Silver Nanoparticle Chains for Ultra-Long-Range Plasmonic Waveguides for Nd3+ Fluorescence.

Plasmonic waveguides have been shown to be a promising approach to confine and transport electromagnetic energy beyond the diffraction limit. However, ohmic losses generally prevent their integration at micrometric or millimetric scales. Here, we present a gain-compensated plasmonic waveguide based on the integration of linear chains of Ag nanoparticles on an optically active Nd3+-doped solid-state gain medium. By means of dual confocal fluorescence microscopy, we demonstrate long-range optical energy propagation due to the near-field coupling between the plasmonic nanostructures and the Nd3+ ions. The subwavelength fluorescence guiding is monitored at distances of around 100 µm from the excitation source for two different emission ranges centered at around 900 nm and 1080 nm. In both cases, the guided fluorescence exhibits a strong polarization dependence, consistent with the polarization behavior of the plasmon resonance supported by the chain. The experimental results are interpreted through numerical simulations in quasi-infinite long chains, which corroborate the propagation features of the Ag nanoparticle chains at both excitation (λexc = 590 nm) and emission wavelengths. The obtained results exceed by an order of magnitude that of previous reports on electromagnetic energy transport using linear plasmonic chains. The work points out the potential of combining Ag nanoparticle chains with a small interparticle distance (~2 nm) with rare-earth-based optical gain media as ultra-long-range waveguides with extreme light confinement. The results offer new perspectives for the design of integrated hybrid plasmonic-photonic circuits based on rare-earth-activated solid-state platforms.

Spatial coherence from Nd3+ quantum emitters mediated by a plasmonic chain.

Controlling the coherence properties of rare earth emitters in solid-state platforms in the absence of an optical cavity is highly desirable for quantum light-matter interfaces and photonic networks. Here, we demonstrate the possibility of generating directional and spatially coherent light from Nd3+ ions coupled to the longitudinal plasmonic mode of a chain of interacting Ag nanoparticles. The effect of the plasmonic chain on the Nd3+ emission is analyzed by Fourier microscopy. The results reveal the presence of an interference pattern in which the Nd3+ emission is enhanced at specific directions, as a distinctive signature of spatial coherence. Numerical simulations corroborate the need of near-field coherent coupling of the emitting ions with the plasmonic chain mode. The work provides fundamental insights for controlling the coherence properties of quantum emitters at room temperature and opens new avenues towards rare earth based nanoscale hybrid devices for quantum information or optical communication in nanocircuits.

Giant Second Harmonic Generation Enhancement by Ag Nanoparticles Compactly Distributed on Hexagonal Arrangements.

The association of plasmonic nanostructures with nonlinear dielectric systems has been shown to provide useful platforms for boosting frequency conversion processes at metal-dielectric interfaces. Here, we report on an efficient route for engineering light-matter interaction processes in hybrid plasmonic-χ(2) dielectric systems to enhance second harmonic generation (SHG) processes confined in small spatial regions. By means of ferroelectric lithography, we have fabricated scalable micrometric arrangements of interacting silver nanoparticles compactly distributed on hexagonal regions. The fabricated polygonal microstructures support both localized and extended plasmonic modes, providing large spatial regions of field enhancement at the optical frequencies involved in the SHG process. We experimentally demonstrate that the resonant excitation of the plasmonic modes supported by the Ag nanoparticle-filled hexagons in the near infrared region produces an extraordinary 104-fold enhancement of the blue second harmonic intensity generated in the surface of a LiNbO3 crystal. The results open new perspectives for the design of efficient hybrid plasmonic frequency converters in miniaturized devices.

Plasmon-induced dual-wavelength operation in a Yb3+ laser.

Expanding the functionalities of plasmon-assisted lasers is essential for emergent applications in nanoscience and nanotechnology. Here, we report on a novel ability of plasmonic structures to induce dual-wavelength lasing in the near-infrared region in a Yb3+ solid-state laser. By means of the effects of disordered plasmonic networks deposited on the surface of a Yb3+-doped nonlinear RTP crystal, room-temperature dual-wavelength lasing, with a frequency difference between the lines in the THz range, is realized. The dual-wavelength laser is produced by the simultaneous activation of two lasing channels, namely, an electronic- and a phonon-terminated laser transition. The latter is enabled by the out-of-plane field components that are generated by the plasmonic structures, which excite specific Raman modes. Additionally, multiline radiation at three different wavelengths is demonstrated in the visible spectral region via two self-frequency conversion processes, which occur in the vicinities of the plasmonic structures. The results demonstrate the potential of plasmonic nanostructures for inducing drastic modifications in the operational mode of a solid-state laser and hold promise for applications in a variety of fields, including multiplexing, precise spectroscopies, and THz radiation generation via a simple and cost-effective procedure.

