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Hybrid organic-inorganic perovskites (HOIPs) offer long carrier diffusion lengths, high absorption coefficients, tunable bandgaps and long spin lifetimes. The flexible crystal structure and ionic nature of HOIPs makes it possible to allow tune their material properties through rational design, including the incorporation of chiral organic ligands. Recently, chiral HOIPs have emerged as promising materials for chiroptoelectronics, spintronics and ferroelectricity. They exhibit high photoluminescence polarization (17% without an external magnetic field), good device performance (a circularly polarized photodetector had 100 times higher responsivity than one based on chiral metasurface) and high saturated polarization (~2 times higher than that of barium titanate). Here we review the latest advances in chiral HOIPs and investigate the specific benefits of combining chiral organic and inorganic components in perovskites. We discuss demonstrations of chiroptical and ferroelectric applications, and conclude with our perspective on the future opportunities for chiral HOIPs.

Atomically thin magnets are the key element to build up spintronics based on twodimensional materials. The surface nature of two-dimensional ferromagnet opens up opportunities to improve the device performance efficiently. Here, we report the intrinsic ferromagnetism in atomically thin monolayer CrBr3, directly probed by polarization resolved magnetophotoluminescence. The spontaneous magnetization persists in monolayer CrBr3 with a Curie temperature of 34 K. The development of magnons by the thermal excitation is in line with the spin-wave theory. We attribute the layer-number dependent hysteresis loops in thick layers to the 2 magnetic domain structures. As a stable monolayer material in air, CrBr3 provides a convenient platform for fundamental physics and pushes the potential applications of the two-dimensional ferromagnetism.Ferromagnetism in atomically thin magnet has been studied in a variety of van der Waals materials 1, 2 , including metallic Fe3GeTe2 3, 4 , semiconducting Cr2Ge2Te6 5 and insulating CrI3 6 .Even though the long-range magnetic order is highly suppressed by the thermal excitation of magnons in a two-dimensional (2D) magnet at finite temperature 7 , the magnetic anisotropy opens an energy gap in the magnon spectra and therefore, protects the ferromagnetism in two dimensions.The magnon-magnon interaction in such van der Waals ferromagnets also provides a platform to study the fundamental topological spin excitation, for example, Dirac magnon 8 and topological magnon surface state 9 . Moreover, in contrast to the three-dimensional ferromagnet, magnetic 2D materials show tunable magnetic properties due to their surface nature 1-3, 10-13 . Particularly the layer-number dependent 4, 6, 14 and gate-tunable magnetism 3, 10-13 opens a new way to build spintronic devices with high accuracy and efficiency 15-20 .Among various van der Waals ferromagnets, CrBr3 is an interesting platform to study the magnetism in low dimensions and light matter interactions in magnetic materials. The neutron scattering has revealed the Dirac points in bulk CrBr3 21, 22 , formed by acoustic and optical spinwave modes, where both intralayer and interlayer exchange interactions play an important role.On the other hand, optical absorption spectra in CrBr3 have shown the out-of-plane magnetic field dependence 23 , suggesting potential applications in optoelectronics. However, magnetism in atomically thin CrBr3, especially in monolayer limit, is still unknown.

Atomically thin layered two-dimensional (2D) materials have provided a rich library for both fundamental research and device applications. Bandgap engineering and controlled material response can be achieved from artificial heterostructures. Recently, excitonic lasers have been reported using transition metal dichalcogenides; however, the emission is still the intrinsic energy bandgap of the monolayers. Here, we report a room temperature interlayer exciton laser with MoS2/WSe2 heterostructures. The onset of lasing was identified by the distinct kink in the “L-L” curve and the noticeable spectral linewidth collapse. Different from visible emission of intralayer excitons in monolayer components, our laser works in the infrared range, which is fully compatible with the well-established technologies in silicon photonics. Long lifetime of interlayer excitons relaxes the requirement of the cavity quality factor by orders of magnitude. Room temperature interlayer exciton lasers might open new perspectives for developing coherent light sources with tailored optical properties on silicon photonics platforms.

Transition metal dichalcogenides have valley degree of freedom, which features optical selection rule and spin-valley locking, making them promising for valleytronics devices and quantum computation. For either application, a long valley polarization lifetime is crucial. Previous results showed that it is around picosecond in monolayer excitons, nanosecond for local excitons and tens of nanosecond for interlayer excitons. Here we show that the dark excitons in two-dimensional heterostructures provide a microsecond valley polarization memory thanks to the magnetic field induced suppression of valley mixing. The lifetime of the dark excitons shows magnetic field and temperature dependence. The long lifetime and valley polarization lifetime of the dark exciton in two-dimensional heterostructures make them promising for long-distance exciton transport and macroscopic quantum state generations.