Research ArticleNANOMATERIALS

A customizable class of colloidal-quantum-dot spasers and plasmonic amplifiers

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Science Advances  22 Sep 2017:
Vol. 3, no. 9, e1700688
DOI: 10.1126/sciadv.1700688
  • Fig. 1 Structure of colloidal quantum dot spaser.

    (A) Top-view electron micrograph of a plasmonic cavity: Two ~600-nm-tall Ag block reflectors positioned 10 μm apart on an ultrasmooth Ag surface. The blocks can be designed and placed at will. Plasmons propagate between the reflectors to create a cavity. (B) Tilted view of a functional device. NanoDrip printing is used to deposit a stripe (~100 nm thick and ~2 μm wide) of colloidal quantum dots between the reflectors. (C) Tilted view of a square array of nine plasmonic cavities (50 μm apart). The three cavities in the right column are empty. The rest contain a quantum dot stripe, as in (B).

  • Fig. 2 Design and characterization of plasmonic cavities containing quantum dots.

    (A) The plasmonic reflectors are designed with a radius of curvature twice the cavity length (R = 2L cavity) and include a parabolic correction. Plasmons emitted into the cavity are spatially confined in a stable cavity mode, depicted in red. (B) When ~100 quantum dots are placed in the cavity and weakly photoexcited, they emit into plasmonic cavity modes. The experimental cavity spectrum, measured by detecting plasmons scattered into the far field at the outer edge of one of the reflectors, is shown in red. The calculated spectra of the stable plasmonic cavity modes are in gray (see sections S4 and S5). The slight offset is due to a slightly smaller experimental cavity length (9.965 μm). (C) When a cavity filled with quantum dots as in (D) is weakly photoexcited, modes appear as ripples in the cavity spectrum [collected as in (B)] only at longer wavelengths due to losses from the quantum dot film. Calculated modes (neglecting quantum dot absorption) are in gray. a.u., arbitrary units. (D) Top-view electron micrograph of the cavity for comparison with (A). The quantum dot stripe is printed to maximize spatial overlap with the cavity mode. (E) Under pulsed excitation (130 μJ/cm2), a small feature rises on top of the cavity spectrum at the position of a calculated plasmonic mode. Right inset: Real-space image (false color) of the emission from the quantum dot stripe. The two bright spots are due to scattering off the reflectors. (F) At higher excitation (250 μJ/cm2), the small peak in (E) narrows and increases in intensity. Right inset: Real-space image as in (E). The device exhibits decreased emission within the stripe and increased signal at the reflectors. The changes in the spectra and images in (E) and (F) are indicative of the onset of spasing. Cartoons in (B) and (D) to (F) depict the optical excitation and collection processes.

  • Fig. 3 Analysis of quantum dot spasers.

    (A) Quantum dot samples with emission centered at 602, 625, and 633 nm were used to fabricate spasers. Cavity spectra for each are plotted for three excitation intensities, one below and two above threshold. (B) The 633 device from (A) exhibited a single spasing mode above threshold (red curve). Typical linewidths were 2 meV (0.65 nm, Q ~ 1000). The broad photoluminescence spectrum from quantum dots on flat Ag (outside the cavity) is shown for comparison (gray curve). (C) The input-output power plot of the 633 device (red points) reveals an inflection at ~180 μJ/cm2. Spasing thresholds as low as ~100 μJ/cm2 were observed. The spectral full-width at half-maximum (blue points) decreases dramatically at the same inflection point, consistent with the onset of spasing. (D) When the 633 device is excited at very high intensities (~1000 μJ/cm2), a progression of spasing modes is observed. These occur within the gain envelopes of the exciton/biexciton (X/BX) centered at ~640 nm and the multiexcitons (MX) centered at ~590 nm. The ~160-meV spacing between the envelopes is consistent with previous measurements (35). The broad quantum dot spectrum (solid gray curve) from (B) and the expected plasmonic modes for a film with n = 1.60 (dashed gray curve) are shown for comparison.

  • Fig. 4 Extraction, amplification, and focusing of plasmons generated by quantum dot spasers.

    (A) Top-view electron micrograph of a spaser with a printed quantum dot stripe and an elongated reflector (11.5° taper) to guide and nanofocus the spaser signal. (B) Real-space image (false color) of the device in (A) above threshold. The contrast of the image to the right of the vertical dashed line is enhanced (104) to show the plasmons focused at the tip. (C and D) Spectra measured by collecting plasmons scattered at the outer reflector edge (C) and the tip (D) confirm that the spasing signal is guided and focused. (E and F) Real-space images (false color) of a cavity with an elongated reflector (14° taper) coated with a ~150-nm-thick drop-casted quantum dot film, excited below and above threshold, respectively. The intensity of (E), which is multiplied by 3, is fairly uniform across the device with some broadband focusing at the tip. In (F), plasmons generated by the cavity are amplified and focused at the tip. (G and H) Spectra measured by collecting plasmons scattered at the inner reflector edge (G) and the tip (H) confirm that a single spasing mode is selectively guided, amplified, and focused. The signal in (H) (with amplification) is 1800 times greater than that in (D) (without amplification).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/9/e1700688/DC1

    section S1. Computation of mode stability

    section S2. Beam waist calculation and ray tracing

    section S3. Cavity design

    section S4. Plasmonic Fabry-Perot calculation for cavities

    section S5. Calculation of propagating modes in silver/quantum dot/air stack

    section S6. Optical characterization: Lifetime measurements

    section S7. Spasing thresholds

    section S8. Modal gain, plasmonic loss, and quantum dot material gain

    section S9. Estimation of spasing threshold

    table S1. Parameters for threshold calculation.

    fig. S1. Calculated electric field profiles.

    fig. S2. Calculated effective indices for plasmonic and photonic modes.

    fig. S3. AFM topographic image of a printed quantum dot stripe.

    fig. S4. Lifetime measurements.

    fig. S5. Measuring amplifier gain.

    fig. S6. Amplified spontaneous emission at high excitation intensity.

    References (3840)

  • Supplementary Materials

    This PDF file includes:

    • section S1. Computation of mode stability
    • section S2. Beam waist calculation and ray tracing
    • section S3. Cavity design
    • section S4. Plasmonic Fabry-Perot calculation for cavities
    • section S5. Calculation of propagating modes in silver/quantum dot/air stack
    • section S6. Optical characterization: Lifetime measurements
    • section S7. Spasing thresholds
    • section S8. Modal gain, plasmonic loss, and quantum dot material gain
    • section S9. Estimation of spasing threshold
    • table S1. Parameters for threshold calculation.
    • fig. S1. Calculated electric field profiles.
    • fig. S2. Calculated effective indices for plasmonic and photonic modes.
    • fig. S3. AFM topographic image of a printed quantum dot stripe.
    • fig. S4. Lifetime measurements.
    • fig. S5. Measuring amplifier gain.
    • fig. S6. Amplified spontaneous emission at high excitation intensity.
    • References (38–40)

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