Projects per year
Metamaterial technologies have matured over the past decade for a variety of applications such as super-resolution imaging, perfect absorption, enhancement of the Purcell factor and so on. The class of artificial media which has emerged as one of the most important at optical frequencies are hyperbolic metamaterials (HMMs). These non-magnetic media have a dielectric tensor which is extremely anisotropic taking values both positive and negative. The reason for their widespread interest is due to the relative ease of fabrication, broadband non-resonant response, wavelength tunability, bulk three-dimensional response and high figure of merit. The goal of this PhD project is to develop the technology to fabricate and then measure the characteristics of metal-dielectric multilayer hyperbolic metamaterials as well as comparing the obtained characteristics with the theoretical ones in the visible and near-infrared wavelengths. The main topics within the project are: (a) optimizing the deposition technique for obtaining ultra-thin (below 10 nm) Au layers (b) developing the obtained technique for fabricating multi-layer structures of metal-dielectric (c) measuring the obtained structures using total internal reflection setups (d) patterning of the metal-dielectric multilayer thin film and (e) investigation of ultra-thin silver film for plasmonics. The fabrication process started with optimization of depositing a single thin smooth layer of gold of thickness around 6 nm-10 nm on top of non-metallic aminosilane adhesion layer (APTMS) using DC sputtering deposition technique. We then compared the optical performance based on surface plasmon polariton propagation on the gold film for various adhesion layers namely Cr, APTMS and no-adhesion layer for the gold films of thickness ranging from 8 nm to 24 nm. Our findings show that the gold films with APTMS adhesion layer gives the better performance in comparison to metallic adhesion layer or no adhesion layer in terms of good surface roughness and the optical property close to the theoretical predictions. This helps us to use APTMS as an alternative to metallic adhesion layers. The next fabrication step is to develop the obtained technique for obtaining gold and alumina multilayer structures. The optimization of deposition of ultra-thin alumina films in the range of 6-10 nm on top of APTMS is done using atomic layer deposition (ALD). We fabricated our multilayer Au-alumina HMM of thickness 10 nm each using the two optimized processes. We fabricated various periods of the HMMs from 1 upto 10. The final roughness of the 10th period sample is 0.80 nm. The samples are optically characterized using Otto configuration to excite the modes supported by the structures and the obtained reflection response are compared with the ones predicted by effective medium approach (EMA). With the comparison, we found that there is a mimimum period of the multilayer thin film for the EMA to be applicable. The experimental response difference for the fourth period is as close as 4\% as compared to the EMA. After obtaining a reproducible optimized process, we continued our investigation on developing a technique to pattern the structures in order to enlarge the applicability of the device. We started our patterning procedure for ultra-thin gold film of thickness 10 nm based on combination of e-beam lithography and lift-off techniques. However, the lift-off technique becomes harder and difficult for more number of periods of the multilayer thin film due to the conformal depositions of sputtering which limited us to a maximum of two to three periods. As part of patterning HMM, we also investigated direct laser writing as an alternative to e-beam lithography and we observed a possibility of writing sub-diffraction linewidth upto 220 nm using a low power continuous wave laser. Finally, the fabrication of ultra-thin silver films using various metallic adhesion layers, non-metallic silane based adhesion layers and also Al-doped silver films for the deposition of ultra-thin silver films has been investigated. Al doped silver films can be deposited upto 6 nm thick.
|Publisher||Technical University of Denmark|
|Publication status||Published - 2019|