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Monolayer semiconductor nanocavity lasers with ultralow thresholds

Abstract

Engineering the electromagnetic environment of a nanometre-scale light emitter by use of a photonic cavity can significantly enhance its spontaneous emission rate, through cavity quantum electrodynamics in the Purcell regime. This effect can greatly reduce the lasing threshold of the emitter1,2,3,4,5, providing a low-threshold laser system with small footprint, low power consumption and ultrafast modulation. An ultralow-threshold nanoscale laser has been successfully developed by embedding quantum dots into a photonic crystal cavity (PCC)6,7,8. However, several challenges impede the practical application of this architecture, including the random positions and compositional fluctuations of the dots7, extreme difficulty in current injection8, and lack of compatibility with electronic circuits7,8. Here we report a new lasing strategy: an atomically thin crystalline semiconductor—that is, a tungsten diselenide monolayer—is non-destructively and deterministically introduced as a gain medium at the surface of a pre-fabricated PCC. A continuous-wave nanolaser operating in the visible regime is thereby achieved with an optical pumping threshold as low as 27 nanowatts at 130 kelvin, similar to the value achieved in quantum-dot PCC lasers7. The key to the lasing action lies in the monolayer nature of the gain medium, which confines direct-gap excitons to within one nanometre of the PCC surface. The surface-gain geometry gives unprecedented accessibility and hence the ability to tailor gain properties via external controls such as electrostatic gating and current injection, enabling electrically pumped operation. Our scheme is scalable and compatible with integrated photonics for on-chip optical communication technologies.

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Figure 1: Hybrid monolayer WSe2–PCC nanolasers.
Figure 2: Lasing characteristics.
Figure 3: Spatially resolved emission and temperature-dependent device behaviour.
Figure 4: Reproducibility and scalability of the 2D nanolasers.

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Acknowledgements

We thank C. Dodson for helping with reflection measurements of nanocavities. This work was mainly supported by AFOSR (FA9550-14-1-0277). A.M. is supported by NSF-EFRI-1433496. Photonic crystal fabrication was performed in part at the Stanford Nanofabrication Facility of NNIN supported by the NSF under grant no. ECS-9731293, and at the Stanford Nano Center. S.W. was partially supported by the State of Washington through the University of Washington Clean Energy Institute. S.B. and J.V. were supported by the Presidential Early Award for Scientists and Engineers (PECASE) administered through the Office of Naval Research, under grant number N00014-08-1-0561. S.B. was also supported by a Stanford Graduate Fellowship. J.Y. and D.G.M. were supported by US DoE, BES, Materials Sciences and Engineering Division. F.H. acknowledges support from the European Commission (FP7-ICT-2013-613024-GRASP).

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Authors

Contributions

X.X. and A.M. conceived the experiments. S.B. and A.M. fabricated and characterized PCCs under the supervision of J.V. S.W. fabricated the hybrid devices and performed the measurements with assistance from J.R.S. and L.F., under the supervision of X.X. S.W., X.X., A.M. and S.B. analysed the data, and acknowledge discussions with W.Y. and J.V. J.Y. and D.G.M. provided the bulk WSe2. F.H. grew the GaP membrane. S.W. wrote the paper with input from all authors.

Corresponding authors

Correspondence to Arka Majumdar or Xiaodong Xu.

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Competing interests

The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Cavity Q-factor determination.

a, SEM image of a typical PCC. b, c, Room-temperature cross-polarized reflection taken from this cavity, before (b) and after (c) monolayer WSe2 transfer. As-fabricated cavities (before transfer) of our lasing devices typically have Q-factors ranging from 5,000 to 14,000. After monolayer transfer, the Q-factor is reduced from 8,000 to 1,300 in this device. After cooling down to cryogenic temperatures, the Q-factor recovers to 2500.

Extended Data Figure 2 Behaviour of device with Q-factor reduced by poly(methyl methacrylate).

a, Photoluminescence spectra taken from the PMMA covered device at different pumping powers (30 K), showing pronounced cavity peaks. b, Magnified view of cavity peaks ringed in a. c, Power dependence of the integrated peak intensity. A nonlinear ‘kink’ appears around 100 μW. The PMMA layer reduces the Q-factor to 500, and also shifts the resonance to lower energy (750.7 nm). This supports the conclusion that the ultralow lasing threshold in our device results from the high Q-factor, by significantly enhancing the spontaneous emission rate into the lasing mode.

Extended Data Figure 3 Nonlinear ‘kinks’ in plots of device properties at 80 K.

a, b, Plots show pump power dependence of integrated emission intensity (a) and line width (b). The same set of data are shown here as in Fig. 2b.

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Wu, S., Buckley, S., Schaibley, J. et al. Monolayer semiconductor nanocavity lasers with ultralow thresholds. Nature 520, 69–72 (2015). https://doi.org/10.1038/nature14290

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