Going Negative in the Visible

Hank Hogan

Negative refraction — the recently realized opposite of positive refraction — is of interest because, among other applications, it promises a superlens that can image far below the diffraction limit. To date, negative refraction has been demonstrated for various wavelengths but not seen.

Now researchers from California Institute of Technology in Pasadena have achieved negative refraction that is readily visible, with a beam emerging from their structure at a negative angle. That direct visualization is appropriate, said Henri J. Lezec, a visiting research associate at Caltech and a research director at the Centre National de la Recherche Scientifique in Paris. “To me, refraction is really about bending light.”

Using waveguides, researchers achieved negative refraction in the visible region. Light injected in one slot traveled through a prism-shaped waveguide, with only a single mode appearing on the other end (left). In the red, the result was positive refraction (top right). In the green, the result was negative refraction (bottom right). As seen in the schematic of the device (left), the negative refraction caused a negative angle of deflection, making a direct geometric visualization of the effect. Courtesy of Henri J. Lezec, California Institute of Technology and Centre National de la Recherche Scientifique.

The researchers used suitably designed and carefully constructed metal-insulator-metal waveguides in which the effect of surface plasmons — electron oscillations found at the metal-dielectric interface — makes some transmission modes behave as if the material had negative refraction.

Waveguide construction

For their demonstration, they fabricated waveguides with silicon nitride dielectric cores 50 to 150 nm thick, cladding the core with silver on one side and gold on the other. Prism-shaped segments of these waveguides, which were predicted to have negative refraction in the blue-green region of the visible, were embedded in a thick waveguide that consisted of a 500-nm silicon nitride core clad with silver and that was predicted to have positive refraction throughout the visible wavelength range.

Lezec noted that the waveguide construction was helped because the researchers used a focused ion beam system from FEI Corp. of Hillsboro, Ore., to mill the dielectric material. The device allowed them to work on freestanding dielectric membranes and enabled them to have a variety of prism core thicknesses on the same sample. “We did a matrix, and we found a window where it worked,” Lezec said.

Once the structure was complete, the investigators injected a beam into it through a subwavelength-width input slot on one side. They characterized the angle of refraction of the beam after it emerged, visualizing the projected spot with an optical microscope. At 685 nm, the refractive index was small and positive. At 514 and 476 nm, it was –5 and –1, respectively.

Science, April 20, 2007, pp. 430-432.