Spectral and directional reshaping of fluorescence in large area self-assembled plasmonic-photonic crystals

Realizing zero-threshold population inversion via plasmonic doping

Lowering the population inversion threshold is key to leveraging quantum dots (QDs) for nanoscale lasing and laser miniaturization. However, optical realization of population inversion in QDs has an inherent limitation: the number of excited electrons per QD is bound by the absorbed photons. Here we show that one can break this population limit and realize near-zero threshold inversion via plasmonic doping. Specifically, we integrate QDs into a grating-like plasmonic resonator, which, upon optical excitation, can transiently dope the QDs with numerous highly energetic electrons and make excited electrons in the QDs outnumber absorbed photons. This high population under low excitation blocks QD absorption and reduces the population-inversion threshold over 100 times compared to neutrally populated QDs. Our findings not only reveal new understanding of cavity-emitter interactions but also provide practical avenues for zero-threshold lasing, nanolasing and amplification devices.

Charge transfer excitons and directional fluorescence of in-plane lateral MoSe2-WSe2 heterostructures

In this article, the linear and nonlinear optical properties of in-plane lateral MoSe2-WSe2 heterostructures are theoretically investigated. The polarization-dependent strongest optical absorption in one-photon absorption occurs in charge transfer excited states, where electrons transfer from WSe2 to MoSe2. This phenomenon is supported by the LUMO (lowest unoccupied molecular orbital) and HUMO (highest occupied molecular orbital) imaging obtained through scanning tunneling microscopy. The charge difference density and transition density matrix are used to interpret the electronic transitions, and these interpretations rely on the concept of transition density. The optical properties of two-photon absorption in its nonlinear optical process are significantly different from the excitation in one-photon absorption, where the strongest optical absorption is contributed from direct transition from the ground state to the final state without going through an intermediate excited state, due to the very large difference of permanent dipole moments between the excited and ground states. Our results also reveal directional fluorescence and physical mechanism of in-plane lateral MoSe2-WSe2 heterostructures. Our work can provide insights into the physical mechanism of the optical properties of in-plane lateral MoSe2-WSe2 heterostructures.

Refractive Index Dependence of Fluorescence Enhancement in a Nanostructured Plasmonic Grating

Plasmonic gratings are attracting huge interest in the context of realizing sensors based on surface-enhanced fluorescence. The grating features control the plasmonic modes and consequently have a strong effect on the fluorescence response. Within this framework, we focused on the use of a buffer solution flowing across the grating active surface to mimic a real measurement. The refractive index of the surrounding medium is therefore altered, with a consequent modification of the resonance conditions. The result is a shift in the spectral features of the fluorescence emission accompanied by a reshaping of the fluorescence emission in terms of spectral weight and direction.

Plasmonic Modes and Fluorescence Enhancement Coupling Mechanism: A Case with a Nanostructured Grating

The recent development and technological improvement in dealing with plasmonic metasurfaces has triggered a series of interesting applications related to sensing challenges. Fluorescence has been one of the most studied tools within such a context. With this in mind, we used some well characterized structures supporting plasmonic resonances to study their influence on the emission efficiency of a fluorophore. An extended optical analysis and a complementary investigation through finite-difference time-domain (FDTD) simulations have been combined to understand the coupling mechanism between the excitation of plasmonic modes and the fluorescence absorption and emission processes. The results provide evidence of the spectral shape dependence of fluorescence on the plasmonic field distribution together with a further relationship connected with the enhancement of its signal. It has made evident that the spectral region characterized by the largest relative enhancement closely corresponds to the strongest signatures of the plasmonic modes, as described by both the optical measurements and the FDTD findings.

