Research ArticleAPPLIED OPTICS

Surface plasmon polariton laser based on a metallic trench Fabry-Perot resonator

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Science Advances  06 Oct 2017:
Vol. 3, no. 10, e1700909
DOI: 10.1126/sciadv.1700909
  • Fig. 1 Design, fabrication, and characterization of the metallic trench SPP Fabry-Perot resonator.

    (A) Schematic diagram of a plasmonic trench resonator, consisting of an opaque Ag film forming two vertical Ag sidewalls (separation l) and a horizontal floor and illuminated with white light from free space. The schematic standing wave (red) illustrates the trapping of a resonant SPP mode. A recessed subwavelength-width slit, offset from the cavity center by distance ζ, evanescently samples the resonant SPP mode and transmits a wave of proportional intensity, IT, into the far field via a transparent substrate. (B) Finite-difference time-domain (FDTD) simulated normalized Hy-field distribution at λ0 = 647 nm assuming a cavity length of l = 2.5 μm and a vertical mirror height of h = 2 μm, along with a sampling slit (slit-sidewall taper angle = 15°, δ = 50 nm, and σ = 10 nm) positioned at ζ = 465 nm. For ease of visualization, Hy is multiplied by 100 in the plane below the cup floor (z < 0). The slit samples a small fraction of the time-varying electromagnetic field at the surface of the cavity floor by coupling to the x-directed electric field component of the SPP standing wave, also minimal at this position, converting it into a guided mode, and radiating it into the far field on the back side of the cavity. (C) Scanning electron microscope (SEM) image of the silicon mesa used for the template-stripping method to fabricate the resonator with a sidewall height of h = 3 μm and sidewall taper angles of ≤0.2°. (D) SEM image of the focused ion beam (FIB) cross section of the fabricated resonator, exposing the profile of the fabricated recessed slit underneath the cavity floor. Inset: Magnified view of the cross section of the recessed slit with an ultranarrow slit end width of σ ≈ 10 nm and a natural slit sidewall taper angle of ≈5°. (E) Measured spectrum of the transmitted light intensity outcoupled by the slit aperture IT under illumination of the open side of the cavity with incoherent white light. The measured spectrum was fitted with a Lorentzian shape to estimate the linewidth Δ and quality factor Q. (F) FDTD-simulated cross section of the magnetic field intensity |Hy(z)|2 at the center of the cavity (ζ = 0 nm), for a cavity coated with a layer of Al2O3 of a thickness of t = 8 nm and illuminated at λ0 = 653 nm. (G) Experimentally measured transmission spectra of the cavity resonator described in (D) successively coated with Al2O3 layers of increasing thickness from 0 to 8 nm, displayed in thickness increments of 1.6 nm. a.u., arbitrary units.

  • Fig. 2 Experimental characterization of the SPP Fabry-Perot laser.

    (A) Schematic diagram of the metallic trench SPP laser, consisting of a gain medium–decorated Fabry-Perot resonator, illuminated with a pump beam (blue-green) of intensity IP. The schematic standing wave (red) illustrates a trapped SPP lasing mode. A recessed slit, offset from the cavity center by distance ζ′, evanescently samples the lasing SPP mode and transmits a wave of proportional intensity, IE, into the far field. (B) Evolution of the spectrum of IE with increasing pump intensity IP. Inset: SEM image of the FIB cross section of the fabricated SPP laser. A thin layer of platinum was deposited on top of the device to assist with the FIB cross section. The orange dashes outline the boundaries of the spin-coated gain medium. (C) Emission linewidth, Δ, and (D) log scale light-light curve of the outcoupled light emission, IE, versus IP. The uncertainties in the measurements of IP are 1 SD based on the measurement of pump energy from 100 consecutive pulses. (C) Inset: IE versus IP on a linear scale indicating the lasing threshold Ith.

  • Fig. 3 Experimental characterization of grating-decorated SPP-pumped SPP laser.

    (A) Schematic diagram of an SPP laser decorated with a low-profile grating of periodicity p, peak height d with respect to cavity floor, and offset ξ from the cavity center, illuminated with a pump beam (blue-green) of intensity IP. The schematic blue-green and red standing waves illustrate the grating-coupled pump SPPs and cavity-trapped lasing SPPs, respectively. A recessed slit, offset from the cavity center by distance ζ″, evanescently samples the lasing SPP mode and transmits a wave of proportional intensity, IE, into the far field. (B) Tilted-view SEM image of the template-stripped cavity resonator illustrating the grating-decorated cavity floor. Inset: Top-view SEM image of the device, where the nominal location of the buried recessed sampling slit is indicated by the dotted black line. (C) Evolution of the SPP emission linewidth Δ with increasing pump intensity IP. Inset: Representative emission spectra of IE for IP below (IP = 4.0 MW/cm2) and above (IP = 6.0 MW/cm2) the lasing threshold (Ith = 5.6 MW/cm2). (D) SPP lasing turn-on characteristics IE versus IP on a linear scale for grating-decorated and grating-free devices as a function of in-plane pump polarization.

