Ultrafast Coherent Imaging Spectroscopy of Van-der-Waals Materials

Transition metal dichalcogenides (TMDs) have received considerable attention in the past decade for their optoelectronic applications in photovoltaics, lasers, and quantum information. In the monolayer limit, these materials exhibit extraordinary properties, including efficient light-matter coupling, ultrafast charge transfer, long-lived interlayer excitons with high binding energies, and many-body excitonic interactions. In this thesis, we present the development and application of multidimensional coherent imaging spectroscopy (MDCIS), a four-wave mixing (FWM) based nonlinear spectroscopic imaging technique, to TMD monolayers and heterostructures. Based on multidimensional coherent spectroscopy (MDCS), MDCIS allows us to distinguish between homogeneous and inhomogeneous contributions to the material linewidths and distinguish coherent and incoherent coupling mechanisms in TMD heterostructures. The imaging aspect allows us to capture the spatial variation of the aforementioned physical processes across TMDs. We first discuss our results applying MDCS to an MoSe2/WSe2 heterostructure, for which we characterize the coherent and incoherent coupling mechanisms present in these materials. We quantify the timescales of rapid electron (91+/-9 fs) and hole (148+/-28 fs) transfer between the two materials. Furthermore, we visualize strong coherent coupling between excitons in the MoSe2 and WSe2 layers by observing oscillations of the coupling peaks in one-quantum MDCS and measuring a mixing energy of 73 meV in zero-Quantum MDCS. We also observe many-body signatures of the interlayer excitons and, in conjunction with photoluminescence measurements, measure their binding energy to be 254 meV. To accelerate nonlinear imaging, we develop a lock-in amplifier that uses a box-weighted instead of an exponentially-weighted lowpass filter. The transfer function of the box lock-in has tunable notches in the frequency domain that enable sufficient suppression of adjacent modulations present on the detector. We use Monte-Carlo simulations to quantify the signal-to-noise ratio and suppression of adjacent modulations, demonstrating the superiority of the box lock-in over the conventional exponential lock-in at short pixel-dwell times. We further experimentally demonstrate this advantage by imaging a monolayer of MoSe2 on a distributed Bragg reflector. Furthermore, we present results using MDCIS to study the potential of MoSe2 monolayers and MoSe2/WSe2 heterostructures for quantum information applications. We map the distribution of homogeneous and inhomogeneous linewidths across an MoSe2 monolayer, identifying promising areas with low inhomogeneity and long dephasing times that bear the potential for qubits. We also visualize the strain across the MoSe2 monolayer and comment on the detrimental effects strain may have in device applications. Similarly, we map strain across an MoSe2/WSe2 heterostructure and quantify the spatial homogeneity of coherent coupling (81 % of the sample) and charge transfer (91 % of the sample). We further map the distribution of interlayer exciton lifetimes. These quantities display a surprising robustness in the presence of strain, strengthening the case for TMD heterostructures as an applications platform for quantum information and photovoltaics. Lastly, we demonstrate how to further accelerate the nonlinear imaging techniques in this thesis by smart scanning and sampling schemes in the time-domain. We obtain FWM images, dephasing maps, and decay maps within minutes, opening the avenue for moving these techniques out of the lab and into a fabrication/manufacturing setting for advanced materials characterization.PHDPhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/175689/1/purz_1.pd

Dispersal, location of bloom initiation, and nutrient conditions determine the dominance of the harmful dinoflagellate Alexandrium catenella: A meta‐ecosystem study

Harmful algal blooms (HABs) are globally increasing in number and spatial extent. However, their propagation dynamics along environmental gradients and the associated interplay of abiotic factors and biotic interactions are still poorly understood. In this study, a nutrient gradient was established in a linear meta‐ecosystem setup of five interconnected flasks containing an artificially assembled phytoplankton community. The harmful dinoflagellate Alexandrium catenella was introduced into different positions along the nutrient gradient to investigate dispersal and spatial community dynamics. Overall, total algal biovolume increased, while community evenness decreased with increasing nutrient concentrations along the gradient. Alexandrium was able to disperse through all flasks. On the regional scale, diatoms dominated the community, whereas on the local scale the dinoflagellate showed higher contributions at low nutrient concentrations and dominated the community at the lowest nutrient concentration, but only when initiated into this flask. A control treatment without dispersal revealed an even stronger dominance of Alexandrium at the lowest nutrient concentration, indicating that dispersal and the associated nutrient exchange may weaken dinoflagellate dominance under low nutrient conditions. This study presents a first approach to experimentally investigate spatial dynamics and ecological interactions of a harmful dinoflagellate along an environmental gradient in a meta‐ecosystem setup, which has the potential to substantially enhance our understanding of the relevance of dispersal for HAB formation and propagation in combination with local environmental factors.Volkswagen Foundation http://dx.doi.org/10.13039/50110000166

Relationships between histogram analysis of ADC values and complex 18F-FDG-PET parameters in head and neck squamous cell carcinoma.

PURPOSE:Histogram analysis is an emergent imaging technique to further analyze radiological images and to obtain imaging biomarker. In head and neck cancer, MRI and PET are routinely used in clinical practice. The aim of this study was to analyze associations between histogram based ADC parameters and complex FDG-PET derived parameters in head and neck squamous cell carcinoma (HNSCC). METHODS:34 patients (26% female, mean age, 56.7 ± 10.2 years) with primary HNSCC were prospectively included into the study. ADC histogram parameters were calculated by inhouse made matlab software using a whole lesion measurement. For each tumor, maximum and mean standardized uptake values (SUVmax, SUVmean), Total Lesion Glycolysis (TLG) and Metabolic Tumor Volume (MTV) were determined on PET-images. Spearman's correlation coefficient (ρ) was used to analyze associations between investigated parameters. Benjamini-Hochberg correction was used to adjust for multiple testing. Mann-Whitney test was used for group discrimination. P-values < 0.05 were taken to indicate statistical significance. RESULTS:The correlation analysis in the whole tumor group revealed a statistically significant correlation between entropy and MTV as well as TLG (ρ = 0.67, P<0.0001 and ρ = 0.61, P = 0.0002 respectively). There were statistically significant differences between T1/2 and T3/4 tumors in the following parameters: entropy (2.07 ± 0.36 vs 2.61 ± 0.43, P = 0.007), SUVmax (10.79 ± 4.13 vs 17.93 ± 5.89, P = 0.007), SUVmean (6.39 ± 2.48 vs 9.81 ± 4.49, P = 0.01), SUVmin (4.09 ± 1.57 vs 6.34 ± 2.59, P = 0.03), MTV (9.50 ± 7.92 vs 20.36 ± 13.30, P = 0.02), TGU (55.97 ± 39.09 vs 212.3 ± 186.3, P = 0.002). CONCLUSION:This study showed that entropy derived from ADC maps is strongly associated with MTV and TLG in HNSCC. Entropy, SUVmax, SUVmean, TLG and MTV were statistically significant higher in T3/4 tumors in comparison to T1/2 carcinomas