Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - SmartGraphene (Graphene based smart surfaces: from visible to microwave)

Teaser

Nature provides fascinating examples of adaptive camouflage. Flounders and reptiles, for example, have developed various mechanisms to tune the scattering and absorption of light on their skin. In modern technology, active camouflage is an emerging field of research aiming to...

Summary

Nature provides fascinating examples of adaptive camouflage. Flounders and reptiles, for example, have developed various mechanisms to tune the scattering and absorption of light on their skin. In modern technology, active camouflage is an emerging field of research aiming to develop surfaces with reconfigurable reflectance and transmittance in a very broad spectral window. These “smart” surfaces are an essential part of new adaptive camouflage systems such as; active radar shields used for concealment of objects from radar detection or flexible and wearable display devices. The ability to control interaction of electromagnetic waves with matter forms the heart of these emerging applications. These technologies would benefit from an enabling material that is (1) tuneable in a broad spectrum, (2) electrically reconfigurable, (3) mechanically flexible, and (4) low cost. None of the existing materials provide these challenging requirements. Realization of smart surfaces would enable new type of displays, wearable devices and smart building.
The aim of SmartGraphene project is to develop graphene based smart surfaces operating over the whole electromagnetic spectrum. In this projec,t we are developing new class of active surfaces capable of real-time electrical-control of its appearance in a very broad spectrum ranging from visible to microwave covering 6 orders of magnitude in wavelength. The our method relies on controlling electromagnetic waves by tuning density of high-mobility charges on single or multilayers of atomically thin graphene electrodes. We realize this goal by efficient electrolyte gating of large-area graphene which yields unprecedented ability to control intensity and phase of the reflected and transmitted electromagnetic waves.
The project has 3 main objectives;
1. Developing active surfaces in microwave and THz,
2. Developing active thermal (infrared) devices
3. Developing smart surfaces in the visible

Work performed

From the beginning of the project, we have performed research in three main wavelength range, (1) THz and microwave, (2) infrared and (3) visible. We have demonstrated various type of graphene based devices to control light in a broad spectral range. The core idea of the of these devices is based on a capacitor structure formed by an electrolyte medium sandwiched between two large area graphene electrodes. The voltage bias applied between the graphene electrodes polarizes the electrolyte and yield very large charge densities on the electrodes. Combining large scale chemical synthesis of graphene with novel device architectures, we have developed new tools to understand and control light-matter interaction in a very broad spectrum. Then we have used these tools to fabricate new camouflage and display technologies on unconventional substrates; such as flexible polymers and paper which cannot be realized by conventional semiconducting materials.
Here we will summarize the work performed in the first half of the project.

1 Active surfaces in microwave and THz,
The general aim of the first part of the proposal is to fabricate active surfaces to control intensity, phase and polarization of electromagnetic waves in microwave and terahertz frequencies. We have demonstrated four specific work in this part.

1.1 Graphene based Electrically Switchable Metadevices:
Metamaterials bring sub-wavelength resonating structures together to overcome the limitations of conventional matter. The realization of active metadevices has been an outstanding challenge that requires electrically reconfigurable components operating over a broad spectrum with a wide dynamic range. The existing capability of metamaterials, however, is not sufficient to realize this goal. Here, integrating passive metamaterials with active graphene devices, we demonstrated a new class of electrically controlled metadevices. These metadevices enable efficient control of both amplitude (> 50 dB) and phase (> 90°) of electromagnetic waves. In this hybrid system, graphene operates a tunable Drude metal that controls the radiation of the passive metamaterials. Furthermore, by integrating individually addressable arrays of metadevices, we demonstrated a new class of spatially varying digital metasurfaces where the local dielectric constant can be reconfigured with bias voltage. Our approach is general enough to implement various metamaterial systems that could yield new applications ranging from electrically switchable cloaking devices to adaptive camouflage systems.
This work has been published in Science Advances, (DOI: 10.1126/sciadv.aao1749)

1.2 Controlling phase of microwaves with active graphene surfaces
In this work, we developed a method to control reflection phase of microwaves using electrically tunable graphene devices. The device consists of mutually gated large-area graphene layers placed at a quarter-wave distance from a metallic surface. This device structure yields electrically tunable resonance absorbance and step-like phase shift around the resonance frequency when the impedance of graphene matches with the free space impedance. Electrostatic control of charge density on graphene yields an unprecedented ability to control both intensity (> 50 dB) and phase (~pi) of the reflected electromagnetic waves with voltage. Furthermore, using the asymmetry of the doping at opposite polarity of the bias voltages, we showed bidirectional phase control with the applied voltage. We anticipate that our results will pave a new directions to control interaction of electromagnetic waves with matter for long wavelengths and could open a new avenue for microwave devices.
This work has been published in Applied Physics Letters, (doi: 10.1063/1.4980087)

1.3 Observation of Gate-Tunable Coherent Perfect Absorption of Terahertz Radiation in Graphene
We report experimental observation of electrically tunable coherent perfect absorption (CPA) of terahertz (THz) radiation in grap

Final results

In this project, we challenge our scientific understanding and technological ability to realize adaptive optical surfaces operating over the whole electromagnetic spectrum. We are developing novel device architectures that enable us to control electromagnetic waves in an unprecedented way. Ability to control visible light has enabled many optical devices, such as displays, light sources and imaging systems. Fabrication of optical devices in other wavelengths (i.e. terahertz ) are limited due to lack of an active material. In this project we have utilised exceptional optical properties of graphene to fabricate novel optical devices that enables manipulation of light and its physical properties.
At the basic science level, this project revisits and challenges our basic understanding of light-matter interaction, in parallel, the proposed graphene-based smart surfaces will serve as a tool for developing new technologies. The ability to fabricate smart surfaces with the proposed degree of capability could yield new technologies with high potential for commercialization.
In the first half of the project, we have developed new tools to control intensity and phase of light from visible to microwave frequencies. In the second half of the project, we will continue developing our tool set and integrate these tools to form more sophisticated active imaging systems.