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Enhancing monolayer photoluminescence on optical micro/nanofibers for low-threshold lasing

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Science Advances  22 Nov 2019:
Vol. 5, no. 11, eaax7398
DOI: 10.1126/sciadv.aax7398
  • Fig. 1 Mechanism of photoactivation-induced PL enhancement.

    (A) Schematic illustration of generation of oxygen dangling bands from a tapered fiber. First, the CO2 laser heating and drawing process breaks six-membered siloxane rings in a standard fiber (125 μm) into highly strained three-membered rings in a MNF (~1 μm), and then, the irradiation using a 532-nm CW laser activates the highly strained rings to generate oxygen dangling bonds (≡Si─O·). The tapering process greatly lowers the activation energy from ≡Si─O─Si≡ to ≡Si─O·. The generated oxygen dangling atoms are reactive and can fill or bride with outer atoms. Inset: Optical image of a MNF tapered from a standard fiber. (B) PL spectra from different microfibers and a planar substrate under excitation intensity of 2.1 × 105 W cm−2. a.u., arbitrary units. (C) PL spectra of the microfiber (Dfiber = 3.1 μm) under different excitation intensity. (D) Raman spectra collected from different silica substrates. The intensity of the 606 cm−1 peak reflects the numbers of strained three-membered rings. (E) Surface roughness of different as-drawn original microfibers in the tapering (blue histogram) and high-intensity light irradiation (green histogram) process. (F) The generated oxygen dangling atoms can fill the sulfur vacancies, bridge with neighboring sulfur atoms, and form localized sites by transferring electrons from MoS2, leading to enhanced PL emission.

  • Fig. 2 Monolayer MoS2 grown on MNFs.

    (A) Conceptual illustration of monolayer MoS2 grown on MNFs with uniform diameters and with microbottle structures. A single-crystal triangular domain first nucleates on the MNF surface and then laterally grows into a large-area monolayer structure. A 532-nm CW laser is used to excite the PL emissions, some of which can be guided along the MNF or resonate in the MNF and microbottle structures. (B) Optical and PL images of a triangular layered MoS2 on a microfiber (Dfiber = 6.1 μm), with (C) a thickness of ~0.8 nm confirmed by an AFM scan. (D) Optical and PL images showing clean surface and two sharp boundary edge of a MoS2 monolayer/microfiber structure (Dfiber = 5.5 μm). (E) Optical and PL images showing a large-area monolayer/microfiber (Dfiber = 5.4 μm). Some guided emission is emitted at the distal end of the microfiber. (F) Raman spectra and (G) PL spectra of monolayer MoS2 grown on different substrates. Both the strain-sensitive E12g mode and PL spectral peaks exhibit red shifts as the MNF diameters decrease.

  • Fig. 3 Photoactivation-enhanced PL QYs.

    (A) Time-dependent PL intensity of MoS2 monolayers grown on different substrates, under activation intensity of 2 × 105 W cm−2 using a 532-nm CW laser. (B) PL spectra of the monolayer/microfiber (Dfiber = 3.3 μm) at different irradiation time, which are deconvoluted into three emissions of A0 at 1.85 eV, A at 1.81 eV, and AD at 1.75 eV. (C) Steady-state PL spectra of the sample after photoactivation with increasing Ipump from 10−1 to 104 W cm−2, all of which are dominant by the A0 peaks (~1.85 eV). (D) Pump-dependent peak positions (black data) and PL QYs (red data) in the as-photoactivated monolayer MoS2 (solid circles). A planar sample is also provided for reference (open squares). (E) Long-term stability of an as-photoactivated MoS2 monolayer. Inset: Normalized PL spectra collected at different time.

  • Fig. 4 Room-temperature CW lasing in as-photoactivated monolayer MoS2.

    (A) Steady-state PL spectra in an as-photoactivated MoS2 monolayer/microfiber (Dfiber = 5.4 μm) with increasing Ipump using the 532-nm CW laser, in which a strong 688.1-nm peak (TM311 mode) increases quickly from spontaneous emission to lasing. Inset: Optical and PL images of the monolayer/microfiber. Scale bar, 5 μm. (B) Steady-state PL spectra in an as-photoactivated MoS2 monolayer/microbottle (outer diameter, 5.5 μm) with increasing Ipump, in which WGM oscillations are observed. (C) Light in–light out curve for a 682.5-nm lasing peak (TM311 mode) in the monolayer/microbottle on a log-log scale, in which a β of 0.5 is the best fit to the experimental data. Inset: Optical and PL images the monolayer/microbottle. Scale bar, 5 μm. (D) Pump-dependent FWHM of the 682.5-nm peak, which narrows quickly as Ipump exceeds the lasing threshold (5 W cm−2).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/11/eaax7398/DC1

    Supplementary Text

    Section S1. Fabrication of MNF and microbottle structures

    Section S2. Experimental setup for Raman, PL, and lasing spectra measurements

    Section S3. Oxygen dangling bonds and charge transfer

    Section S4. Synthesis of TMD monolayers

    Section S5. Single-crystal nature

    Section S6. Monolayer MoS2 grown on nanofibers

    Section S7. PL enhancement on a microbottle structure

    Section S8. Absorption spectra

    Section S9. High-resolution AFM scan

    Section S10. Calculation of the band structure

    Section S11. Determination of neutral excitons, trions, and defect-related localized excitons

    Section S12. Spectral changes in monolayer MoS2 without photoactivation

    Section S13. Investigation of cavity effect in the PL enhancement

    Section S14. Optical properties in monolayer MoS2 removed from MNFs

    Section S15. Photoactivation threshold

    Section S16. PL enhancement in monolayer MoSe2 grown on MNFs

    Section S17. Calibration of PL QYs

    Section S18. Simulation of lasing cavity modes

    Section S19. Rate equation analysis

    Section S20. Lasing performance in other resonant modes

    Section S21. Reproducibility of our approach

    Fig. S1. Fabrication of MNF and microbottle structures.

