One-Way Smoke—Janus Particle-Engineered Material for Unidirectional Imaging

Janus particles, with their unique properties on each side, present a promising solution for directional scattering. By harnessing these properties, we can design Janus particles that interact with photons in a direction-dependent manner. This opens the possibility of creating a medium composed of Janus particles that can obscure an image from one side while maintaining clear vision from the other. In practical terms, we can develop a material of such engineered particles that functions like one-way smoke, a concept with significant potential applications in various fields, including surveillance, imaging, security, and nanophotonics.<br/>We demonstrate that meticulous material design and optimization for controlling the scattering and absorption cross-sections are essential to accomplish this task. For obscuring the image from one side, the particle must direct photons from the illumination source towards that side, effectively blurring the image. To achieve this, we have optimized two sets of Janus nanoparticles: a silica-rod gold sphere matchstick for illumination from behind the medium and a half-coated silica sphere for illumination from the front. These Janus particles are highly asymmetric, with dielectric properties on one side and plasmonic properties on the other. When the matchstick particle is excited from the rod side (dielectric), the probability of a photon being forward scattered is 46 times higher than backscattered. This occurs because the rod guides the electromagnetic waves toward the gold sphere, enhancing the scattering effect. Near-field analysis supports this conclusion, showing high-field concentration on the plasmonic side, indicating significant light guidance and absorption. For the half-coated silica sphere, the particle primarily backscatters incident photons when illuminated from the gold-coated side due to localized plasmonic resonance. When illuminated from the silica side, the sphere acts as a lens, focusing light onto the gold cap and increasing absorption. The high-intensity electric field resulting from localized plasmonic resonance increases absorption when excited from the dielectric side while enhancing reflection from the gold side.<br/>Due to their engineered asymmetric scattering properties, we can create smoke, an engineered medium of Janus particles, by suspending and aligning them in the air. Using this configuration, we have a random array of these particles, where we model the visualization throughout the media using a Monte Carlo-based approach, where the scattering properties of the high asymmetrical particles are exploited. In this medium, a target can be identified more than 50% of the time when seen from one side, while the target is not even detected more than 50% from the other side. In addition, we present a practical quasi-electrostatic approach to align the particles in free space, where we leverage the metallic parts of the Janus particles using an electrical field to manipulate the free charges of the metal. It generates a torque and controls the particle's position and orientation.<br/>These findings highlight the potential of Janus particle-engineered material to enable asymmetric imaging with simultaneous concealment from one side and clear visualization from the other, and they also open up new possibilities for exploring engineered materials for nanophotonics. In conclusion, we present an unexplored application for Janus particles based on its highly asymmetric scattering properties, having dielectric properties on one side and plasmonic on the other. This research has significant implications for various fields, including surveillance, imaging, security, and nanophotonics.