{"id":{"repo_id":"cork","oai_identifier":"oai:cora.ucc.ie:10468/18909"},"canonical_url":"https://search.dev.ndltd.org/etd/cork/oai:cora.ucc.ie:10468/18909","repository":{"repo_id":"cork","name":"University College Cork","base_url":"https://cora.ucc.ie/server/oai/request"},"display":{"title":"Theory and simulation of quantum nanostructures for integrated photonics","abstract":"Quantum nanostructures such as quantum wells (QWs) and quantum dots (QDs) are employed in a variety of integrated photonic devices, including lasers, electro-absorption modulators (EAMs), single-photon sources, and photodetectors. Theoretical analysis of these devices is required to enable performance improvements and support development for novel applications. In this thesis, we employ the semi-empirical continuum-based k · p method to analyse the electronic and optical properties of QWs and QDs to enhance the capabilities of these devices. First, we present the QUDOS code: a free, open-source software package to underpin predictive analysis and design of QD heterostructures. Using a semi-analytical plane wave approach, QUDOS allows to compute the electronic structure of QD heterostructures. QUDOS includes strain and piezoelectric effects on the QD electronic structure, and allows to apply an arbitrarily oriented electric field to enable analysis of electric-field dependent QD properties including the quantum-confined Stark effect (QCSE) and its implications for device applications. QUDOS computes the strain and piezoelectric fields, constructs the Hamiltonian for a supercell containing an arbitrary QD-based heterostructure, and exactly diagonalises the Hamiltonian to yield the single-particle QD eigenstates. These eigenstates are then used to compute optical (momentum) and Coulomb matrix elements, thereby providing the inputs required to perform semi-classical or many-body calculations of classical and quantum QD properties. QUDOS provides a high degree of flexibility, allowing to simulate a variety of heterostructures, including numerous QD shapes both with and without wetting layers, QD molecules and QD-in-a-well structures, and composition-graded or arbitrarily oriented QDs. The core physics library is written in C, employs hybrid OpenMP-MPI parallelisation, and exploits various routines from Intel’s oneMKL to accelerate performance. Second, we apply the QUDOS code to investigate the viability of lateral-field QD-EAMs. Advances in III-V on Si QD growth have enabled monolithic integration of high-performance electrically-pumped lasers on Si, as a critical enabling component for Si photonics. Another critical component is the EAM, which exploits the QCSE to achieve high-speed modulation of laser signals. Conventional QW-EAMs exploit a “vertical” QCSE via top and bottom electrical contacts. Rapid advancements in planar photonic integrated circuit technology motivate development of laterally-contacted EAMs, which offer performance benefits including reduced parasitic capacitance. The QCSE cannot be achieved via a lateral field in a QW, but can in a QD due to the three-dimensional carrier confinement. We undertake theoretical analysis of the lateral-field QCSE in 1.3 μm InGaAs/GaAs QDs. Comparing the QCSE produced by vertical and lateral electric fields for realistic QD morphology a robust lateral field QCSE is demonstrated, with the optical absorption edge redshifting more rapidly vs. field strength than in a conventional QW-EAM. It is shown that lateral-field QD-EAM performance is expected to be strongly sensitive to the spectral linewidth of the band edge absorption, and can also depend upon the in-plane orientation of the lateral electric field. The impact of QD morphology – the base shape, aspect ratio, and composition profile – is also quantified. It is demonstrated that InGaAs/GaAs QDs possessing high aspect ratios and low absorption linewidths are well-suited to develop lateral-field 1.3 μm QD-EAMs. This suggests leveraging III-V on Si epitaxy to integrate EAMs with lasers or single-photon sources, to realise high-speed Si photonic integrated circuits for applications in datacomms and linear optical quantum computing. Third, we develop a multi-component diffusion theory of QW intermixing for quaternary alloy systems. QW intermixing via post-growth annealing enables integration of active and passive components in photonic integrated circuits. QW intermixing generally produces a blueshift of the QW band gap, rendering the intermixed section of a QW transparent to a laser fabricated from a non-intermixed section of the same structure. Experimental studies