Spontaneous emission and nonlinear response enhancement by silver nanoparticles in a Nd³⁺-doped periodically poled LiNbO₃ laser crystal

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 (λ_exc = 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.

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.

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.

Enhancement of the second harmonic signal of nonlinear crystals by self-assembled gold nanoparticles

In second harmonic generation (SHG), the energy of two incoming photons, e.g., from a femtosecond laser, can be combined in one outgoing photon of twice the energy, e.g., by means of a nonlinear crystal. The SHG efficiency, however, is limited. In this work, the harvested signal is maximized by composing a hybrid system consisting of a nonlinear crystal with a dense coverage of plasmonic nanostructures separated by narrow gaps. The method of self-assembled diblock-copolymer-based micellar lithography with subsequent electroless deposition is employed to cover the whole surface of a lithium niobate (LiNbO₃) crystal. The interaction of plasmonic nanostructures with light leads to a strong electric near-field in the adjacent crystal. This near-field is harnessed to enhance the near-surface SHG signal from the nonlinear crystal. At the plasmon resonance of the gold nanoparticles, a pronounced enhancement of about 60-fold SHG is observed compared to the bare crystal within the confocal volume of a laser spot.

Hybrid Plasmonic-Ferroelectric Architectures for Lasing and SHG Processes at the Nanoscale

Coherent light sources providing sub-wavelength confined modes are in ever more demand to face new challenges in a variety of disciplines. Scalability and cost-effective production of these systems are also highly desired. The use of ferroelectrics in functional optical platforms, on which plasmonic arrangements can be formed, is revealed as a simple and powerful method to develop coherent light sources with improved and novel functionalities at the nanoscale. Two types of sources with sub-diffraction spatial confinement and improved performances are presented: i) plasmon-assisted solid-state nanolasers based on the interaction between metallic nanostructures and optically active rare earth doped ferroelectric crystals and ii) nonlinear radiation sources based on quadratic frequency mixing processes that are enhanced by means of localized surface plasmon (LSP) resonances. The mechanisms responsible for the intensification of the radiation-matter interaction processes by LSP resonances are discussed in each case. The challenges, potential applications, and future perspectives of the field are highlighted.

Tailoring optical nonlinearities of LiNbO3 crystals by plasmonic silver nanoparticles for broadband saturable absorbers

We report on the synthesis of plasmonic Ag nanoparticles (NPs) embedded in a LiNbO₃ crystal (AgNP:LN) by ion implantation and its application as an efficient broadband saturable absorber (SA) to realize Q-switched pulsed laser generation at both visible and near-infrared wavelength bands. The nonlinear optical response of AgNP:LN is considered as a synergistic effect between Ag NPs and LiNbO₃. We apply the AgNP:LN as visible-near-infrared broadband saturable absorbers (SAs) into Pr:LuLiF₄ bulk and Nd:YVO₄ waveguide laser cavity, achieving efficient passively Q-switched laser at 639 nm and 1064 nm, respectively. This work paves a new way to tailor the nonlinear optical response of LiNbO₃ crystals by using plasmonic Ag NPs, manifesting the significant potential as broadband SAs in the aspect of pulsed lasing.

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.

Anisotropic enhancement of Yb3+ luminescence by disordered plasmonic networks self-assembled on RbTiOPO4 ferroelectric crystals

Increasing Yb³⁺ absorption efficiency is currently desired in a number of applications including bio-imaging, photovoltaics, near infrared driven photocatalysis or ultra-short pulsed solid-state lasers. In this work, silver nanoparticles, which are connected forming disordered networks, have been self-assembled on Yb³⁺ doped RbTiOPO₄ crystals to produce a remarkable enhancement of Yb³⁺ absorption, and hence in the photoluminescence of this ion. The results are interpreted taking into account the near-field response of the plasmonic networks, which display strong amplification of the electric field at the maximum of Yb³⁺ excitation at around 900 nm, together with the anisotropic character of the Yb³⁺ transitions in RbTiOPO₄. We show that in the near field regime, the scattering of the plasmonic networks produces additional polarization field components to those of the incident field, which allows access to the largest transition dipolar moment of Yb³⁺ ions in RbTiOPO₄. As a result, a much more efficient route for Yb³⁺ excitation takes place at the immediacy of the plasmonic networks. This work provides fundamental insights for improving the optical properties of rare earth ions by the suitable design of metallic nanoparticle arrangements, and constitutes a promising step towards the development of new multifunctional solid-state lasers.

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.

Hybrid coordination-network-engineering for bridging cascaded channels to activate long persistent phosphorescence in the second biological window

We present a novel “Top-down” strategy to design the long phosphorescent phosphors in the second biological transparency window via energy transfer. Inherence in this approach to material design involves an ingenious engineering for hybridizing the coordination networks of hosts, tailoring the topochemical configuration of dopants, and bridging a cascaded tunnel for transferring the persistent energy from traps, to sensitizers and then to acceptors. Another significance of this endeavour is to highlight a rational scheme for functionally important hosts and dopants, Cr/Nd co-doped Zn_1−xCa_xGa₂O₄ solid solutions. Such solid-solution is employed as an optimized host to take advantage of its characteristic trap site level to establish an electron reservoir and network parameters for the precipitation of activators Nd³⁺ and Cr³⁺. The results reveal that the strategy employed here has the great potential, as well as opens new opportunities for future new-wavelength, NIR phosphorescent phosphors fabrication with many potential multifunctional bio-imaging 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.

Spontaneous assembly of carbon-based chains in polymer matrixes through surface charge templates

Stable chains of carbon-based nanoparticles were formed directly in polymer matrixes through an electrode-free approach. Spontaneous surface charges were generated pyroelectrically onto functionalized ferroelectric crystals, enabling the formation of electric field gradients that triggered the dipole-dipole interactions responsible for the alignment of the particles, while embedded in the polymer solution. The phenomenon is similar to the dielectrophoretic alignment of carbon nanotubes reported in the literature. However, here the electric fields are generated spontaneously by a simple heat treatment that, simultaneously, aligns the particles and provides the energy necessary for curing the host polymer. The result is a polymer sheet reinforced with well-aligned chains of carbon-based particles, avoiding the invasive implementation of appropriate electrodes and circuits. Because polymers with anisotropic features are of great interest for enhancing the thermal and/or the electrical conductivity, the electrode-free nature of this technique would improve the scaling down and the versatility of those interconnections that find applications in many fields, such as electronics, sensors, and biomedicine. Theoretical simulations of the interactions between the particles and the charge templates were implemented and appear in good agreement with the experimental results. The chain formation was characterized by controlling different parameters, including surface charge configuration, particle concentration, and polymer viscosity, thus demonstrating the reliability of the technique. Moreover, micro-Raman spectroscopy and scanning electron microscopy were used for a thorough inspection of the assembled chains.

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.