Letter pubs.acs.org/NanoLett
Optically Pumped Two-Dimensional MoS2 Lasers Operating at RoomTemperature Omid Salehzadeh, Mehrdad Djavid, Nhung Hong Tran, Ishiang Shih, and Zetian Mi* Department of Electrical and Computer Engineering, McGill University, 3480 University Street, Montreal, Quebec H3A 0E9, Canada S Supporting Information *
ABSTRACT: The discovery of direct bandgap semiconducting two-dimensional (2D) transition metal dichalcogenides (TMDCs) has opened a new era in flexible optoelectronic devices. Critical to this development is the realization of a semiconductor laser using the emerging 2D TMDCs. Here, by embedding 2D MoS2 at the interface between a free-standing microdisk and microsphere, we have demonstrated, for the first time, room-temperature lasing from 2D TMDCs. The devices exhibit multiple lasing peaks in the wavelength range of ∼600 to 800 nm. The threshold is measured to be ∼5 μW under continuous wave operation at room temperature. No saturation in the output power is measured for pump powers more than 2 orders of magnitude larger than the threshold. The superior performance is attributed to the large gain of 2D TMDCs and the strong coupling between the 2D MoS2 gain medium and optical modes in the unique optical cavity. KEYWORDS: MoS2, lasing, photoluminescence, gain, whispering gallery mode
D
the realization of a room temperature 2D TMDC based semiconductor laser has hitherto not been reported.27 There are several critical challenges for achieving lasing using 2D TMDCs. For example, the large effective masses of charge carriers in MoS2 (me*, mh* ≈ 0.3−0.4m0)28,29 results in high densities of states in both the valence and conduction bands (m*/πℏ2). This fact, coupled with the fairly large bandgap of MoS2, requires carrier concentrations larger than ∼5 × 1018 cm−3 to push the quasi-Fermi levels into the corresponding bands to achieve population inversion (Supplementary Section 1). The surface-defect related inhomogeneous broadening also reduces the peak gain and differential gain in monolayer-based devices. The modal gain of the emerging 2D TMDC lasers may also suffer severely from the poor coupling between the atomically thin gain medium and the evanescent field of the confined optical modes. In this context, we have considered the use of 4L MoS2 as the gain medium in a vertically coupled, freestanding SiOx microdisk and microsphere cavity, which can significantly enhance the coupling between the MoS2 gain medium and the optical cavity modes, leading to ultralow threshold lasing under continuous wave operation at room temperature. It has been well understood that few-layer MoS2 (
