Performance of adaptive infrared camouflage is usually parameterized in terms of cycle-ability, response time, actuation mechanism, stability etc., however, one of the key components that has not been addressed so far is the spatial density of infrared information that can be encoded and actively manipulated for camouflaging.
We report an adaptive infrared camouflage system that can be engineered to operate at any desired wavelength in the technologically relevant, infrared transparent 3 – 5 µm and 8 – 12 µm bands. We exploit the metal-insulator phase transition in VO2 to design an optical cavity coupled infrared absorber where the cavity length can be altered by controlling the VO2 phase. Cavity tuning is done by strategically placing the VO2 layer inside the optical cavity composed of a tri-layer architecture. In its insulating state VO2 is transparent to infrared such that incident light couples to the entire cavity length, however in the metallic state, VO2 behaves like a mirror and shortens the cavity length by reflecting ~80% of incident light. The Maxwell Garnett EMT describes the phase transition dependent optical response of the absorber better than the Bruggeman EMT when compared to the experimental results. We tailor the device parameters to demonstrate adaptive thermal camouflage of multispectral encoded infrared information on a pixelated designer surface with a pixel resolution (~20 µm) and density comparable to the industry standard for infrared sensors. We envision this work will pave the way for novel tunable optical devices for technological advancements in infrared tagging, camouflaging and anti-counterfeiting efforts.
In this work, we demonstrate superchiral light generation based on achiral plasmonic surfaces. At resonance, the symmetric cavity-coupled plasmonic system generates single-sign chiral near-field whose helicity is determined solely by the handedness of the incident light. We elucidate the mechanism for such unique superchiral near field generation and find its origin in coherent and synergetic interactions between plasmonic and photonic cavity modes. The cavity-coupling enhances otherwise weak plasmonic chiral near-field by many folds. Furthermore, the system in a unique way suppresses the far field chirality due to its totally symmetric geometry providing a route for surface-enhanced chiroptic spectroscopy on a single surface.
Plasmonic color originating from metallic nanostructures has many advantages over traditional pigmentation based color and have demonstrated sub wavelength resolution, tolerance to high intensity light, and scalability of the structure's optical response with dimensions and surrounding media. The later of these attributes, post-fabrication tunability, is a unique advantage of plasmonic structures that may enable it to reach niche applications. However, previous attempts of plasmonic tuning have yet to span an entire color space with a single nanostructure dimension. Here, we demonstrate a full red-green-blue (RGB) color changing surface enabled by a high birefringent liquid crystal (LC) and with a single nanostructure. This is achieved through the onset of a surface roughness induced polarization dependence and a combination of bulk and surface LC effects which manifest at different voltages. To further show the feasibility of such a system for display applications, we integrate the LC-plasmonic device with an actively addressed thin film transistor array (TFT) to display arbitrary images and video. Such a color changing surface may also find applications in wearables and active camouflage.
Plasmonic structural color has recently garnered significant interest as an alternative to the organic dyes standard in print media and liquid crystal displays. These nanostructured metallic systems can produce diffraction limited images, be made polarization dependent, and exhibit resistance to color bleaching. Perhaps even more advantageous, their optical characteristics can also be tuned, post-fabrication, by altering the surrounding media's refractive index parallel to the local plasmonic fields. A common material with which to achieve this is liquid crystal. By reorienting the liquid crystal molecules through external electric fields, the optical resonances of the plasmonic filters can be dynamically controlled. Demonstrations of this phenomenon, however, have been limited to modest shifts in plasmon resonance. Here, we report a liquid crystal-plasmonic system with an enhanced tuning range through the use of a shallow array of nano-wells and high birefringent liquid crystal. The continuous metallic nanostructure maximizes the overlap between plasmonic fields and liquid crystal while also allowing full reorientation of the liquid crystal upon an applied electric field. Sweeping over structural dimensions and voltages results in a color palette for these dynamic reflective pixels that can further be exploited to create color tunable images. These advances make plasmonic-liquid crystal systems more attractive candidates for filter, display, and other tunable optical technologies.