The ability to integrate metamaterials, which exhibit useful and unique optical properties, with semiconductor optoelectronics could be about to become easier. Researchers from Princeton University, Oregon State University and Alcatel-Lucent have now fabricated an all-semiconductor metamaterial that exhibits negative refraction in the long-wavelength region of the infrared (Nature Mater. 6, 946–950; 2007).

Although metamaterials with similar properties in the visible and infrared have been demonstrated previously, they rely on metallic nanostructures and often incur a large optical loss. They are also relatively complex to manufacture and potentially difficult to directly integrate with semiconductor devices.

The new material is composed of 80-nm-thick interleaved layers of heavily doped InGaAs and AlInAs, which give it a strongly anisotropic dielectric function. The layers are grown by molecular-beam epitaxy on lattice-matched InP substrates.

According to the researchers, the transition wavelength from a positive to a negative refractive index is determined by the electron density of the doped InGaAs layers. By controlling the level of doping, the team successfully fabricated four samples with transition wavelengths of 8.8, 9.1, 10.1 and 13.1 μm. In all cases, the spectral bandwidth of the region of negative index was around 27%–30% wide, with the short-wavelength limit marked by a discontinuity of the Brewster angle, and the long-wavelength limit marked by a large increase in reflectivity.

To confirm the negative-index properties of the material, the team performed a series of reflection measurements (see right image, for a map of reflection measurements at different incidence angles and wavelengths, where regions of negative index are shown in blue). The team also conducted theoretical simulations of the beam propagation across an air–metamaterial boundary (left image) and geometric beam-blocking experiments that indicate the angle of refraction.

The researchers are confident that the material will prove to be useful for waveguiding and imaging applications.