Search

In two dimensional materials, arbitrary combinations of stacks are possible where each layer constitutes an atomically thin building block to engineer unprecedented properties. In monolayer form, 2D semiconductors show outstanding light-matter interactions, stemming from the confined nature of Coulomb-bound complexes known as excitons. When two Transition Metal Dichalcogenide (TMD) layers are brought together, new excitonic species of interlayer excitons (IXs) can be formed. Their properties can, hence, be engineered with material choice and stacking order. In Chapter 4, we stack pure monolayers of MoSe2 to single-layer MoxW1−xSe2 alloys, which allows a continuous tuning of the IX energy by more than 50 meV introducing a chemical degree of freedom. Profound modifications of the electronic band structure occur when the alloy composition become increasingly similar to the adjacent pure TMD. In addition, we are able to observe optical signatures of interlayer exciton hybridisation and moiré superlattice potentials. We therefore expand the design possibilities for atomically thin devices. In the next chapters, we employ encapsulated homobilayers of MoS2 to observe hybridised interlayer excitons (hIX) that uniquely combine high oscillator strength and promise for increased nonlinear interactions. Compared to excitons in MoS2 monolayers, hIX exhibit ≈ 8 times higher nonlinearity, which is further strongly enhanced when hIX and intralayer excitons, sharing the same valence band, are excited simultaneously. This provides access to an unprecedented highly nonlinear regime which, through collaborative work, we describe theoretically by introducing the concept of hole crowding. In Chapter 6, the aforementioned system is embedded in energy-tunable optical microcavities, in which the TMD excitons strongly couple to photonic modes, forming composite exciton-polaritons. This allows the realisation of dipolar polaritons (dipolaritons).We find dipolaritons to be an order of magnitude more nonlinear than monolayer polaritons, showing a rapid quenching of the Rabi splitting with increased density. The presented insight into many-body interactions can provide new avenues for accessing sought-after few-polariton quantum correlations.

Semiconductor quantum dots (QDs) are a promising source of quantum light, combining sub-Poissonian photon statistics, high entanglement fidelities, and compact integration. This thesis examines the temporal driving methodology of electrically-driven semiconductor QDs and its impact on the quantum optical characteristics of the emitted light. To this end, we introduce the design and fabrication of an ultra-high-bandwidth QD light-emitting diode (LED). We demonstrate fast control of charge carrier injection and tunnelling, and report generation of single photons at a record 3.05 GHz clock rate. Importantly, we introduce a novel driving scheme for entangled-photon sources that rely on radiative cascades of atom-like few-level systems. Using a rate-equation model, we show that an early reinitialisation of the quantum system-before the radiative cascade has completed-may produce entangled photon pairs at a higher rate compared to either conventional pulsed operation or continuous driving of the system. This is subsequently implemented in practice, demonstrating an entangled-pair rate enhanced by (21 ± 3) % compared to the same device when driven continuously. Employing this novel driving scheme, we also demonstrate the electrical generation of entangled-photons at a record 1.15 GHz clock rate with an overall entanglement fidelity of (79.5 ± 1.1) %. Finally, we present a theoretical model describing the integration of a QD entangled-light-emitting diode (E-LED) in entanglement-based measurement-device-independent quantum key distribution (MDI-QKD). We conclude that a proof-of-principle experimental demonstration is feasible, yielding a peak bit-exchange fidelity of up to 84.3 % for a four-state BB84 protocol. These results may particularly benefit the performance of future long-distance quantum networks using atom-like quantum light sources. Furthermore, the ultra-high-bandwidth device and early reinitialisation scheme may be combined with other techniques to accelerate the radiative lifetime of the emitter, such as Purcell enhancement, further enhancing the quantum light emission for photonic applications.

