Two-dimensional semiconductors have emerged as a new class of materials for nanophotonics for their strong exciton-photon interaction and flexibility for engineering and integration.Taking advantage of these properties, we engineer an efficient lasing medium based on dipolar interlayer excitons, in rotationally aligned atomically thin heterostructures. Lasing is measured from a transition metal dichalcogenide hetero-bilayer integrated in a silicon nitride grating resonator. A sharp increase in the spatial coherence of the emission was observed across the lasing threshold. The work establishes interlayer excitons in two-dimensional heterostructures as a silicon-compatible coherent medium. With electrically tunable lightmatter interaction strength and long-range dipolar interactions, these interlayer excitons promise both applications to low-power, ultrafast laser and modulators and rich many-body quantum phenomena.Semiconductor lasers are ubiquitous in today's technology because they are compact, cover a 1 arXiv:1901.00598v1 [cond-mat.mes-hall] 3 Jan 2019 wide range of wavelengths, and allow efficient electrical pumping and fast electrical modulation 1 .They are predominantly based on traditional III-V quantum wells. To achieve lower power consumption, more compact size, and a higher degree of integration on the silicon platform, there has been tremendous effort to develop alternative gain materials and structures, such as nanowire lasers 2 , spasers 3 , and photonic crystal lasers 4 . However, tunability, electrical pumping and siliconchip integration remain common challenges for all these approaches.Recently, monolayer transition metal dichalcogenide crystals (TMDCs) have emerged as a new class of materials for semiconductor lasers, for they are atomically thin, feature strong exciton emission 5, 6 and can be integrated with diverse materials, including silicon compounds 7 . Previous studies have assessed lasing in monolayer TMDCs from the non-linear intensity dependence and linewidth reductions as a function of pump power [8][9][10][11][12][13] . However, the photon flux appears to be below the stimulated scattering threshold 14 . The emission often saturates soon after reaching the linear regime of operation. Spatial coherence -one of the quintessential characters of laser 15has not been studied. Hence it is difficult to exclude localized excitons, such as point-defects, as the source of the observed nonlinear power dependence. Moreover, with only a monolayer as the gain medium, tunability is limited and vertical p-n junctions are not possible without contacting with other doped semiconductors.In contrast, heterostructures open the door to engineering of the band structures and exciton states. In particular, spatially indirect excitons in heterostructures have been intensively studied 16, 17 for they feature a static dipole with long-range diople interactions that may give rise to

Two-dimensional semiconductors feature valleytronics phenomena due to locking of the spin and momentum valley of the electrons. However, the valley polarization is intrinsically limited in monolayer crystals by the fast intervalley electron-hole exchange. Hetero-bilayer crystals have been shown to have a longer exciton lifetime and valley depolarization time. But the reported valley polarization was low; the valley selection rules and mechanisms of valley depolarization remains controversial. Here, we report singlet and brightened triplet interlayer excitons both with over 80% valley polarizations, cross-and co-polarized with the pump laser, respectively.This is achieved in WSe 2 /MoSe 2 hetero-bilayers with precise momentum valley alignment and narrow emission linewidth. The high valley polarizations allow us to identify the band minima in a hetero-structure and confirm unambiguously the direct band-gap exciton transition, ultrafast charge separation, strongly suppressed valley depolarization. Our results pave the way for using semiconductor heterobilayers to control valley selection rules for valleytronic applications. *

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Interlayer excitons are observed coexisting with intralayer excitons in bi-layer, few-layer, and bulk MoSe2 single crystals by confocal reflection contrast spectroscopy. Quantitative analysis using the Dirac-Bloch-Equations provides unambiguous state assignment of all the measured resonances. The interlayer excitons in bilayer MoSe2 have a large binding energy of 153 meV, narrow linewidth of 20 meV, and their spectral weight is comparable to the commonly studied higher-order intralayer excitons. At the same time, the interlayer excitons are characterized by distinct transition energies and permanent dipole moments providing a promising high temperature and optically accessible platform for dipolar exciton physics. arXiv:1712.04485v1 [cond-mat.mtrl-sci]

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