Metamaterials and recently metasurfaces have been a powerful tool to control and manipulate electromagnetic waves and their interaction with matter. Active control of metamaterials is an expanding new direction, which is promising for the realization of novel active devices, such as optical switches, transducers, modulators, filters, and phase shifters at different wavelengths. The integration of passive metamaterials with a variety of tuning mechanisms has been extensively examined to generate active metamaterials that have novel functionalities. In general, there are two major schemes to implement active plasmonic systems. One is based on the integration of active media, that is, phase-transition materials, graphene and carrier-modulated semiconductors, which can respond to thermal, electrical and optical stimuli. The other is based on geometrical reconfiguration, that is, structural tuning of metamaterials.
Although the demonstrated devices provide some degree of tunability, their performances are limited to narrow spectra with a small dynamic range due to the material and fabrication limitations. Therefore, these technologies would greatly benefit from a material that yields large tunability over broad spectra. None of the existing materials provides these challenging requirements. Furthermore, the requirement for electrically controlled tunability places another challenge for practical applications of metamaterials.
Integrating metamaterial (a split ring resonator, SRR, in this work) in close proximity to graphene surface yields a new type of hybrid metamaterial whose resonance amplitude can be tuned. Previous attempts to integrate graphene with metamaterials yielded very limited modulation in IR and terahertz frequencies. Here, to tune the electrical resonance of metamaterials, we varied the charge density on graphene layer via ionic gating. It should be emphasized here that the technical challenge for graphene-based microwave devices is the requirement of large-area devices owing to the centimeter scale wavelength. To overcome this challenge, large-area graphene by chemical vapor deposition (CVD) on copper foils is used, which enables the realization of the microwave metamaterials. At 0 V, the device yields a resonance at 11.82 GHz with a resonance transmittance of −60 dB. When we applied a bias voltage, electrons and holes accumulate on the graphene electrodes and yield significant damping that diminishes the resonant behavior. For example, at 1.5 V, the resonance transmittance is -12 dB. Figure 2 shows the voltage dependence of the amplitude of transmittance at resonance and the phase at 11.82 GHz. The phase of the transmitted signal varies from -30° to 70°.
These active metamaterials enable efficient control of both amplitude (>50 dB) and phase (>90°) of electromagnetic waves. The operation frequency of these metamaterials can be easily scaled up to the terahertz and higher frequencies. Large modulation depth, simple device architecture, and mechanical flexibility are the key attributes of the graphene-enabled active metamaterials. We anticipate that the presented approach could lead to new applications ranging from electrically switchable cloaking devices to adaptive camouflage systems in microwave and terahertz frequencies.
In this work, we have designed and fabricated an array of plasmonic nano-ellipse that interacts with different types of QEs in the visible range of wavelength. The proper geometry of our design provides such absorption-reflection properties which spectrally overlap with the emission spectrum of the QE. Alongside such spectral overlap, a thin layer of the dielectric layer between the plasmonic structures and a gain medium provides the possibility of spatial overlap. The interaction between the strong subwavelength localized field at the edges of the gold nano-ellipses and QEs, enhances Purcell factor towards the modification of the fluorescence and decay time of QEs. This approach allows enhanced emission from different emitters embedded in hybrid quantum systems. In this work, we study the energy transfer between the fluorescent dye molecules and CdSe/ZnS hydrophobic QDs with the array of plasmonic nano-ellipses.
Sunlight can be used a source of light in buildings and automobiles, however infrared wavelengths in sunlight result in heating. In this work, Infrared Reflective Coatings are designed using thin films to transmit visible wavelengths 400~700 nm while reflecting infrared wavelengths above 700 nm. Three different design approaches have been used, namely single layer of metal, sandwich structure and multilayer design. Four metals (Ag, Au, Al and Cu) and two dielectrics (TiO2 and SiO2) are used in this study. Designs with Ag show maximum reflection of Infrared wavelengths in all designs. Sandwich structures of TiO2-Ag-TiO2 on substrate with 22 nm of thickness for each layer show the maximum transmission of 87% in the visible region and maximum reflection of Infrared wavelengths.