, 5, and 8 superperiods and 7 unit cells in the PhC layers. Both gaps become deeper as the number of stacks increases. Inset: Example of near-infrared top view image of 3 superperiods, under transmission measurement at 1550 nm. In all plots, the shaded region illustrates the negative-index region.