Abstract
Mirror symmetries are of particular importance because they are connected to fundamental properties and conservation laws. Spatial inversion and time reversal are typically associated to charge and spin phenomena, respectively. When both are broken, magnetoelectric cross-coupling can arise. In the optical regime, a difference between forward and backward propagation of light may result. Usually, this nonreciprocal response is small. We show that a giant nonreciprocal optical response can occur when transferring from linear to nonlinear optics, specifically second harmonic generation (SHG). CuB2O4 exhibits SHG transmission changes by almost 100% upon reversal of a magnetic field of just ±10 mT. The observed nonreciprocity results from an interference between magnetic-dipole and electric-dipole SHG. Although the former is inherently weaker than the latter, a resonantly enhanced magnetic-dipole transition has a comparable amplitude as a nonresonant electric-dipole transition, thus maximizing the nonreciprocity. Multiferroics and magnetoelectrics are an obvious materials platform to exhibit nonreciprocal nonlinear optical functionalities.
Highlights
Unidirectional manipulation of photons is a key issue in modern information technology as exemplified by optical isolators used for lasers and in optical networks
While conventional optical isolators are the composite of a magneto-optical medium, magnets, and polarizers, recent studies show that such a one-way flow of photons can be achieved in single bulk magnetoelectric materials, where time-
The energy of the fundamental light was tuned by an optical parametric amplifier (OPA) to ħ = 0.703 eV, for which the Second harmonic generation (SHG) energy 2ħ = 1.406 eV was resonant with the d-d transition of Cu2+ holes between the dx2-y2 and dxy orbitals [4, 18, 21,22,23]
Summary
Unidirectional manipulation of photons is a key issue in modern information technology as exemplified by optical isolators used for lasers and in optical networks. ED-SHG is an i-type tensor and MD-SHG is a c-type tensor (see the Supplementary Materials); the latter changes sign under the reversal of the magnetic field. In the canted antiferromagnetic phase, where the time-reversal and space-inversion symmetries are simultaneously broken, CuB2O4 shows a magnetoelectric effect, which is explained by the modification of the metal-ligand hybridization with the Cu2+ moments [19, 20].
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