No abstract

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. *

We report the experimental observation of strongly enhanced tunneling between graphene bilayers through a WSe_{2} barrier when the graphene bilayers are populated with carriers of opposite polarity and equal density. The enhanced tunneling increases sharply in strength with decreasing temperature, and the tunneling current exhibits a vertical onset as a function of interlayer voltage at a temperature of 1.5 K. The strongly enhanced tunneling at overall neutrality departs markedly from single-particle model calculations that otherwise match the measured tunneling current-voltage characteristics well, and suggests the emergence of a many-body state with condensed interbilayer excitons when electrons and holes of equal densities populate the two layers.

We demonstrate gate-tunable resonant tunneling and negative differential resistance between two rotationally aligned bilayer graphene sheets separated by bilayer WSe. We observe large interlayer current densities of 2 and 2.5 μA/μm and peak-to-valley ratios approaching 4 and 6 at room temperature and 1.5 K, respectively, values that are comparable to epitaxially grown resonant tunneling heterostructures. An excellent agreement between theoretical calculations using a Lorentzian spectral function for the two-dimensional (2D) quasiparticle states, and the experimental data indicates that the interlayer current stems primarily from energy and in-plane momentum conserving 2D-2D tunneling, with minimal contributions from inelastic or non-momentum-conserving tunneling. We demonstrate narrow tunneling resonances with intrinsic half-widths of 4 and 6 meV at 1.5 and 300 K, respectively.