Control of Exciton Photon Coupling in Nano-structures

In this thesis, we study the interaction of excitons with photons and plasmons and methods to control and enhance this interaction. This study is categorized in three parts: light-matter interaction in microcavity structures, direct dipole-dipole interactions, and plasmon-exciton interaction in metal-semiconductor systems. In the microcavity structures, the light-matter interactions become significant when the excitonic energy is in resonance with microcavity photons. New hybrid quantum states named polariton states will be formed if the strong coupling regime is achieved, where the interaction rate is faster than the average decay rate of the excitons and photons. Polaritons have been investigated in zinc oxide (ZnO) nanoparticles based microcavity at room temperature and stimulated emission of the polaritons has also been observed with a low optical pump threshold. Exictons in organic semiconductors (modeled as Frenkel excitons) are tightly bound to molecular sites, and differ considerably from loosely bound hydrogen atom-like inorganic excitons (modeled as Wannier-Mott excitons). This fundamental difference results in distinct optoelectronic properties. Not only strongly coupled to Wannier-Mott excitons in ZnO, the microcavity photons have also been observed to be simultaneously coupled to Frenkel excitons in 3,4,7,8-naphthalene tetracarboxylic dianhydride (NTCDA). The photons here act like a glue combining Wannier-Mott and Frenkel excitons into new hybrid polaritons taking the best from both constituents. Two-dimensional (2D) excitons in monolayer transition metal dichalcogenides (TMDs) have emerged as a new and fascinating type of Wannier-Mott-like excitons due to direct bandgap transition, huge oscillator strength and large binding energy. Monolayer molybdenum disulfide (MoS2) has been incorporated into the microcavity structure and 2D exciton-polaritons have been observed for the first time with directional emission in the strong coupling regime. Valley polarization has also been discussed in this MoS2 microcavity for the possible applications in spin switches and logic gates. The direct dipole-dipole type excitonic interactions have also been studied in inorganic-organic nanocomposites, where ZnO nanowire is taken as the inorganic constituent and NTCDA thin films as the organic constituent. The excitonic interactions can be classified into weak coupling regime and strong coupling regime. Forster Resonant Energy Transfer (FRET), which is in the weak coupling regime, has been observed in this hybrid system. The optimized optical nonlinearity has also been determined in the hybrid system via Z-scan measurements. Exciton-plasmon polariton, another example of strongly coupled state which results from the interaction between excitons and plasmons when they are in resonance, has also been investigated in this thesis. Two rhodamine dyes spincoated on the silver thin films have separately been observed to be strongly coupled to the surface plasmon modes. With observed new polariton states, energy transfer mechanism has been discussed for nonlinear optical applications

Ultrafast fluorescent decay induced by metal-mediated dipole-dipole interaction in two-dimensional molecular aggregates

Two-dimensional molecular aggregate (2DMA), a thin sheet of strongly interacting dipole molecules self-assembled at close distance on an ordered lattice, is a fascinating fluorescent material. It is distinctively different from the single or colloidal dye molecules or quantum dots in most previous research. In this paper, we verify for the first time that when a 2DMA is placed at a nanometric distance from a metallic substrate, the strong and coherent interaction between the dipoles inside the 2DMA dominates its fluorescent decay at picosecond timescale. Our streak-camera lifetime measurement and interacting lattice-dipole calculation reveal that the metal-mediated dipole-dipole interaction shortens the fluorescent lifetime to about one half and increases the energy dissipation rate by ten times than expected from the noninteracting single-dipole picture. Our finding can enrich our understanding of nanoscale energy transfer in molecular excitonic systems and may designate a new direction for developing fast and efficient optoelectronic devices.Comment: 9 pages, 6 figure

Numerical Simulation of Plate Evaporators in Multi-effect Distillation Seawater Desalination

AbstractOwning to the high heat transfer coefficient and relatively small heat transfer areas, plate evaporator is now becoming more dominant in the desalination market. However, literatures on plate evaporator performance for seawater desalination are rather limited. Physical and mathematics model for plate evaporator with corrugated plate was developed for computational fluid dynamics (CFD) simulation. Two-phase flow and heat transfer characteristics in the channel of plate evaporator were analyzed in detail. The effects of different geometric parameters of corrugated plate inclination angle on seawater evaporating performance were revealed and discussed. In order to verify the simulation results, the experimental correlation for flow performance characteristics of plate evaporator was employed to compare with that of CFD results. The results may benefit the design of plate evaporators for multi effect distillation seawater desalination

Valley Bosonic Stimulation of Exciton-Polaritons in a Monolayer Semiconductor

The newly discovered valley degree of freedom (DOF) in atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) offers a promising platform to explore rich nonlinear physics, such as spinor Bose-Einstein condensate (BEC) and novel valleytronics applications. However, the critical nonlinear effect, such as valley polariton bosonic stimulation (BS), has long remained an unresolved challenge due to the generation of limited polariton ground state densities necessary to induce the stimulated scattering of polaritons in specific valleys. Here, we report, for the first time, the valley bosonic stimulation of exciton-polaritons via spin-valley locking in a WS2 monolayer microcavity. This is achieved by the resonant injection of valley polaritons at specific energy and wavevector, which allows spin-polarized polaritons to efficiently populate their ground state and induce a valley-dependent bosonic stimulation. As a result, we observe the nonlinear self-amplification of polariton emission from the valley-dependent ground state. Our finding paves the way for both fundamental study of valley polariton BEC physics and non-linear optoelectronic devices such as spin-dependent parametric oscillators and spin-lasers.Comment: Article + Supplementary Information (tot. 21 pages

