Coherent and Incoherent Dynamics of Quasiparticles in Monolayer Molybdenum Diselenide

Monolayer Materials, especially single-layer graphite, called graphene, as the first synthesized and most prominent representative, have attracted significant research interest since its discovery in 2004. The efforts were rewarded with a Nobel prize in 2010 for the discovery of graphene, the same year in which the first monolayer transition metal dichalcogenide (ML-TMD) was found to have a direct bandgap. In contrast to graphene ML-TMDs have a direct bandgap in the visible or near-infrared spectral range, making them ideally suited for optoelectronic device applications. Explicit inversion symmetry breaking of the unit cell in ML-TMDs furthermore leads to a new interesting property, called valley pseudo-spin. Electrons excited within one valley are restricted to this valley due to momentum trapping. Investigating the valley pseudo-spin dynamics is of importance for both understanding of the fundamental physics as well as device applications since the valley pseudo-spin is a potential information carrier and has potential use for information storage or computing application. Additionally, the confinement to two dimensions leads to enhanced Coulomb interaction and increased dielectric screening between electron and hole. Interestingly, the two-dimensional screening effects were already studied before the first two-dimensional materials were synthesized on quasi-two-dimensional systems. The screening of the Coulomb interaction in turn leads to a significantly increased binding energy between electron and hole, such that the bound electron-hole state, so-called exciton, is stable up to room temperature and above. The same reasoning leads to an enhanced stability of charged excitons, so-called trions, which are the main focus of this dissertation. The optical response of ML-TMDs is therefore completely dominated by excitons and trions, requiring an in-depth understanding of these quasiparticles for device performance optimization. Time-resolved techniques can offer rich information compared to steady-state measurements. While steady-state measurements can resolve things such as the bandgap of a semiconductor or the fact that valley spin exists, time-resolved techniques allow the access of transients and reveal the lifetime of unstable or metastable states, which may be invisible in steady-state measurements. Coherent techniques are known for their ability of probing many-body effects and microscopic inhomogeneity. The technique used to investigate the coherent trion dynamics in this dissertation is two-dimensional coherent spectroscopy, a nonlinear coherent technique, that resolves the signal as a function of two time delays. Using two-dimensional spectroscopy, it is possible to measure the homogeneous linewidth, which is related to the coherence time, even in a strongly inhomogeneously broadened system. The measurement of the coherence time marks the first step in evaluating a material for possible quantum computation applications

Long range dipole-dipole interaction in low-density atomic vapors probed by double-quantum two-dimensional coherent spectroscopy

Optical double-quantum two-dimensional coherent spectroscopy (2DCS) was implemented to probe interatomic dipole-dipole interactions in both potassium and rubidium atomic vapors. The dipole-dipole interaction was detected at densities of $4.81 \times 10^8$ cm$^{-3}$ and $8.40 \times 10^9$ cm$^{-3}$ for potassium and rubidium, respectively, corresponding to a mean interatomic separation of 15.8 $\mu$m or $3.0\times 10^5a_0$ for potassium and 6.1 $\mu$m or $1.2\times 10^5a_0$ for rubidium, where $a_0$ is the Bohr radius. We report the lowest atomic density at which dipole-dipole interactions are detected. The experimental results confirm the long range nature of the dipole-dipole interaction which is critical for understanding many-body physics in atoms/molecules. The long range interaction also has implications in atom-based applications involving many-body interactions. Additionally, we demonstrated that double-quantum 2DCS is sufficiently sensitive to probe dipole-dipole interaction at densities that can be achieved with cold atom in a magneto-optical trap, paving the way for double-quantum 2DCS studies of cold atoms and molecules. The method can also open a new avenue to study long-range interactions in solid states systems such as quantum dots and color centers in diamonds.Comment: 5 pages, 4 figure

Valley trion dynamics in monolayerﾠMoSe2

Charged excitons called trions play an important role in the fundamental valley dynamics in newly emerging two-dimensional semiconductor materials. We use ultrafast pump-probe spectroscopy to study the valley trion dynamics in aﾠMoSe2ﾠmonolayer grown by using chemical vapor deposition. The dynamics displays an ultrafast trion formation followed by a nonexponential decay. The measurements at different pump fluences show that the trion decay dynamics becomes slower as the excitation density increases. The observed trion dynamics and the associated density dependence are a result of the trapping by two defect states as being the dominating decay mechanism. The simulation based on a set of rate equations reproduces the experimental data for different pump fluences. Our results reveal the important trion dynamics and identify the trapping by defect states as the primary trion decay mechanism in monolayerﾠMoSe2ﾠunder the excitation densities used in our experiment

Mitigation of Nitrogen Vacancy Ionization from Material Integration for Quantum Sensing

The nitrogen-vacancy (NV) color center in diamond has demonstrated great promise in a wide range of quantum sensing. Recently, there have been a series of proposals and experiments using NV centers to detect spin noise of quantum materials near the diamond surface. This is a rich complex area of study with novel nano-magnetism and electronic behavior, that the NV center would be ideal for sensing. However, due to the electronic properties of the NV itself and its host material, getting high quality NV centers within nanometers of such systems is challenging. Band bending caused by space charges formed at the metal-semiconductor interface force the NV center into its insensitive charge states. Here, we investigate optimizing this interface by depositing thin metal films and thin insulating layers on a series of NV ensembles at different depths to characterize the impact of metal films on different ensemble depths. We find an improvement of coherence and dephasing times we attribute to ionization of other paramagnetic defects. The insulating layer of alumina between the metal and diamond provide improved photoluminescence and higher sensitivity in all modes of sensing as compared to direct contact with the metal, providing as much as a factor of 2 increase in sensitivity, decrease of integration time by a factor of 4, for NV $T_1$ relaxometry measurements

