Optical diode effect at telecom wavelengths in a polar magnet

npj | quantum materials Article Published in partnership with Nanjing University https://doi.org/10.1038/s41535-026-00848-w Optical diode effect at telecom wavelengths in a polar magnet Check for updates 1234567890():,; 1234567890():,; Kevin A. Smith 1, Yanhong Gu1, Xianghan Xu2, Heung-Sik Kim3, Sang-Wook Cheong2,4, Scott A. Crooker5 & Janice L. Musfeldt 1,6 Magnetoelectric multiferroics such as rare earth manganites host nonreciprocal behavior driven by low symmetry, spin-orbit coupling, and toroidal moments, although less has been done to explore whether lanthanides like Er3+ might extend functionality into the hard infrared for optical communications purposes. In this work, we reveal nonreciprocity in the f-manifold crystal field excitations of h-Lu0.9Er0.1MnO3. In addition to contrast in the highest fields, we demonstrate nonreciprocity at technologically-relevant energy scales--specifically in the E-, S-, and C-bands of the telecom wavelength range--and at low magnetic fields and room temperature. In fact, the low field behavior is consistent with possible altermagnetism. These findings advance the overall understanding of localized excitations in rare earth-containing systems and pave the way for entirely new types of telecom applications. Nonreciprocal directional dichroism is a type of asymmetric light absorption that depends upon propagation direction1,2. It is a defining feature of materials that simultaneously break spatial inversion and time-reversal symmetries3,4. Although discovered in magnetoelectrics including CuB2O45–8, FeZnMo3O89, LiNiPO410, LiCoPO411, Nd2Ti2O712, Pb(TiO) Cu4(PO4)413, and Ni3TeO614–16, nonrecprocity is under-explored in rare earth-containing systems. This is because f-manifold crystal field excitations (which are both spin- and parity-forbidden) have been presumed not to host large nonreciprocal effects even though mixing of electric and magnetic dipoles at noncentrosymmetric rare earth sites can generate significant magnetoelectric coupling1,17. Properties arising from Er3+ are especially relevant to amplifiers, isolators, modulators, and rectifiers at optical communications wavelengths18–27. The prospect of integrating additional functionality in the form of nonreciprocity to the rare earth excitations that power these telecommunications technologies is therefore both challenging and potentially transformative. Rather than testing drawn quartz fibers sprinkled with powdered Er22,23, we employed h-Lu0.9Er0.1MnO3 as a platform for examining whether rare earth f-manifold excitations have the potential to host nonreciprocal behavior in the hard infrared. This system sports a dilute ensemble of Er3+ ions within a P63mc matrix that combines spontaneous polarization along c (arising from improper ferroelectricity involving the Mn centers) with antiferromagnetism due to Mn3+ ordering (TN ≈ 80 K) and a rare earth-related transition near 30 K (Fig. 1a)28,29. In fact, h-Lu0.9Er0.1MnO3 is likely an altermagnet due to antisymmetric spin splitting in the 60 mm0 magnetic ground state30. Our work is enabled by the development of monopolar domain single crystals, which grow in a characteristic canopy-like shape (Fig. 1b). Such a material—if functioning as a secure communications element—should host higher fidelity and lower loss than glass fibers with Er randomly distributed throughout. The symmetry requirements for toroidal nonreciprocity in h-Lu0.9Er0.1MnO3 dictate that polarization, magnetic field, and light propagation direction must be mutually orthogonal1,4,16. We therefore performed magneto-optical spectroscopy in this fashion. Strikingly, these measurements reveal strong nonreciprocal behavior in the f-manifold crystal field excitations of Er3+ that persists not only at high fields but also at modest magnetic fields and even up to room temperature. At 1525 nm, we find a contrast of 3.2% at 1.2 T and 296 K. These findings challenge the conventional wisdom about localized excitations, opening the door to entirely new types of nonreciprocal behavior and applications. Results and discussion Er3+ crystal field excitations in the hard infrared Figure 1c, d displays the near infrared absorption of h-Lu0.9Er0.1MnO3 as a function of temperature. This particular wavelength range focuses on the Er3+ f-manifold crystal field excitations in the E-, S-, and C-bands of the telecom range. These excitations are well-known to be sharp and highly localized. The clusters of peaks between 1440 and 1540 nm can be assigned as 4I15/2 → 4I13/2. This set of excitations is well-studied in Er-containing 1 Department of Chemistry, University of Tennessee, Knoxville, TN, USA. 2Department of Physics and Astronomy, Rutgers University, Piscataway, NJ, USA. Department of Semiconductor Physics and Institute of Quantum Convergence Technology, Kangwon National University, Chuncheon, Republic of Korea. 4Keck Center for Quantum Magnetism, Rutgers University, Piscataway, NJ, USA. 5National High Magnetic Field Laboratory, Los Alamos, NM, USA. 6Department of e-mail: Physics and Astronomy, University of Tennessee, Knoxville, TN, USA. 3 npj Quantum Materials | (2026)11:18 1 Article https://doi.org/10.1038/s41535-026-00848-w E-band S-band C-band d b c b H a P 106.8 182 89.5 100 80 70 40 200 72.3 150 55.0 Tc, Er 4K 1440 1460 1480 1500 1520 1540 1510 Wavelength (nm) 1520 1525 1530 1535 1540 the ⊥c direction. Additional information about the orientation (Fig. S1), monopolar domain character (Fig. S5), and magnetic properties (Fig. S6) is provided in the Supplementary Information. c Absorption of the 4I15/2 → 4I13/2 rare earth crystal field excitations in h-Lu0.9Er0.1MnO3 as a function of temperature with the corresponding telecom wavelength ranges indicated in green (E-band), yellow (S-band), and orange (C-band). The spectra are offset for clarity. d Contour plot of the spectra in c with focus on the S- and C-band features. The horizontal lines indicate Mn3+ antiferromagnetic ordering and the Er3+-related transition. NDD effect at 5.5 K ( H,k) NDD effect at 296 K ( H, k) 10.0 10.00 50 1515 Wavelength (nm) Fig. 1 | Structure, measurement conditions, and temperature effects in hLu0.9Er0.1MnO3. a Crystal structure of h-Lu0.9Er0.1MnO3 in the P63mc space group41. The Lu/Er, Mn, and O sites are indicated by teal, magenta, and red spheres, respectively. b Schematic of the measurement geometry indicating the light propagation k, applied field H, and polarization P directions (left) and optical microscope image (right) of the ab-plane of the single crystal revealing the natural canopylike structure that is indicative of a monopolar domain sample. Our single crystals were polished to expose the c-axis (which is the direction of polarization) as well as a 55.0 100 T N, Mn 50 20 k 110.0 250 Temperature (K) Absorbance (Arb. units) 298 110.0 -1 c Absorption (cm ) a b 5.0 50 7.500 4.100 'DNDD(cm ) 'DNDD(cm ) 40 0.000 -2.500 30 -7.500 -10.00 (...truncated)