3-D Optical Cavities Made from Metamaterials

Three-dimensional nanoscale optical cavities made from metamaterials have the potential to generate the most intense nanolaser beams to date and hold promise for a range of other technologies, including LEDs, optical sensing, nonlinear optics, quantum optics and photonic integrated circuits.

Optical cavities are the major components of most lasers. Light confined within these cavities reflects back and forth between two opposing mirrors to produce a standing wave at a specific resonant frequency. It is from this standing light wave that a laser beam is generated.

When made from natural materials, optical cavities can be no smaller than the wavelength of the light propagating through them. Metamaterials, however, allow for electromagnetic behavior that is not found in nature. These materials are engineered by combining metals and dielectrics, and they derive their optical properties from their structure rather than from their chemical composition.

Indefinite optical cavities feature a hyperboloid iso-frequency contour that supports ultrahigh optical refractive indices. This cross section shows the IFC (bronze curves) for a silver/germanium metamaterial, with yellow circles representing cavity wave vectors and the green circle representing the light cone of air. (Images: Xiang Zhang group)

Scientists from the US Department of Energy’s Lawrence Berkeley National Laboratory and the University of California used this information to create the 3-D optical cavities using an “indefinite metamaterial” that alternates superthin multiple layers of silver and germanium.

“Our work opens up a new approach for designing a truly nanoscale optical cavity,” said Xiang Zhang, a principal investigator with Berkeley Lab’s Materials Science Div. and director of UC Berkeley’s Nano-scale Science and Engineering Center. “By using metamaterials, we show intriguing cavity physics that counters conventional wisdom. For example, the quality factor of our optical mode rapidly increases with the decrease of cavity size. The results of this study provide us with a tremendous opportunity to develop high-performance photonic devices for communications.”

In natural materials, light behaves the same no matter in what direction it propagates. In indefinite metamaterials, however, light can be bent backward in some directions, a property known as negative refraction. Using this indefinite metamaterial enabled the researchers to scale down the 3-D optical cavities to extremely deep subwavelegnth size, resulting in a “hyperboloid iso-frequency contour” of light wave vectors that supported the highest optical refractive indices ever reported.

“Due to the unnaturally high refractive index supported in the metamaterials, our 3-D cavities can be smaller than one-tenth of the optical wavelength,” said Xiaodong Yang, lead author of the paper and who is now with the Missouri University of Science and Technology. “At these nanoscale dimensions, optical cavities compress the optical mode into a tiny space, increasing the photon density of states and thereby enhancing the interactions between light and matter.”

This schematic shows (a) an indefinite metamaterial structure with alternating silver and germanium multilayers; and (b) its iso-frequency contour of light wave vectors with negative refractions along the X- and Y-directions, and positive along the Z-direction.

Using indefinite metamaterials to make 3-D optical cavities is also advantageous because they offer more flexibility in cavity design, Yang said. Cavities with different sizes can have the same resonance frequency. Another advantage is that the photons lost when light is reflected back and forth — a problem for optical cavities from natural materials — is reduced as the cavity size gets smaller. This could benefit the design of future nanoscale lasers, Yang said.

The scientists used the dielectric germanium to fabricate their metamaterial because it has a relatively high refractive index (about 4.0), compared with air (1.0), which is the dielectric most typically used to make a metamaterial. They alternated layers of 20-nm-thick silver and 30-nm-thick germanium that were cut into various size cubes, depending on the number of metal/dielectric layers. The cube walls tilted into the shape of a trapezoid during the final stage of fabrication, with a nanosize optical cavity in the core.

Electron micrograph showing arrays of indefinite optical cavities composed of silver/germanium multilayers.

“The hyperboloid iso-frequency contour of wave vector space in these cavities allowed us to reach very high wave vector values,” Yang said. “As wave vector values are proportional to the refractive index, we were able to record optical refractive indices as large as 17.4, which is far beyond that found in natural materials.”

The research appeared in Nature Photonics and was supported by the US Department of Air Force Office of Scientific Research.

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