Ex Vivo High Salt Activated Tumor-Primed CD4+T Lymphocytes Exert a Potent Anti-Cancer Response

Cell based immunotherapy is rapidly emerging as a promising cancer treatment. A modest increase in salt (sodium chloride) concentration in immune cell cultures is known to induce inflammatory phenotypic differentiation. In our current study, we analyzed the ability of salt treatment to induce ex vivo expansion of tumor-primed CD4 (cluster of differentiation 4)+T cells to an effector phenotype. CD4+T cells were isolated using immunomagnetic beads from draining lymph nodes and spleens from tumor bearing C57Bl/6 mice, 28 days post-injection of Py230 syngeneic breast cancer cells. CD4+T cells from non-tumor bearing mice were isolated from splenocytes of 12-week-old C57Bl/6 mice. These CD4+T cells were expanded ex vivo with five stimulation cycles, and each cycle comprised of treatment with high salt (Δ0.035 M NaCl) or equimolar mannitol controls along with anti-CD3/CD28 monoclonal antibodies for the first 3 days, followed by the addition of interleukin (IL)-2/IL-7 cytokines and heat killed Py230 for 4 days. Ex vivo high salt treatment induced a two-fold higher Th1 (T helper type 1) expansion and four-fold higher Th17 expansion compared to equimolar mannitol treatment. Importantly, the high salt expanded CD4+T cells retained tumor-specificity, as demonstrated by higher in vitro cytotoxicity against Py230 breast cancer cells and reduced in vivo syngeneic tumor growth. Metabolic studies revealed that high salt treatment enhanced the glycolytic reserve and basal mitochondrial oxidation of CD4+T cells, suggesting a role of high salt in enhanced pro-growth anabolic metabolism needed for inflammatory differentiation. Mechanistic studies demonstrated that the high salt induced switch to the effector phenotype was mediated by tonicity-dependent transcription factor, TonEBP/NFAT5. Using a transgenic murine model, we demonstrated that CD4 specific TonEBP/NFAT5 knock out (CD4cre/creNFAT5flox/flox) abrogated the induction of the effector phenotype and anti-tumor efficiency of CD4+T cells following high salt treatment. Taken together, our data suggest that high salt-mediated ex vivo expansion of tumor-primed CD4+T cells could induce effective tumor specific anti-cancer responses, which may have a novel cell-based cancer immunotherapeutic application

High salt induces P-glycoprotein mediated treatment resistance in breast cancer cells through store operated calcium influx

Recent evidence from our laboratory has demonstrated that high salt (Δ0.05 M NaCl) induced inflammatory response and cancer cell proliferation through salt inducible kinase-3 (SIK3) upregulation. As calcium influx is known to effect inflammatory response and drug resistance, we examined the impact of high salt on calcium influx in breast cancer cells. Treatment of MCF-7 and MDA-MB-231 cells with high salt induced an enhanced intracellular calcium intensity, which was significantly decreased by store operated calcium entry (SOCE) inhibitor co-treatment. Further, high salt induced P-glycoprotein (P-gp) mediated paclitaxel drug resistance in breast cancer cells. Murine tumor studies demonstrated that injection of MCF-7 cells cultured in high salt, exerted higher tumorigenicity compared to the basal cultured counterpart. Knock down of SIK3 by specific shRNA inhibited tumorigenicty, expression of SOCE regulators and P-gp activity, suggesting SIK3 is an upstream mediator of SOCE induced calcium influx. Furthermore, small molecule inhibitor, prostratin, exerted anti-tumor effect in murine models through SIK3 inhibition. Taken together, we conclude that SIK3 is an upstream regulator of store operated calcium entry proteins, Orai1 and STIM1, and mediates high salt induced inflammatory cytokine responses and P-gp mediated drug resistance. Therefore, small molecule inhibitors, such as prostratin, could offer novel anti-cancer approaches

Fabrication of thin diamond membranes by Ne+^+ implantation

Color centers in diamond are one of the most promising tools for quantum information science. Of particular interest is the use of single-crystal diamond membranes with nanoscale-thickness as hosts for color centers. Indeed, such structures guarantee a better integration with a variety of other quantum materials or devices, which can aid the development of diamond-based quantum technologies, from nanophotonics to quantum sensing. A common approach for membrane production is what is known as "smart-cut", a process where membranes are exfoliated from a diamond substrate after the creation of a thin sub-surface amorphous carbon layer by He$^+$ implantation. Due to the high ion fluence required, this process can be time-consuming. In this work, we demonstrated the production of thin diamond membranes by neon implantation of diamond substrates. With the target of obtaining membranes of $\sim$ 200 nm thickness and finding the critical damage threshold, we implanted different diamonds with 300 keV Ne$^+$ ions at different fluences. We characterized the structural properties of the implanted diamonds and the resulting membranes through SEM, Raman spectroscopy, and photoluminescence spectroscopy. We also found that a SRIM model based on a two-layer diamond/sp$^2$-carbon target better describes ion implantation, allowing us to estimate the diamond critical damage threshold for Ne$^+$ implantation. Compared to He$^+$ smart-cut, the use of a heavier ion like Ne$^+$ results in a ten-fold decrease in the ion fluence required to obtain diamond membranes and allows to obtain shallower smart-cuts, i.e. thinner membranes, at the same ion energy