One of the untapped energy sources is the thermal energy available either from solar irradiance which is still not fully utilized or from the ambient heat temperature. Both resources share the nature of infrared (IR) radiation but with different range of wavelengths. The rectenna (rectifying antenna) concept is presented to harvest these IR radiations. The rectenna is simply an antenna connected to a diode. The diode has to be able to follow and rectify the ultra-fast received AC signal. This condition promotes the use of metal-insulator-metal (MIM) diodes due to their ultra-fast tunneling mechanism. The impedance matching between the diode an antenna is to be considered. The resistance practical ranges of both nano-antenna and MIM diode are generally far. The diode responsivity determines the MIM rectification capability. By building MIM diodes with multiple insulator layers the trade-off between the resistance and responsivity can be resolved. An optimization algorithm to select the qualified materials to build an MIIM diode with high responsivity and low resistance is introduced. A Ti-TiO2/ZnO-Al MIIM diode with ultra-thin oxide layers is fabricated. Also, a global optimization approach is carried out to maximize the impedance matching between the diode and the nano-antenna while improving the capacitance effect on the device’s cut-off frequency. The optimal results reveal a maximum coupling efficiency of 5.5%, a responsivity of 6.4 A/W, and a cut-off frequency of ~34 THz. A symmetric MIM metamaterial perfect absorber is introduced. The design has larger resistance than conventional nano-antennas. The near unity absorptivity is achieved through an optimization approach. A novel Chand-Bali nano-antenna that supports dual polarization and wide angle of reception is presented. The rectenna based on this nano-antenna is expected to achieve more than one magnitude of efficiency higher than ones fabricated in literature. / Thesis / Doctor of Philosophy (PhD)

We have studied several metamaterials structures with multiple layers by explaining them theoretically and verifying experimentally. The engineered structures we have designed work either as a perfect absorber or antireflection coating. The multilayer model as we call it Three Layer Model (TLM) has been developed, which gives the total reflection and transmission as a function of reflection and transmission of individual layers. By manipulating the amplitude and phase of the reflection and the transmission of the individual layers, we can get the required functionality of the optoelectronic devices. To get zero reflection in the both perfect absorber and the antireflection coating, the amplitude and phase conditions should be satisfied simultaneously. We have employed the numerical simulation of the structures to verify those conditions for all of the work presented here. As the theoretical retrieval method to extract the effective permittivity and effective permeability of the metamaterial contains air on the both side of the structure, we have dielectric at least on one side practically, that gives a little bit deviated result. We have modified the retrieval method to better fit with the multilayer structure by introducing air on the both side of the resonator using transfer matrix method and use it throughout all the works. We have explained the perfect absorption of the EM wave through Fabry-Perot cavity bounded by the resonator mirror and the metallic film. The metallic film acts as the close boundary whereas the resonator acts as the quasi-open boundary with very high effective permittivity, which leads to the characteristic feature of subwavelength thickness. We have shown numerically that the ultra-thin thickness makes the perfect absorber angular independent. We have also explained the phenomenon of perfect absorption through Impedance Matched Theory and Transmission Line Theory, and showed their matching with TLM. We have also developed the Meta Film Model by considering the resonator as a homogeneous thin film characterized by the effective permittivity and permeability giving rise to the same behavior as the original multilayer structure. We have shown that the resonance of the metamaterial resonator is very far from the resonance of the absorber, it behaves as the medium of high refractive index and very low loss. We have also shown that the density of states of the absorber is increased as compared to the resonator itself. We have investigated that the resonance peaks of the absorber arise from the combination of Fabry- Perot cavity modes and surface plasmon resonance modes. All the modes with increased spacer thickness are assigned with specific names describing the mode profiles. We have shown the application of perfect absorber as a refractive index sensor. It is used as a plasmonic sensor to detect the refractive index change of the chemical and biological samples. To increase the sensitivity, we have etched the dielectric spacer below the resonator, where electric field is localized and enhanced. We have found that the sensitivity (wavelength shift per refractive index change) and the Figure of Merit (FOM*) as an indicator of performance of the device both are enhanced significantly. We have employed metamaterial (MM) anti-reflection (AR) coating to avoid the shortcomings of the conventional thin film coating in three different cases of the structures. At first, we have deployed metamaterial Metal Disk Array (MDA) on the top of conventional coating material (BCB) with homogeneous substrate to enhance the transmission of EM wave. Then conventional AR coating is employed to the dispersive media (metal Hole Array) to enhance the transmission. We have shown that Impedance matched condition has been satisfied not only for homogeneous media, but for dispersive media also. At the end, we have employed the MM AR coating to the MM dispersive media (MHA). The two MM layers may interact with each other and may degrade the SPP wave of the MHA, which is essential to enhance the performance of the devices. To investigate the effect of interaction, we perform the simulation of the MDA, which shows that the resonance of the MDA is far from the antireflection resonance and hence the electric field of the SPP is significantly increased (~30%). With an improved retrieval method, the metasurface is proved to exhibit a high effective permittivity (εeff~30) and extremely low loss (tanδ~0.005). For all of the three AR structures, a classical thin film AR coating mechanism is identified through analytical derivations and numerical simulations. The properly designed εeff and μeff of the meta surface lead to the required phase and amplitude conditions for the AR coating, thereby paving the way for the improved performance of the optoelectronic devices. We have used MHA as a dispersive media to get extraordinary optical transmission (EOT). To understand the behavior of the SPP peaks, we have investigated the shifting and splitting of the spoof SPP resonance by varying the polar angle and azimuthal angle. The amplitude of extraordinary optical transmission also shows angle dependence and exhibits mirror-image or translational symmetries. Our measurements and simulations of the THz spoof SPP waves match very well with the theoretical predictions. The angle dependence results provide the important information for designing THz plasmonic devices in sensor and detector applications.

