Research ArticlePLASMONICS

Imaging the dark emission of spasers

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Science Advances  14 Apr 2017:
Vol. 3, no. 4, e1601962
DOI: 10.1126/sciadv.1601962
  • Fig. 1 Imaging the dark emission of spasers by leakage radiation microscopy.

    (A) Left: Electric field distribution of a spaser in a two-dimensional simulation, where we can see that most emission of the spaser couples to propagating surface plasmons. Right: The surface plasmon mode profiles in and out of the spaser cavity. (B) Schematic of leakage radiation microscopy for imaging emission of spasers in spatial (top) and momentum (bottom) spaces.

  • Fig. 2 Simultaneously obtained emission images of a spaser in spatial, frequency, and momentum spaces.

    (A) SEM image (inset) and the atomic force microscopy–measured profile of the spaser device. The length, width, and thickness are about 4.06 μm, 2 μm, and 70 nm, respectively. (B) Emission spectrum of the spaser shows that the device experiences a transition from spontaneous emission to full laser oscillation with an increase in pump power. (C) Integrated light output versus pump response. (D and E) Spatial space (D) and momentum space (E) images of the spaser emission obtained via leakage radiation microscopy (false color). (F) Azimuthal distribution of the spaser emission obtained from spatial space (black) and momentum space (red) images.

  • Fig. 3 Lasing plasmonic mode of a spaser.

    (A and B) Three-dimensional full wave simulated surface plasmon mode (A) supported by the plasmon cavity depicted in Fig. 2 that matched the experimental one (B) (false color). (C) Azimuthal distribution of the spaser emission obtained from spatial space images (black) and the full wave simulation (blue). (D) Electric distribution of the simulated mode on the zy plane in log scale, which exhibits the signature of a plasmonic mode.

  • Fig. 4 Determinants of surface plasmon generation efficiency.

    (A and B) The full wave simulated Ex (A) and Ez (B) field distribution of a spaser cavity with a length of 4.06 μm. (C) The fast Fourier–transformed Ex and Ez fields of the leakage surface plasmons 5 μm below the Ag film, indicated by the green broken lines in (A) and (B). (D) The fast Fourier–transformed total electric field (black) and experimental result obtained from the momentum space image of the spaser depicted in Fig. 2 (red). (E) Amplitude and phase of Ez/Ex of the fundamental plasmonic cavity mode at various nanowire diameters (black curves) and the surface plasmon mode supported by the air/Ag interface outside the cavity (red curves). (F) Surface plasmon generation efficiency (η) of nanowire cavities with different lengths, which have diameters of 80 and 130 nm. η is calculated by averaging the different orders of longitudinal modes of fundamental plasmonic modes around a wavelength of 700 nm. Error bar represents the SD of η of the different longitudinal modes. Inset: The simulated electric field distribution of the fundamental plasmonic mode.

  • Fig. 5 Nanowire spasers with high surface plasmon generation efficiency.

    (A) Surface plasmon generation efficiencies of nanowire spasers with varied diameters operated at the fundamental plasmonic mode. The length of the cavity is 8.4 μm. (B) Spectrum, SEM image, and light-light curve of a nanowire spaser with a length of 8.4 μm and a diameter of 212 nm. (C) Spatial space image of the nanowire spaser emission depicted in (B). (D to G) Electric field distribution of four transverse modes supported by the 212-nm-diameter and 8.4-μm-long cavity in three-dimensional simulation. The fundamental plasmonic mode has a similar emission pattern to the experimental one, whereas the other three modes differ in their pronounced emission perpendicular to the nanowire long axis. (H) Simulated electric field distribution (yz plane) of the four modes depicted in (D) to (G); the white arrow shows their polarization distribution. (I) Effective refractive index with various diameters in two-dimensional simulation of the four modes depicted in (D) to (G).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/4/e1601962/DC1

    section S1. Schematic of spasers

    section S2. Imaging principle

    section S3. Optical setup

    section S4. Calibration in momentum space

    section S5. Lasing spectra and emission patterns at varied pump powers

    section S6. Interference in the momentum space image

    section S7. Interference in the spatial space image

    section S8. Surface plasmon generation efficiency

    section S9. Preferentially scattering polarization

    section S10. Peak value of η at a diameter of around 210 nm

    section S11. Transverse modes supported by nanowire spaser cavity

    section S12. Momentum space image of a nanowire spaser

    fig. S1. Schematics of spaser configurations studied in this work.

    fig. S2. Schematics of imaging principles.

    fig. S3. Schematic of leakage radiation microscopy setup.

    fig. S4. Calibration in the momentum spaces.

    fig. S5. Spectrum and spatial emission patterns of the spaser shown in Fig. 2 under different pump powers.

    fig. S6. Interference of scattered photon emission in the momentum spaces.

    fig. S7. Interference in the spatial space image.

    fig. S8. Calculation of surface plasmon generation efficiency.

    fig. S9. Effective index for the calculation of the Ez/Ex ratio of a surface plasmon cavity mode.

    fig. S10. The proportion of surface plasmon mode in HSP1 mode versus nanowire diameter.

    fig. S11. Four transverse modes supported by a nanowire cavity with a diameter of 212 nm on the Ag film in the two- and three-dimensional simulation.

    fig. S12. Spatial, momentum, and frequency space images of a nanowire spaser.

  • Supplementary Materials

    This PDF file includes:

    • section S1. Schematic of spasers
    • section S2. Imaging principle
    • section S3. Optical setup
    • section S4. Calibration in momentum space
    • section S5. Lasing spectra and emission patterns at varied pump powers
    • section S6. Interference in the momentum space image
    • section S7. Interference in the spatial space image
    • section S8. Surface plasmon generation efficiency
    • section S9. Preferentially scattering polarization
    • section S10. Peak value of η at a diameter of around 210 nm
    • section S11. Transverse modes supported by nanowire spaser cavity
    • section S12. Momentum space image of a nanowire spaser
    • fig. S1. Schematics of spaser configurations studied in this work.
    • fig. S2. Schematics of imaging principles.
    • fig. S3. Schematic of leakage radiation microscopy setup.
    • fig. S4. Calibration in the momentum spaces.
    • fig. S5. Spectrum and spatial emission patterns of the spaser shown in Fig. 2 under different pump powers.
    • fig. S6. Interference of scattered photon emission in the momentum spaces.
    • fig. S7. Interference in the spatial space image.
    • fig. S8. Calculation of surface plasmon generation efficiency.
    • fig. S9. Effective index for the calculation of the Ez/Ex ratio of a surface plasmon cavity mode.
    • fig. S10. The proportion in surface plasmon mode with HSP1 mode versus nanowire diameter.
    • fig. S11. Four transverse modes supported by a nanowire cavity with a diameter of 212 nm on the Ag film in the two- and three-dimensional simulation.
    • fig. S12. Spatial, momentum, and frequency space images of a nanowire spaser.

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