Theses and Dissertations from UMD
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Item Magneto-optical response of WSe2 excitons in sub-Kelvin regime(2022) Vannucci, Jonathan; Hafezi, Mohammad; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Interest in correlated electron states in strongly correlated materials, such as transition metal dichalcogenides (TMDs), has been growing in recent years thanks to significant technological advancements in cryogenic quantum optics measurement techniques that allow for their experimental realization. Milli-kelvin temperatures are required to eliminate thermalization effects that otherwise mask the interesting correlated physics in these materials. In an effort to contribute to the understanding of these states, we developed an ultra-low temperature dilution refrigerator measurement system with free-space optical access that is able to achieve a base temperature of $<$30 milli-kelvin and apply a $\pm$12 T perpendicular magnetic field. We utilize a 100x cryogenic objective with 0.82 NA to make highly localized optical measurements on our samples. Our custom stage system allows for quick sample exchange and rapid characterization of our devices. TMDs have been shown to be suitable materials for probing new and interesting exciton physics. In monolayer form, their direct band gap in the visible range and high binding energies open the possibility to optically explore higher energy Rydberg states. As the understanding of the neutral exciton's Rydberg series has grown in recent years, interest has started to shift toward the less understood charged excitons. Using photoluminescence excitation (PLE), we sweep a laser's energy across the spectral range of the $n>1$ Rydberg exciton states and monitor the emission intensities of the 1$s$ excitons. Through this technique, we were able to measure the precise spectral profile and carrier density dependence of the 3$s$ neutral (X$_0^{3s}$), 2$s$ neutral (X$_0^{2s}$), and 2$s$ negatively charged (X$_-^{2s}$) excitons in monolayer WSe$_2$. By applying a perpendicular magnetic field to the sample, we were also able to measure the valley dependent Zeeman splitting of both X$_0^{2s}$ and X$_-^{2s}$ and extracted their corresponding g-factors.Item EXCITED STATES IN MONOLAYER TRANSITION METAL DICHALCOGENIDES(2022) Sell, Julia C.; Hafezi, Mohammad; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Monolayer two-dimensional transition metal dichalcogenides (2D TMDs) represent a class of atomically thin semiconductors with unique optical properties. Similar to graphene, but with a three-layer (staggered) honeycomb lattice, TMDs host direct-gap transitions at their $\pm$K valleys that exhibit circular-dichroism due to their finite Berry curvature. The reduced dimensionality of materials in this system, combined with large effective carrier masses, leads to enhanced Coulomb interaction and extremely tightly bound excitons ($E_{\rm{B}} \approx 150-300\:\rm{meV})$. Here, we seek to exploit the unusually tight binding of the excitons to probe two different types of higher energy exciton species in TMDs. First, we experimentally probe the magneto-optical properties of 2$s$ Rydberg exciton species in WSe$_2$. The magnetic response of excitons gives information on their spin and valley configurations, nuanced carrier interactions, and insight into the underlying band structure. Recently, there have been several reports of 2$s$/3$s$ charged excitons in TMDs, but very little is still known about their response to external magnetic fields. Using photoluminescence excitation spectroscopy, we verify the 2$s$ charged exciton and report for the first time its response to an applied magnetic field. We benchmark this response against the neutral exciton and find that both the 2$s$ neutral and charged excitons exhibit similar behavior with $g$-factors of g$_{\rm{X_0^{2s}}}$=-5.20$\pm0.11$ $ \mu_{\rm{B}}$ and g$_{\rm{X_-^{2s}}}$=-4.98$\pm0.11$ $ \mu_{\rm{B}}$, respectively. Second, via theoretical calculations, we investigate the exciton spectrum generated in 2D semiconductors under illumination by twisted light. Twisted light carries orbital angular momentum (OAM) which can act as an additional tunable degree of freedom in the system. We demonstrate that twisted light does not have the ability to modify the exciton spectrum and induce dipole-forbidden excitons, in contrast to atoms. This result stems from the fact that the additional OAM is transferred preferentially to the center-of-mass (COM) of the exciton, without modifying the relative coordinate which would allow dipole-forbidden, higher energy excitons to form.Item Quantum Light Generation from Bound Excitons in ZnSe(2022) Karasahin, Aziz; Waks, Edo; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quantum light sources and spin-based qubits are essential building blocks for on-chip scalable quantum computation and information processing. To achieve scalability, information-storing qubits should exhibit long coherence times. These qubits should also be efficiently interfaced with information-carrying single photons. Semiconductors are not only able to host such qubits and single photon sources but also, they offer a platform to interface them with the help of photonic structures. Hence, optically active solid-state qubits such as quantum dots, crystal defects and color centers have been extensively studied to date in various semiconductors. However, we still lack a suitable platform to satisfy all the requirements needed to realize a scalable quantum technology. Impurities in epitaxially grown ZnSe are particularly promising single photon sources and qubit candidates due to the direct bandgap of the material and potential for isotopic purification to achieve nuclear spin-zero background. These impurities possess impurity bound electrons that can serve as spin-qubit. They also form impurity-bound excitons that can generate single photons. Various impurities have been studied in ZnSe, but only F impurities have been isolated as single emitters to date. Despite the great potential suggested by previous results, there are many impurities waiting to be explored for their quantum capabilities. In this thesis, we study isolated Cl impurities in ZnSe for their photon emission and spin properties. We utilize a ZnMgSe/ZnSe/ZnMgSe quantum well to increase