A topological Hund nodal line antiferromagnet.

The interplay of topology, magnetism, and correlations gives rise to intriguing phases of matter. In this study, through state-of-the-art angle-resolved photoemission spectroscopy, density functional theory, and dynamical mean-field theory calculations, we visualize a fourfold degenerate Dirac nodal line at the boundary of the bulk Brillouin zone in the antiferromagnet YMn2Ge2. We further demonstrate that this gapless, antiferromagnetic Dirac nodal line is enforced by the combination of magnetism, space-time inversion symmetry, and nonsymmorphic lattice symmetry. The corresponding drumhead surface states traverse the whole surface Brillouin zone. YMn2Ge2 thus serves as a platform to exhibit the interplay of multiple degenerate nodal physics and antiferromagnetism. Interestingly, the magnetic nodal line displays a d-orbital dependent renormalization along its trajectory in momentum space, thereby manifesting Hund's coupling. Our findings offer insights into the effect of electronic correlations on magnetic Dirac nodal lines, leading to an antiferromagnetic Hund nodal line.

Demonstration of an 8-Gbit/s quadrature-phase-shift-keying coherent underwater wireless optical communication link using coherent heterodyne detection under scattering conditions.

In this paper, we experimentally demonstrate an 8-Gbit/s quadrature-phase-shift-keying (QPSK) coherent underwater wireless optical communication (UWOC) link under scattering conditions at 532 nm. At the transmitter, we generate the 532-nm QPSK signal using second-harmonic generation (SHG), where the 1064-nm signal modulated with four phase levels of an 8-phase-shift-keying (8-PSK) format is phase doubled to produce the 532-nm QPSK signal. To enhance the receiver sensitivity, we utilize a local oscillator (LO) at the receiver from an independent laser source. The received QPSK data beam is mixed with the independent LO for coherent heterodyne detection. Results show that the bit error rates (BERs) of the received QPSK signal can reach below the 7% forward error correction (FEC) limit under turbid water with attenuation lengths (γL) up to 7.4 and 6.1 for 2- and 8-Gbit/s QPSK, respectively. The corresponding receiver sensitivities are -34.0 and -28.4 dBm for 2- and 8-Gbit/s QPSK, respectively.

Turbulence mitigation of four mode-division-multiplexed QPSK channels in a pilot-assisted self-coherent free-space optical link using a photodetector array and DSP-based channel demultiplexing.

In this Letter, we demonstrate turbulence mitigation of four mode-division-multiplexing (MDM) quadrature-phase-shift-keying (QPSK) channels in a pilot-assisted self-coherent free-space optical (FSO) link using a photodetector (PD) array and digital signal processing (DSP)-based channel demultiplexing. A Gaussian pilot beam is co-transmitted with four 1-Gbaud QPSK channels carried by four orbital angular momentum (OAM) modes. The pilot beam experiences similar turbulence-induced wavefront distortion to the data beams. At the receiver, the turbulence distortion is mitigated by its conjugate during the pilot-data mixing in a PD array. Subsequently, we demultiplex the four channels by applying in DSP a fixed matrix on the signals. Results show that our approach has <3-dB turbulence-induced power penalty at a 7% forward error correction (FEC) limit under a turbulence strength of 2w0/r0 = ∼4.4. The same turbulence can cause >18-dB penalties for a local oscillator (LO)-based coherent MDM system.

Adaptive-optics-based turbulence mitigation in a 400 Gbit/s free-space optical link by multiplexing Laguerre-Gaussian modes varying both radial and azimuthal spatial indices: publisher's note.

This publisher's note contains a correction to Opt. Lett.48, 6452 (2023)10.1364/OL.506270.

Adaptive-optics-based turbulence mitigation in a 400†Git/s free-space optical link by multiplexing Laguerre-Gaussian modes varying both radial and azimuthal spatial indices.

In general, atmospheric turbulence can degrade the performance of free-space optical (FSO) communication systems by coupling light from one spatial mode to other modes. In this Letter, we experimentally demonstrate a 400 Gbit/s quadrature-phase-shift-keyed (QPSK) FSO mode-division-multiplexing (MDM) coherent communication link through emulated turbulence using four Laguerre Gaussian (LG) modes with different radial and azimuthal indices (L G 10, L G 11, L G -10, and L G -11). To mitigate turbulence-induced channel cross talk and power loss, we implement an adaptive optics (AO) system at the receiver end. A Gaussian beam at a slightly different wavelength is co-propagated with the data beams as the probe beam. We use a wavefront sensor (WFS) to measure the wavefront distortion of this probe beam, and this information is used to tune a spatial light modulator (SLM) to adaptively correct the four distorted data-beam wavefronts. Using this adaptive-optics approach, the power loss and cross talk are reduced by ∼10 and ∼18 dB, respectively.