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.

Plasmonic lasers exploit strong electromagnetic field confinement at dimensions well below the diffraction limit. However, lasing from an electromagnetic hot spot supported by discrete, coupled metal nanoparticles (NPs) has not been explicitly demonstrated to date. We present a new design for a room-temperature nanolaser based on three-dimensional (3D) Au bowtie NPs supported by an organic gain material. The extreme field compression, and thus ultrasmall mode volume, within the bowtie gaps produced laser oscillations at the localized plasmon resonance gap mode of the 3D bowties. Transient absorption measurements confirmed ultrafast resonant energy transfer between photoexcited dye molecules and gap plasmons on the picosecond time scale. These plasmonic nanolasers are anticipated to be readily integrated into Si-based photonic devices, all-optical circuits, and nanoscale biosensors.

The optical and morphological characteristics of vanadium dioxide nanoparticles and thin films during their nucleation and growth phases have been studied by correlating the temperature and sharpness of the transition with the processing parameters. Thermal annealing results in grain growth and improved crystallinity. Normally, larger crystallites show smaller hysteresis, as there is a greater probability of finding a nucleating defect in the larger volume. But at the same time, this improved crystal perfection, which accompanies the thermal annealing and grain growth, tends to a larger hysteresis, as there are fewer nucleating defects within the volume. We show that the width and shape of the hysteresis cycle are thus determined by the competing effects of crystallinity and grain size.

We investigate the dielectric properties of a thin VO 2 film in the terahertz frequency range in the vicinity of the semiconductor-metal phase transition. Phase-sensitive broadband spectroscopy in the frequency region below the phonon bands of VO 2 gives insight into the conductive properties of the film during the phase transition. We compare our experimental data with models proposed for the evolution of the phase transition. The experimental data show that the phase transition occurs via the gradual growth of metallic domains in the film, and that the dielectric properties of the film in the vicinity of the transition temperature must be described by effective-medium theory. The simultaneous measurement of both transmission and phase shift allows us to show that Maxwell-Garnett effective-medium theory, coupled with the Drude conductivity model, can account for the observed behavior, whereas the widely used Bruggeman effective-medium theory is not consistent with our findings. Our results show that even at temperatures significantly above the transition temperature the formation of a uniform metallic phase is not complete.

This paper describes 3D nanohole arrays whose high optical transmission is mediated more by localized surface plasmon (LSP) excitations than by surface plasmon polaritons (SPPs). First, LSPs on 3D hole arrays lead to optical transmission an order of magnitude higher than 2D planar hole arrays. Second, LSP-mediated transmission is broadband and more tunable than SPPenhanced transmission which is restricted by Bragg coupling. Third, for the first time, two types of surface plasmons can be selectively excited and manipulated on the same plasmonic substrate. This new plasmonic substrate fabricated by high-throughput nanolithography techniques paves the way for cutting-edge optoelectronic and biomedical applications.Keywords nanohole array; enhanced optical transmission; plasmonics; molding; surface plasmon polariton; localized surface plasmon Surface plasmon polaritons (SPPs) mediate light-matter interactions in subwavelength hole arrays and are the dominant mechanism for enhanced optical transmission,1 , 2 plasmonic focusing,3 and the plasmon Talbot effect.4 , 5 Most work to date has focused on manipulating SPPs by tuning the hole shape,6 , 7 the array symmetry,8 , 9 or the superlattice geometry.10 , 11 However, the tailoring of only the in-plane, two-dimensional (2D) structure has limited the excitation of strong localized surface plasmons (LSPs) within individual holes. Here we report that LSPs from three-dimensional (3D) nanoholes enhance transmission more than SPPs from the same nanohole array. Subwavelength hole arrays with anisotropic hole shapes were fabricated by metal deposition at oblique angles on nanopyramidal templates. Localized resonances supported by 3D holes produced broadband optical transmission with intensities an order of magnitude higher than that from 2D planar nanohole arrays. Moreover, in contrast to SPP-mediated enhanced transmission,12 , 13 LSPenhanced optical transmission at different wavelengths and with different dispersion properties can be tuned by controlling the shape of the 3D hole. The discovery of broadband-tunable LSP and SPP resonances on the same plasmonic substrate provides new opportunities in optoelectronics and optical biosensing.* To whom correspondence should be addressed. todom@northwestern.edu. Figure 1 summarizes how metal films perforated with 3D nanoholes were fabricated from templates of nanopyramidal pits followed by metal deposition at oblique angles. Protruding 3D nanohole arrays (tapered ends pointing away from the substrate) were fabricated by deposition of gold (Au) at specific angles (α) on a silicon (Si) template followed by template-stripping14 using UV-curable polyurethane (PU) as the transfer material (Fig. 1a). PU replicas of the Si template were formed by first molding poly(dimethylsiloxane) (PDMS) against the Si nanopatterns and then molding the patterned PDMS against PU (Fig. 1b). Recessed 3D hole arrays (tapered ends pointing toward the substrate) were generated by the angled deposition of Au directly on the PU template (Fig. 1b). For bot...

This paper reports the manipulation of surface plasmon polaritons (SPPs) in a liquid plasmonic metal by changing its physical phase. Dynamic properties were controlled by solid-to-liquid phase transitions in 1D Ga gratings that were fabricated using a simple molding process. Solid and liquid phases were found to exhibit different plasmonic properties, where light coupled to SPPs more efficiently in the liquid phase. We exploited the supercooling characteristics of Ga to access plasmonic properties associated with the liquid phase over a wider temperature range (up to 30 °C below the melting point of bulk Ga). Ab initio density functional theory-molecular dynamic calculations showed that the broadening of the solid-state electronic band structure was responsible for the superior plasmonic properties of the liquid metal.

This article describes the angle-dependent optical responses of 2D metal−insulator−metal (MIM) nanocavity arrays. Through a combination of soft nanolithography and template stripping, we fabricated arrays of plasmonic MIM nanostructures with subwavelength spacings over square centimeter areas. We controlled the coupling between the localized surface plasmon and guided modes as well as engineered the optical band structure by tuning the insulator thickness. Rabi splitting of hybridized modes strongly depended on the spatial overlap of the near-fields of the localized and guided modes.

We demonstrate the modulation of the transmission of near-infrared light through a periodic array of subwavelength apertures in Ag–VO2 and Au–VO2 double-layer films using the semiconductor-to-metal phase transition in VO2. The transmitted intensity ratio increases by a factor of 8 as the VO2 goes from the semiconductor to the metal phase. We attribute this modulation to the switchable dielectric-permittivity contrast between the air-filled holes in the array and the surrounding VO2 material, a conjecture that is semiquantitatively confirmed by simulation.