the binding energies bound excitons and to better separate donor bound exciton emission from the free excitons. In the PL spectrum, we observe narrow emission lines around 440 nm, which are originated from the single bound excitons. We calculate the average binding energy as 15 meV (at least 2 times higher than bulk values) and inhomogeneous broadening as 6 meV. We confirm the single photon emission by observing clear photon antibunching in the second order autocorrelation measurements. The time-resolved photoluminescence measurements show short radiative lifetimes of 192 ps. Our results demonstrate first time isolation of donor impurities in an unstructured ZnSe and provide complete characterization of radiative properties single Cl bound excitons. The bound electron of a donor impurity atom can serve as a spin qubit. We verify that the presence of ground state electron of the Cl donor complex by observing two electron satellite emission. We also characterize the Zeeman splitting of the exciton transitions by performing polarization-resolved magnetic spectroscopy on the single emitters. We also discover the presence of single biexcitons bound to Cl impurities. We demonstrate a radiative cascade from the decay of bound biexcitons. The emission exhibits both single photon statistics and clear temporal correlations revealing the time–ordering of the cascade. Finally, we discuss the design of nanophotonic cavities in the ZnSe platform. We develop a nanofabrication recipe to create suspended photonic crystal cavities. Then, we optically characterize the fabricated cavities. The results presented in this thesis provide the first complete study of single Cl impurities in ZnSe. Based on the results discussed, single Cl impurities in ZnSe manifest themselves as promising quantum light sources and appealing solid-state qubit candidates.Item Exciton Photophysics at Fluorescent Quantum Defects(2018) Kim, Mijin; Wang, YuHuang; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fluorescent quantum defect is an emerging synthetic structure that can be covalently attached to a semiconducting single-walled carbon nanotube. Incorporation of fluorescent quantum defect breaks the symmetry of carbon nanotubes at a defect center, creating new optically allowed, low-lying states in the electronic structure of carbon nanotube. Exciting electronic and optical properties arise from the defects, including the generation of new photoluminescence features, which can be used for applications, such as chemical sensing, bioimaging, and quantum light source. As excitons dominate the optical properties of carbon nanotubes, understanding the exciton photophysics in a defect-tailored carbon nanotube is essential to efficiently harness the emission properties of fluorescent quantum defects. In this dissertation, I aim to understand the exciton photophysics in fluorescent quantum defects in order to explain the origins and behavior of novel phenomena arising from them. First, the structure-property relationships of fluorescent quantum defects are discussed; these guide the systematic tuning of defect-induced emission and the binding energy of defect-trapped excitons. Then, the discussion moves to the exciton dynamics at fluorescent quantum defects. Particularly, I describe how the chemical nature of defects or the density of defects influences the thermal detrapping energy of excitons. The exciton-electron interaction at a fluorescent defect is also discussed. Our results suggest that a fluorescent quantum defect colocalizes an exciton and an electron as a tri-charge carrier and the brightening at the defect can be chemically tuned. Finally, I introduce super-resolved, hyperspectral photoluminescence spectroscopy, enabling both direct probing of a single fluorescent defect and the quantitative evaluation of the brightening of dark excitons.Item Electronically tailored functionalization of carbon nanotubes(2014) Piao, Yanmei; Wang, YuHuang; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Carbon nanotubes (CNTs) were chemically tailored on the electronic level to enhance their optical and electrical properties. Incorporation of sp3 defects into the sidewalls of CNTs significantly improved quantum efficiency of CNT photoluminescence (PL). Nanotube PL is intrinsically inefficient, usually less than 1%, due to the presence of dark excitons. This low efficiency makes nanotubes impractical for many applications, especially bio-imaging and optoelectronics. The nanotube PL was increased by up to 28 times through the chemical creation of a new defect induced state. This new state is optically allowed and resides below the predicted energy levels of the dark excitons, allowing the dark excitons to be harvested from this new defect state. Emission from the new state generates a distinct, structure-specific, and chemically tunable photoluminescence peak. This new peak is red-shifted by as much as 254 meV from the original excitonic transition and located within the tissue transparent window, which merits bio-imaging and bio-sensing. This work opens the door to harnessing dark excitons and lays the foundation for chemical control of defect quantum states in low dimensional carbon materials. Unlike atom-thick materials such as SWCNTs and graphene which are prone to chemical attacks because all constituent atoms are exposed, double-walled carbon nanotubes (DWCNTs) provide a chemically tailorable surface and an inner-tube with intact electronic properties. Even when the outer walls were selectively functionalized up to 6.9% (percent of carbon that are covalently modified), the inner tubes were electrically intact. Correlated Raman and optical absorption spectroscopy unambiguously confirm that the covalent modification was outer wall-selective. Nearly 50% of the electrical conductivity was retained in thin films of covalently functionalized nanotubes owing to the protected inner-tube conducting channels. Lacking such channels, SWCNTs became insulators after similar functionalization. Further experiments demonstrated that the covalently attached aryl groups could be selectively removed by optical annealing. These results suggest the possibility of high performance DWCNT electronics with important capabilities of tailored surface chemistry on the outer walls while the inner walls are chemically protected.