Heikki T. Rekola

Plasmonic Surface Lattice Resonances at the Strong Coupling Regime

We show strong coupling involving three different types of resonances in plasmonic nanoarrays: surface lattice resonances (SLRs), localized surface plasmon resonances on single nanoparticles, and excitations of organic dye molecules. The measured transmission spectra show splittings that depend on the molecule concentration. The results are analyzed using finite-difference time-domain simulations, a coupled-dipole approximation, coupled-modes models, and Fano theory. The delocalized nature of the collective SLR modes suggests that in the strong coupling regime molecules near distant nanoparticles are coherently coupled.

Lasing in dark and bright modes of a finite-sized plasmonic lattice

Lasing at the nanometre scale promises strong light-matter interactions and ultrafast operation. Plasmonic resonances supported by metallic nanoparticles have extremely small mode volumes and high field enhancements, making them an ideal platform for studying nanoscale lasing. At visible frequencies, however, the applicability of plasmon resonances is limited due to strong ohmic and radiative losses. Intriguingly, plasmonic nanoparticle arrays support non-radiative dark modes that offer longer life-times but are inaccessible to far-field radiation. Here, we show lasing both in dark and bright modes of an array of silver nanoparticles combined with optically pumped dye molecules. Linewidths of 0.2 nm at visible wavelengths and room temperature are observed. Access to the dark modes is provided by a coherent out-coupling mechanism based on the finite size of the array. The results open a route to utilize all modes of plasmonic lattices, also the high-Q ones, for studies of strong light-matter interactions, condensation and photon fluids.

Spatial Coherence Properties of Organic Molecules Coupled to Plasmonic Surface Lattice Resonances in the Weak and Strong Coupling Regimes

We study spatial coherence properties of a system composed of periodic silver nanoparticle arrays covered with a fluorescent organic molecule (DiD) film. The evolution of spatial coherence of this composite structure from the weak to the strong coupling regime is investigated by systematically varying the coupling strength between the localized DiD excitons and the collective, delocalized modes of the nanoparticle array known as surface lattice resonances. A gradual evolution of coherence from the weak to the strong coupling regime is observed, with the strong coupling features clearly visible in interference fringes. A high degree of spatial coherence is demonstrated in the strong coupling regime, even when the mode is very excitonlike (80%), in contrast to the purely localized nature of molecular excitons. We show that coherence appears in proportion to the weight of the plasmonic component of the mode throughout the weak-to-strong coupling crossover, providing evidence for the hybrid nature of the normal modes.

Chiral Plasmonics Using Twisting along Cellulose Nanocrystals as a Template for Gold Nanoparticles

The right-handed twist along aqueous dispersed cellulose nanocrystals allows right-handed chiral plasmonics upon electrostatic binding of gold nanoparticles in dilute environment, through tuning the particle sizes and concentrations. Simulations using nanoparticle coordinates from cryo-electron tomography confirm the experimental results. The finding suggests generalization for other chiral and helical colloidal templates for nanoscale chiral plasmonics.

Surface plasmon polariton-controlled tunable quantum-dot emission

The unique properties of surface plasmon polaritons, such as strong field confinement and local field enhancement effects, make them ideal candidates to enhance and shape the emission of luminescent nanoparticles. Of these nanoparticles, quantum dots are highly versatile, suitable for vastly different applications due to their size and material tunability. In many cases however, the emission wavelength of the quantum dots is fixed after manufacturing, allowing no control over the in situ emission properties. Here, we show fully optical, in situ tunability of the emission wavelength of quantum dots, with shifts of over 30 nm, employing surface plasmon polaritons to control the emission wavelength.

Cooperative colloidal self-assembly of metal-protein superlattice wires

Material properties depend critically on the packing and order of constituent units throughout length scales. Beyond classically explored molecular self-assembly, structure formation in the nanoparticle and colloidal length scales have recently been actively explored for new functions. Structure of colloidal assemblies depends strongly on the assembly process, and higher structural control can be reliably achieved only if the process is deterministic. Here we show that self-assembly of cationic spherical metal nanoparticles and anionic rod-like viruses yields well-defined binary superlattice wires. The superlattice structures are explained by a cooperative assembly pathway that proceeds in a zipper-like manner after nucleation. Curiously, the formed superstructure shows right-handed helical twisting due to the right-handed structure of the virus. This leads to structure-dependent chiral plasmonic function of the material. The work highlights the importance of well-defined colloidal units when pursuing unforeseen and complex assemblies.Colloidal self-assembly is a unique method to produce three-dimensional materials with well-defined hierarchical structures and functionalities. Liljeström et al. show controlled preparation of macroscopic chiral wires with helical plasmonic superlattice structure composed of metal nanoparticles and viruses.

Bose–Einstein condensation in a plasmonic lattice

Bose–Einstein condensation is a remarkable manifestation of quantum statistics and macroscopic quantum coherence. Superconductivity and superfluidity have their origin in Bose–Einstein condensation. Ultracold quantum gases have provided condensates close to the original ideas of Bose and Einstein, while condensation of polaritons and magnons has introduced novel concepts of non-equilibrium condensation. Here, we demonstrate a Bose–Einstein condensate of surface plasmon polaritons in lattice modes of a metal nanoparticle array. Interaction of the nanoscale-confined surface plasmons with a room-temperature bath of dye molecules enables thermalization and condensation in picoseconds. The ultrafast thermalization and condensation dynamics are revealed by an experiment that exploits thermalization under propagation and the open-cavity character of the system. A crossover from a Bose–Einstein condensate to usual lasing is realized by tailoring the band structure. This new condensate of surface plasmon lattice excitations has promise for future technologies due to its ultrafast, room-temperature and on-chip nature.Surface plasmon polaritons in an array of metallic nanoparticles evolve quickly into the band minimum by interacting with a molecule bath, forming a Bose–Einstein condensate at room temperature within picoseconds.

Strong Coupling Between Organic Molecules and Plasmonic Nanostructures

This chapter introduces the theory behind strong coupling of plasmonic modes, such as surface plasmon polaritons, with electronic transitions that are typical for quantum emitters, such as dye molecules and quantum dots. A brief historical overview of the experimental endeavor of measuring such strong coupling is provided, after which we look more carefully at the dynamics in such systems. We proceed with a more in-depth discussion of strong coupling between emitters and delocalized plasmonic modes, called surface lattice resonances, but not before devoting some space to the ideas and theory behind surface lattice resonances for the readers who might not be familiar with the topic. We end the chapter with an outlook on the potential strong coupling has for new and exciting fundamental phenomena and applications of light on the nanoscale.

Modelling lasing in plasmonic nanoparticle arrays

We study lasing in arrays of metallic nanoparticles combined with an active medium of dye molecules. We use a model which reduces the generally complicated spatial and temporal dynamics of electromagnetic fields in the array structure to a single ordinary differential equation for an oscillating dipole. By focusing on the most relevant processes, this approximation allows direct physical insight while reducing the computational effort. We benchmark our model against recent experiments and find satisfactory agreement.

From vacuum Rabi splitting towards stimulated emission with surface plasmon polaritons

We report on strong coupling between surface plasmon polaritons and Rhodamine 6G molecules at room temperature. As a reference to compare with, we first determine the dispersion curve of (uncoupled) surface plasmon polaritons on a 50 nm thick film of silver. Consequently, we determine the dispersion curve of surface plasmon polaritons strongly coupled to Rhodamine 6G molecules, which exhibits vacuum Rabi splitting. Furthermore, we present spontaneous emission spectra of Rhodamine 6G on silver, which are shown to change with detector angle due to surface plasmon polariton generation by Rhodamine 6G molecules.