3D Two-Photon Microprinting of Nanoporous Architectures.

A photoresist system for 3D two-photon microprinting is presented, which enables the printing of inherently nanoporous structures with mean pore sizes around 50 nm by means of self-organization on the nanoscale. A phase separation between polymerizable and chemically inert photoresist components leads to the formation of 3D co-continuous structures. Subsequent washing-out of the unpolymerized phase reveals the porous polymer structures. To characterize the volume properties of the printed structures, scanning electron microscopy images are recorded from ultramicrotome sections. In addition, the light-scattering properties of the 3D-printed material are analyzed. By adjusting the printing parameters, the porosity can be controlled during 3D printing. As an application example, a functioning miniaturized Ulbricht light-collection sphere is 3D printed and tested.

On the limits of laminates in diffusive optics.

Laminate metamaterials lead to anisotropic material properties, which can be tailored by the contrast between the two ingredient materials within the laminate. Such tailored anisotropies are, for example, required to realize advanced invisibility cloaks, wormhole architectures, or analogues of negative refraction. The physics and mathematics of laminates is very well established in the context of the diffusion equation and mathematical equivalents thereof, such as the heat conduction equation, the electrical conduction equation, electrostatics, magnetostatics, and laminar fluid dynamics. However, the validity of the diffusion equation is often stressed for disordered optical media, because sufficiently large transmission of light is requested. As a result, the condition that all relevant transport mean free path lengths need to be small compared to all relevant geometrical dimensions, may not be fulfilled. Monte Carlo simulations can grasp the physics of this transition regime between diffusive and ballistic optics. Here, we present corresponding numerical simulations for laminates. On this basis, we discuss the resulting fundamental limitations and trade-offs for laminates.

Uncloaking diffusive-light invisibility cloaks by speckle analysis.

Within the range of validity of the stationary diffusion equation, an ideal diffusive-light invisibility cloak can make an arbitrary macroscopic object hidden inside of the cloak indistinguishable from the surroundings for all colors, polarizations, and directions of incident visible light. However, the diffusion equation for light is an approximation which becomes exact only in the limit of small coherence length. Thus, one expects that the cloak can be revealed by illumination with coherent light. The experiments presented here show that the cloaks are robust in the limit of large coherence length but can be revealed by analysis of the speckle patterns under illumination with partially coherent light. Experiments on cylindrical core-shell cloaks and corresponding theory are in good agreement.

Diffuse-light all-solid-state invisibility cloak.

An ideal invisibility cloak makes arbitrary macroscopic objects within the cloak indistinguishable from its surrounding­for all directions, illumination patterns, polarizations, and colors of visible light. Recently, we have approached such an ideal cloak for the diffusive regime of light propagation using a core-shell geometry and a mixture of water and white wall paint as the surrounding. Here, we present an all-solid-state version based on polydimethylsiloxane doped with titania nanoparticles for the surrounding/shell and on a high-reflectivity microporous ceramic for the core. By virtue of reduced effects of absorption, especially from the core, the cloaking performance and the overall light throughput are improved significantly.