Recent Advances in the Photoreactions Triggered by Porphyrin-Based Triplet–Triplet Annihilation Upconversion Systems: Molecular Innovations and Nanoarchitectonics

Triplet–triplet annihilation upconversion (TTA-UC) is a very promising technology that could be used to convert low-energy photons to high-energy ones and has been proven to be of great value in various areas. Porphyrins have the characteristics of high molar absorbance, can form a complex with different metal ions and a high proportion of triplet states as well as tunable structures, and thus they are important sensitizers for TTA-UC. Porphyrin-based TTA-UC plays a pivotal role in the TTA-UC systems and has been widely used in many fields such as solar cells, sensing and circularly polarized luminescence. In recent years, applications of porphyrin-based TTA-UC systems for photoinduced reactions have emerged, but have been paid little attention. As a consequence, this review paid close attention to the recent advances in the photoreactions triggered by porphyrin-based TTA-UC systems. First of all, the photochemistry of porphyrin-based TTA-UC for chemical transformations, such as photoisomerization, photocatalytic synthesis, photopolymerization, photodegradation and photochemical/photoelectrochemical water splitting, was discussed in detail, which revealed the different mechanisms of TTA-UC and methods with which to carry out reasonable molecular innovations and nanoarchitectonics to solve the existing problems in practical application. Subsequently, photoreactions driven by porphyrin-based TTA-UC for biomedical applications were demonstrated. Finally, the future developments of porphyrin-based TTA-UC systems for photoreactions were briefly discussed.

Generation of Complex Tunable Multispectral Signatures with Reconfigurable Protein-Based, Plasmonic-Photonic Crystal Hybrid Nanostructures

Structurally colored materials, which rely on the interaction between visible light and nanostructures, produce brilliant color displays through fine control of light interference, diffraction, scattering, or absorption. Rationally combining different color-selective functions into a single form offers a powerful strategy to create programmable optical functions which are otherwise difficult, if not impossible to obtain. By leveraging structural protein templates, specifically silk fibroin, nanostructured materials that combine plasmonic and photonic crystal paradigms are shown here. This confluence of function enables directional, tunable, and multiple co-located optical responses derived from the interplay between surface plasmon resonance and photonic bandgap effects. Several demonstrations are shown with programmable coloration at varying viewing sides, angle, and by solvent infiltration, opening avenues for smart displays and multi-mode information encoding applications.

Bandgap control in two-dimensional semiconductors via coherent doping of plasmonic hot electrons

Bandgap control is of central importance for semiconductor technologies. The traditional means of control is to dope the lattice chemically, electrically or optically with charge carriers. Here, we demonstrate a widely tunable bandgap (renormalisation up to 550 meV at room-temperature) in two-dimensional (2D) semiconductors by coherently doping the lattice with plasmonic hot electrons. In particular, we integrate tungsten-disulfide (WS₂) monolayers into a self-assembled plasmonic crystal, which enables coherent coupling between semiconductor excitons and plasmon resonances. Accompanying this process, the plasmon-induced hot electrons can repeatedly fill the WS₂ conduction band, leading to population inversion and a significant reconstruction in band structures and exciton relaxations. Our findings provide an effective measure to engineer optical responses of 2D semiconductors, allowing flexibilities in design and optimisation of photonic and optoelectronic devices.

Surface-enhanced fluorescence imaging on linear arrays of plasmonic half-shells

Here, we perform a Surface-Enhanced Fluorescence (SEF) intensity and lifetime imaging study on linear arrays of silver half-shells (LASHSs), a class of polarization-sensitive hybrid colloidal photonic-plasmonic crystal unexplored previously in SEF. By combining fluorescence lifetime imaging microscopy, scanning confocal fluorescence imaging, Rayleigh scattering imaging, optical microscopy, and finite difference time domain simulations, we identify with high accuracy the spatial locations where SEF effects (intensity increase and lifetime decrease) take place. These locations are the junctions/crevices between adjacent half-shells in the LASHS and locations of high electromagnetic field enhancement and strong emitter-plasmon interactions, as confirmed also by simulated field maps. Such detailed knowledge of the distributed SEF enhancements and lifetime modification distribution, with respect to topography, should prove useful for improved future evaluations of SEF enhancement factors and a more rational design of efficiency-optimized SEF substrates. These linear arrays of metal-coated microspheres expand the family of hybrid colloidal photonic-plasmonic crystals, platforms with potential for applications in optoelectronic devices, fluorescence-based (bio)chemical sensing, or medical assays. In particular, due to the polarized optical response of these LASHSs, specific applications such as hidden tags for anti-counterfeiting or plasmon-enhanced photodetection can be foreseen.

