Tunable Plasmonic Metamaterial

Plasmonic metamaterials are artificial materials typically composed of noble metals in which the features of photonics and electronics are linked by coupling photons to conduction electrons of metal (known as surface plasmon). These rationally designed structures have spurred interest noticeably since they demonstrate some fascinating properties which are unattainable with naturally occurring materials. Complete absorption of light is one of the recent exotic properties of plasmonic metamaterials which has broadened its application area considerably. However, up to date all of the applied methods (perforated metallic films, grating structured systems, and conventional metamaterials) are costly and suffer from a lack of flexibility. Furthermore, their absorbance is mainly limited to a narrow spectral range or their fabrication is costly. So, such drawbacks make their vast application almost impossible. Here, in this dissertation, we design, fabricate and characterize a novel perfect absorbers based on nanocomposites whose total thickness is only a few tens of nanometers and its absorption band is broad, tunable and insensitive to the angle of incidence. The nanocomposites consist of metal nanoparticles embedded in a dielectric matrix with a high filling factor close to the percolation threshold. The filling factor can be tailored by vapor phase co-deposition of the metallic and dielectric components. Accordingly, three types of metals (gold, silver and copper) as the inclusions of the nanocomposite and four different mirrors (gold, silver, copper and aluminum) are used as the base layer. The high absorption of these metamaterials are originated from the huge absorption capability of the metallic nanoparticles (smaller than 5 nanometer in diameter) via localized plasmon resonance, confinement of the light within the tiny gap between nanoparticles as well as interference of the light by reflection through the layers. To functionalize the system, polymer-photoswitchable molecules were added as the top or spacer layer which enable us to demonstrate a photodriven perfect absorber in which the absorption band can be broadened or narrowed by ultraviolet or visible light illumination, respectively. In this approach, the absorption tuning is originated from the bond-breakage of the molecules which can be activated by irradiation. Due to the strong interaction of the molecules and metal mirror, plasmon-exciton coupling happens which not only enhances the absorption but also shifts or splits the absorption band. Also as the specific highlight of the idea, we show that a thin plasmonic nanocomposite film on a silicon wafer covered by a silicon dioxide film would diminish the reflection in a broad range of frequency and make a new class of plasmonic anti-reflection coating. Our novel concept (called hybrid ARC) combines two possible arrangements for the layers in an anti-reflection coating into a single structure; albeit at two different wavelengths. Its performance originates from the strong dispersive nature of the nanocomposite. Furthermore, we show that the current metamaterial on a metal reflector can be used for visualization of different colorations as a plasmonic rainbow despite its sub-wavelength thickness

Multifunctional waveguide interferometer sensor: simultaneous detection of refraction and absorption with size-exclusion function

A waveguide Young interferometer is presented with simultaneous detection of complex refractive index of a liquid sample. The real part of the refractive index change (refraction) is detected by tracing phase shifts of the interferogram generated by a sensing and reference waveguide. The imaginary part of the refractive index (absorption) is determined by the attenuation of the transmitted signal at certain wavelength. Furthermore, nano-filters are fabricated atop the sensing waveguide, which enables size-exclusion filtering of species to the evanescent field. It shows capability of distinguishing small and large particles from 100 nm to 500 nm in diameter, which is further confirmed by fluorescent excitation experiments. The present sensor could find broad application in optical characterization of complex turbid media with regard to their complex refractive index

Ultra-stretchable active metasurfaces for high-performance structural color

Metamaterials as artificially structural materials exhibit customized properties unattainable in nature. While dynamic response is highly desired, metamaterials are usually passive and cannot be tuned post-fabrication. A conventional active metamaterial consists of rigid resonators mounted on flexible substrates that permit a limited amount of mechanical tuning. Given that rigid resonators permanently deform or debond under large strains (above 30%), the range of flexibility that is possible with tunable metamaterials is limited. Here, we propose a kirigami-inspired geometry that overcomes this limitation. The proposed design enhances stretchability exceeding 100% when compared with the existing design. A high degree of flexibility is achieved through “stress engineering” at the interface between rigid resonators and flexible substrates. Our design shows that the resonance modes shift at a rate of 3.32 ± 0.1 nm for every 1% change in strain, which is the highest tunability reported thus far. We demonstrate how this new concept can be applied to structural color. Using a single design, we demonstrated the full range of colors for the first time. The novel concept of highly stretchable metamaterials may revolutionize the field and enable its use in applications such as wearable sensors, smart displays, and switchable devices requiring extremely dynamic properties

