Composite materials with vanishing effective permittivity
Creators: name: Akpan, Usenobong Benjamin
Publisher: name: The University of St Andrews
Year: 2023

description: My PhD research thesis, entitled "Composite materials with vanishing effective permittivity" is presented here. Epsilon Near Zero (ENZ) Metamaterials (MMs) are photonic media in which the real part of the relative dielectric permittivity is vanishing and are useful for spatial light control such as squeezing and directive emission, enhanced light-matter interaction, sub-wavelength imaging, and nonlinear optics. This thesis demonstrates the use of mixed sub-wavelength metal and dielectric materials, which relies on the effective medium theory approximation to fine-tune the ENZ operation wavelength over wider ranges. First, we synthesized by chemical vapor deposition (CVD) a nonconductive bilayer graphene, and then design and show by a model of a semi-metal graphene/amorphous-germanium (α-Ge) bilayer and multilayer ENZ MM in the mid-infrared (IR) region. We then fabricate and experiment on its bilayer, with a discussion of its dynamic tunability via external control of the graphene’s potential. One intention is to see how this enables practicable photonic switching at the 10.6µm, where the carrier is the photon and the control mechanism is the back-gated voltage. Another goal is to electro-optically investigate the experimental compatibility of the basic bilayer of graphene/α-Ge structure in a MOSFET-based architecture which holds promise for a holographic imaging application. We found a gate resistance of 5.1MΩ, which met the requirement of a MOSFET device; and graphene’s sheet resistance of 8.9kΩ in our GFET device which is comparable to previously reported values in literature. We found out that graphene/α-Ge FET device lacks electrically compatibility with the MOSFET geometry as it failed to reproduce the usual ambipolar transconductance typical of a graphene/silicon geometry. We found that the optical transmission and reflection visibility of the graphene changes nonlinearly with the gated voltage; and the reflection visibility of graphene with gate voltage decreases when the source voltage is increased. Model results showed that reflection decreases with increasing voltage. Secondly, we design, fabricate and experiment by investigating the coupling loss, due to impedance mismatch, in anti-reflection coated/metal/dielectric, and the absorption loss in metal/dielectric with gain ENZ composite 1-dimensional anisotropic structures. We found the antireflection coating enhancing the transmission to around 20% and reducing the reflection by approximately 50%, in the ENZ (682nm) region. With the gain-doped dielectric, we tailored the thickness of the Alq₃:DCM dye dielectric in our Ag/Alq₃:DCM multilayer to achieve the ENZ wavelength close to the gain medium central emission wavelength and a reduction of the imaginary permittivity, from which future investigations of stimulated emission effect and other 8 nonlinear properties by all-optical pump-probe mechanism are indispensable. Finally, we design, fabricate, and experiment a graded ENZ composite which finds application as an optical aperture element. We also modeled various standard and ENZ apertures and compared with our experiment, which we observed not to agree completely with the model.