Selectively enhanced Raman/fluorescence spectra in photonic-plasmonic hybrid structures

The manipulation of the interaction between molecules and photonic-plasmonic hybrid structures is critical for the application of surface-enhanced spectroscopy (SES). Herein, we report a study on the mode coupling mechanism and SES performance in a typical optoplasmonic system constructed with a polystyrene microsphere (PS MS) resonator and gold nanoparticles (Au NPs). The mode coupling mechanism was found to be closely dependent on the relative positions of PS MS, Au NPs, and molecules in the optoplasmonic system, based on which selectively enhanced Raman and fluorescence signals of molecules can be realized via the collaboration of enhancement and quenching channels of the PS MS and Au NPs. We demonstrate two arrangements of the photonic-plasmonic hybrid structure, which can support fluorescence signals with sharp whispering-gallery modes and apparently enhanced Raman signals with relatively low detection limits and good robustness, respectively.

Plasmon-Assisted Direction- and Polarization-Sensitive Organic Thin-Film Detector

Utilizing Bragg surface plasmon polaritons (SPPs) on metal nanostructures for the use in optical devices has been intensively investigated in recent years. Here, we demonstrate the integration of nanostructured metal electrodes into an ITO-free thin film bulk heterojunction organic solar cell, by direct fabrication on a nanoimprinted substrate. The nanostructured device shows interesting optical and electrical behavior, depending on angle and polarization of incidence and the side of excitation. Remarkably, for incidence through the top electrode, a dependency on linear polarization and angle of incidence can be observed. We show that these peculiar characteristics can be attributed to the excitation of dispersive and non-dispersive Bragg SPPs on the metal–dielectric interface on the top electrode and compare it with incidence through the bottom electrode. Furthermore, the optical and electrical response can be controlled by the organic photoactive material, the nanostructures, the materials used for the electrodes and the epoxy encapsulation. Our device can be used as a detector, which generates a direct electrical readout and therefore enables the measuring of the angle of incidence of up to 60° or the linear polarization state of light, in a spectral region, which is determined by the active material. Our results could furthermore lead to novel organic Bragg SPP-based sensor for a number of applications.

Polarization-sensitive anisotropic plasmonic properties of quantum dots and Au nanorod composites

Polarization-sensitive anisotropic plasmonic interaction between gold nanorods (AuNRs) and quantum dots (QDs) encapsulated in an epoxy resin polymer has been experimentally investigated. The anisotropic plasmonic interaction utilized the polarization-dependent plasmonic properties of aligned AuNR in AuNR-QD composite. AuNRs were aligned by an external AC electric field of 3.5 ×10⁵ Vm^-1. The plasmonic interaction modified QD absorption and emission dependent on excitation light polarization and maximum enchantment of 10% and 59%, respectively. Moreover, anisotropic plasmonic interaction induced directional emission of QDs has improved emission decay rate by 20% and modulated emission polarization ratio of out-of-plane (vertical) and in-plane (horizontal) from 1 to 0.84.

Enhanced triplet–triplet annihilation upconversion by photonic crystals and Au plasma resonance for efficient photocatalysis

The coupling electromagnetic field of AVS structure effect and AuNPs LSPR can synergistically improve TTA-UC efficiency, thereby enhancing the photocatalytic activity of g-C₃N₄@CdS.

