Self-assembled tunable photonic hypercrystals
Over the last few decades, considerable progress has been made in developing artificial optical materials with novel and often counterintuitive properties. Revolutionary research by Yablonovitch and John on photonic crystals1,2 was followed by the development of the electromagnetic metamaterial paradigm by Pendry.3 Both photonic crystals and metamaterials are artificially structured materials. While periodicity of photonic crystals is comparable to the wavelength of light, metamaterials are structured on a scale much smaller than light's wavelength. Metamaterials offer novel ways for controlling electromagnetic, acoustic, elastic, and thermal properties of matter. However, although 2D lithography techniques are very well developed, it is very difficult to apply them to fabricate 3D photonic crystals and metamaterials.
Recently, we have demonstrated a novel artificial optical material, a ‘photonic hypercrystal,’ which combines the most interesting features of hyperbolic metamaterials and photonic crystals.4 In hyperbolic metamaterials, the dielectric permittivity has different signs along different orthogonal directions. We achieved 3D self-assembly of these photonic hypercrystals by applying an external magnetic field to a diluted cobalt-nanoparticle-based ferrofluid. Our approach enables fabrication of large 3D hyperbolic metamaterials with relatively low-loss, broadband performance in the long-wavelength infrared (LWIR) frequency range.
Moreover, the metamaterial's properties are easily tunable by an external magnetic field, while the sample size in all three dimensions is limited only by the size of the magnet used. Similar to hyperbolic metamaterials, the lack of the usual diffraction limit on the photon wave vector enables photonic hypercrystals to exhibit broadband divergence in their photonic density of states, the number of photons that may exist at a given frequency. In ordinary materials the wavelength of light cannot be smaller than the diffraction limit. In contrast, hyperbolic metamaterials have photonic states with wavelengths much smaller than the diffraction limit, permitting a much larger number of photons.
At the same time, similar to photonic crystals, the hyperbolic dispersion law of extraordinary photons is modulated by forbidden gaps, frequency ranges where there are no photons, when the photon wavelength is close to the spatial period of the metamaterial. (Extraordinary photons have a component of their electric field vector parallel to the optical axis of the metamaterial.) The reason for such modulation is that a diluted ferrofluid develops a very pronounced phase separation into cobalt-rich and cobalt-poor phases if subjected to an external magnetic field.
Optical microscope images of the diluted ferrofluid before and after applying an external magnetic field show a periodic pattern of self-assembled stripes in a magnetic field (see Figure 1). This pattern is caused by phase separation and the stripes are oriented along the direction of magnetic field. The stripe periodicity appears to be much smaller than the free-space wavelength in the hyperbolic frequency range, showing that the self-assembled optical medium is a photonic hypercrystal.
The unique spectral properties of photonic hypercrystals lead to extreme sensitivity of the material to monolayer coatings of cobalt nanoparticles, which should find numerous applications in biological and chemical sensing. We have combined our fabrication approach with well-established Fourier-transform infrared (FTIR) spectroscopy techniques, and explored the sensing potential of our newly developed photonic hypercrystal-based FTIR spectroscopy, as illustrated in Figure 2. Since LWIR is well known as a ‘chemical fingerprint’ region, this research direction may find numerous applications in homeland security, for instance in detecting drugs and explosives. Our technique appears to enhance the spectral features of interest and may enable improvements in the sensitivity of LWIR detection techniques.
In summary, we have fabricated large 3D hyperbolic metamaterials with relatively low-loss, broadband LWIR performance that may find applications in homeland security. We are now experimentally exploring theoretically predicted phenomena in hyperbolic metamaterials and photonic hypercrystals, such as subwavelength spatial solitons and spacetime cloaks.5 Spatial solitons are narrow beams of light that may appear as a result of self-focusing in a nonlinear optical medium. The beam diameter of solitons in ordinary media is limited by diffraction. Since the diffraction limit is violated in hyperbolic metamaterials, solitons may be much more narrow in such metamaterials. The nature of the dielectric permittivity in a hyperbolic metamaterial means that the wave equation describing light propagation inside the metamaterial is formally equivalent to the equation of motion in a vacuum (which is called Minkowski spacetime). Considering the proposed hyperbolic metamaterial as a Minkowski spacetime analog, we find that solitons may experience ‘gravitational collapse.’ Such extremely narrow solitons would be useful for microscopy.
A spacetime cloak, which hides events for specific places and times, was suggested recently by McCall and colleagues.6 Only a limited version of such cloak (a time cloak) has been demonstrated in recent experiments. The spacetime analogy described above enables experimental testing of the complete spacetime version of such a cloak.