Electrically tunable radiative cooling performance of a photonic structure with thermal infrared applications

Thermal infrared (IR) radiation has attracted considerable attention due to its applications ranging from radiative cooling to thermal management. In this paper, we design a multi-band graphene-based metamaterial absorber compatible with infrared applications and radiative cooling performance. The proposed structure consists of the single-sized metal-insulator-metal (MIM) grating deposited on metal/insulator substrate and single-layer graphene. The system realizes a broadband perfect absorption ranging from 940 nm to 1498 nm and a narrowband perfect absorption at the resonance wavelength of 5800 nm. Meanwhile, the absorptivity of the structure is suppressed within the mid-wave infrared (MWIR) and long-wave infrared (LWIR) ranges. Furthermore, to demonstrate the tunability of the structure, an external voltage gate is applied to the single-layer graphene. It is shown that, by varying the chemical potential of graphene layer from 0 eV to 1 eV , the absorption resonances at the mid-infrared (MIR) range can shift toward the shorter wavelengths. It is also observed that the structure can possess an average net cooling power over 18 at the ambient temperature, when is varied from 0 eV to 1 eV. Finally, we investigate the overall performances of the structure as a function of temperature to realize thermal infrared applications.Comment: 11 pages, 6 figure

Electromagnetic Metamaterial Absorbers: From Narrowband to Broadband

Metamaterial perfect absorbers have received significant attention owing to their ability of achieving complete absorption of the electromagnetic waves with deeply subwavelength profiles. In this chapter, we will present a general review of the recent progress on theories, designs, and characterizations of metamaterial absorbers. We will first review the fundamental theories and design guidelines for achieving perfect absorption in subwavelength metamaterials. Several typical narrowband metamaterial absorbers are then presented with nearly 100% absorptivities. Next, we will focus on the realizations of broadband and frequency-tunable metamaterial absorbers. Coherent perfect absorbers, whose absorption performances are controllable via the interference of two counter-propagating electromagnetic waves, will also been introduced. In particular, we will also focus on the recent achievements of metamaterial absorbers in our research group

Antenna-assisted picosecond control of nanoscale phase transition in vanadium dioxide

Nanoscale devices in which the interaction with light can be configured using external control signals hold great interest for next-generation optoelectronic circuits. Materials exhibiting a structural or electronic phase transition offer a large modulation contrast with multi-level optical switching and memory functionalities. In addition, plasmonic nanoantennas can provide an efficient enhancement mechanism for both the optically induced excitation and the readout of materials strategically positioned in their local environment. Here, we demonstrate picosecond all-optical switching of the local phase transition in plasmonic antenna-vanadium dioxide (VO2) hybrids, exploiting strong resonant field enhancement and selective optical pumping in plasmonic hotspots. Polarization- and wavelength-dependent pump-probe spectroscopy of multifrequency crossed antenna arrays shows that nanoscale optical switching in plasmonic hotspots does not affect neighboring antennas placed within 100 nm of the excited antennas. The antenna-assisted pumping mechanism is confirmed by numerical model calculations of the resonant, antenna-mediated local heating on a picosecond time scale. The hybrid, nanoscale excitation mechanism results in 20 times reduced switching energies and 5 times faster recovery times than a VO2 film without antennas, enabling fully reversible switching at over two million cycles per second and at local switching energies in the picojoule range. The hybrid solution of antennas and VO2 provides a conceptual framework to merge the field localization and phase-transition response, enabling precise, nanoscale optical memory functionalities

Tunable Terahertz Metamaterials with Germanium Telluride Components

Terahertz (THz) technology is an emerging field with many exciting applications. THz waves can be used to locate explosives and illicit drugs in security applications, or DNA and other molecule resonances in medical applications. THz frequencies represent the next level of modern, high-speed computing, but they also can be used for covert battlefield communications links. Metamaterials are an integral part of THz technology because they can be used to create exotic material properties—permittivities and permeabilities—in a part of the frequency spectrum that is otherwise rather empty and passive. This work aims to acquire a fuller understanding of THz metamaterials in terms of background and theory, and then use this understanding to create a few novel, actively tunable structures using the phase-change material germanium telluride

