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Physics Colloquium - "The Persistence of Spin Coherence in a Crystalline Environment" By Professor Ivan Biaggio



Lewis Lab 316

The colloquium will focus on the entanglement of triplet exciton pairs as it can be observed in some organic molecular crystals. In such crystals, triplet excitons (quasiparticles with a spin of one) can be created as a spin-entangled pair via fission of a photoinduced singlet exciton, an analogous system to the entangled photon pairs used in many aspects of quantum information technology. But this is also a unique realization of entanglement in a condensed matter system. Exciton fission and fusion in organic semiconductors, and consequently entangled triplet pairs, are currently enjoying a widespread interest, both in fundamental science and for applications. However, questions remain, in particular in regards to how a coherent triplet-pair state behaves when its two excitons move independently in the crystal lattice. As an example, when the two entangled excitons have the good fortune to meet, they can annihilate each other and emit a photon with a probability that depends on the how the triplet-pair wavefunction has evolved over time. This effect should always lead to quantum beats in the time-dependent fluorescence induced by a short later pulse, but until now it was a bit of a mystery  why such quantum beats can be seen in some molecular crystals but not in others, or why they can sometimes be seen with an applied magnetic field, but not without it. As an example, it has never been understood why quantum beats cannot be observed in rubrene crystals without an applied magnetic field, while they can always be seen in tetracene crystals. In this talk I will  discuss our recent progress towards resolving this mystery. I'll show how  the spin wavefunction of the triplet-pair can be affected by stochastic phase fluctuations during triplet exciton transport, and how this transport-induced dephasing not only depends on the presence or absence of a magnetic field, but it also depends on its direction within the crystallographic lattice. This allows to experimentally tune the global quantum coherence in a population via the direction of the applied magnetic field, while maintaining at all times the local spin coherence (in individual entangled pairs). In addition to their fundamental interest, these effects open up new opportunities for experimentally studying quasiparticle transport, and in our recent experiments we demonstrated the determination of the picosecond-scale anisotropic hopping rate of triplet excitons in a crystal by measuring the decay rate of the quantum beat envelope as a function of magnetic field direction. Dr. Biaggio is professor in the Physics Department at Lehigh University, where he established a research program dedicated to condensed matter physics, nonlinear optics, and materials for photonics. His expertise is in time-resolved laser spectroscopy, the use laser light to investigate electronic processes, nonlinear optical materials and effects. He developed laser-based techniques for condensed matter studies and new molecular materials for integrated nonlinear optics. Recently, his research group has worked on uncovering some exciting properties of singlet and triplet excitons in molecular crystals and organic semiconductors, developing novel new methods to study triplet exciton transport in these materials. Biaggio holds the Joseph A. Waldschmitt Chair in Physics and is an Optical Fellow.