Mechanically tunable metasurface with large gamut of color: Lateral hybrid system

Hybrid metasurfaces are made of metals and dielectrics in which dielectrics (metals) are sandwiched between metals (dielectrics) to control the reflection and transmission of light. The existing designs have low sensitivity, little color coverage, and a lack of flexibility. Here, a new structural color design is proposed in which metals and dielectric resonators are arranged spatially in 2D to form a lateral hybrid system, instead of being placed as layers. Such a design exhibits a high level of sensitivity to mechanical forces because it works via 2D optical coupling and light confinement between adjacent resonators. Our study shows that in-planar coupling of two dissimilar resonators can enhance sensitivity by an order of magnitude in comparison to stacking them. Metasurfaces with our design would have unprecedented mechanical tunability without compromising either the materials choice or processing. Using the proposed hybrid system, we demonstrate large tunability across the full range of colors with only a 10% change in the size of the lattice, which further proves its superiority over existing designs. This concept could find application in wearable devices that require high sensitivity to small mechanical fluctuations

Inverse Design of Distributed Bragg Reflectors Using Deep Learning

Distributed Bragg Reflectors are optical structures capable of manipulating light behaviour, which are formed by stacking layers of thin-film materials. The inverse design of such structures is desirable, but not straightforward using conventional numerical methods. This study explores the application of Deep Learning to the design of a six-layer system, through the implementation of a Tandem Neural Network. The challenge is split into three sections: the generation of training data using the Transfer Matrix method, the design of a Simulation Neural Network (SNN) which maps structural geometry to spectral output, and finally an Inverse Design Neural Network (IDNN) which predicts the geometry required to produce target spectra. The latter enables the designer to develop custom multilayer systems with desired reflection properties. The SNN achieved an average accuracy of 97% across the dataset, with the IDNN achieving 94%. By using this inverse design method, custom-made reflectors can be manufactured in milliseconds, significantly reducing the cost of generating photonic devices and thin-film optics

Antireflective Coatings: Conventional Stacking Layers and Ultrathin Plasmonic Metasurfaces, A Mini-Review

Reduction of unwanted light reflection from a surface of a substance is very essential for improvement of the performance of optical and photonic devices. Antireflective coatings (ARCs) made of single or stacking layers of dielectrics, nano/microstructures or a mixture of both are the conventional design geometry for suppression of reflection. Recent progress in theoretical nanophotonics and nanofabrication has enabled more flexibility in design and fabrication of miniaturized coatings which has in turn advanced the field of ARCs considerably. In particular, the emergence of plasmonic and metasurfaces allows for the realization of broadband and angular-insensitive ARC coatings at an order of magnitude thinner than the operational wavelengths. In this review, a short overview of the development of ARCs, with particular attention paid to the state-of-the-art plasmonic- and metasurface-based antireflective surfaces, is presented

A deep learning approach to the forward prediction and inverse design of plasmonic metasurface structural color

This report details a deep learning approach to the forward and inverse designs of plasmonic metasurface structural color. Here, optimized Deep Neural Network models are presented to enable the forward and inverse mapping between metamaterial structure and corresponding color. The forward model is capable of predicting color with >96% accuracy, with a 105 order of magnitude decrease in computational time when compared to finite-difference time-domain simulations used in conventional design workflows. An inverse model is trained using a tandem autoencoder, employing the pre-trained forward model. Here, the use of synthetic training data for self-learning is reported, which results in an ≈15% improvement in training accuracy. The tightly constrained inverse model allows for the instantaneous design of metasurfaces, given a desired color, with an accuracy of >86%, making it suitable for commercial use as well as the acceleration of photonics research