Metamaterials: Tunable hypercrystals merge best of photonic crystals and metamaterials

Researchers have combined the most interesting properties of photonic crystals and hyperbolic metamaterials into an entirely new material.

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Researchers at the University of Maryland, College Park (College Park, MD) and Towson University (Towson, MD) have combined the most interesting properties of photonic crystals and hyperbolic metamaterials into an entirely new material dubbed a hypercrystal.1 This new material offers relatively low-loss, broadband performance in the long-wavelength infrared (LWIR) region and is also fabricated via self-assembly—an important attribute considering that artificial 3D materials are not easily fabricated using 2D lithography techniques.

Best of both worlds
Photonic crystals typically have a periodicity on the order of the wavelength of light, while metamaterials can be fabricated with periodicity on a much smaller scale. The dielectric permittivity of hyperbolic metamaterials has different signs along different orthogonal directions. Because the fabrication process for these hypercrystals involves applying an external magnetic field to a diluted cobalt-nanoparticle-based ferrofluid, this permittivity function is maintained and wavelengths of light much smaller than the diffraction limit are allowed.

Like a photonic crystal, the hypercrystals also have photonic bandgaps due to the fabrication process. The magnetic field induces a periodic pattern of self-assembled stripes caused by phase separation into nanoparticle-rich and nanoparticle-poor phases, and the stripe periodicity is much smaller than the free-space wavelength in the hyperbolic frequency range, showing that the self-assembled optical medium is a photonic hypercrystal (see figure).

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Microscope images show the diluted cobalt-nanoparticle-based ferrofluid before (a) and after (b) applying an external magnetic field; the fluid separates into cobalt-rich and cobalt-poor phases creating self-assembled stripes oriented along the direction of the magnetic field. (Courtesy Towson University)

Furthermore, the hypercrystal is tunable using an external magnetic field, which leads to tunability of its periodicity (of the order of 1-2 μm) and very unusual polarization properties. Due to phase separation, the hypercrystal exhibits “polarization notch” behavior, which is much sharper than the polarizing properties of the usual polarizers defined by the Malus law (the standard cosine-squared transmission angular dependence for a polarizer in a polarized field).

The researchers say that their hypercrystals are extremely sensitive to monolayer coatings of cobalt nanoparticles, a property that could lead to interesting biological and chemical sensing applications. In fact, they have explored their hypercrystals in Fourier-transform infrared (FTIR) spectroscopy in the well-known LWIR fingerprint region. They are also exploring theoretical phenomena in hyperbolic metamaterials and photonic hypercrystals including subwavelength spatial solitons (ultranarrow beams of light that form due to self-focusing in nonlinear optical media) and space-time cloaks (that hide events in both specific places and times).

“Tunable photonic hypercrystals show great promise in chemical and polarization sensing,” says Vera Smolyaninova, professor of physics at Towson University. “They open up new directions in metamaterials research.”

REFERENCE
1. I. Smolyaninov and V. Smolyaninova, “Self-assembled tunable photonic hypercrystals,” SPIE Newsroom (January 9, 2015); http://www.spie.org/x111854.xml.

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