on AlInGaAs/InP QWs have, however, identified structures in which intermixing produces a redshift of the band gap. This redshift is anomalous insofar as it is not explained by previous models of QW intermixing based on Fick’s law. To overcome this limitation we apply a generalised (multi-component) form of Fick’s law to describe inter-diffusion of Ga, In, and Al in AlInGaAs/InP. Based on the experimental observation that annealing can drive strong Ga and In intermixing but Al diffuses minimally, we demonstrate that intermixing in AlInGaAs QWs can be formulated as a coupled inter-diffusion problem characterised by independent Ga- and In-related diffusivities, which increase with increasing strain in the QW, and suppressed Al diffusion. The intermixing is governed by the ratio of these diffusivities, which can drive a counterintuitive process that explains the experimentally observed redshift: namely, diffusion of additional In into the well upon annealing of a structure having equal well and barrier In compositions. The model is validated via comparison to experiment. First, by showing that it can quantitatively describe measured composition profiles. Second, via empirical parameterisation of the diffusion coefficients, which we demonstrate as sufficient to quantitatively reproduce the measured QW band gap shift vs. annealing temperature.","abstract_html":"Quantum nanostructures such as quantum wells (QWs) and quantum dots (QDs) are employed in a variety of integrated photonic devices, including lasers, electro-absorption modulators (EAMs), single-photon sources, and photodetectors. Theoretical analysis of these devices is required to enable performance improvements and support development for novel applications. In this thesis, we employ the semi-empirical continuum-based k · p method to analyse the electronic and optical properties of QWs and QDs to enhance the capabilities of these devices. First, we present the QUDOS code: a free, open-source software package to underpin predictive analysis and design of QD heterostructures. Using a semi-analytical plane wave approach, QUDOS allows to compute the electronic structure of QD heterostructures. QUDOS includes strain and piezoelectric effects on the QD electronic structure, and allows to apply an arbitrarily oriented electric field to enable analysis of electric-field dependent QD properties including the quantum-confined Stark effect (QCSE) and its implications for device applications. QUDOS computes the strain and piezoelectric fields, constructs the Hamiltonian for a supercell containing an arbitrary QD-based heterostructure, and exactly diagonalises the Hamiltonian to yield the single-particle QD eigenstates. These eigenstates are then used to compute optical (momentum) and Coulomb matrix elements, thereby providing the inputs required to perform semi-classical or many-body calculations of classical and quantum QD properties. QUDOS provides a high degree of flexibility, allowing to simulate a variety of heterostructures, including numerous QD shapes both with and without wetting layers, QD molecules and QD-in-a-well structures, and composition-graded or arbitrarily oriented QDs. The core physics library is written in C, employs hybrid OpenMP-MPI parallelisation, and exploits various routines from Intel’s oneMKL to accelerate performance. Second, we apply the QUDOS code to investigate the viability of lateral-field QD-EAMs. Advances in III-V on Si QD growth have enabled monolithic integration of high-performance electrically-pumped lasers on Si, as a critical enabling component for Si photonics. Another critical component is the EAM, which exploits the QCSE to achieve high-speed modulation of laser signals. Conventional QW-EAMs exploit a “vertical” QCSE via top and bottom electrical contacts. Rapid advancements in planar photonic integrated circuit technology motivate development of laterally-contacted EAMs, which offer performance benefits including reduced parasitic capacitance. The QCSE cannot be achieved via a lateral field in a QW, but can in a QD due to the three-dimensional carrier confinement. We undertake theoretical analysis of the lateral-field QCSE in 1.3 μm InGaAs/GaAs QDs. Comparing the QCSE produced by vertical and lateral electric fields for realistic QD morphology a robust lateral field QCSE is demonstrated, with the optical absorption edge redshifting more rapidly vs. field strength than in a conventional QW-EAM. It is shown that lateral-field QD-EAM performance is expected to be strongly sensitive to the spectral linewidth of the band edge absorption, and can