Diffuse-light all-solid-state invisibility cloak

Tunable Rectification of Diffusion-Wave Fields by Spatiotemporal Metamaterials

The diffusion process is the basis of many branches of science and engineering, and generally obeys reciprocity between two ports of a linear time-invariant medium. Recent research on classical wave dynamics has explored the spatiotemporal modulation to exhibit preferred directions in photons and plasmons. Here we report a distinct rectification effect on diffusion-wave fields by modulating the conductivity and observe nonreciprocal transport of charges. We experimentally create a spatiotemporal diffusion metamaterial, in which a mode transition to zero frequency is realized. A direct current component thereby emerges, showcasing a biased effect on the charge diffusion when the incident fundamental frequency is a multiple of the system modulation frequency. These results may find applications spanning a plethora of diffusive fields in general.

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.

Frequency domain transformation optics for diffusive photon density waves' cloaking

We make use of transformation optics technique to realize cloaking operation in the light diffusive regime, for spherical objects. The cloak requires spatially heterogeneous anisotropic diffusivity, as well as spatially varying speed of light and absorption. Analytic calculations of Photon's fluence confirm minor role of absorption in reduction of far-field scattering, and a monopole fluence field converging to a constant in the static regime in the invisibility region. The latter is in contrast to acoustic and electromagnetic cloaks, for which the field vanishes inside the core. These results are finally discussed in the context of mass diffusion, where cloaking can be achieved with a heterogeneous anisotropic diffusivity.

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.

Cloaking through cancellation of diffusive wave scattering

A new cloaking mechanism, which makes enclosed objects invisible to diffusive photon density waves, is proposed. First, diffusive scattering from a basic core-shell geometry, which represents the cloaked structure, is studied. The conditions of scattering cancellation in a quasi-static scattering regime are derived. These allow for tailoring the diffusivity constant of the shell enclosing the object so that the fields scattered from the shell and the object cancel each other. This means that the photon flow outside the cloak behaves as if the cloaked object were not present. Diffusive light invisibility may have potential applications in hiding hot spots in infrared thermography or tissue imaging.

Detection of a diffusive cloak via second-order statistics

We propose a scheme to detect the diffusive cloak proposed by Schittny et al. [Science345, 427 (2014).SCIEAS0036-807510.1126/science.1254524]. We exploit the fact that diffusion of light is an approximation that disregards wave interference. The long-range contribution to intensity correlation is sensitive to the locations of path crossings and the interference inside the medium, allowing one to detect the size and position, including the depth, of the diffusive cloak. Our results also suggest that it is possible to separately manipulate the first- and the second-order statistics of wave propagation in turbid media.