  Plasmonic nanoparticles resonantly couple to and confine light below the
diffraction limit. This mechanism has enabled a modern renaissance in optical
materials, with potential applications ranging from sensing and circuitry to
renewable energies and medicines. However, these plasmonic materials are
typically constrained to dilute liquids or solid two-dimensional surfaces,
thereby limiting their possibilities. Here, we experimentally demonstrate a
plasmonic aerosol by transitioning liquid suspensions of gold nanorods into the
gas phase and simultaneously measuring their optical spectra. By measuring and
modeling the evolution of the longitudinal absorbance peak of the nanorods from
the liquid to the gas phase, we find that the aerosols are optically
homogeneous and thermodynamically stable. We show that by tailoring the aspect
ratio of the nanorods, the aerosol absorbance peak is tunable from visible to
mid-wave infrared wavelengths. We find the sensitivity of the absorbance peak
wavelength to changes in the refractive index of the gas depends linearly on
the aspect ratio and can be estimated from the geometric and material
properties of the nanorod. For high aspect ratio nanorods the sensitivity
becomes extremely large, which may be useful in aiding geoengineering
challenges. This work establishes plasmonic aerosols, potentially enabling
exciting opportunities for fundamental and applied research.
