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    EXC 3120 BlueMat - Research Topic C4: Scattering of Visible Light by Control of Water Distribution
    Porous media with periodic or random structures comparable to the wavelength of light can provide spectrally selective scattering. Here, the optical response or color is no longer dictated by the base materials but by the pore shape, orientation, and arrangement, as well as by pore filling. Embedding water into the pore space and controlling its distribution at the nm- and ?m-scale allows for modifying the scattering, thus opening new possibilities to dynamically adjust or switch optical appearance. Previously, water has only been used to slightly shift the spectral response of periodic media upon water infiltration. However, here, we plan to go beyond this state of the art, achieving broadband control of the spectral response and even switching the color on and off. In this respect, we aim to achieve a similar functionality as in the skin of chameleons or cephalopods, but using water in porous media. The key scientific questions are: How can we design and fabricate structures with self-organized multiscale porosity to have defined optical appearance? How can we control water distribution in multiscale porous framework structures to switch this optical appearance? How can we utilize these effects for switchable structural color and opacity, including adaptive functionality?
      25
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    EXC 3120 BlueMat - Research Topic C1: Tunable Microwave and MIR Permittivity by Control of Water Distribution
    The nanoporous structures considered in BlueMat are orders of magnitude smaller than the wavelength of microwave and MIR radiation. Thus, we will utilize water-driven materials in the effective medium regime. In the microwave range, bulk water has a high permittivity of 80, attributed to the reorientation of water dipoles. The permittivity is expected to become anisotropic at interfaces, with the component orthogonal to the interface reducing to 2. Utilizing samples with anisotropic porosity, e.g., porous silica and alumina, we envisage for the first time experimentally clarifying this water anisotropy. In the MIR range, the permittivity of bulk water reduces to 2, but water is characterized by significant absorption with an attenuation length in the order of 10 ?m. We intend to employ polyolefin aerogels, which have intrinsically high transmittance in the MIR range, and add optical tunability via reversible adsorption of water on the surface of the aerogel. The key scientific questions are: How can we control the complex permittivity of water and its anisotropy by nanoconfinement? How can we utilize water and its confinement to tune the effective permittivity of nanostructured media? How can we utilize these effects to switch microwaves and thermal radiation?
      63
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    EXC 3120 BlueMat - Research Topic C2: Tunable MIR and NIR Permittivity by Electrochemical Oxidation and Hydrogen Insertion
    In this RT, we will electrochemically modify the optical properties of materials interacting with water. Specifically, the reversible partial oxidation of Cu photonic structures in aqueous electrolytes will be studied. Cu has IR reflectivity comparable to gold and can be reversibly oxidized. In this case, the complex effective permittivity of porous Cu will be tuned via changes in the metal filling fraction and metal electron collision frequency, known as chemical interface damping (CID). Furthermore, we will investigate the reversible and controlled hydrogen insertion from water into porous photonic hybrid structures. We will induce the formation of hydrogen bronzes within vanadium dioxide, changing it from a dielectric to metallic state. Later, hydrogen-induced reduction will be also explored to tune the optical properties of titanium dioxide. The hydrogen insertion will be driven by water-based electrochemistry and by thermal treatments in water-containing vapor. The structures considered will be much smaller than the wavelength and, thus, will work in an effective medium regime. The change of the effective permittivity from a metallic to dielectric state in both cases will allow the control of NIR and MIR absorption. The key scientific questions are: How can we strongly and reversibly tune IR absorption of porous Cu via electrochemical oxidation? How can we induce a reversible phase transition in VO2 and TiO2 via hydrogen insertion? How can we utilize nanoscale structures to increase the tuning rate and amplitude?
      38
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    EXC 3120 BlueMat - Research Topic C3: Scattering of NIR and Visible Light by Reversible Plasmonic Cluster Formation
    This research topic aims to develop self-regulating and switchable materials based on thermo-reversible assemblies of plasmonic nanoparticles (NPs) within purpose-designed and 3D-nanoprinted porous structures filled with water. Single spherical plasmonic gold particles have their resonance in the visible range, while plasmonic dimers and larger clusters exhibit broad absorption into the NIR. Thus, reversible (dis)assembly can be used to switch NIR transmission. The NPs will be functionalized with programmable oligonucleotide ligands and placed in 3D-nanoprinted capillaries filled with water, where a fraction of the particles will be bound to the pore walls by thermal treatment or surface chemistry. This design will create energy-selective and adaptive materials that can be used as responsive and switchable optical filters in the visible and NIR range with tailored response times, bringing plasmonic switchability into rigid scaffolds. The key scientific questions are: How do the size, shape, and distribution of nanopores influence the reversible assembly of plasmonic particles? How can we design an engineered environment in which the diffusion and assembly of NPs is controlled? How can we improve the switching time and reproducibility of assembly?
      18
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    Fourier optics and light stopping with nonlinear fronts
    In this proposal we draft the route towards experimental realisation of different index front-induced optical transitions close to the band edge of a periodical waveguides. Namely, light stopping, time lens, and ultrafast arbitrary waveform generation are considered. For that we propose to utilise silicon waveguides with Bragg gratings and operate them close to the band edge. The moving refractive index front will be generated in the same waveguide via instantaneous Kerr effect from the copropagating switching/pump pulse. To avoid the losses via free carrier generation we plan to use mid-infrared (MIR) radiation for the pump pulse.The proposed effects are utilising the theoretical concepts developed by our group in recent year which we now plan to demonstrate experimentally. In case of light stopping, like in a mechanical collision, the light pulse hits a propagating front and stops. Here, the light energy can be transferred to the zero group velocity mode and later released by a second front with an opposite slope. In case of a so-called optical push broom, the signal pulse is trapped by the index front with a simultaneous frequency change and compression. As we have recently shown theoretically the trapping in the front is equivalent to Fourier optics observed at the focal plane of conventional lenses. This effect allows not only pulse compression but also the transfer of information from real to reciprocal space and vice versa. Thus, a defined spectral composition of the signal can be converted in a designed short temporal pulse, allowing for customizable wave form generation.The proposed system will provide signal temporal compression, light stopping, and pulse structuring in integrated optics technology. The extension to the MIR pump pulse allows utilisation of established silicon waveguide technology with large nonlinearity and strong field confinement.
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