  • Fig. 4 Active refractive index sensing using the SPP lasing mode and beaming of the lasing emission.

    (A) FDTD-simulated cross section of magnetic field intensity |Hy(z)|2 at the center of the lasing cavity described in section S7, coated with layers of gain medium with respective thicknesses of dG = 50 nm (at λpeak = 645 nm) and 260 nm (at λpeak = 630 nm). (B) FDTD-simulated lasing wavelength shifts, δλpeak (relative to λpeak at n = 1), normalized to the corresponding experimentally measured lasing linewidth Δ = 0.24 nm (Fig. 2C), are plotted as a function of n. For reference, the simulated measured peak resonance wavelength shift (relative to λpeak at n = 1) of a passive cavity (dye-free), δλpeak, normalized to the corresponding simulated resonance linewidth Δ = 1.8 nm, is also plotted. (C) FDTD-simulated normalized magnetic field intensity distribution |Hy|2 at λE = 630 nm for the lasing cavity described in section S8 (dG = 260 nm) pumped at λP = 480 nm with an intensity of IP = 2.0 MW/cm2. For ease of visualization, |Hy|2 is multiplied by 103 in the plane below the cup floor (z < 0). (D) Same as (C), with the addition of an outcoupling beaming grating.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/10/e1700909/DC1

    section S1. Minimizing SPP propagation losses

    section S2. Minimizing SPP reflection losses

    section S3. Optimization of the length of the SPP Fabry-Perot cavity

    section S4. Optimization of the sampling-slit profile

    section S5. Fabrication process and optical characterization

    section S6. DCM material gain and spontaneous emission spectrum

    section S7. Fitting laser rate equation to experimental SPP emission

    section S8. Numerical simulation of the SPP Fabry-Perot laser

    section S9. Modal analysis of SPP modes in the presence of a thin dielectric layer

    section S10. Morphology characterization of the tapered coupling grating

    section S11. Optimization of the SPP-pumped SPP laser

    section S12. Simulation of grating-coupled pump SPPs

    fig. S1. Surface morphology characterization of a template-stripped Ag film.

    fig. S2. Propagation decay length of SPPs propagating on a template-stripped Ag-air interface.

    fig. S3. Optimization of SPP reflection losses.

    fig. S4. Design of an SPP resonator based on a lossy Fabry-Perot cavity model.

    fig. S5. Optimization of the sampling-slit profile.

    fig. S6. Spontaneous emission spectrum of PMMA:DCM.

    fig. S7. Laser emission as a function of varying the spontaneous emission factor.

    fig. S8. FDTD simulation of the SPP Fabry-Perot laser.

    fig. S9. Dispersion of the lasing SPP mode.

    fig. S10. Morphology characterization of the inverse grating structure patterned on the Si mesa.

    fig. S11. Grating optimization of the SPP-pumped SPP laser.

    fig. S12. Simulated steady-state intensity profiles for grating-decorated and grating-free Ag surfaces coated with a 260-nm-thick four-level gain medium.

    References (36, 37)

  • Supplementary Materials

    This PDF file includes:

    • section S1. Minimizing SPP propagation losses
    • section S2. Minimizing SPP reflection losses
    • section S3. Optimization of the length of the SPP Fabry-Perot cavity
    • section S4. Optimization of the sampling-slit profile
    • section S5. Fabrication process and optical characterization
    • section S6. DCM material gain and spontaneous emission spectrum
    • section S7. Fitting laser rate equation to experimental SPP emission
    • section S8. Numerical simulation of the SPP Fabry-Perot laser
    • section S9. Modal analysis of SPP modes in the presence of a thin dielectric layer
    • section S10. Morphology characterization of the tapered coupling grating
    • section S11. Optimization of the SPP-pumped SPP laser
    • section S12. Simulation of grating-coupled pump SPPs
    • fig. S1. Surface morphology characterization of a template-stripped Ag film.
    • fig. S2. Propagation decay length of SPPs propagating on a template-stripped Ag-air interface.
    • fig. S3. Optimization of SPP reflection losses.
    • fig. S4. Design of an SPP resonator based on a lossy Fabry-Perot cavity model.
    • fig. S5. Optimization of the sampling-slit profile.
    • fig. S6. Spontaneous emission spectrum of PMMA:DCM.
    • fig. S7. Laser emission as a function of varying the spontaneous emission factor.
    • fig. S8. FDTD simulation of the SPP Fabry-Perot laser.
    • fig. S9. Dispersion of the lasing SPP mode.
    • fig. S10. Morphology characterization of the inverse grating structure patterned on the Si mesa.
    • fig. S11. Grating optimization of the SPP-pumped SPP laser.
    • fig. S12. Simulated steady-state intensity profiles for grating-decorated and grating-free Ag surfaces coated with a 260-nm-thick four-level gain medium.
    • References (36, 37)

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