The integration of single photon sources with resonators for Purcell enhancement of spontaneous emission has drawn tremendous research efforts from many researchers around the world over the past 20 years. The desire to increase the radiative emission rate of quantum light sources arises due to new and exciting applications in quantum information processing which require emitters with ideal properties. Most light sources characterized as exhibiting single photons, however, do not meet many of the requirements of these new applications and so the enhancement of their emission rate via the Purcell effect is one method of suppressing unwanted properties and improving desired functions. Previous works involving micrometer-scale cavities fabricated from one material system have been integrated with quantum emitters from another, which has presented many new challenges. The use of transition metal dichalcogenides (TMDs) may provide a solution as these materials not only host single photon emission, but also exhibit a large refractive index and transparency window in the visible making them ideal for the fabrication of resonant structures. As 2D material, TMDs are also intrinsically compatible with a wide variety of substrates due to their weak van-der-Waals attractive forces. In this work, we demonstrate an approach to precisely position strain-induced single photon sources in WSe₂ monolayers (g⁽²⁾(0) = 0.26±0.03) and simultaneously couple their emission to broad Mie resonances of high refractive index gallium phosphide (GaP) dimer nano-antennas. This leads to large quantum efficiency enhancements yielding values of 21% on average when compared to 4% for emitters formed on SiO2 nano-pillars. A maximum value of 86% corresponds to a 69 MHz single photon emission rate. The GaP substrate also enables more stable emission over time as well as reduced non-radiative recombination channels leading to lifetime values of up to 200 ns, which suggests much longer radiative decay times than previously reported. The enhanced quantum efficiency allows the study of the emission rise time which enables the first report of a dark exciton dwelling time before trapping into a 0D state in WSe₂ to be τt ≈ 1.7 ns. Increased excitation powers are shown to lead to efficient exciton-exciton annihilation rates which may lead to photoluminescence saturation that does not occur as a result of the filling of the quantum state but instead represents a depletion of the dark exciton population. We also perform the first systematic study of the coherence time of WSe₂ single photon emitters (SPEs) yielding values of 3.12±0.4 ps which we assert to be limited by pure dephasing via phonon interactions. In order to complete the system of TMD single photon emitters coupled to nano-photonic resonators in the same material family, our work leads to the fabrication of monomer and dimer nano-antenna Mie resonators in bulk crystals of WS₂. The unique crystal symmetry of TMDs allows the fabrication of circular, hexagonal and square nano-antenna geometries with potentially atomically sharp vertices for the latter two. The high refractive index and large transparency window of the material enable the formation of highly confined anapole resonances which we show to enhance second harmonic generation (SHG) in nano-meter scale monomer and dimer nano-antennas with the latter yielding linearly polarized signal. The weak van-der-Waals forces responsible for the compatibility with a wide variety of substrates also enable the use of atomic force microscopy (AFM) for the repositioning of constituent nano-pillars in dimer nano-antennas. This enables a reduction of the dimer gap to values as low as 10±5 nm which has so far only been achieved in plasmonic bowtie antennas or in dielectric structures using non-repeatable and damadging methods such as focused ion beam milling. We study the applications enabled by these TMD nano-resonators numerically. A near unity degree of linear polarization of the SHG enhancement factor is expected for WS2 dimer nano-antennas with small gaps. For the minimum achieved dimer gap of 10 nm, we simulate electric field hotspots with intensities of more than 103 when compared with vacuum for the novel hexagonal and square dimer geometries. This large enhancement leads to single photon emission Purcell enhancements of more than 150 for emitters positioned at the inner top vertices of the hexagonal and square dimer nano-antennas. The AFM repositioning technique and to a lesser degree the fabrication procedure enables the modulation of the electric field intensity and the Purcell enhancement through increases in the dimer gap as well as the rotation of individual nano-pillars in the dimer. Another application expected to benefit from such large electric field intensities is the optical trapping of nano-particles which we show to yield attractive forces of more than 350 fN for colloidal quantum dots and more than 70 fN for polystyrene beads, the latter of which closely resembles the refractive index and size of a large protein. The stable trapping which we simulate surpasses previous reports by a factor of more than 80 for quantum dots and more than 40 for polystyrene beads.

