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Charge-depletion-enhanced WSe2 quantum emitters on gold nanogap arrays with near-unity quantum efficiency

Abstract

Achieving unity quantum efficiency in single-photon emitters (SPEs) is a holy grail in quantum information science. Through plasmonic coupling it is possible to increase the quantum efficiency of SPEs by increasing the radiative decay rate, but to approach unity quantum efficiency, non-radiative decay must be mitigated. Here we show that non-radiative decay in two-dimensional WSe2 quantum emitters can be electrically suppressed through charge depletion by using dual gate configurations under a large electric field. In this condition, for site-controlled SPEs in WSe2 coupled to gold nanogaps, the SPE transition quantum efficiency after gating is increased to 76.4 ± 14.6% on average, with some SPEs reaching near-unity (more than 90%) quantum efficiency. This study provides a new approach for tuning SPEs with an applied gate voltage and motivates further theoretical and experimental studies of SPE enhancement on vertically aligned nanogaps.

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Fig. 1: Design of site-controlled WSe2 emitters with plasmonic enhancement and gate tunability.
Fig. 2: Dynamic optical characterization of strain-induced SPEs coupled to metallic nanogap arrays.
Fig. 3: Gate dependence of SPEs on nanogaps.
Fig. 4: Dual gate characterization and non-radiative decay suppression mechanism.

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Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request. Source data are provided with this paper.

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Acknowledgements

We thank I. Yudhistira, A. Ling and J. Aizpurua for useful discussions. This work is supported by the Singapore National Research Foundation through the QEP programme (grant numbers NRF2021-QEP2-01-P01, NRF2021-QEP2-01-P02, NRF2021-QEP2-03-P01, NRF2021-QEP2-03-P10 and NRF2022-QEP2-02-P13) and the Singapore Ministry of Education (grant number MOE-T2EP50222-0018). W.G. and Z.D. acknowledge funding from an A*STAR MTC IRG grant (project number M21K2c0116). C.-W.Q. acknowledges support from the National Research Foundation (grant number NRF2021-QEP2-03-P09 with WBS number A-8000708-00-00 and grant number NRF-CRP26-2021-0063). This work was partially carried out at the USTC Center for Micro and Nanoscale Research and Fabrication.

Author information

Authors and Affiliations

Authors

Contributions

H.C. and W.G. conceived the project. H.C., Z.Z., Q.T., D.C., N.W., Z.M., J.J.H.E., Y.S. and W.G. performed the measurements. H.C., Q.T., N.W. and Q.W. fabricated the devices. H.C., A.R., R.H., Z.D., X.W., J.W., R.S., C.-W.Q. and W.G. analysed the data. A.R. and Q.T. performed the theoretical analysis and simulations. H.C., A.R., R.H. and W.G. wrote the paper with input from all authors. N.P., X.W., Y.M., X.L. and W.G. supervised the project. All authors contributed to discussion of the results.

Corresponding author

Correspondence to Weibo Gao.

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The authors declare no competing interests.

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Nature Photonics thanks Je-Hyung Kim and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Fabrication of the nanogap array with sharp edges covered with h-BN by the peeling-off method.

a, Deposition of a continuous gold film on the SiO2 substrate by standard thermal evaporation with a gold deposition rate of ~0.06 nm/s. b, Transfer of a mechanically exfoliated h-BN thin layer on the gold film by the common dry method with PDMS. c, FIB milling of nanogaps with designed size and shapes, using an ion current of 6.0 pA at 30 kV to achieve high resolution. d, Fabrication of nanogap array with the peeling off method. The unpatterned gold film was peeled off while the as-prepared nanogap array was left behind. e, Schematic of the fabricated nanogap array with h-BN on top.

Extended Data Fig. 2 Time trace of CW PL mapping of one emitter under different top gate voltages.

The PL time traces at the top gate voltage of a, 0 V, b, −0.5 V, and c, −1.2 V, are shown. A bandpass filter was used to choose the emission peak with an energy of ~1.605 eV. While the PL intensity shows a gate dependence, the PL intensity remains stable in the time trace, indicating the absence of spectral jump and a stable photon emission.

Source data

Extended Data Fig. 3 Gate dependence of plasmonic resonance.

a, PL spectrum of the WSe2 monolayer on a nanogap array under white light illumination. The ΔPL spectrum is defined as \({\boldsymbol{\Delta }}{\rm{PL}}=({I}_{{\rm{PL}}}^{{\,{\rm{nanogap}}}}-{I}_{{\rm{PL}}}^{{\,{\rm{SiO}}}_{2}})/({I}_{{\rm{PL}}}^{{\,{\rm{nanogap}}}}+{I}_{{\rm{PL}}}^{{\,{\rm{SiO}}}_{2}})\), where \({I}_{{\rm{PL}}}^{{\,{\rm{nanogap}}({\rm{SiO}}_{2}})}\) is the PL spectrum at the nanogap (SiO2) area. b and c, ΔPL spectrum as a function of the top gate and back gate, respectively.

