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Keynote Speaker

Assist. Prof. Alexander McLeod

mcLeod nano-Optics and Photonics (mnOP) Lab, University of Minnesota, Twin Cities, US

E-mail

mcleoda@umn.edu

Nano-infrared visualization and tuning of propagating polaritons in free-standing oxide membranes

Membranes of complex oxides like perovskite SrTiO3 extend the multi-functional promise of oxide electronics into the nanoscale regime of two-dimensional materials (1). Here we demonstrate that free-standing oxide membranes supply a reconfigurable platform for nano-photonics based on propagating surface phonon polaritons. Using a home-built nanoscope based on a Park Systems AFM to bypass the diffraction limit of light, we apply infrared near-field imaging and -spectroscopy enabled by a tunable ultrafast laser to study pristine nano-thick SrTiO3 membranes prepared by hybrid molecular beam epitaxy. As predicted by coupled mode theory, we find that strong coupling of interfacial polaritons realizes symmetric and antisymmetric hybridized modes with simultaneously tunable negative and positive group velocities. By resolving reflection of these propagating modes from membrane edges, defects, and substrate structures, we quantify their dispersion with position-resolved nano-spectroscopy. Remarkably, we find polariton negative dispersion is both robust and tunable through choice of membrane dielectric environment and thickness and propose a novel design for in-plane Veselago lensing harnessing this control. Our work lays the foundation for tunable transformation optics at the nanoscale using polaritons in a wide range of freestanding complex oxide membranes. (2)

References:

(1) Varshney, S.; Choo, S.; Thompson, L.; Yang, Z.; Shah, J.; Wen, J.; Koester, S. J.; Mkhoyan, K. A.; McLeod, A. S.; Jalan, B. Hybrid Molecular Beam Epitaxy for Single-Crystalline Oxide Membranes with Binary Oxide Sacrificial Layers. ACS Nano 2024, 18 (8), 6348–6358. https://doi.org/10.1021/acsnano.3c11192.

(2) Lukaskawcez, B.; Varshney, S.; Choo, S.; Park, S. H.; Seo, D.; Thompson, L.; Hirshberg, N.; Garber, M.; Uram, D.; Binger, H.; Koester, S.; Oh, S.-H.; Low, T.; Jalan, B.; McLeod, A.S. “Interfacial Strong Coupling and Negative Dispersion of Propagating Polaritons in Freestanding Oxide Membranes.” Advanced Optical Materials 2025. https://doi.org/10.1002/adom.202501452.

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