Experimental Tests of DC SFCL under Low Impedance and High Impedance Fault Conditions

DC system protection is more challenging than that for AC system due to the rapid rate of rise of the fault current and absence of natural current zero-crossing in DC systems. Superconducting fault current limiter (SFCL) in DC systems is a promising technology to reduce the fault current level and the rate of rise of the fault current, and also SFCLs have no resistance during normal operation. In this paper, the behaviors of an SFCL coil are investigated under both low impedance and high impedance fault conditions in DC systems. In the low impedance fault condition system, the SFCL coil performs effective limitation of the fault current level under different prospective fault current levels. The application of SFCLs with limited inductance in the DC system can be a potential solution to effectively suppress the fault current under low impedance short-circuit faults. The SFCL coil under the high impedance fault condition can only limit the prospective fault current when it is much higher than the critical current of the coil

Ultrafast fluorescent decay induced by metal-mediated dipole–dipole interaction in two-dimensional molecular aggregates

Two-dimensional molecular aggregate (2DMA), a thin sheet of strongly interacting dipole molecules self-assembled at close distance on an ordered lattice, is a fascinating fluorescent material. It is distinctively different from the conventional (single or colloidal) dye molecules and quantum dots. In this paper, we verify that when a 2DMA is placed at a nanometric distance from a metallic substrate, the strong and coherent interaction between the dipoles inside the 2DMA dominates its fluorescent decay at a picosecond timescale. Our streak-camera lifetime measurement and interacting lattice–dipole calculation reveal that the metal-mediated dipole–dipole interaction shortens the fluorescent lifetime to about one-half and increases the energy dissipation rate by 10 times that expected from the noninteracting single-dipole picture. Our finding can enrich our understanding of nanoscale energy transfer in molecular excitonic systems and may designate a unique direction for developing fast and efficient optoelectronic devices. Keywords: molecular aggregate; fluorescence; nonradiative decay; dipole–dipole interaction; surface plasmonNational Science Foundation (U.S.) (Grant CMMI-1120724)United States. Air Force Office of Scientific Research (Award FA9550-12-1-0488

Universal Multi-modal Entity Alignment via Iteratively Fusing Modality Similarity Paths

The objective of Entity Alignment (EA) is to identify equivalent entity pairs from multiple Knowledge Graphs (KGs) and create a more comprehensive and unified KG. The majority of EA methods have primarily focused on the structural modality of KGs, lacking exploration of multi-modal information. A few multi-modal EA methods have made good attempts in this field. Still, they have two shortcomings: (1) inconsistent and inefficient modality modeling that designs complex and distinct models for each modality; (2) ineffective modality fusion due to the heterogeneous nature of modalities in EA. To tackle these challenges, we propose PathFusion, consisting of two main components: (1) MSP, a unified modeling approach that simplifies the alignment process by constructing paths connecting entities and modality nodes to represent multiple modalities; (2) IRF, an iterative fusion method that effectively combines information from different modalities using the path as an information carrier. Experimental results on real-world datasets demonstrate the superiority of PathFusion over state-of-the-art methods, with 22.4%-28.9% absolute improvement on Hits@1, and 0.194-0.245 absolute improvement on MRR

Effect of Arc Chute on DC Current Interruption by Liquid Nitrogen in HTS Electrical System of Distributed Propulsion Aircraft

The distributed propulsion aircraft with HTS electrical system is a novel concept for future airliners, which can reduce by more than 70% fuel burn and NO x emissions. The circuit breakers ensure the security of this novel aircraft by isolating electrical faults timely. Solid-state circuit breakers (SSCBs) are preferred due to their fast response and high performance in the cryogenic circumstance. However, the high conduction loss of SSCBs impedes their further application. A mechanical switch using liquid nitrogen (LN 2 ) as an arc extinguishing medium shows excellent DC current interruption performance. The LN 2 switch is characterized with extremely low contact resistance, and the proper use may reduce the conduction loss of power switches significantly. Nevertheless, the effect of metal type arc chutes on the arcing process in the LN 2 is still not clear. Thus the objective of this paper is to understand the effect of metal type arc chutes on the current interruption performance of LN 2 . Silicon iron arc chutes are employed. Neodymium (NdFeB) magnets are used to stretch the arc into the arc chutes. The maximum interrupting current is 1 kV/ 2 kA when only magnets are applied. Further applying the arc chutes leads to a significant drop in the arc voltage and interruption performance. Since the high relative permeability of silicon iron weakens the magnetic field acting on the arc, metal type arc chutes are not recommended. 1 kV / 10 kA fault current is successfully cleared by the combination of resistance type superconducting fault current limiter (R-SFCL) and LN 2 switch with magnets, during which the R-SFCL responds to the fault within 420 μs, compensating the long clear time of the LN 2 switch

Pole-to-Pole Fault Management for Electric Aircraft DC Network with HTS Cables

Full-electric propulsion aircraft is attracting a lot of interests in recent years. To improve the power density of electric propulsion systems, superconducting power devices are attractive due to their high current density and high efficiency. Fault analysis and fault management techniques with the combination of superconducting power devices are critically needed to ensure the safety and reliability of electric propulsion systems. In this paper, pole-to-pole fault analysis is carried out for the DC network in electric aircraft. High temperature superconducting (HTS) cable modeling is taken into consideration for the fault characterization. A system-level pole-to-pole fault management strategy is proposed to detect and isolate the faults at different locations. The analytical results are verified by the simulation models using Matlab/Simscape. The studies in this paper provide valuable guidance for the design and setting of protection systems in electric aircraft