Self-Collimation of Light over Millimeter-Scale Distance in a Quasi-Zero-Average-Index Metamaterial

Inspired by the concept of complementary media, we experimentally demonstrate that an engineered metamaterial made of alternating, stripe layers of negatively refracting (photonic crystals) and positively refracting (air) materials strongly collimates a beam of near-infrared light. This quasi-zero-average-index metamaterial fully preserves the beam spot size throughout the sample for a light beam traveling through the metamaterial a distance of 2 mm—more than 1000 times the input wavelength $\lambda =1.55\mu \mathrm{m}$. These results demonstrate the first explicit experimental verification of optical antimatter as proposed by Pendry and Ramakrishna [J. Pendry and S. Ramakrishna, J. Phys. Condens. Matter 15, 6345 (2003)], using two complementary media in which each $n_{\mathrm{eff}}=-1$ layer appears to annihilate an equal thickness layer of air.

Figure 1

Negative refraction by a PhC and zero-average-index band gap. (a) The EFSs for TM polarization show a negative ${\overrightarrow{v}}_g$. The dashed circle is the air dispersion curve for ${\omega }_n=0.305$ that is practically superposed to the corresponding EFS of the PhC. (b) The band diagram of the supercell, composed of equal lengths of negative refracting PhC and air. The zero-average band gap appears at ${\omega }_n=0.305$.

Figure 2

The field propagating inside the QZAI metamaterials calculated using a FDTD code. (a) The intensity of the electric field component ${\mathrm{E}}_{\mathrm{y}}$ (TM polarization). (b) The time-average Poynting vector. (c) Surface termination helps match the air and negatively refracting PhCs.

Figure 3

SEM images of the nanofabricated device. (a) Alternating of space and photonic crystal material, with different termination on each PhC structure. (b) Magnified image.

Figure 4

Experimental setup and grating coupling. (a) Schematic of experimental setup. (b) Out-of-plane scattering couples with grating direction ${\beta }_{-1}$, ${\beta }_{-2}$. (c) Angular scan in logarithmic scale in the ($y$, $z$) plane.

Figure 5

The experimental images. (a) The image of the scattered radiation along the ${\beta }_{-1}$ direction reveals a well-collimated beam along the whole 2 mm length of the sample, (b) compared to the propagation over the unpatterned SOI. (c) A magnified image of the scattered radiation in the first part, (d) in the middle, and (e) in the final part of the quasi-zero-average metamaterial. (f)–(h) The corresponding beam profile.