The experiments detailed in this thesis bring together two well established fields within solid state physics, namely the optical properties of Coulomb-bound complexes known as excitons, and the application of novel quantum degrees of freedom towards information processing, storage and communication. The material basis for these investigations are the monolayer transition metal dichalcogenides (TMDs), atomically thin direct band gap semiconductors which exhibit exceptional coupling to light, via tightly bound exciton states which are stable beyond room temperature. These materials may be easily thinned down to single atomic monolayers, in which a unique regime of broken crystal inversion symmetry, time reversal symmetry, and strong spin-orbit coupling together give rise to a valley pseudospin, which acts as a spin-1/2 degree of freedom for charge carriers and excitons alike. Robust optical selection rules allow selective addressability of the exciton valley pseudospin via circularly polarised light, paving the way towards all-optical valley computation or memory. Here, various ways in which the exciton valley pseudospin may be enhanced or controlled are explored. In Chapter 4, the interaction between negatively charged exciton valley pseudospin and strong magnetic fields is discussed, where lifting of degeneracy of opposite valley states is demonstrated, leading to new understanding of the valley magnetism. In Chapter 5, monolayer TMDs are embedded in energy-tunable zero dimensional optical microcavities, in which the TMD excitons strongly couple to the confined photonic modes, forming composite quasiparticles known as exciton-polaritons. By control of the optical density of states via tuning of cavity length, the signatures of exciton valley pseudospin in emission may be enhanced by several times relative to the uncoupled monolayer. Finally, in Chapter 6, a monolayer TMD is placed in direct proximity to a ferromagnetic van der Waals layered material, realising an exciton valley pseudospin switch controllable via milliTesla range magnetic fields.

Recent progress in the fabrication of two-dimensional (2D) materials attracts our attention to the study of such structures. Most of them are typically van-der-Waals-like with weak interlayer and strong intralayer bonding. Nowadays, single-layer semiconducting transition metal dichalcogenides (TMDCs), namely molybdenum and tungsten selenides and sulphides, have the biggest interest in optoelectonics due to the remarkable optical properties: the giant spin-orbit coupling, the inversion symmetry breaking, the large exciton binding energy and large oscillator strength. This thesis discusses the investigations in the samples based on the above compounds using photoluminescence (PL), photoluminescence excitation (PLE) and reflectance contrast (RC) spectroscopies. We observe prominent changes in the PL spectral parameters, such as integrated intensities, linewidths, energy positions of peaks and distances between peak maxima, with the alloy composition in the mono-, bi- and trilayer MoxW1-xS2 crystals at room temperature. The bowing parameters from parabolic dependences of the band-edge PL and RC on the Mo concentration are extracted. The modifications of the PL and RC spectral peaks with the number of layers and the indirect-to-direct band-gap crossover are also first demonstrated in those materials. Analysing gate-voltage- and temperature-dependent PL spectra, we make evaluations of exciton-electron and exciton-phonon interactions in the single-layer MoSe2. Additionally, the influence of localization and recharging processes is established. In the MoSe2/WSe2 heterostructures (HSs), the behaviour of localized interlayer excitonic (IX) complexes is investigated by means of different experimental techniques, specifically polarization-resolved, power-, gate-voltage-, temperature-dependent PL and PLE. The impact of the HfO2/hBN encapsulation on the structure and polarization of the IX emission is studied for the first time. Finally, the specific band alignment of the MoSe2/WS2 HS appears to cause the extraordinary PL signal emitted from the states which are likely originated from the interlayer electron hybridization.

Presented herein are results of optical studies with emphasis on the Raman response, providing significant contribution to the knowledge of the field. In Mox W(1−x) S2 , confirmation of the behaviour of the excitonic properties is made. Raman measurements performed in this system allow investigation with unprecedented resolution, highlighting deviations in the high frequency A1g optical phonon mode from theoretical predictions, and previous experimental studies. In the low frequency, data confirms the trends in the shear and breathing interlayer modes in alloys between WS2 and MoS2 are well described by the modification in the square density. Resonant excitation for [Mo] < 0.4, highlights new evidence for the understanding of the hitherto unexplained ‘Peak X’ resonant feature. Diverse indium-selenium compounds isolated by novel means are studied. The ULF Raman modes of PDMS exfoliated InSe are documented for the first time, demonstrating the ε-phase with ABA stacking, with flake of thickness N manifesting (N − 1) shear modes owing to resonant excitation of few layer samples. InSe flakes encapsulated in hexagonal boron nitride manifest different stacking orders to those of PDMS exfoliated InSe, and were found to have significant contamination, with crystalline degradation of the monolayer flake, and peaks corresponding N2 & O2 rotational modes present. In2Se3 films grown epitaxially on GaSe display substrate-selective polymorphism, where α-, β-, & γ- phases are identified, in addition to regions of InSe. Laser photo-annealing is shown to drive a phase change from the groundstate β → α phase, which is against the thermodynamic gradient. MoSe2/WS2 twisted hetero-bi-layer structures are studied, where shear modes showing a linear softening from AA′ stacking towards the AB at 60° indicating reduced interlayer coupling, as expected from the difference in interlayer spacing of AA′ and AB ordering. High frequency modes in the heterobilayer also demonstrate some sensitivity in the relative angle, and are analysed in detail.

