Monolayer excitonic laser

Two-dimensional van der Waals materials have opened a new paradigm for fundamental physics exploration and device applications because of their emerging physical properties. Unlike gapless graphene, monolayer transition-metal dichalcogenides (TMDCs) are two-dimensional semiconductors that undergo an indirect-to-direct bandgap transition1–5, creating new optical functionalities for next-generation ultra-compact photonics and optoelectronics. Although the enhancement of spontaneous emission has been reported on TMDC monolayers integrated with photonic crystals6,7 and distributed Bragg reflector microcavities8,9, coherent light emission from a TMDC monolayer has not been demonstrated. Here, we report the realization of a two-dimensional excitonic laser by embedding monolayer WS2 in a microdisk resonator. Using a whispering gallery mode with a high quality factor and optical confinement, we observe bright excitonic lasing at visible wavelengths. This demonstration of a two-dimensional excitonic laser marks a major step towards two-dimensional on-chip optoelectronics for high-performance optical communication and computing applications. As a direct bandgap semiconductor, transition-metal dichalcogenide (TMDC) monolayers have attracted increasing attention for electronic and optoelectronic applications due to their strong light emission accompanied with unique access to spin and valley degrees of freedom10–13. These properties arise from the quantum confinement and crystal symmetry effect on the electronic band structure as the material is thinned down to a monolayer configuration. The demonstrations of excellent on–off ratio transistors14, valley-Hall physics15, large exciton binding energy16–18, lightemitting diodes3–5, superconductivity19, sensors20 and piezoelectricity21,22 show the diverse physics and applications of this material system. However, coherent light emission, or lasing, an essential step towards the realization of on-chip photonic applications, has not been realized. The design and fabrication of microcavities is crucial for a two-dimensional laser, which requires a high optical mode confinement factor and high quality factor Q. Here, we demonstrate a two-dimensional excitonic laser using monolayer WS2 coupled to a microdisk resonator, which has high quantum yield, small footprint and low power consumption. TMDCs such as WS2 (Fig. 1a) evolve from indirectto directbandgap semiconductors as the number of layers is reduced from bulk to monolayer, with sizable bandgaps around 2.0 eV at visible wavelengths. The direct bandgaps sit at the K and K′ valleys (Fig. 1b), two non-equivalent momentum valleys in the reciprocal space of the monolayer WS2 protected by the crystal’s broken inversion symmetry, providing rich valley-contrasting physics10–13. The transition between valence band and conduction band-edge is excitonic in nature in such a monolayer system. Strong excitonic features, including neutral and redshifted charged excitons, have been observed and studied23. The exciton in two-dimensional TMDCs not only governs the emissions properties, but also allows for the long-lived population inversion required to achieve optical gain and possible stimulated emissions. Although Purcell enhancement of spontaneous emission has been achieved in photonic crystals and distributed Bragg reflector microcavities6–9, coherent light emission or lasing from a two-dimensional semiconducting TMDC has not been demonstrated due to the limited material gain volume, and the lack of optical confinement and feedback within the atomic monolayer. Here, we report a monolayer excitonic laser in a microdisk resonator (Fig. 1c). By integrating the monolayer WS2 into a strongfeedback photonic cavity, the build-up of stimulated emission can eventually exceed the lasing threshold. Microdisks feature lowloss, high-quality whispering gallery modes (WGMs) that offer the potential for ultralow-threshold lasing24. Embedding the monolayer between two dielectric layers (Si3N4/WS2/hydrogen silsesquioxane (HSQ)) enables strong optical confinement and leads to a larger modal gain, necessary for an atomically thin monolayer gain medium. The scanning electron microscope (SEM) image in Fig. 1d of the undercut Si3N4/WS2/HSQ microdisk shows the low sidewall roughness that is essential to obtain a high cavity Q. The diameter of the HSQ layer is slightly smaller than that of the Si3N4 layer due to the finite etching selectivity between the HSQ and Si3N4. The cavity resonance is designed to overlap with the gain spectrum of monolayer WS2 (Supplementary Section Ib), with the electric field polarized in the plane of the TMDC monolayer to efficiently couple with the in-plane dipoles of the excitons25. For a Si3N4/HSQ microdisk structure with a diameter of 3.3 μm, we expect a strong transverse electric (TE)-polarized WGM at a wavelength of 612 nm. The small diameter of the microdisk ensures other resonance modes are widely separated in wavelength to avoid mode competition. Reducing the number of modes will also increase the spontaneous emission factor and contribute to the improvement of the lasing threshold. The electric field distribution of the TE1,24 resonance (radial mode number l = 1, azimuthal mode number m = 24) is shown in Fig. 2a (top view) and Fig. 2b (crosssectional view). The resonant wavelength matches the dominant peak of the measured lasing spectrum from a WS2 monolayer embedded within a 3.3-μm-diameter microdisk at 612.2 nm (Fig. 2c), with a measuredQ = λ/Δλ of ∼2,604. The sandwiched configuration provides two advantages: (1) enhanced optical mode overlap, (2) material protection. It is important to note that the optical confinement factor of our Si3N4/WS2/HSQ structure is ∼30% higher than in the case where the monolayer is directly transferred onto the top of a pre-built microdisk (Supplementary Section Ic). The enhanced confinement factor is one order larger than those of low-threshold quantum-dot microdisk lasers26. It is worth