# Calendar

Ohm’s Law, which says that electrical current depends linearly on applied electric field, is not a law of nature after all, merely a lowest-order approximation. Recently there has been an upsurge of interest in the response of crystals beyond the linear approximation, in particular 2^{nd} order nonlinearity, which generates currents at the sum and difference frequencies of the applied fields. Materials with a large 2^{nd} order response are candidates for applications involving frequency generation and the conversion of light to *dc* current (photogalvanic effects). However, despite a long history of research in harmonic generation and photogalvanic effects, understanding their origin and ultimate limits has been gained only recently. The clue is that the condition that allows a nonzero 2^{nd} order response – breaking of inversion symmetry – is the same as the condition for the existence of a curved bandstructure geometry. In this talk I will describe the deep connections between photogalvanic effects, second-harmonic generation, and quantum geometry. I will also describe how our attempt to test these ideas experimentally led to the accidental discovery that the Weyl semimetal TaAs has the largest 2^{nd} order susceptibility of any previously known crystal.

## Physics Colloquium: "Nonlinear Optics and the Quantum Geometry of Crystals"

Speaker |
(sign-up)
Joe Orenstein, University of California Berkeley |
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Date: | 10/17/2018 |

Time: | 4 p.m. |

Location: | 141 Loomis Lab |

Event Contact: | Suzanne Hallihan 217-244-7151 shalliha@illinois.edu |

Cost: | none |

Sponsor: | Department of Physics |

Event Type: | Other |

Ohm’s Law, which says that electrical current depends linearly on applied electric field, is not a law of nature after all, merely a lowest-order approximation. Recently there has been an upsurge of interest in the response of crystals beyond the linear approximation, in particular 2 |

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