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5 March 2019 Coherent oscillations in the THz response of single crystal methylammonium lead triiodide (Conference Presentation)
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Lead halide perovskites have been under intense research focus due to their potential for efficient solar cells and other devices. Among their outstanding optoelectronic properties is a long carrier lifetime which is difficult to reconcile within a simple direct band gap semiconductor picture. Recent studies point to Rashba band splitting, charge separation and the formation of a large polaron as possible explanations. Understanding band edge excitations and dynamics is therefore critical to developing a working model of these materials. Time-resolved THz spectroscopy (TRTS) is an ideal tool to interrogate these meV level excitations on a sub-100 fs time scale. Here we use an air-plasma based TRTS covering the 4 – 120 meV range to measure the THz differential reflectivity from a facet of a methylammonium lead triiodide single crystal following femtosecond optical excitation at the band edge. Full two–dimensional time/energy reflectivity maps reveal a beat between the THz probe light frequency and the crystal photoconductivity, which coherently oscillates at the LO phonon frequency and vanishes with a sub-picosecond lifetime. This indicates some population of carriers are impulsively formed upon excitation and are strong coupling to the polar lattice, in good agreement with the polaron picture. To our knowledge, this is the first observation of a coherent oscillation in the transport parameters in the lead halide perovskites.
Conference Presentation
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Yang Lan, Carlito S. Ponseca Jr., Benjamin Dringoli, David Valverde-Chavez, Mark Sutton, Yihui He, Mercouri G. Kanatzidis, and David G. Cooke "Coherent oscillations in the THz response of single crystal methylammonium lead triiodide (Conference Presentation)", Proc. SPIE 10916, Ultrafast Phenomena and Nanophotonics XXIII, 109161S (5 March 2019);

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