Photonic crystals color-tunable at low voltages
Photonic crystals are peculiar structures that show periodic variations in refractive index on a length scale comparable to the wavelength of light. This periodicity means that, for certain ranges of energies and wave vectors, light is not allowed to propagate through the medium.1 Such disallowed groups of wavelengths are called photonic band gaps. The coloration thereby imparted to photonic structures is called structural color, since it is not due to the presence of a dye or pigment, but rather to the conformation of the material itself. Photonic crystals are found in nature, e.g. in beetle scales, butterfly wings and parrot feathers, and can also be fabricated using a wide range of techniques, with dielectric periodicity in one, two, or three dimensions.
Periodicity in one dimension can be visualized as a stack of planar layers, typically with an alternation of two materials that transmit light differently. One-dimensional photonic crystals (1DPCs) are of great practical interest because they are easy to fabricate by various methods, and their optical properties can be predicted using simple but powerful theoretical tools. This theoretical computation of the crystal's optical behavior remains reliable even for structures more complicated than the standard alternation of two materials.
With thin-film deposition processes such as spin coating, it is possible, in principle, to obtain a monolithic photonic stack composed of layers of multiple materials. Such multilayer systems have not been extensively investigated, so it would be beneficial to better understand their optical behavior. To this end, we have recently conducted a theoretical study of the light transmission properties of 1DPCs composed of four materials.2 We investigated these properties using the transfer matrix method, a general technique widely used in optics for the description of stacked layers. Compared to a conventional 1DPC (composed of two alternating layers), the four-material system features an additional photonic band gap at longer wavelengths. This new gap could be ascribed to a longer-order periodicity that does not occur in conventional 1DPCs. Another interesting feature of the four-material system is that the intensity and shape of the photonic band gaps are strongly dependent on how the four materials are sequentially arranged within the unit cell (i.e., the simplest repeating unit) of the photonic lattice. This could prove to be a useful property for applications such as optical cryptography, where an optical measure (in this case, the transmission spectrum) is associated with a particular unit cell sequence.2
Other types of photonic crystals made with more than two materials are infiltrated porous 1DPCs. These porous multilayer structures have gaps between their constituent nanoparticles, which can, in principle, be infiltrated by any compound. By infiltrating into these pores a material possessing optical gain, it is possible to obtain distributed feedback lasers in which the optical gain material is embedded in the photonic structure.3 If the porous photonic crystal, coupled with a titanium dioxide photo-anode, is infiltrated with a dye, the result is an efficient dye-sensitized solar cell with improved light harvesting.4
To experimentally verify the behavior of these systems, we recently investigated the fabrication and optical characterization of a photonic crystal made up of metal oxide nanoparticles (silicon dioxide and titanium dioxide) infiltrated with a nematic liquid crystal.5 The photonic crystal was deposited by spin coating on indium tin oxide (ITO)-coated glass, starting from a colloidal dispersions of the metal oxide nanoparticles. The structure was then infiltrated with the E7 commercial liquid crystal mixture simply by drop casting (at a temperature of 60°C to facilitate the process). We observed that infiltrating the liquid crystal into the porous structure produced a red shift in the photonic band gap due to an increase in the effective refractive index of the multilayer system.
To tune the photonic band gap with an electric field, we constructed a device by placing a second ITO-coated glass on the sample and applying electrical contacts: see Figure 1 (top). We found that an 8nm shift toward shorter wavelengths could be achieved at the relatively low voltage of 8V, corresponding to an electric field of approximately 3.4V/μm. This is one order of magnitude smaller than the previously reported values for tunable liquid-crystal infiltrated photonic crystals. By further increasing the applied voltage, we obtained an additional blue shift that reached a saturation level of 10nm at 32V: see Figure 1 (bottom).
The performance of this device could be further improved by optimizing the photonic structure and by selecting the most suitable liquid crystals for low-voltage operation, and this is something we plan to pursue in future work. We intend to corroborate this experimental optimization with theoretical analysis by the transfer matrix method, also taking scattering losses into account. These results could have very interesting applications in optoelectronic switches and low-cost displays. The tunability of structural color by means of a low applied electric voltage could be an important step toward achieving electrically-driven color changing devices that can also run on portable batteries.
Politecnico di Milano (POLIMI)
Francesco Scotognella is an assistant professor of physics in the Physics Department. He has a degree in materials science from the University of Parma (2004) and the University of Milan-Bicocca (2006). His research is focused on the photophysics of nanostructures and the optical properties of photonic structures.
Italian Institute of Technology
Luigino Criante is a senior postdoc. He has a cum laude degree in electronic engineering from Marche Polytechnic University. His research is focused on the physics of liquid crystals, solid-state organic lasers, and optical tweezers.
