Verschiebung von Nanoantennen-Resonanzen im mittleren infraroten Spektralbereich mittels Phasenwechselmaterialien (2016)

In nanooptics, the field of metamaterials (MMs) is emerging. Its focus lies on the e ective optical properties of resonant subwavelength metal nanoparticles, which allow for a variety of artificial effects, such as negative refraction, nanofocusing of light or sensing and imaging.This thesis investigates hybrid nanophotonic structures, which combine passive noble metal nanos- tructures, so-called nanoantennas, with tunable phase-change materials (PCMs) to obtain novel functionalities. This combination allows for a flexible post-fabrication control of the resonant properties of these nanoantennas, which can be understood as the building blocks of MMs. The control of their optical properties is based on the fact that the spectral resonance position of the antennas can, among other parameters, be influenced by the refractive index of its surrounding.A large change in the refractive index of the antenna surrounding is directly linked to a large shift of the antenna resonance frequency position. This can be achieved by an external stimulus, such as the insulator-to-metal transition of vanadium dioxide (VO2), the change of the carrier concentration in graphene, as well as the amorphous to crystalline structural transition in PCMs. Furthermore, these tunable materials should not introduce additional dielectric losses to the hybrid nanophotonic structures. In the mid-infrared (MIR) spectral range, the dielectric losses introduced by various PCMs, which are related to the imaginary part of their complex permittivity, are close to zero. Moreover, PCMs o er a non-volatile structural transition, which can be triggered reversibly. In comparison, VO2 and graphene lack of this non-volatily, since the switched state (and thus, the refractive index change) vanishs after the external stimulus is turned o . As a consequence, PCMs are one of the most promising candidates for applications in tunable (active) plasmonics and photonics.In this thesis, arrays of nanoantennas are designed for the MIR spectral range, while PCMs with a dielectric function best-suited for the MIR spectral range are investigated. The crystallization of the PCMs by annealing changes the dielectric surrounding of the nanoantennas and thus, the resonance frequency position of the nanoantennas is shifted by up to nearly 20%. A reversible shift of the resonance position is realized by applying laser pulses to re-crystallize and re-amorphize the used PCM. To do so, laser pulses on the femtosecond, as well as on the nanosecond timescale are applied. The first allows for a large-area change of the structural state of the PCM, while the latter is applied to address single nanoantennas. This can allow for a multistep gradual change of the dielectric surrounding of the antennas and thus, of a gradual shift of the resonance position, pushing the controllability of the resonance shifting.The presented concept is promising regarding a flexible post-fabrication tuning of the nanoantenna resonance position. Further applications to more complex nanostructures, e.g. a chiral metamaterial, show the reproducibility. Besides, demonstrated polarization control can bridge the gap of optical devices for the MIR spectral range. By understanding the physics of the PCMs the best-suited materials can be chosen to optimize the functionality of the nanostructure-PCM hybrids and thus, it leads towards programmable metamaterials.