Better Holograms for Higher Security

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Image generated by the hologram in the visible range (wavelength of 530 nm). Credit: Capasso Lab, Harvard SEAS

Security holograms have been part of credit cards for more than three decades, but scientists are always seeking to advance the technology. Now, using nanotechnology, researchers in OSA Fellow Federico Capasso’s laboratory at Harvard University (USA) have incorporated polarization into holography to boost its efficiency and security (Sci. Adv., doi: 10.1126/sciadv.1501258).
The Capasso lab’s holograms operate throughout the visible spectrum and into the near-infrared region, where they achieved efficiencies as high as 75 percent under illumination with 1.0- to 1.4-μm- wavelength light.
The Harvard team revisited the concept of “detour phase,” in which the displacement of adjacent elements in a grating shapes the wavefront and changes the interference pattern of light. The basic notion dates back to Lord Rayleigh and OSA Honorary Member A.A. Michelson, but it is used to generate binary holograms from an aperture array; it also played a role in the development of computer-generated holograms.
Using electron-beam lithography, the scientists in Capasso's lab, led by Mohammadreza Khorasaninejad and Antonio Ambrosio, built subwavelength silicon structures onto a transparent glass substrate. The tiny dielectric ridges used in their first experiments can distinguish between linear polarization states of incident light without a multiplicity of optical components in the light path.
Next, the researchers designed a chiral hologram consisting of tiny silicon “nanofins,” which display different information depending on the handedness of the circularly polarized reference light beam.
Such thin, lightweight, nanopatterned holograms could be integrated into wearable optical systems such as near-to-eye displays. They could also be deployed as anti-counterfeiting measures that display a message only when a recipient knows the correct light polarization to use.
The eight-petaled flower-shaped far-field intensity design in some of the Harvard group’s holographic images may look familiar—it is a monochromatic rendition of the logo for the 2015 International Year of Light.

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