Integrated compound semiconductor lab-on-chip optical biosensors

Monim, Nadhia 2023. Integrated compound semiconductor lab-on-chip optical biosensors. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

The primary objective of this thesis is to review, design and evaluate features of Photonic Crystal Slabs (PCS), including Photonic Crystal Cavities (PCC) and Photonic Crystal Waveguides (PCWG), for their application in Labon- a-Chip (LOC) biosensors. After conducting a review of current literature on PCS, initial studies employed a first principles approach to develop relevant simulations using COMSOL. Specifically, the semiconductor material GaAs was investigated due to its direct band gap allowing for the integration of excitation elements in the LOC device. Preliminary simulations aimed at optimising the photonic band gap frequency range of a GaAs PCS with a triangular hole structure suggest that a slab thickness of 0.8a and hole radii of 0.35a are suitable for PCSs operating in water, approximating biological specimen, where a represents the lattice constant of 325 nm. This specific parameter settings established a substantial bandgap ranging from approximately 234 THz to 312 THz within a GaAs PCS with a refractive index set to 3.444 and surrounded by water, all while maintaining the structural integrity of the slab. In addition to further minimising calculation errors, the water/air layers surrounding the slab were refined to a minimum thickness to prevent reflections of calculated fields whilst balancing computational time. After fine-tuning the simulation parameters for the PCS, the manual optimisation of PCWG was performed, leveraging their simpler optimisation protocols arising from their inherent symmetry. This optimisation focused on adjusting PCWG guided mode positions within the band gap, thereby facilitating their capacity to couple with a multitude of sensor PCCs resonating at various 2 frequencies. Special consideration was given to controlling the high-dispersion segment of the guided PCWG mode, aligning it with the central part of the band gap to enhance the Wavelength Division Multiplexing (WDM) capabilities of LOC PCS devices. Research demonstrated that enlarging the radii of adjacent holes while displacing them away from the central axis of the waveguide resulted in the repositioning of the two PCWG guided modes towards the centre of the band gap, whilst simultaneously producing a separation between the two modes. Notably neighbouring hole radii of 0.35a and 0.4a exhibited the most suitable guided mode dispersions for PCC coupling. Simultaneous to the development of the PCWG optimisation protocol, a more complex approach was taken to address the optimisation of PCCs, adjusting the position and size of cavity adjacent holes. To account for this complexity an algorithmic protocol was developed and deployed via MATLAB with COMSOL Livelink. The L3 cavity was employed as an initial algorithm development starting point, with a range of methodologies explored to identify an appropriate approach. Among these, Gradient Descent (GD) was selected, satisfying the conditions of striking a balance between computational efficiency and the inherent complexity of the cavity optimisation, which could potentially extend the duration of the optimisation protocol. While all PCC optimisation protocols used a GD method, slab mode comparison algorithms varied amongst the tested PCC cavities: L3, H0, H1 with radial symmetry. Due to the nature of the intended use of the PCCs the algorithmic protocols developed not only optimised for high Quality factor (Q) but also targeted specific Eigenfrequencies. The nominal L3, H0, H1 fundamental resonant modes (M1) were initially optimised, followed by an additional iterative second stage optimisation utilising previous high Q M1 optimised modes to explore high Q attainable target frequencies across the band gap in 5% increments, whilst the cross functional utility of the GD method was evaluated via optimisation runs for Si in Air L3 PCC with target frequencies set to a common telecommunication wavelength 1.55 μm [1]. Successful optimisation of L3 PCC were achieved from nominal M1 of Q 103 to Q between 104 and ∼ 105 for various target 3 frequencies. Secondary band gap iterative stages reaching 106. H1 M1 modes showed even greater nominal mode comparative improvements in Q, starting with modes of 102 Q and achieving optimised modes of Q ∼104 for differing energies. Moreover, probing Si in Air L3 PCC optimisation runs targeting telecommunication wavelengths successfully achieved first stage optimisation results Q 105 from nominal M1 Q of 103.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Biosciences
Subjects:	Q Science > Q Science (General)
Date of First Compliant Deposit:	6 December 2024
Last Modified:	10 Jan 2025 11:34
URI:	https://orca.cardiff.ac.uk/id/eprint/174560

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