Observation of Bulk Fermi Arc and Polarization Half Charge from Paired Exceptional Points

The ideas of topology have found tremendous success in Hermitian physical systems, but even richer properties exist in the more general non-Hermitian framework. Here, we theoretically propose and experimentally demonstrate a new topologically-protected bulk Fermi arc which---unlike the well-known surface Fermi arcs arising from Weyl points in Hermitian systems---develops from non-Hermitian radiative losses in photonic crystal slabs. Moreover, we discover half-integer topological charges in the polarization of far-field radiation around the Fermi arc. We show that both phenomena are direct consequences of the non-Hermitian topological properties of exceptional points, where resonances coincide in their frequencies and linewidths. Our work connects the fields of topological photonics, non-Hermitian physics and singular optics, and paves the way for future exploration of non-Hermitian topological systems.Comment: 7 pages, 4 figure

Bose-Einstein Condensation of Long-Lifetime Polaritons in Thermal Equilibrium

Exciton-polaritons in semiconductor microcavities have been used to demonstrate quantum effects such as Bose-Einstein condensation, superfluity, and quantized vortices. However, in these experiments, the polaritons have not reached thermal equilibrium when they undergo the transition to a coherent state. This has prevented the verification of one of the canonical predictions for condensation, namely the phase diagram. In this work, we have created a polariton gas in a semiconductor microcavity in which the quasiparticles have a lifetime much longer than their thermalization time. This allows them to reach thermal equilibrium in a laser-generated confining trap. Their energy distributions are well fit by equilibrium Bose-Einstein distributions over a broad range of densities and temperatures from very low densities all the way up to the threshold for Bose-Einstein condensation. The good fits of the Bose-Einstein distribution over a broad range of density and temperature imply that the particles obey the predicted power law for the phase boundary of Bose-Einstein condensation

Effects of interactions on correlation, thermalization, and transport of exciton-polaritons

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemistry, 2019Cataloged from PDF version of thesis.Includes bibliographical references (pages 249-271).Light-matter interactions are fundamental processes that allow us not only to interrogate material properties but also to coherently control material phases that cannot be reached otherwise. Matter-matter interactions, on the other hand, result in strong correlations and emergent behavior that cannot be explained in terms of single-particle physics. Exciton-polaritons (hereafter "polaritons") are hybrid quasiparticles in a semiconductor quantum-well microcavity that exhibit both light-matter and matter-matter interactions. Polaritons have the effective mass inherited from the ultralight cavity photon mass, which sets polariton transport phenomena to be photon-like and allows macroscopic quantum phenomena such as Bose-Einstein condensation and superfluidity up to room temperature. Meanwhile, the effect of photon dressing only reduces the exciton-exciton interaction strength by the Hopfield coefficient, which sets the polariton-polariton interaction strength to be exciton-like.Along with the narrow linewidth protected from inhomogeneously broadening, polaritons are an excellent platform to study interaction-induced physics and nonlinear device applications such as ultralow-power optical switches. In this thesis, we investigated the effects of light-matter and matter-matter interactions on various aspects of polaritons. In the first part, we first measured the polariton-polariton interaction strength by tracking the energy blueshift as a function of polariton density. This was enabled by separating and trapping polaritons away from a pumped region, where the measurement of polariton interactions can be obscured by much heavier particles such as a dark exciton reservoir. We provided possible mechanisms that explain the observed anomalously large blueshifts. In the second part, we addressed a long-standing debate on whether polaritons can reach thermal equilibrium.We used a long-lifetime microcavity structure to achieve Bose-Einstein distributions of polaritons, which was the first demonstration of polaritons in equilibrium. This allowed us to measure equilibrium properties, such as temperature and chemical potential, and to map out the phase diagram of Bose-Einstein condensation in a quasi-two-dimensional system. We further investigated how all-optical trapping and polariton interactions enhance relaxation and thermalization processes. In particular, we found that a significant redistribution of polaritons occurs through the reduced density of states and polariton interactions. In the third part, we studied trapped eigenstates and interference patterns of polariton condensates in various trapping and pump geometries. Competition between eigenstates and selection of one of them have been well explained by the overlap of real-space, monientum-space, and energy distributions between the pump and the eigenstate.A mismatch between the pump-induced potential profile and the polariton source profile was a key factor in determining the distribution of transported polaritons. In the last part, we extended the polariton physics to study topological and cooperative effects in open quantum systems. We demonstrated bulk Fermi arcs by connecting two exceptional points arising from the engineered non-Hermitian properties of a photonic crystal. In addition, we theoretically showed that a cascaded-cavity system can outperform a single-cavity system in terms of the single-photon indistinguishability and efficiency, which works even with bad quantum emitters and practical cavity quality factors. Our work provides invaluable insights into the fundamental light-matter and matter-matter interactions, as well as many-body physics of condensed matter and photonic systems.by Yoseob Yoon.Ph. D.Ph.D. Massachusetts Institute of Technology, Department of Chemistr

