Light-Matter Interactions
Selected Publications
Quantum geometry of optical responses
The modern understanding of quantum transport relies on geometric concepts such as the Berry phase. The geometric approach has now been extended to the theory of optical transitions.
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Junyeong Ahn, G.-Y. Guo, N. Nagaosa, and A. Vishwanath, “Riemannian geometry of resonant optical responses”, Nature Physics 18, 290 (2022).​
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Junyeong Ahn, G.-Y. Guo, and N. Nagaosa, “Low-frequency divergence and quantum geometry of the bulk photovoltaic effect in topological semimetals”, Physical Review X 10, 041041 (2020).
Optical axion electrodynamics
​Electromagnetic response of topological materials is described the so called axion electrodynamics which contains additional relations between the fields. Here we extend the theory of axion electrodynamics to general optical frequencies and apply it to a realistic topological antiferromagnet.
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Junyeong Ahn, S.-Y. Xu, and A. Vishwanath, “Theory of optical axion electrodynamics and application to the Kerr effect in topological antiferromagnets”, Nature Communications 13, 7615 (2022).
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J.-X. Qi, C. Tzschaschel, Junyeong Ahn, A. Gao, H. Li, X.-Y. Zhang, C. Hu, Y.-X. Wang, B. Ghosh, D. Bérubé, Z. Gong, K. Watanabe, T. Taniguchi, D. C. Bell, H.-Z. Lu, A. Bansil, H. Lin, T.-R. Chang, Q. Ma, A. Vishwanath, N. Ni, and S.-Y. Xu, “Axion optical induction of antiferromagnetic order”, Nature Materials 22, 583 (2023).
Unconventional optical responses in superconductors
The optical response of a clean momentum-conserved conventional superconductor is known to be forbidden. Here, we propose three conditions that intrinsic optical excitations can occur in clean multi-band superconductors by satisfying one of the three.
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Junyeong Ahn and N. Nagaosa, “Theory of optical responses in clean multiband superconductors”, Nature Communications 12, 1617 (2021).
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Junyeong Ahn and N. Nagaosa, “Many-body selection rule for quasiparticle pair creations in centrosymmetric superconductors”, arxiv:2108.04846.
Topological circular dichroism
This paper explores topological circular dichroism in chiral multifold semimetals, where materials absorb left- and right-circularly polarized light differently due to their crystal structure. We show that this effect is unique to multifold fermions and does not occur in Weyl fermions, offering new insights into the optical properties of chiral materials.
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Junyeong Ahn and B. Ghosh, “Topological circular dichroism in chiral multifold fermions”, Physical Review Letters 131, 116603 (2023).