Near-Infrared Metamaterials Go Beyond Metals
Authors: Gururaj V. Naik and Alexandra Boltasseva
Affiliation: School of Electrical and Computer Engineering and Birck Nanotechnology Center, Purdue University, USA
Engineering the flow of light at the nanoscale is enabled by plasmonics and metamaterials. Research in metamaterials has progressed rapidly in the past decade, producing many breakthroughs that have changed our fundamental understanding of light propagation and interactions and pushed the frontiers of possible applications. The enormous potential of metamaterials is clogged by the limitations arising from the materials, particularly metals that constitute these metamaterials. These limitations of metal building blocks are particular detrimental to the operation of metamaterial devices in the optical range .
Metals are the bottleneck of performance in many classes of optical metamaterials. The limitations arise from undesirable properties of metals such as high losses, large magnitude of permittivity, lack of tunability of optical properties, and challenges associated with nanofabrication and integration . A possible alternative to metals that overcomes most of these problems is a semiconductor-based metal. It is well known that heavily doping semiconductors can exhibit metal-like optical properties. GaAs was demonstrated to work as a metal substitute in the mid-IR range when heavily doped (about 1018-19 cm-3) .
However, achieving metal-like optical properties in semiconductors in the near-infrared range is a tough challenge. The required very high doping (up to 1021 cm-3) can hardly be accomplished in conventional semiconductors. However, some semiconductors such as zinc oxide allow ultra-high doping. Heavily doped zinc oxide for example aluminum-zinc-oxide (Al:ZnO ) belong to the class of materials called transparent conducting oxides (TCOs) that show metal-like optical properties in the near-infrared range .
Figure 1. Field map obtained from simulations showing negative refraction occurring in a metamaterial built by stacking sixteen alternating layers of Al:ZnO and ZnO. The incident beam is TM-polarized and impinges the sample at an angle 40 degrees away from normal incidence.
Recently, we showed that Al:ZnO can be utilized as a metal substitute in a near-infrared metamaterial device and demonstrated negative refraction in this device . The device consisted a stack of sixteen alternating layers of ZnO and Al:ZnO. The thickness of each layer was much smaller than the incident wavelength. Such a metamaterial produces extreme anisotropy in its dispersion, which can lead to negative refraction of the incident light. Simulations showed that the light should bend on the ‘wrong’ side of the sample normal for TM-polarized incident light. An experimental set-up was built to verify this phenomenon. The transmittance of light through the sample was measured with a blade blocking half of the transmitted beam. When negative refraction occurred, the beam shifted such that more of the beam was blocked by the blade, which led to a dip in the transmitted light intensity. This observation not only confirmed negative refraction, but it also allowed us to assess the performance of this metamaterial. We found that the performance of this metamaterial device is three orders of magnitude higher than metal-based designs.
Figure 2. a) The experiment schematic used to observe negative refraction. A blade blocks the transmitted beam partially such that the lateral shift of the beam due to refraction modulates the intensity of unblocked portion of the beam. b) The relative transmittance measured for different angles of incidence from the Al:ZnO/ZnO metamaterial. In the wavelength range 1.8-2.4 μm, the metamaterial shows negative refraction, which results in the dips in the curves.
The demonstration of a metal-free plasmonic metamaterial in the near-infrared range with super-high performance is a technologically important step. The transition from metals to doped semiconductor materials enables the efficient and practical implementation of metamaterial devices for applications such as light concentrators for solar cells, optical invisibility cloaks and super-resolution lenses. This demonstration heralds the field of metal-free optical metamaterials.
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 Gururaj V. Naik, Jingjing Liua, Alexander V. Kildishev, Vladimir M. Shalaev and Alexandra Boltasseva, “Demonstration of Al:ZnO as a plasmonic component of near-infrared metamaterials,” Proceedings of the National Academy of Sciences of the United States of America,(published online May 16, 2012) DOI: 10.1073/pnas.112151710. Abstract.