Cascaded Cavities Boost the Indistinguishability of Imperfect Quantum Emitters

Recently, Grange et al. [Phys. Rev. Lett. 114, 193601 (2015)PRLTAO0031-900710.1103/PhysRevLett.114.193601] showed the possibility of single-photon generation with a high indistinguishability from a quantum emitter despite strong pure dephasing, by "funneling" emission into a photonic cavity. Here, we show that a cascaded two-cavity system can further improve the photon characteristics and greatly reduce the Q factor requirement to levels achievable with present-day technology. Our approach leverages recent advances in nanocavities with an ultrasmall mode volume and does not require ultrafast excitation of the emitter. These results were obtained by numerical and closed-form analytical models with strong emitter dephasing, representing roomerature quantum emitters. ©2019 American Physical Society.Air Force Office of Scientific Research program - AFOSR (FA9550-16-1-0391)Air Force Office of Scientific Research (AFOSR) Multidisciplinary University Research Initiative (MURI)National Science Scholarship from Agency for Science, Technology and Research (A*STAR), SingaporeArmy Research Laboratory Center for Distributed Quantum Information (CDQI

Monitoring driver stress during highway driving using wearable bio-signal sensors

Various functions for improving automobile seat comfort have been developed to reduce driver stress. However, how drivers respond to the seat comfort functions in real driving has not yet been studied. This experiment evaluated the effect of a seat air-conditioning system (ACS) on driver stress by tracking changes in various bio-signals during 60-min highway driving. Heart rate, heart rate variability, skin conductance, and respiration rates were quantified using non-invasive sensors from 13 drivers while varying the seat ACS condition in hot weather. Study results show that the high-frequency power of the heart rate variability, mean skin conductance level, and mean respiration rate detected the changes in the seat ACS over 60%, implying the possibility of seat comfort monitoring using the bio-signals. Further research should be conducted with various environmental or driver conditions to improve detection performance

Charge Transfer Dynamics in MoSe2/hBN/WSe2 Heterostructures.

Ultrafast charge transfer processes provide a facile way to create interlayer excitons in directly contacted transition metal dichalcogenide (TMD) layers. More sophisticated heterostructures composed of TMD/hBN/TMD enable new ways to control interlayer exciton properties and achieve novel exciton phenomena, such as exciton insulators and condensates, where longer lifetimes are desired. In this work, we experimentally study the charge transfer dynamics in a heterostructure composed of a 1 nm thick hBN spacer between MoSe2 and WSe2 monolayers. We observe the hole transfer from MoSe2 to WSe2 through the hBN barrier with a time constant of 500 ps, which is over 3 orders of magnitude slower than that between TMD layers without a spacer. Furthermore, we observe strong competition between the interlayer charge transfer and intralayer exciton-exciton annihilation processes at high excitation densities. Our work opens possibilities to understand charge transfer pathways in TMD/hBN/TMD heterostructures for the efficient generation and control of interlayer excitons