Real-time tunable lasing from plasmonic nanocavity arrays.

Plasmon lasers can support ultrasmall mode confinement and ultrafast dynamics with device feature sizes below the diffraction limit. However, most plasmon-based nanolasers rely on solid gain materials (inorganic semiconducting nanowire or organic dye in a solid matrix) that preclude the possibility of dynamic tuning. Here we report an approach to achieve real-time, tunable lattice plasmon lasing based on arrays of gold nanoparticles and liquid gain materials. Optically pumped arrays of gold nanoparticles surrounded by liquid dye molecules exhibit lasing emission that can be tuned as a function of the dielectric environment. Wavelength-dependent time-resolved experiments show distinct lifetime characteristics below and above the lasing threshold. By integrating gold nanoparticle arrays within microfluidic channels and flowing in liquid gain materials with different refractive indices, we achieve dynamic tuning of the plasmon lasing wavelength. Tunable lattice plasmon lasers offer prospects to enhance and detect weak physical and chemical processes on the nanoscale in real time.

Lasing action in strongly coupled plasmonic nanocavity arrays.

Periodic dielectric structures are typically integrated with a planar waveguide to create photonic band-edge modes for feedback in one-dimensional distributed feedback lasers and two-dimensional photonic-crystal lasers. Although photonic band-edge lasers are widely used in optics and biological applications, drawbacks include low modulation speeds and diffraction-limited mode confinement. In contrast, plasmonic nanolasers can support ultrafast dynamics and ultrasmall mode volumes. However, because of the large momentum mismatch between their nanolocalized lasing fields and free-space light, they suffer from large radiative losses and lack beam directionality. Here, we report lasing action from band-edge lattice plasmons in arrays of plasmonic nanocavities in a homogeneous dielectric environment. We find that optically pumped, two-dimensional arrays of plasmonic Au or Ag nanoparticles surrounded by an organic gain medium show directional beam emission (divergence angle <1.5° and linewidth <1.3 nm) characteristic of lasing action in the far-field, and behave as arrays of nanoscale light sources in the near-field. Using a semi-quantum electromagnetic approach to simulate the active optical responses, we show that lasing is achieved through stimulated energy transfer from the gain to the band-edge lattice plasmons in the deep subwavelength vicinity of the individual nanoparticles. Using femtosecond-transient absorption spectroscopy, we verified that lattice plasmons in plasmonic nanoparticle arrays could reach a 200-fold enhancement of the spontaneous emission rate of the dye because of their large local density of optical states.

Spatial confinement of electromagnetic hot and cold spots in gold nanocubes.

We report a near-field imaging study of colloidal gold nanocubes. This is accomplished through a photochemical imaging method in which molecular displacements are vectorial in nature, enabling sensitivity to the polarization of the optical near-field of the nanocubes. We analyze the confinement of both electromagnetic hot and "cold" spots with a resolution of λ/35 and emphasize the particularly high spatial confinement of cold spots. The concept of a cold spot complements the well-known electromagnetic hot spot but can have significant advantages. The application of the ultraconfined cold spots to high resolution imaging and spectroscopy is discussed.

Modeling of metallic nanostructures embedded in liquid crystals: application to the tuning of their plasmon resonance.

We numerically investigate arrays of metallic nanoparticles deposited on a glass substrate and covered by a liquid-crystal material. Extinction spectra at normal incidence are computed using the finite-difference time-domain method, and we show that by rotating the optical axis around an axis orthogonal to the main direction of illumination, it is possible to tune the resonance of the system according to a simple law. The spectral width of the tunability is studied as a function of different parameters.