also depend upon the in-plane orientation of the lateral electric field. The impact of QD morphology – the base shape, aspect ratio, and composition profile – is also quantified. It is demonstrated that InGaAs/GaAs QDs possessing high aspect ratios and low absorption linewidths are well-suited to develop lateral-field 1.3 μm QD-EAMs. This suggests leveraging III-V on Si epitaxy to integrate EAMs with lasers or single-photon sources, to realise high-speed Si photonic integrated circuits for applications in datacomms and linear optical quantum computing. Third, we develop a multi-component diffusion theory of QW intermixing for quaternary alloy systems. QW intermixing via post-growth annealing enables integration of active and passive components in photonic integrated circuits. QW intermixing generally produces a blueshift of the QW band gap, rendering the intermixed section of a QW transparent to a laser fabricated from a non-intermixed section of the same structure. Experimental studies on AlInGaAs/InP QWs have, however, identified structures in which intermixing produces a redshift of the band gap. This redshift is anomalous insofar as it is not explained by previous models of QW intermixing based on Fick’s law. To overcome this limitation we apply a generalised (multi-component) form of Fick’s law to describe inter-diffusion of Ga, In, and Al in AlInGaAs/InP. Based on the experimental observation that annealing can drive strong Ga and In intermixing but Al diffuses minimally, we demonstrate that intermixing in AlInGaAs QWs can be formulated as a coupled inter-diffusion problem characterised by independent Ga- and In-related diffusivities, which increase with increasing strain in the QW, and suppressed Al diffusion. The intermixing is governed by the ratio of these diffusivities, which can drive a counterintuitive process that explains the experimentally observed redshift: namely, diffusion of additional In into the well upon annealing of a structure having equal well and barrier In compositions. The model is validated via comparison to experiment. First, by showing that it can quantitatively describe measured composition profiles. Second, via empirical parameterisation of the diffusion coefficients, which we demonstrate as sufficient to quantitatively reproduce the measured QW band gap shift vs. annealing temperature.","abstract_has_math":false,"creators":["Murphy, Tommy"],"institution":"University College Cork","degree_name":null,"degree_level":null,"degree_discipline":null,"degree_department":null,"school":null,"contributors":[],"advisors":["O'Reilly, Eoin","Broderick, Christopher","Peters, Frank H."],"committee_chairs":[],"committee_members":[],"year":2026,"date_issued":"2026-05-01","date_published":"2026-05-01","updated_at":"2026-07-24T01:48:31Z","subjects":["Quantum well","Quantum dot","Electro-absorption modulators","Photonics","Quantum-confined Stark effect","Lateral field","Photonic circuits","Integrated photonics","Silicon photonics","Quantum well intermixing","III-V materials","Quaternary materials","Fick's law","Computational physics","Electronic structure","QUDOS","Strain","Piezoelectric potential","Momentum matrix elements","Coulomb matrix elements","Coulomb integrals","MPI","OpenMP","Plane-wave expansion method","8-band k.p","Quantum nanostructure","Open-source code"],"languages":["en"],"rights":["© 2026, Tommy Murphy."],"rights_urls":["https://creativecommons.org/licenses/by/4.0/"],"identifier_entries":[]},"links":{"outbound_url":"https://hdl.handle.net/10468/18909","outbound_label":"Handle","outbound_source":"dc:identifier.uri"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor.advisor","label":"Advisor","values":["O'Reilly, Eoin","Broderick, Christopher","Peters, Frank H."]},{"key":"dc:creator","label":"Author","values":["Murphy, Tommy"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:date.accessioned","label":"Dc Date Accessioned","values":["2026-05-27T11:19:47Z"]},{"key":"dc:date.available","label":"Dc Date Available","values":["2026-05-27T11:19:47Z"]},{"key":"dc:date.issued","label":"Date","values":["2026-05-01"]},{"key":"dc:publisher","label":"Institution","values":["University College Cork"]},{"key":"dc:type","label":"Dc Type","values":["Doctoral thesis"]},{"key":"dc:type.qualificationlevel","label":"Dc Type Qualificationlevel","values":["Doctoral"]},{"key":"dc:type.qualificationname","label":"Dc Type Qualificationname","values":["PhD - Doctor of Philosophy"]}]},{"id":"subjects_keywords","label":"Subjects and Keywords","entries":[{"key":"dc:subject","label":"Dc Subject","values":["Quantum well","Quantum