ABSTRACT
This chapter describes opportunities for the development of nanostructured metamaterials with unique light sensitivity for applications in information technologies. In particular we demon-strate experimentally the all-optical tuning of such metamaterials infiltrated with liquid crystals and show a fivefold enhancement of their nonlinear properties. Further applications of such materials in security or renewable energy are also discussed. 7.1 IntroductionInformation technologies have been dramatically transformed in the last two decades with the development of the Internet and optical
communications. Currently almost the entire communications technology relies on light for information transfer. Even our mobile phone conversations are eventually transmitted through an optical fiber after being received by mobile communication towers. However, something very important that is usually neglected is the fact that when information is being transmitted all-optically, eventually the vast amount of data hits a wall, often referred to as the “yellow wall.” At this point all optical signals have to be converted into electrical in order for the information to be processed and routed to the desired directions. Inevitably this conversion from optical to electrical signals results in large energy dissipation and indirectly to green-gas emission. As an example, in 2010 approximately 1% of the US energy consumption was attributed to electrical routers on the Internet. Hence, one can imagine that with the current exponential growth of the Internet, one can soon expect significant problems in the energy sector only because of the growth of Internet traffic. The conversion of optical signals into electrical and back is considered the bottleneck of information transfer that slows down all our communications. And this is the problem that a large number of scientists aim to solve, including our research team at the Australian Research Council Centre of Excellence, Centre for Ultrahigh-Bandwidth Devices for Optical Systems (CUDOS). The goal of CUDOS researchers is to realize efficient signal processing of light entirely optically, thus avoiding the need for conversion of optical signals to electrical. The aim in such all-optical signal processing is to be able to perform the processing and routing of information on a smaller scale, much faster and more energy efficient. However, to be able to process light all-optically, one needs to be able to control light with light, which is only possible through the use of the so-called optical nonlinearity. Importantly, nonlinear optical effects need to occur at low power in order to improve the efficiency of the signal processing. The key in the development of all-optical control of light signals is to make them interact with each other. However, because light photons are bosonic particles they do not interact in free space. One photon can pass through another one without feeling it. Therefore, to enable interaction between the photons, one needs to introduce
a “soft” material in which the photons propagate. In such a material the presence of photons will create an optical potential, which will act on all other photons, making them interact with each other. The induction of an optical potential in the material is the essence of optical nonlinearity. Physically this is a result of the deformation of the electronic orbits in the atoms of the material under the action of the incident light intensity. As such, the oscillations of the electrons in the nuclei will be anharmonic, resulting in a nonlinear dependence of the material polarization on the external electric field. This nonlinear dependence on an external electric field is the key to realizing any kind of all-optical functionalities for processing of information. For example, the effect of nonlinear refraction enables us to perform light switching, gating, and modulation. The ultrafast nonlinearities of atoms and molecules enable conversion of light from one color to another in a nonlinear frequency conversion process, or parametric nonlinear interactions allow for light amplification without the need of active gain materials. However, nonlinear processes usually happen at high laser intensities and require high-power laser sources that consume a lot of energy. Therefore the important question is, how do you achieve such alloptical functionalities at low power and with high efficiency? A key solution to this fundamental power consumption problem comes from the use of photonic nanostructures. There are several ways by which the nanostructuring of optical materials can strongly enhance the nonlinear effects. The first possibility is to enable light confinement to the nanoscale. Because the nonlinear effects are proportional to the light intensity, the concentration of light to a very small volume will result in a multifold increase in the light intensity at the same laser powers. As such large research efforts have been focused on the increase of light confinement by using dielectric nanowire waveguides, photonic crystal waveguides, or even metal-dielectric-metal waveguides for subwavelength light concentration. Another possibility for utilizing nanostructuring is to engineer the light dispersion in nanoscale photonic waveguides in order to achieve phase matching of the desired nonlinear processes, including frequency light conversion. However, something that is really at the frontier of the modern nonlinear optics is the new opportunities for ultrastrong nonlinear
response of materials with artificial magnetism at optical frequencies. In such materials, the nonlinear interaction between photons arises not only due to the nonlinear response of the electric polarization of the material but also due to the nonlinearity in the magnetic polarizability of the medium. This magnetic-type nonlinear optics is reliant on the development of a new class of artificial optical materials-metamaterials-that exhibit a strong magnetic response at optical frequencies. Due to the strong magnetic nonlinearity of such materials it is possible to observe new nonlinear phenomena and apply them for power-stringent all-optical signal-processing applications. 7.2 Optical Metamaterials
Every material is composed of atoms and molecules, whose properties define the properties of the entire material. In nature, the properties of a material are constrained by the different chemical compositions of its compounds. However, with the development of nanotechnologies, it became possible to engineer artificial atoms with properties defined mostly by the geometry of the atom rather than by its chemical compounds. Such nanoscale artificial atoms are called meta-atoms and are typically much smaller than the wavelength of light. When many such meta-atoms are placed close together they form a composite material, called a metamaterial. Importantly, when the distance between the artificial atoms is much smaller than the wavelength of light, the material appears homogeneous for the incident light. 7.2.1 Split-Ring Resonator as an Artificial Meta-Atom
The key element for designing artificial atoms with a magnetic response is the split-ring resonator (Fig. 7.1a). The important feature of the split-ring resonator meta-atom is that when light is incident on the split ring, it can induce currents along the metal ring. These induced currents correspondingly cause an induced magnetic dipole moment. This dipole moment results in an induced magnetic field that is reversed with respect to the incident magnetic field at optical
frequencies just above the resonant frequency of the split ring (see Fig. 7.1a,b), thus creating strong magnetic resonance. The splitring resonator can be interpreted as a liquid crystal (LC) circuit (see Fig. 7.1b) with a resonant frequency ω0 = 1/LC such that above this resonant frequency, the magnetic polarization of the meta-atom becomes negative. This negative magnetic polarization is the cause of the artificial magnetism of such meta-atoms.
