Explosives detection in a lasing plasmon nanocavity

Journal name:
Nature Nanotechnology
Volume:
9,
Pages:
600–604
Year published:
DOI:
doi:10.1038/nnano.2014.135
Received
Accepted
Published online

Perhaps the most successful application of plasmonics to date has been in sensing, where the interaction of a nanoscale localized field with analytes leads to high-sensitivity detection in real time and in a label-free fashion1, 2, 3, 4, 5, 6, 7, 8, 9. However, all previous designs have been based on passively excited surface plasmons, in which sensitivity is intrinsically limited by the low quality factors induced by metal losses. It has recently been proposed theoretically that surface plasmon sensors with active excitation (gain-enhanced) can achieve much higher sensitivities due to the amplification of the surface plasmons10, 11, 12. Here, we experimentally demonstrate an active plasmon sensor that is free of metal losses and operating deep below the diffraction limit for visible light. Loss compensation leads to an intense and sharp lasing emission that is ultrasensitive to adsorbed molecules. We validated the efficacy of our sensor to detect explosives in air under normal conditions and have achieved a sub-part-per-billion detection limit, the lowest reported to date for plasmonic sensors7, 13, 14, 15, 16, 17, 18 with 2,4-dinitrotoluene and ammonium nitrate. The selectivity between 2,4-dinitrotoluene, ammonium nitrate and nitrobenzene is on a par with other state-of-the-art explosives detectors19, 20. Our results show that monitoring the change of the lasing intensity is a superior method than monitoring the wavelength shift, as is widely used in passive surface plasmon sensors. We therefore envisage that nanoscopic sensors that make use of plasmonic lasing could become an important tool in security screening and biomolecular diagnostics.

At a glance

Figures

  1. Schematic, SEM image, simulated field distribution and transmission electron microscope (TEM) image of an active plasmon nanosensor.
    Figure 1: Schematic, SEM image, simulated field distribution and transmission electron microscope (TEM) image of an active plasmon nanosensor.

    a, Sensing is based on the intensity change in stimulated emission from a lasing plasmon nanocavity with subwavelength electromagnetic field confinement, where the semiconductor slab provides optical gain as well as acts as a sensing material. b, SEM image of the device, which consists of a CdS nanoslab (thickness, d1, 50 nm; length, 600 nm) on top of a Ag film, separated by an 8-nm d2 low-permittivity MgF2 layer. c, Left: Electric field distribution in a cross-section of the electromagnetic nanoslab cavity mode simulated in three-dimensional space. Scale bar, 100 nm. Right: Electric field amplitude |E| distribution along the dashed red line in the left panel. d, High-resolution TEM image (top view) of the CdS slab, showing the single-crystal structure and atomic-level smooth surface, which are crucial for the optical performance of the device. Scale bar, 2 nm. DNT, 2,4-dinitrotoluene.

  2. Characterization of the active plasmon sensor.
    Figure 2: Characterization of the active plasmon sensor.

    a, Experimental set-up. The device is placed in a sealed chamber, with two ports for gas exchange controlled by mass flow controllers (MFCs), and an optical window for both pumping and signal collection. b, Pump intensity dependence of the total output power and linewidth of the device. The stimulated emission above the lasing threshold has stronger intensity, higher slope and much narrower linewidth than the spontaneous emission. c, Measured spectra of the lasing plasmon cavity under N2 and 8 ppb 2,4-dinitrotoluene (DNT). d, Red diamond: continuous trace of emission intensities of the active plasmon nanosensor when delivering DNT vapour at concentrations of 1, 2, 4 and 8 ppb. Black line: guide to the eye. Green circles: the tracked lasing peak wavelength obtained by Gaussian fitting of the spectra. There is no appreciable change in the peak wavelength at various DNT concentrations, which indicates that directly monitoring the lasing intensity has superior performance than monitoring the index-change-induced peak wavelength shift in active plasmon sensors.

  3. Time-resolved emission of the sensor measured at the spontaneous emission region to investigate dynamic processes of the photon-excited carrier relaxation.
    Figure 3: Time-resolved emission of the sensor measured at the spontaneous emission region to investigate dynamic processes of the photon-excited carrier relaxation.

    Two typical time-resolved spontaneous emissions under N2 and 100 ppb 2,4-dinitrotoluene (DNT). After the introduction of DNT vapour, the emission intensity from the device increased, following the same trend as the stimulated emission region. Meanwhile, the measured emission lifetime became longer. Both the intensity and lifetime changes to the spontaneous emission with DNT suggest that the intensity increase is mainly due to the surface recombination velocity modification.

  4. Detection of 2,4-dinitrotoluene, ammonium nitrate and nitrobenzene in air.
    Figure 4: Detection of 2,4-dinitrotoluene, ammonium nitrate and nitrobenzene in air.

    ac, Continuous traces of lasing intensities at different vapour concentrations of DNT (a), AN (b) and NB (c), diluted by air. d, Calibration curves for the three analytes. The sensitivities defined as the slope of the calibration curves for DNT, AN and NB are 1.2%/ppb, 6.1%/ppb and 0.4%/ppm, respectively. The detection limits obtained for DNT, AN and NB are 0.67 ppb, 0.4 ppb and 7.2 ppm, respectively. The device has a specific response to different target molecules depending on their specific electron deficiencies, because our sensing is mainly based on the surface recombination velocity modification, which is sensitive to the electron deficiency of adsorbed molecules.

  5. Detection of explosive molecules via spontaneous emission.
    Figure 5: Detection of explosive molecules via spontaneous emission.

    a, Continuous trace of spontaneous emission intensities at different 2,4-dinitrotoluene (DNT) vapour concentrations diluted by air. b, Calibration curve for DNT detection via spontaneous emission. The sensitivity and detection limit are ∼0.23%/ppb and ∼14 ppb, respectively. c, Spontaneous emission and lasing emission of the sensor device studied in Figs 4 and 5. When ΔI is used as a direct measure of the signal, the sensitivity of lasing emission is approximately 300 times higher than that of spontaneous emission.

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Author information

  1. These authors contributed equally to this work

    • Ren-Min Ma &
    • Sadao Ota

Affiliations

  1. NSF Nanoscale Science and Engineering Centre, 3112 Etcheverry Hall, University of California, Berkeley, California 94720, USA

    • Ren-Min Ma,
    • Sadao Ota,
    • Yimin Li,
    • Sui Yang &
    • Xiang Zhang
  2. Materials Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA

    • Xiang Zhang

Contributions

R-M.M. conducted theoretical simulations. R-M.M. and S.O. performed device fabrication and optical measurements. R-M.M. and S.O. wrote the manuscript. All authors discussed the results and contributed to the manuscript revision. X.Z. guided the research.

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The authors declare no competing financial interests.

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