dot","Electro-absorption modulators","Photonics","Quantum-confined Stark effect","Lateral field","Photonic circuits","Integrated photonics","Silicon photonics","Quantum well intermixing","III-V materials","Quaternary materials","Fick's law","Computational physics","Electronic structure","QUDOS","Strain","Piezoelectric potential","Momentum matrix elements","Coulomb matrix elements","Coulomb integrals","MPI","OpenMP","Plane-wave expansion method","8-band k.p","Quantum nanostructure","Open-source code"]}]},{"id":"language_rights","label":"Language and Rights","entries":[{"key":"dc:language.iso","label":"Language (ISO)","values":["en"]},{"key":"dc:rights","label":"Dc Rights","values":["© 2026, Tommy Murphy."]},{"key":"dc:rights.uri","label":"Rights URI","values":["https://creativecommons.org/licenses/by/4.0/"]}]},{"id":"identifiers","label":"Identifiers","entries":[{"key":"dc:identifier.uri","label":"Identifier URI","values":["https://hdl.handle.net/10468/18909"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description.abstract","label":"Abstract","values":["Quantum nanostructures such as quantum wells (QWs) and quantum dots (QDs) are employed in a variety of integrated photonic devices, including lasers, electro-absorption modulators (EAMs), single-photon sources, and photodetectors. Theoretical analysis of these devices is required to enable performance improvements and support development for novel applications. In this thesis, we employ the semi-empirical continuum-based k · p method to analyse the electronic and optical properties of QWs and QDs to enhance the capabilities of these devices. First, we present the QUDOS code: a free, open-source software package to underpin predictive analysis and design of QD heterostructures. Using a semi-analytical plane wave approach, QUDOS allows to compute the electronic structure of QD heterostructures. QUDOS includes strain and piezoelectric effects on the QD electronic structure, and allows to apply an arbitrarily oriented electric field to enable analysis of electric-field dependent QD properties including the quantum-confined Stark effect (QCSE) and its implications for device applications. QUDOS computes the strain and piezoelectric fields, constructs the Hamiltonian for a supercell containing an arbitrary QD-based heterostructure, and exactly diagonalises the Hamiltonian to yield the single-particle QD eigenstates. These eigenstates are then used to compute optical (momentum) and Coulomb matrix elements, thereby providing the inputs required to perform semi-classical or many-body calculations of classical and quantum QD properties. QUDOS provides a high degree of flexibility, allowing to simulate a variety of heterostructures, including numerous QD shapes both with and without wetting layers, QD molecules and QD-in-a-well structures, and composition-graded or arbitrarily oriented QDs. The core physics library is written in C, employs hybrid OpenMP-MPI parallelisation, and exploits various routines from Intel’s oneMKL to accelerate performance. Second, we apply the QUDOS code to investigate the viability of lateral-field QD-EAMs. Advances in III-V on Si QD growth have enabled monolithic integration of high-performance electrically-pumped lasers on Si, as a critical enabling component for Si photonics. Another critical component is the EAM, which exploits the QCSE to achieve high-speed modulation of laser signals. Conventional QW-EAMs exploit a “vertical” QCSE via top and bottom electrical contacts. Rapid advancements in planar photonic integrated circuit technology motivate development of laterally-contacted EAMs, which offer performance benefits including reduced parasitic capacitance. The QCSE cannot be achieved via a lateral field in a QW, but can in a QD due to the three-dimensional carrier confinement. We undertake theoretical analysis of the lateral-field QCSE in 1.3 μm InGaAs/GaAs QDs. Comparing the QCSE produced by vertical and lateral electric fields for realistic QD morphology a robust lateral field QCSE is demonstrated, with the optical absorption edge redshifting more rapidly vs. field strength than in a conventional QW-EAM. It is shown that lateral-field QD-EAM performance is expected to be strongly sensitive to the spectral linewidth of the band edge absorption, and can also depend upon the in-plane orientation of the lateral electric field. The impact of QD morphology – the base shape, aspect ratio, and composition profile – is also quantified. It is demonstrated that InGaAs/GaAs QDs possessing high aspect ratios and low absorption linewidths are well-suited to develop