This thesis describes experiments using optically detected nuclear magnetic resonance (NMR) techniques to probe and control the nuclear spin bath evolution in single self-assembled InGaAs quantum dots at large external magnetic fields. Strong non-resonant optical pumping is used to create spin bath polarisations ≳ 60% in dynamic nuclear polarisation. Changes of the nuclear magnetisation under resonant radio frequency (rf) excitation of nuclear Zeeman transitions are detected in low-power photoluminescence spectroscopy measurements of the hyperfine shifts of the exciton transition lines. Continuous wave rf excitation shows strong inhomogeneous broadening of the NMR spectra in a quantum dot owing to strain-induced quadrupolar interactions. Hahn echo measurements reveal comparatively long nuclear spin phase memory times T^HE_ M,n ~ 1-4 ms which are attributed to strong suppression of nuclear spin fluctuations due to the inhomogeneous broadening. Further progress towards full control of the spin bath evolution is made using a set of new multiple pulse NMR sequences combining features of Hahn and solid echoes. Measurements demonstrate that spin bath coherence times of up to ~ 20 ms can be achieved within the experimental limitations of this work. Use of these NMR sequences in combination with electron/hole spin control techniques is expected to increase central spin coherence times significantly. The equilibrium spin bath coherence properties are probed using a novel, weakly-invasive rf frequency comb NMR method. Frequency combs with varying tooth spacing are used to determine the homogeneous NMR lineshapes. The sensitivity of the homogeneous linewidths to fluctuations in the spin bath is used to probe the bath dynamics. Few-second-long spin flip-flop correlation times are revealed, demonstrating the potential of self-assembled InGaAs quantum dots to serve as a highly stable host system for spin qubits.

The recently emerging layered two-dimensional materials provide a new material class for novel opto-electronic devices. These materials have a unique crystal structure with strong intra-layer bonding and weak van der Waals inter-layer bonding. This allows to thin down the crystal to single atomic layer thickness using an adhesive tape. With the discovery of this method to produce stable monolayer sheets of graphene and the observation of its remarkable properties, a new research area started to develop. Besides graphene there is a whole class of two-dimensional materials with similar crystal structure. One of the most prominent are transition-metal dichalcogenides, molybdenum and tungsten selenide and sulphide. They are semiconducting materials that experience an indirect-to-direct bandgap transition when the material is thinned down to monolayer thickness. This change of the bandstructure leads to a remarkable increase in the emission efficiency of those materials in monolayer form. Strong spin-orbit coupling, inversion symmetry breaking, large exciton binding energy and large oscillator strength means that this class of materials are very promising for future room temperature opto-electronic devices. In this work monolayer sheets of transition-metal dichalcogenides, as well as vertically stacked heterostructure of two-dimensional materials, are coupled to microcavity structures in order to study lightmatter interaction of these materials. A tunable open-acces microcavity structure has been developed to have full control of the light-matter interaction. In this system monolayer sheets of molybdenum disulphide have been studied, where the weak coupling regime with a Purcell enhancement of a factor of 10 has been observed. Monolayer sheets of molybdenum diselenide have been investigated where the first conclusive demonstration of strong exciton-photon coupling is demonstrated. Finally, a light emitting diode, produced by a heterostructure consisting of graphene, boron nitride and tungsten diselenide has been embedded in a microcavity structure where a significant change in the emission pattern of photo- and electroluminescence has been demonstrated.

