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Squeezing Optical Cavities to the Nanoscale

SEM images (above) and measured transmission spectra (below) of optical cavities with differing dimensions. The transmission spectra indicate that resonant frequency is not dictated by cavity dimensions.

The Materials Science Division’s Xiang Zhang and colleagues have created the world’s smallest optical cavities, the light-amplification chamber at the heart of laser technology. The researchers circumvented the usual size limitations on optical cavities by harnessing exotic properties of metamaterials, and the unprecedented size and performance of the resulting cavities open exciting possibilities in nanophotonic applications.

Light confined within an optical cavity can resonate at specific frequencies to produce a standing wave but cavities made from natural materials are limited in size by the wavelength of light propagating through them, which usually keeps infrared-range optical cavities much larger than micro-scale.

Here, the researchers laid down alternating, ultra-thin layers of silver and germanium, which behave as a composite metamaterial capable of bending light backwards. The metal-dielectric stacks form square-based pyramids 100–200 nanometers across and exhibit an extremely large refractive index of 17.4, which causes total internal optical reflection and can resonate at any prescribed frequencies. The team found higher quality optical resonance in smaller cavities—a surprising result. This unusual phenomenon indicates an enhanced light-matter interaction, which may make these cavities valuable for high-performance optical devices.

“Experimental realization of three-dimensional indefinite cavities at the nanoscale with anomalous scaling laws.” Xiaodong Yang, Jie Yao, Junsuk Rho, Xiaobo Yin & Xiang Zhang. Nature Photonics 6, 450–454 (2012). doi:10.1038/nphoton.2012.124