lateral-field 1.3 μm QD-EAMs. This suggests leveraging III-V on Si epitaxy to integrate EAMs with lasers or single-photon sources, to realise high-speed Si photonic integrated circuits for applications in datacomms and linear optical quantum computing. Third, we develop a multi-component diffusion theory of QW intermixing for quaternary alloy systems. QW intermixing via post-growth annealing enables integration of active and passive components in photonic integrated circuits. QW intermixing generally produces a blueshift of the QW band gap, rendering the intermixed section of a QW transparent to a laser fabricated from a non-intermixed section of the same structure. Experimental studies on AlInGaAs/InP QWs have, however, identified structures in which intermixing produces a redshift of the band gap. This redshift is anomalous insofar as it is not explained by previous models of QW intermixing based on Fick’s law. To overcome this limitation we apply a generalised (multi-component) form of Fick’s law to describe inter-diffusion of Ga, In, and Al in AlInGaAs/InP. Based on the experimental observation that annealing can drive strong Ga and In intermixing but Al diffuses minimally, we demonstrate that intermixing in AlInGaAs QWs can be formulated as a coupled inter-diffusion problem characterised by independent Ga- and In-related diffusivities, which increase with increasing strain in the QW, and suppressed Al diffusion. The intermixing is governed by the ratio of these diffusivities, which can drive a counterintuitive process that explains the experimentally observed redshift: namely, diffusion of additional In into the well upon annealing of a structure having equal well and barrier In compositions. The model is validated via comparison to experiment. First, by showing that it can quantitatively describe measured composition profiles. Second, via empirical parameterisation of the diffusion coefficients, which we demonstrate as sufficient to quantitatively reproduce the measured QW band gap shift vs. annealing temperature."]},{"key":"dc:format.mimetype","label":"Dc Format Mimetype","values":["application/pdf"]},{"key":"dc:title","label":"Title","values":["Theory and simulation of quantum nanostructures for integrated photonics"]}]}],"canonical_facts":{"dc:contributor.advisor":["O'Reilly, Eoin","Broderick, Christopher","Peters, Frank H."],"dc:creator":["Murphy, Tommy"],"dc:date.accessioned":["2026-05-27T11:19:47Z"],"dc:date.available":["2026-05-27T11:19:47Z"],"dc:date.issued":["2026-05-01"],"dc:description.abstract":["Quantum nanostructures such as quantum wells (QWs) and quantum dots (QDs) are employed in a variety of integrated photonic devices, including lasers, electro-absorption modulators (EAMs), single-photon sources, and photodetectors. Theoretical analysis of these devices is required to enable performance improvements and support development for novel applications. In this thesis, we employ the semi-empirical continuum-based k · p method to analyse the electronic and optical properties of QWs and QDs to enhance the capabilities of these devices. First, we present the QUDOS code: a free, open-source software package to underpin predictive analysis and design of QD heterostructures. Using a semi-analytical plane wave approach, QUDOS allows to compute the electronic structure of QD heterostructures. QUDOS includes strain and piezoelectric effects on the QD electronic structure, and allows to apply an arbitrarily oriented electric field to enable analysis of electric-field dependent QD properties including the quantum-confined Stark effect (QCSE) and its implications for device applications. QUDOS computes the strain and piezoelectric fields, constructs the Hamiltonian for a supercell containing an arbitrary QD-based heterostructure, and exactly diagonalises the Hamiltonian to yield the single-particle QD eigenstates. These eigenstates are then used to compute optical (momentum) and Coulomb matrix elements, thereby providing the inputs required to perform semi-classical or many-body calculations of classical and quantum QD properties. QUDOS provides a high degree of flexibility, allowing to simulate a variety of heterostructures, including numerous QD shapes both with and without wetting layers, QD molecules and QD-in-a-well structures, and composition-graded or arbitrarily oriented QDs. The core physics library is written in C, employs hybrid OpenMP-MPI parallelisation, and exploits various routines from Intel’s oneMKL to accelerate performance. Second, we apply the QUDOS code to investigate the viability of lateral-field