This thesis discusses optical investigations of two-dimensional metal-chalcogenide semiconductor materials and their heterostructures. Topics include a study of continuous wave (cw) and time-resolved photoluminescence (PL) of GaTe and GaSe thin films. Based on experimental evidence, we propose a model explaining the strong PL intensity decrease for thin films as a result of non-radiative carrier escape via surface states. We investigate the stability of thin films of InSe and GaSe using a combination of PL and Raman spectroscopies. By comparing signal intensities in films exposed to ambient conditions for up to 100 hours, we find notable degradation in GaSe and high stability of InSe. We continue our study with the investigation of optical properties of light emitting diodes (LED) made of van der Waals (vdW) heterostructures comprising graphene as transparent contacts, hexagonal boron nitride as tunnel barriers and transition metal dichalcogenides (TMDC), MoS2 and WS2, as the semiconductor active regions. Single and multiple 'quantum well' structures were fabricated with an aim to enhance the external quantum efficiency (EQE) under electrical injection. We also present PL characterisation of LEDs based on vdW heterostructures comprising WSe2 and MoSe2 as active layers. Temperature dependent experiments show unusual enhancement of the EQE with temperature in WSe2 in contrast to MoSe2, where both electroluminescence and PL are reduced with temperature. A theoretical approach to explain this behaviour is proposed, which is based on the strong spin-orbit interaction present in both materials.

This thesis comprises works on novel quantum dot structures. New ways of growing III-V semiconductor quantum dots by integrating a ternary element or by growing on top of a silicon wafer are optically characterized, opening the way to more specific work on those new structures, while furthering our understanding of the epitaxy mechanisms behind them. We study InGaAs/GaAs quantum dot structures monolithically grown on a silicon substrate, without use of germanium virtual substrate nor wafer bonding technique. Optical characterization of the sample with micro photoluminescence is performed and shows very good single quantum dot emission lines. Single photon emission from the InGaAs dots is demonstrated with photon correlation experiment showing clear anti-bunching. Photonic crystal cavities are fabricated for the first time with InGaAs dots monolithically grown on silicon and exhibit very high quality factor up to 13000 with a large percentage of cavities having Q-factors over 9000. This allows observation of Purcell effect for single photon emitting QDs and strong light-matter coupling between InGaAs QDs and cavities. We also investigate unexpected emission lines on the same sample. The lines are identified as interface fluctuations in a GaAs/AlGaAs short period superlattice, making them the first Interface fluctuation quantum dots grown directly on silicon. Further optical characterization confirms the quantum dot nature of the emissions. Polarization measurements allow study of the fine structure splitting of exciton/bi-exciton pairs and the single photon emission of the dots is demonstrated. Finally in a subsequent chapter we investigate InP/GaInP quantum dots with arsenic deposited during the growth process. Magneto-optic PL of samples with different concentrations of As allows to determine how the As changes the characteristics of the dots. Schottky diodes are fabricated and tested to show good characteristics, and electric field experiments demonstrate charge control over this new kind of dots.

This thesis discusses development of new semiconductor quantum dot (QD) devices and materials. Optical spectroscopy of single QDs is employed in order to investigate electronic structure and magnetic properties of these materials. First we realise self-assembled InP/GaInP QDs embedded in Schottky diode structures, with the aim to realise charge control in these nanostructures, which recently provided an important test-bed for spin phenomena on the nano-scale. By varying the bias applied to the diode, we achieve accurate control of charge states in individual QDs, and also characterise the electron-hole alignment and the lateral extent of the exciton wavefunction. Second part of the thesis explores optimum regimes for optically induced dynamic nuclear polarization (DNP) in neutral InGaAs/GaAs QDs. Very efficient DNP under ultra low optical excitation is demonstrated, and its mechanism is explained as the electron-nuclear flip-flop occurring in the second order process of the dark exciton recombination. The final part of the thesis reports on magneto-optical studies of novel individual InPAs/GaInP quantum dots studied in this work for the first time. Here the long-term aim is to realise strong carrier confinement potentially suitable for QD operation at elevated temperatures, e.g. as a single photon emitter. Here we lay foundations for future structural studies of these dots using optically detected nuclear magnetic resonance, and explore regimes for ecient DNP in InPAs dots emitting in a wide range of wavelength 690-920 nm.