Understanding the fundamental physics in light absorption and perfect light absorption is vital for device applications in detector, sensor, solar energy harvesting and imaging. In this research study, a large area fabrication of Al-doped ZnO/Al2O3/graphene/Al2O3/gold/silicon device was enabled by a spin-processable hydrophilic mono-layer graphene oxide. In contrast to the optical properties of noble metals, which cannot be tuned or changed, the permittivity of transparent metal oxides, such as Al-doped ZnO and indium tin oxide, are tunable. Their optical properties can be adjusted via doping or tuned electrically through carrier accumulation and depletion, providing great advantages for designing tunable photonic devices or realizing perfect absorption. A significant shift of Raman frequency up to 360 cm-1 was observed from graphene in the fabricated device reported in this work. The absorption from the device was tunable with a negative voltage applied on the Al-doped ZnO side. The generated absorption change was sustainable when the voltage was off and erasable when a positive voltage was applied. The reflection change was explained by the Fermi level change in graphene. The sustainability of tuned optical property in graphene can lead to a design of device with less power consumption.

Plasmonic properties and the related novel applications are studied on various types of metallic nano-structures in one, two, or three dimensions. For 1D nanostructure, the motion of free electrons in a metal-film with nanoscale thickness is confined in its normal dimension and free in the other two. Describing the free-electron motion at metal-dielectric surfaces, surface plasmon polariton (SPP) is an elementary excitation of such motions and is well known. When further perforated with periodic array of holes, periodicity will introduce degeneracy, incur energy-level splitting, and facilitate the coupling between free-space photon and SPP. We applied this concept to achieve a plasmonic perfect absorber. The experimentally observed reflection dip splitting is qualitatively explained by a perturbation theory based on the above concept. If confined in 2D, the nanostructures become nanowires that intrigue a broad range of research interests. We performed various studies on the resonance and propagation of metal nanowires with different materials, cross-sectional shapes and form factors, in passive or active medium, in support of corresponding experimental works. Finite- Difference Time-Domain (FDTD) simulations show that simulated results agrees well with experiments and makes fundamental mode analysis possible. Confined in 3D, the electron motions in a single metal nanoparticle (NP) leads to localized surface plasmon resonance (LSPR) that enables another novel and important application: plasmon-heating. By exciting the LSPR of a gold particle embedded in liquid, the excited plasmon will decay into heat in the particle and will heat up the surrounding liquid eventually. With sufficient exciting optical intensity, the heat transfer from NP to liquid will undergo an explosive process and make a vapor envelop: nanobubble. We characterized the size, pressure and temperature of the nanobubble by a simple model relying on Mie calculations and continuous medium assumption. A novel effective medium method is also developed to replace the role of Mie calculations. The characterized temperature is in excellent agreement with that by Raman scattering. If fabricated in an ordered cluster, NPs exhibit double-resonance features and the double Fano-resonant structure is demonstrated to most enhance the four-wave mixing efficiency.