Field enhancement and spectral features of hexagonal necklaces of silver nanoparticles for enhanced nonlinear optical processes.

The nonlinear properties of hybrid metallic-dielectric systems are attracting great interest due to their potential for the enhancement of frequency conversion processes at nanoscale dimensions. In this work, we theoretically and experimentally address the correlation between the near field distribution of hexagonal plasmonic necklaces of silver nanoparticles formed on the surface of a LiNbO3 crystal and the second harmonic generation (SHG) produced by this nonlinear crystal in the vicinities of the necklaces. The spectral response of the hexagonal necklaces does not depend on the polarization direction and is characterized by two main modes, the absorptive high-energy mode located in the UV spectral region and the lower energy mode, which is strongly radiant and extends from the visible to the near infrared region. We show that the spatial distribution of the enhanced SHG is consistent with the local field related to the low energy plasmon mode, which spectrally overlaps the fundamental beam. The results are in agreement with the low absorption losses of this mode and the two-photon character of the nonlinear process and provide deeper insight in the connection between the linear and nonlinear optical properties of the hybrid plasmonic-ferroelectric system. The study also highlights the potential of hexagonal necklaces as useful plasmonic platforms for enhanced optical processes at the nanoscale.

Experimental evidence of charged domain walls in lead-free ferroelectric ceramics: light-driven nanodomain switching.

The control of ferroelectric domain walls at the nanometric level leads to novel interfacial properties and functionalities. In particular, the comprehension of charged domain walls, CDWs, lies at the frontier of future nanoelectronic research. Whereas many of the effects have been demonstrated for ideal archetypes, such as single crystals, and/or thin films, a similar control of CDWs on polycrystalline ferroelectrics has not been achieved. Here, we unambiguously show the presence of charged domain walls on a lead-free (K,Na)NbO3 polycrystalline system. The appearance of CDWs is observed in situ by confocal Raman microscopy and second harmonic generation microscopy. CDWs produce an internal strain gradient within each domain. Specifically, the anisotropic strain develops a crucial piece in the ferroelectric domain switching due to the coupling between the polarization of light and the ferroelectric polarization of the nanodomain in the (K,Na)NbO3 ceramic. This effect leads to the tuning of the ferroelectric domain switching by means of the light polarization angle. Our results will help to understand the relevance of charged domain walls on the ferroelectric domain switching process and may facilitate the development of domain wall nanoelectronics by remote light control utilizing polycrystalline ferroelectrics.

2D Arrays of Hexagonal Plasmonic Necklaces for Enhanced Second Harmonic Generation.

Hexagonal plasmonic necklaces of silver nanoparticles organized in 2D superlattices on functional ferroelectric templates are fabricated in large-scale spatial regions by using a surfactant-free photo-deposition process. The plasmonic necklaces support broad radiative plasmonic resonances allowing the enhancement of second harmonic generation (SHG) at the ferroelectric domain boundaries. A 400-fold SHG enhancement is achieved at the near-UV spectral region with subsequent interest for technological applications.

Plasmonic enhancement of second harmonic generation from nonlinear RbTiOPO4 crystals by aggregates of silver nanostructures.

We demonstrate a 60-fold enhancement of the second harmonic generation (SHG) response at the nanoscale in a hybrid metal-dielectric system. By using complex silver nanostructures photochemically deposited on the polar surface of a ferroelectric crystal, we tune the plasmonic resonances from the visible to the near-infrared (NIR) spectral region, matching either the SH or the fundamental frequency. In both cases the SHG signal at the metal-dielectric interface is enhanced, although with substantially different enhancement values: around 5 times when the plasmonic resonance is at the SH frequency or up to 60 times when it matches the fundamental NIR radiation. The results are consistent with the more spatially-extended near-field response of complex metallic nanostructures and can be well explained by taking into account the quadratic character of the SHG process. The work points out the potential of aggregates of silver nanostructures for enhancing optical nonlinearities at the nanoscale and provides an alternative approach for the development of nanometric nonlinear photonic devices in a scalable way.

Plasmon-Assisted Nd(3+)-Based Solid-State Nanolaser.

Solid-state lasers constitute essential tools in a variety of scientific and technological areas, being available in many different designs. However, although nanolasing has been successfully achieved for dyes and semiconductor gain media associated with plasmonic structures, the operation of solid-state lasers beyond the diffraction limit has not been reported yet. Here, we demonstrate room temperature laser action with subwavelength confinement in a Nd(3+)-based solid-state laser by means of the localized surface plasmon resonances supported by chains of metallic nanoparticles. We show a 50% reduction of the pump power at threshold and a remarkable 15-fold improvement of the slope efficiency with respect to the bulk laser operation. The results can be extended to the large diversity of solid-state lasers with the subsequent impact on their applications.