QD-EAMs. Advances in III-V on Si QD growth have enabled monolithic integration of high-performance electrically-pumped lasers on Si, as a critical enabling component for Si photonics. Another critical component is the EAM, which exploits the QCSE to achieve high-speed modulation of laser signals. Conventional QW-EAMs exploit a “vertical” QCSE via top and bottom electrical contacts. Rapid advancements in planar photonic integrated circuit technology motivate development of laterally-contacted EAMs, which offer performance benefits including reduced parasitic capacitance. The QCSE cannot be achieved via a lateral field in a QW, but can in a QD due to the three-dimensional carrier confinement. We undertake theoretical analysis of the lateral-field QCSE in 1.3 μm InGaAs/GaAs QDs. Comparing the QCSE produced by vertical and lateral electric fields for realistic QD morphology a robust lateral field QCSE is demonstrated, with the optical absorption edge redshifting more rapidly vs. field strength than in a conventional QW-EAM. It is shown that lateral-field QD-EAM performance is expected to be strongly sensitive to the spectral linewidth of the band edge absorption, and can also depend upon the in-plane orientation of the lateral electric field. The impact of QD morphology – the base shape, aspect ratio, and composition profile – is also quantified. It is demonstrated that InGaAs/GaAs QDs possessing high aspect ratios and low absorption linewidths are well-suited to develop lateral-field 1.3 μm QD-EAMs. This suggests leveraging III-V on Si epitaxy to integrate EAMs with lasers or single-photon sources, to realise high-speed Si photonic integrated circuits for applications in datacomms and linear optical quantum computing. Third, we develop a multi-component diffusion theory of QW intermixing for quaternary alloy systems. QW intermixing via post-growth annealing enables integration of active and passive components in photonic integrated circuits. QW intermixing generally produces a blueshift of the QW band gap, rendering the intermixed section of a QW transparent to a laser fabricated from a non-intermixed section of the same structure. Experimental studies on AlInGaAs/InP QWs have, however, identified structures in which intermixing produces a redshift of the band gap. This redshift is anomalous insofar as it is not explained by previous models of QW intermixing based on Fick’s law. To overcome this limitation we apply a generalised (multi-component) form of Fick’s law to describe inter-diffusion of Ga, In, and Al in AlInGaAs/InP. Based on the experimental observation that annealing can drive strong Ga and In intermixing but Al diffuses minimally, we demonstrate that intermixing in AlInGaAs QWs can be formulated as a coupled inter-diffusion problem characterised by independent Ga- and In-related diffusivities, which increase with increasing strain in the QW, and suppressed Al diffusion. The intermixing is governed by the ratio of these diffusivities, which can drive a counterintuitive process that explains the experimentally observed redshift: namely, diffusion of additional In into the well upon annealing of a structure having equal well and barrier In compositions. The model is validated via comparison to experiment. First, by showing that it can quantitatively describe measured composition profiles. Second, via empirical parameterisation of the diffusion coefficients, which we demonstrate as sufficient to quantitatively reproduce the measured QW band gap shift vs. annealing temperature."],"dc:format.mimetype":["application/pdf"],"dc:identifier.uri":["https://hdl.handle.net/10468/18909"],"dc:language.iso":["en"],"dc:publisher":["University College Cork"],"dc:rights":["© 2026, Tommy Murphy."],"dc:rights.uri":["https://creativecommons.org/licenses/by/4.0/"],"dc:subject":["Quantum well","Quantum dot","Electro-absorption modulators","Photonics","Quantum-confined Stark effect","Lateral field","Photonic circuits","Integrated photonics","Silicon photonics","Quantum well intermixing","III-V materials","Quaternary materials","Fick's law","Computational physics","Electronic structure","QUDOS","Strain","Piezoelectric potential","Momentum matrix elements","Coulomb matrix elements","Coulomb integrals","MPI","OpenMP","Plane-wave expansion method","8-band k.p","Quantum nanostructure","Open-source code"],"dc:title":["Theory and simulation of quantum nanostructures for integrated photonics"],"dc:type":["Doctoral thesis"],"dc:type.qualificationlevel":["Doctoral"],"dc:type.qualificationname":["PhD - Doctor of Philosophy"]},"updated_at":"2026-07-24T01:48:31Z"}