Optically controlled metamaterial absorbers in the terahetz regime

Thesis (Ph.D.)--Boston UniversityElectromagnetic wave absorbers have been intensely investigated in the last century and found important applications particularly in radar and microwave technologies to provide anechoic test chambers, or vehicle stealth. Adding new features such as dynamic modulation, absorption frequency tunability, and nonlinearity, absorbers gain further functions as spatial light modulators, adjustable protective layers, and saturable absorbers which was a key factor in creation of ultra-fast lasers. These efforts required a rigorous search on various materials to find desired behavior. As a rather recent research field Metamaterials (MM) provide an easier path for creation of such materials by allowing engineering the interaction between electromagnetic radiation and materials. Alongside many exotic applications such as invisibility cloaking or negative refraction, MMs also made perfect, or near-unity, absorbers possible. Thanks to their ability to control electric and magnetic responses, by matching the impedance of the MMs to that of free space and simultaneously increasing the losses in the structure, perfect absorption can be achieved. This has been experimentally demonstrated in various bands of electromagnetic spectrum such as microwave, terahertz (THz), infrared, and visible. As in their earlier counterparts, adding modulation and nonlinearity to MM absorbers will broaden their contribution especially in the THz region which is nascent in terms of optical devices such as switches, modulators or detectors. With the recent developments in the THz lasers, THz nonlinear absorbers will be needed to realize ultra-fast phenomena in this region. The main focus of this thesis is incorporating conventional and novel methods to create some of the initial examples of optically controlled MM THz perfect absorbers using microfabrication tools. [TRUNCATED

Study of thermochromic nature of VO2 for reconfigurable frequency selection applications

The goal of this project is to investigate the use of vanadium dioxide in reconfigurable microwave devices such as antennas or filters. The second phase would see the creation of a more concrete application. The ability of vanadium dioxide (VO2) to change its structure above a certain temperature is of particular interest. Under 68°C, VO2 behaves like a dielectric, but when it reaches and exceeds that temperature, it behaves like a metal. With this in mind, we wanted to demonstrate the possibility of creating a reconfigurable FSS for spatial filtering by selectively heating the VO2 sample's surface area. A laser was used to select which area of the sample to heat: by shaping the beam, we were able to illuminate, and thus heat, only specific areas. This dissertation also describes the use of the time-resolved microwave conductivity (TRMC) technique to characterise vanadium dioxide to design these FSS images projected on the VO2 surface. We show that TRMC is a versatile technique for determining the electromagnetic material properties and conductivity of VO2 compounds. This was used to compare the behaviour of several VO2 samples of varying thicknesses and fabrication technologies.James Watt Scholarshi

Design, Fabrication and Testing of Tunable RF Meta-atoms

Metamaterials are engineered structures designed to alter the propagation of electromagnetic waves incident upon the structure. The focus of this research was the effect of metamaterials on electromagnetic signals at radio frequencies. RF meta-atoms were investigated to further develop the theory, modeling, design and fabrication of metamaterials. Comparing the analytic modeling and experimental testing, the results provide a deeper understanding into metamaterials which could lead to applications for beam steering, invisibility cloaking, negative refraction, super lenses, and hyper lenses. RF meta-atoms integrated with microelectromechanical systems produce tunable meta-atoms in the 10 - 15 GHz and 1 - 4 GHz frequency ranges. RF meta-atoms with structural design changes are developed to show how inductance changes based on structural modifications. RF meta-atoms integrated with gain medium are investigated showing that loss due to material characteristics can be compensated using active elements such as a Low Noise Amplifier. Integrating the amplifier into the split ring resonator causes a deeper null at the resonant frequency. The research results show that the resonant frequency can be tuned using microelectromechanical systems, or by induction with structural designs and reduce loss associated with the material conductivity by compensating with an active gain medium. Proposals that offer future research activities are discussed for inductance and active element meta-atoms. In addition, terahertz (THz), infrared (IR), and optical structures are briefly investigate

Synthesis of strongly correlated oxides and investigation of their electrical and optical properties

Strongly correlated oxides are studied widely for the host of unique applications, such as hightemperature superconductivity, colossal magneto resistance, exotic magnetic, charge and orbital ordering, and insulator-to-metal transitions. Transitional metal oxides which form the majority of the correlated oxide systems and oxides of Vanadium, especially VO2 and V2O5 are the two most favourite systems among researchers for several applications. In this thesis, the growth and characterization of VO2 and V2O5 are discussed along with a special focus on the optical property, especially thermochromic properties. Traditionally SMT behaviour and Infrared reflectively was the focus area for VO2 research, and its only until recently that VO2 is being treated as a much more complex system and investigated as highly responsive naturally disordered metamaterial near the phase transition temperature where the material exhibits semiconducting and metallic phase co-existence. Since each phase of VO2 has a distinct optical and electrical properties, controlling the extent of phase transitions by accurate temperature modulation, enables exploitation of the material for new properties like emissivity modulation in the NIR region and for creating IR visible reversible and rewritable patterns. V2O5 is traditionally seen as a high TCR material and regarded as material of choice for application ranging from catalysis, gas sensors to lithium batteries. In this study, however we focus on the optical properties of the material, especially the visible range thermochromic nature of V2O5 coatings synthesised by oxidative annealing of MOCVD grown VOx coatings. The impact of doping and selective oxygen vacancy generation on the thermochromic property are discusse