Source data

Extended Data Fig. 4 Gate-dependent time-resolved PL of SPEs on the nanogap array.

The time-resolved PL at VB = 0 V and VT = −1 V (red), 0 V(blue), and 1 V (green) are shown. The symbols represent the measured data, and the lines represent the fitting results. The normalization is with respect to the maximum of each time-resolved PL data.

Source data

Extended Data Fig. 5 Gate tunable lifetime and PL intensity of two other SPEs in WSe2 monolayer (device #1).

a, Gate-dependent PL spectra under 726 nm CW excitation (power: 300 nW). The orange arrow and the dashed line indicate the filtered peak studied in b. b, Gate-dependent PL lifetime and normalized integrated PL intensity of filtered emission under 726 nm pulse excitation (power 300 nW). The lifetime is obtained by fitting the time-resolved PL data (~30 data points for every voltage; see Extended Data Fig. 4). The error bars are the standard deviation of the lifetime obtained from the fitting. Inset: filtered PL spectrum at VT = VB = 0 V under 726 nm CW excitation (power 300 nW). The red line shows the Lorentzian fitting of the dominant peak. c-d, Similar to a and b but from another SPE under 726 nm CW excitation (power: 400 nW) and 726 nm pulse excitation (power: 400 nW). The dashed orange box in c indicates the filtered peak studied in d. In all studied SPEs, the PL lifetime and intensity have a qualitatively similar shape, indicating a gate-modulated nonradiative decay process.

Source data

Extended Data Fig. 6 Gate-dependent PL and dynamic characterization of SPEs on nanogaps in another WSe2 monolayer device (device #2).

a, Gate-dependent PL spectra under 726 nm CW excitation (power: 300 nW). b, Gate-dependent PL lifetime and normalized integrated PL intensity of filtered emission under 726 nm pulse excitation. The PL lifetime and intensity have a qualitatively similar shape, indicating a gate-modulated nonradiative decay process. The lifetime is obtained by fitting the time-resolved PL data (~30 data points for every voltage; see Extended Data Fig. 4). The error bars are the standard deviation of the lifetime obtained from the fitting. c, Power dependence for the same SPE at different gate voltages. The lines are fitting with the power saturation function (see Supplementary Information Section 1). The emitter is excited by a 726 nm CW laser and a 726 nm pulse laser with 80 MHz repetition rate, respectively. The data is taken from emitter E1 in device #2. The optical microscopy image and the structure of device #2 are shown in Supplementary Fig. 7.

Source data

Extended Data Fig. 7 Gate dependence of SPE linewidth.

The full width at half maximum (FWHM) of the chosen peak in Extended Data Fig. 5a is plotted against the applied top gate voltage. The red line is a guide for the eye. The reduction of linewidth indicates suppression of blinking due to nonradiative decay pathways. The linewidth is obtained by fitting the PL spectrum (~70 data points for every voltage). The error bars are the standard deviation of the linewidth obtained from the fitting.

Source data

Extended Data Fig. 8 Electrically-gated SPE with near-unity quantum efficiency.

a, Gate-dependent spectrum. A 726 nm 1000 nW pulse laser excitation is used. The emission at 1.636 eV is filtered to be studied further in b. b, Power saturation curve at VT = −1.3 V. The saturation count is ~216 kcps, corresponding to a quantum efficiency of about 90% under pulse excitation. The data is taken from emitter E2 in device #1.

Source data

Extended Data Fig. 9 Electrically-gated SPE quantum efficiency.

Representative results from three different SPEs are shown. The ‘before gating’ refers to VT = 0 V case, and ‘after gating’ refers to the case where VT is applied to reach the maximum count. The value of VB = 0 V is used. Emitter E2 and E3 are in device #1, while emitter E1 is in device #2. The data for emitter E3 is shown in the main text Fig. 3.

Source data

Supplementary information

Supplementary Information

Supplementary Figs. 1–16, Table 1 and Sections 1–5.

Source data

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Cai, H., Rasmita, A., He, R. et al. Charge-depletion-enhanced WSe2 quantum emitters on gold nanogap arrays with near-unity quantum efficiency. Nat. Photon. (2024). https://doi.org/10.1038/s41566-024-01460-9

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