Controlling solid state gain media by deposition of silver nanoparticles: from thermally- quenched to plasmon-enhanced Nd(3+) luminescence.

We show the possibility of controlling the optical properties of Nd(3+) laser ions by using different configurations of metallic nanoparticles (NPs) deposited on a solid state gain medium. In particular, we analyze the effect of two different silver NP arrangements on the optical properties of Nd(3+) ions in LiNbO(3): a two-dimensional (2D) high density and disordered Ag NP distribution and a one-dimensional (1D) long single chain of Ag NPs. We demonstrate that while the 2D disordered distribution produces a thermal quenching of the Nd(3+) luminescence, the 1D single chain leads to the enhancement of the fluorescence from the (4)F(3/2) metastable state. The experimental data are theoretically interpreted by taking into account the different character, radiative or non-radiative, of the localized surface plasmonic modes supported by the Ag nanoparticle distributions at the excitation wavelength. The results point out the capabilities of rare earth ions as optical tools to probe the local plasmonic fields and are relevant to determine the optimal configuration of metallic arrays to improve the performance of potential rare earth ion based sub-micrometer lasers.

Blue SHG enhancement by silver nanocubes photochemically prepared on a RbTiOPO4 ferroelectric crystal.

Silver nanocubes with low size dispersion have been selectively photo-deposited on the positive surface of a periodically poled RbTiOPO4 ferroelectric crystal. The obtained nanocubes show preferential orientations with respect to the substrate suggesting epitaxial growth. The plasmonic resonances supported by the nanocubes are exploited to enhance blue SHG at the domain walls.

Selective plasmon enhancement of the 1.08 µm Nd3+ laser Stark transition by tailoring Ag nanoparticles chains on a PPLN Y-cut.

Selective photoluminescence enhancement of the specific Nd(3+) Stark transition for which laser gain has been obtained in Nd(3+)/LiNbO3 is demonstrated by means of plasmonic resonances with the appropriate symmetry configuration. By using the nonpolar Y-cut of a periodically poled LiNbO3 crystal as platform for photoreduction of metallic nanostructures, periodically distributed chains of Ag nanoparticles oriented parallel to the ferroelectric c-axis are obtained. This alternative metallic nanostructure configuration supports the resonance between the localized surface plasmon and exclusively the π-polarized Stark laser line of Nd(3+) ions at 1.08 µm, while maintaining the remaining crystal field transitions unchanged. The work provides the experimental proof on how plasmonic-based optical antennas can be used to influence selectively rare earth optical Stark transitions to improve the performance of solid state laser gain media.

Simultaneous generation of second to fifth harmonic conical beams in a two dimensional nonlinear photonic crystal.

Broadly tunable multiple high-harmonic conical beams have been generated by means of a multistep χ(2) cascade processes in a two dimensional nonlinear photonic crystal. The nonlinear structure consists of a square lattice of inverted hexagonal domains with diameters and distances between domains as low as 1 µm. The large number of reciprocal lattice vectors provided by both the square nonlinear structure and the hexagonal shaped domains, along with imperfections on the size and shape of the individual domains make possible the simultaneous generation of second up to fifth harmonic conical beams in a single nonlinear structure by using different types of phase matching geometries. The frequency response can be tuned in an extremely large spectral range, and continuous generation of nonlinear conical beams covering the whole visible spectral region can be achieved. Further, the same photon energy can be generated at different orders, so that concentrically emitted conical beams with angular dispersion as large as Δθ = 50° can be observed. The results highlight the significance of highly controlled engineered 2D nonlinear structures to generate advanced multi-photon devices with large spatial and spectral tunable response.

Three-dimensional grain boundary spectroscopy in transparent high power ceramic laser materials.

Using confocal Raman and fluorescence spectroscopic imaging in 3-dimensions, we show direct evidence of inhomogeneous Nd(3+) distribution across grain boundaries (GBs) in Nd(3+):YAG laser ceramics. It is clearly shown that Nd(3+) segregation takes place at GBs leading to self-fluorescence quenching which affects a volume fraction as high as 20%. In addition, we show a clear trend of increasing spatial inhomogeneities in Nd(3+) concentration when the doping levels exceeds 3 at%, which is not detected by standard spectrometry techniques. These results could point the way to further improvements in what is already an impressive class of ceramic laser materials.