    Fig. S2. Experimental setup for Raman, PL, and lasing measurements.

    Fig. S3. Numerical simulation of oxygen dangling bonds and charge transfer.

    Fig. S4. Experimental setup for growing monolayer MoS2 using a CVD method.

    Fig. S5. Single-crystal nature.

    Fig. S6. Monolayer MoS2 grown on an optical nanofiber.

    Fig. S7. PL enhancement on a microbottle structure.

    Fig. S8. Absorption spectra of monolayer MoS2 on a planar silica substrate and different fibers.

    Fig. S9. High-resolution AFM scan.

    Fig. S10. DFT calculations of the band structure of monolayer MoS2.

    Fig. S11. Spectral responses obtained under low temperatures and different atmospheric pressures, and time-resolved PL measurement.

    Fig. S12. Spectral changes in monolayer MoS2 without photoactivation.

    Fig. S13. Investigation of cavity effect in the PL enhancement.

    Fig. S14. Optical properties in monolayer MoS2 removed from MNFs.

    Fig. S15. Pump intensity dependent PL intensity of monolayer MoS2 on different substrates.

    Fig. S16. PL enhancement in monolayer MoSe2 grown on MNFs.

    Fig. S17. Simulation of lasing cavity modes.

    Fig. S18. Rate equation analysis.

    Fig. S19. Lasing performance in other resonant modes.

    Fig. S20. Typical lasing spectra and light in–light out curves of the lasing performance in typical five different samples.

    Table S1. Details of electron transfer in the structure of fig. S3.

    Table S2. Summary of the accuracy in QY measured in typical five samples.

    Table S3. Comparison of the fitting parameters of microfiber and microbottle of rate equation analysis.

    Table S4. Summary of the lasing thresholds of samples shown in fig. S20.

    References (39, 40)

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Text
    • Section S1. Fabrication of MNF and microbottle structures
    • Section S2. Experimental setup for Raman, PL, and lasing spectra measurements
    • Section S3. Oxygen dangling bonds and charge transfer
    • Section S4. Synthesis of TMD monolayers
    • Section S5. Single-crystal nature
    • Section S6. Monolayer MoS2 grown on nanofibers
    • Section S7. PL enhancement on a microbottle structure
    • Section S8. Absorption spectra
    • Section S9. High-resolution AFM scan
    • Section S10. Calculation of the band structure
    • Section S11. Determination of neutral excitons, trions, and defect-related localized excitons
    • Section S12. Spectral changes in monolayer MoS2 without photoactivation
    • Section S13. Investigation of cavity effect in the PL enhancement
    • Section S14. Optical properties in monolayer MoS2 removed from MNFs
    • Section S15. Photoactivation threshold
    • Section S16. PL enhancement in monolayer MoSe2 grown on MNFs
    • Section S17. Calibration of PL QYs
    • Section S18. Simulation of lasing cavity modes
    • Section S19. Rate equation analysis
    • Section S20. Lasing performance in other resonant modes
    • Section S21. Reproducibility of our approach
    • Fig. S1. Fabrication of MNF and microbottle structures.
    • Fig. S2. Experimental setup for Raman, PL, and lasing measurements.
    • Fig. S3. Numerical simulation of oxygen dangling bonds and charge transfer.
    • Fig. S4. Experimental setup for growing monolayer MoS2 using a CVD method.
    • Fig. S5. Single-crystal nature.
    • Fig. S6. Monolayer MoS2 grown on an optical nanofiber.
    • Fig. S7. PL enhancement on a microbottle structure.
    • Fig. S8. Absorption spectra of monolayer MoS2 on a planar silica substrate and different fibers.
    • Fig. S9. High-resolution AFM scan.
    • Fig. S10. DFT calculations of the band structure of monolayer MoS2.
    • Fig. S11. Spectral responses obtained under low temperatures and different atmospheric pressures, and time-resolved PL measurement.
    • Fig. S12. Spectral changes in monolayer MoS2 without photoactivation.
    • Fig. S13. Investigation of cavity effect in the PL enhancement.
    • Fig. S14. Optical properties in monolayer MoS2 removed from MNFs.
    • Fig. S15. Pump intensity dependent PL intensity of monolayer MoS2 on different substrates.
    • Fig. S16. PL enhancement in monolayer MoSe2 grown on MNFs.
    • Fig. S17. Simulation of lasing cavity modes.
    • Fig. S18. Rate equation analysis.
    • Fig. S19. Lasing performance in other resonant modes.
    • Fig. S20. Typical lasing spectra and light in–light out curves of the lasing performance in typical five different samples.
    • Table S1. Details of electron transfer in the structure of fig. S3.
    • Table S2. Summary of the accuracy in QY measured in typical five samples.
    • Table S3. Comparison of the fitting parameters of microfiber and microbottle of rate equation analysis.
    • Table S4. Summary of the lasing thresholds of samples shown in fig. S20.
    • References (39, 40)

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