In 2014, researchers predicted the theoretical existence of a strange new material that altered the interaction between light and matter. The material, dubbed photonic hypercrystal, has now become a reality.
Researchers at the City College of New York (CCNY) say they have actually created some, and the realization of these materials could yield big changes in applications including light-based technologies such as solar cells and Li-Fi, where visible light from light emitting diodes provides a means of communication (essentially doing what Wi-Fi does, but with flashes of light instead of radio waves) and quantum information processing.
In research described in the journal of the Proceedings of the National Academy of Sciences, the researchers demonstrated that the photonic hypercrystal allows unprecedented control over the propagation and confinement of photons.
Traditionally, photonic crystals and metamaterials—a class of materials famed for its ability to manipulate light rendering objects ‘invisible’—have provided control over the propagation of photons. However, these new photonic hypercrystals manage to overcome the drawbacks of bandwidth limitation and poor light emission seen in both metamaterials and photonic crystals.
The photonic hypercrystals improve control over light in two ways. Incident light gets trapped inside the material and stays long enough to have more interaction with matter. This could significantly improve the efficiency of solar cells. And Vinod M. Menon, the CCNY physics professor who led the research, told IEEE Spectrum that the hypercrystal material “significantly enhances the strength of the interaction with matter (such as quantum dots) placed inside it making it useful for light emitters including single photon sources as well as for light harvesting.” What is equally important, according to Menon, is that it is a broadband effect.
While photonic hypercrystals have a photonic crystal structure and a constituent metamaterial, they are quite different from both of these materials, according to Menon.
Hypercrystals are distinct from photonic crystals, according to Menon, because of two material scales involved: the period, which is the time it takes for a particle in a medium to make one complete vibrational cycle, and the repeating structures of the crystals known as unit cells are sub-wavelength. They are also not standard metamaterials since their electromagnetic response is qualitatively different from metamaterials’ dependence on the averaged polarization of subwavelength unit cells.
“These fundamental differences result in a number of non-trivial electromagnetic properties that can be observed in experiment and even lead to practical devices,” says Menon. “[These include] broadband enhancement of spontaneous emission and light out-coupling, which have never before been demonstrated simultaneously in either metamaterials or photonic crystals.”
One of the highlighted applications for the photonic hypercrystals would be so-called Li-Fi technology. The key to enabling Li-Fi is LEDs that can flash on and off very quickly. Menon believes that these photonic hypercrystals could be incorporated into architectures that would allow fast direct modulation of the diodes.
Of course, the design of a device architecture that could integrate a hypercrystal remains one of the key engineering challenges for this material to be used in applications such as LEDs. Menon says that this is the direction the CCNY team is currently working towards.
Menon adds: “We are looking into single photon emitters integrated into hypercrystals as well as structures that operate in near and mid infrared.”