Higher-order correlated excitonic states arise from the mutual interactions of excitons, which generally requires a significant exciton density and therefore high excitation levels. Here, we report the emergence of two biexcitons species, one neutral and one charged, in monolayer tungsten diselenide under moderate continuous-wave excitation. The efficient formation of biexcitons is facilitated by the long lifetime of the dark exciton state associated with a spin-forbidden transition, as well as improved sample quality from encapsulation between hexagonal boron nitride layers. From studies of the polarization and magnetic field dependence of the neutral biexciton, we conclude that this species is composed of a bright and a dark excitons residing in opposite valleys in momentum space. Our observations demonstrate that the distinctive features associated with biexciton states can be accessed at low light intensities and excitation densities.

Visualizing conducting domain walls When a metal undergoes a phase transition and becomes insulating, it sometimes also becomes magnetically ordered. It is possible that some metallicity survives along the boundaries of magnetic domains, the so-called domain walls, but the question is difficult to address directly in experiments. Ma et al. did just that by mapping out the conductance of the material Nd 2 Ir 2 O 7 in its low-temperature magnetic insulating phase, using microwave impedance microscopy. The magnetic domain walls showed up clearly in the images as regions of high conductance. Science , this issue p. 538

Electromagnetic radiation emitted by electrons allows their ultrafast motion to be probed on the atomic length scale.

Silicon photonics is becoming a leading technology in photonics, displacing traditional fiber optic transceivers and enabling new applications. Further improving the density and performance of silicon photonics, however, has been challenging, due to the large size and limited performance of traditional semi-analytically designed components. Automated optimization of photonic devices using inverse design is a promising path forward but has until now faced difficulties in producing designs that can be fabricated reliably at scale. Here we experimentally demonstrate four inverse-designed devices -a spatial mode multiplexer, wavelength demultiplexer, 50-50 directional coupler, and 3-way power splitter -made successfully in a commercial silicon photonics foundry. These devices are efficient, robust to fabrication variability, and compact, 1 arXiv:1911.03535v2 [physics.app-ph] 13 Jan 2020 with footprints only a few micrometers across. They pave the way forward for the widespread practical use of inverse design.