Examensarbeten för masterexamen

Länka till denna samling:

https://odr.chalmers.se/handle/20.500.12380/50

Browse

Visar 1 - 5 av 265

Nanofabrication of 2D photonic crystals for PCSELs emitting in UV region
(2024) Sunil, Vismaya; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap (MC2); Chalmers University of Technology / Department of Microtechnology and Nanoscience (MC2); Haglund, Åsa; Apaydin, Doğukan
Lasers are light sources that produce coherent beams. Semiconductor lasers offer notable advantages such as compact size, lower power consumption, and extended lifespan over other laser types. Among different semiconductor lasers, photonic crystal surface-emitting lasers (PCSELs) are recognized for their high optical output power and low beam divergence, achieved through the use of a two-dimensional photonic crystal. The proposed PCSELs use photonic crystals which offer refractive index contrast in two dimensions. This creates a standing wave with zero group velocity and vertical emission through Bragg diffraction. PCSELs have been successfully demonstrated in the visible and infrared wavelengths, but not yet in the ultraviolet (UV) range. This gap represents a significant opportunity, as UV-emitting PCSELs find potential applications in lithography, sterilization, and processing of materials. This project aims to explore different etching techniques to deep into the cladding layer of the PCSELs. To keep the optical losses low, the vertical field profile should not overlap with the absorbing metal layer used for the p-contact. Therefore the distance between this metal and the QWs needs to be large and on the order of 300-400 nm. The bottom of the photonic crystal holes on the other hand should be very close to the top QW (about 60 nm). To fulfill both these requirements the photonic crystal should have an etch depth of about 300 nm. Etching is preferred over alternative methods, such as over-growth or mass transport, due to its presumed simplicity and cost-effectiveness.
Computational modelling of phospholipids in plasma membranes
(2022) Lanai, Victor; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap (MC2); Chalmers University of Technology / Department of Microtechnology and Nanoscience (MC2); Schröder, Elsebeth; Schröder, Elsebeth
The purpose of this project was to investigate if the constituents of phospholipids in plasma membranes affect how cells interact with graphene (G) and graphene oxide (GO). It has previously been shown that vertically grown graphene flakes are effective in killing bacteria whilst keeping mammalian cells intact. However, the mechanism behind this phenomena is not known, and is at the same time difficult to measure experimentally. Therefore we choose density functional theory as a tool, with the goal to enhance the understanding. This thesis dives into the plasma membranes of bacterial and mammalian cells, and target different phospholipids in these membranes. The project started off by creation of a library of single phospholipids. These were put together into systems of pairs for calculation of bonding between different phospholipids. Further, both a G and a GO flake were created, and incorporated into the systems with the phospholipid pairs. Analysis of the interaction energies between these flakes with the phospholipid pairs was performed, both when the flakes approach the phospholipids, and upon penetration of the membrane. Calculations show that the most abundant phospholipids in mammalian cells have stronger bonding to each other, compared to bacterial phospholipids. Further, when the G/GO flakes enter between the phospholipid pairs, the bacterial pair exhibits less repulsive interactions, and a more stable system with the flakes were found. Therefore, these variables may contribute to the diverse robustness between bacterial and mammalian cells, and thus, the composition of phospholipids can be an important factor in explaining the difference in viability between organisms.
Characterisation of YBCO surfaces and STO/YBCO interfaces using DFT
(2024) Danielsson, Erika; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap (MC2); Chalmers University of Technology / Department of Microtechnology and Nanoscience (MC2); Schröder, Elsebeth; Schröder, Elsebeth
The focus of this project was to investigate the surface structure of the high temperature superconductor YBa2Cu3O7 (YBCO) as well as determining the interface structure when YBCO is capped with a thin film of SrTiO3 (STO). The motivation for STO capping is to see if it can protect YBCO from degradation losing its superconductivity. Here surface and interface terminations were studied with a literature review as well as calculations based on density functional theory (DFT). Together with this, a method of determining the surface energy contribution of a slab structure was used which found the CuO2(Y) surface and the BaO(CuO) surface as most probable surface terminations of YBCO. The change in electron density was also studied that showed a rearrangement of charges at the interface in the STO/YBCO systems. The STO capping itself together with electronic redistribution at the interface made the top YBCO layers become more bulk-like, and this can be a part of the explanation how STO can protect the YBCO surface.
Superconducting flux transformers for the modulation of flux-tunable resonators
(2024) Dakroury, Karim; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap (MC2); Chalmers University of Technology / Department of Microtechnology and Nanoscience (MC2); Wieczorek, Witlef; Paradkar, Achintya; Wieczorek, Witlef
Pushing the limits of quantum mechanics to larger objects is a goal of current research efforts. One approach to test the limits of quantum mechanics is to achieve quantum superposition with a macroscopic object on the order of micrometers. A possible experimental approach in this direction is given by coupling a magnetically levitated particle to a superconducting flux-tunable resonator. This system can allow us to sense the particle’s motion and the flux-tunable resonator will act as the quantum sensor and the readout for the particle. This system exploits flux coupling between the particle and the flux-tunable resonator. An approach of realizing this flux coupling is by implementing a flux transformer which is the main goal of this thesis. In this thesis we demonstrate a theoretical model to optimize the geometry of the flux transformer for maximum flux transfer efficiency. The theoretical analysis is verified with simulations on COMSOL Multiphysics. Then, a reliable fabrication recipe has been developed which had high yield of superconducting flux transformer and flip-chip devices. A novel flip-chip assembly technique was implemented with usage of Indium microspheres as superconducting interconnects. The thin-film of the materials used for the flux transformer and the flip-chip devices were characterized to demonstrate their superconductivity. Finally, a proof-of-principle for flip-chip based modulation of a flux-tunable resonator is demonstrated.
Electrical Modelling of High-speed Photodiodes
(2024) Muppathiyil, Geo Philip; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap (MC2); Chalmers University of Technology / Department of Microtechnology and Nanoscience (MC2); Karlsson, Magnus; Hjort, Filip
Photodiodes are pivotal components in optoelectronic systems, converting light into electrical signals with applications spanning from telecommunications to medical devices. This thesis presents a comprehensive study on the electrical modeling of photodiodes, aiming to enhance their performance when integrated with transimpedance amplifiers (TIAs) in receivers. The research commences with an in-depth analysis of the physical principles governing photodiode operation. Various modeling techniques are examined, with a particular emphasis on equivalent circuit models that accurately represent the photodiode’s behavior under different biasing conditions. The model is constructed based on experimentally measured frequency response and single-port reflection of the device. Devices with varying optical apertures and different biasing pads are analyzed. It is observed that the ground-signal pad geometry introduces additional inductance to the electrical model, enhancing the frequency response by up to 5 GHz compared to the ground-signal-ground pads. The study also compares the performance of several photodiode prototypes, varying parameters such as absorber thickness and extraction layer thickness. Furthermore, the combined response of the photodiode and TIA was simulated. The results indicate that the frequency response, when combined with the TIA for ground-signal pad geometry, remains flat up to 35 GHz, outperforming the ground-signal-ground design. This ensures that the receiver is free from frequency-dependent distortions. The responsivity and dark current characteristics of the devices are also measured. In conclusion, the electrical modeling techniques presented in this thesis offer a powerful framework for understanding the photodiode performance with TIA in optical recievers. The findings contribute to the advancement of optoelectronic technology, paving the way for more efficient and versatile applications in various fields such as data centers and other high-speed communication systems.

Browse

Senast publicerade