Tunable Valley Polarized Plasmon-Exciton Polaritons in Two-Dimensional Semiconductors

Monolayers of transition-metal dicalcogenides have emerged as two-dimensional semiconductors with direct bandgaps at degenerate but inequivalent electronic "valleys", supporting distinct excitons that can be selectively excited by polarized light. These valley-addressable excitons, when strongly coupled with optical resonances, lead to the formation of half-light half-matter quasiparticles, known as polaritons. Here we report self-assembled plasmonic crystals that support tungsten disulfide monolayers, in which the strong coupling of semiconductor excitons and plasmon lattice modes results in a Rabi splitting of ∼160 meV in transmission spectra as well as valley-polarized photoluminescence at room temperature. More importantly we find that one can flexibly tune the degree of valley polarization by changing either the emission angle or the excitation angle of the pump beam. Our results provide a platform that allows the detection, control, and processing of optical spin and valley information at the nanoscale under ambient conditions.

Separating and enhancing the green and red emissions of NaYF4:Yb3+/Er3+ by sandwiching them into photonic crystals with different bandgaps

Rational control of the multiple emission outputs and achieving single-band and strong luminescence of Ln³⁺ doped upconversion nanoparticles is highly desirable for their applications in sensor and display fields. Here, we designed a sandwich structure to separate and enhance the green and red emission of NaYF₄:Yb³⁺/Er³⁺ simultaneously and realized pure strong green and red emissions. NaYF₄:Yb³⁺/Er³⁺ nanocrystals were sandwiched between two layers of photonic crystals, which have bandgaps at 660 nm and 530 nm, respectively. The photonic crystal with a bandgap at 530 nm on top of the NaYF₄:Yb³⁺/Er³⁺ layer can filter the green emission of NaYF₄:Yb³⁺/Er³⁺, prohibiting its emission upward, and at the same time, enhancing its emission downward. Similarly, the photonic crystal with a bandgap at 660 nm can prohibit the transmission of the red emission, and at the same time enhance its reflection in the opposite direction. Consequently, enhanced green emission was observed from the bottom of the sandwich structure and enhanced red emission was observed from the top of the sandwich structure. Thus, the green and red emissions of NaYF₄:Yb³⁺/Er³⁺ were separated and both of them were enhanced. On the other hand, when using a photonic crystal with a bandgap that overlapped with the excitation light of NaYF₄:Yb³⁺/Er³⁺ nanoparticles, their emissions were all greatly enhanced. Our results suggest that photonic crystals are good candidates to separate and enhance the emissions of Ln³⁺ doped luminescent materials.

Surface Plasmon Enhanced Strong Exciton-Photon Coupling in Hybrid Inorganic-Organic Perovskite Nanowires

Manipulating strong light-matter interaction in semiconductor microcavities is crucial for developing high-performance exciton polariton devices with great potential in next-generation all-solid state quantum technologies. In this work, we report surface plasmon enhanced strong exciton-photon interaction in CH₃NH₃PbBr₃ perovskite nanowires. Characteristic anticrossing behaviors, indicating a Rabi splitting energy up to ∼564 meV, are observed near exciton resonance in hybrid perovskite nanowire/SiO₂/Ag cavity at room temperature. The exciton-photon coupling strength is enhanced by ∼35% on average, which is mainly attributed to surface plasmon induced localized excitation field redistribution. Further, systematic studies on SiO₂ thickness and nanowire dimension dependence of exciton-photon interaction are presented. These results provide new avenues to achieve extremely high coupling strengths and push forward the development of electrically pumped and ultralow threshold small lasers.

Differential Wavevector Distribution of Surface-Enhanced Raman Scattering and Fluorescence in a Film-Coupled Plasmonic Nanowire Cavity

We report on the experimental observation of differential wavevector distribution of surface-enhanced Raman scattering (SERS) and fluorescence from dye molecules confined to a gap between plasmonic silver nanowire and a thin, gold mirror. The fluorescence was mainly confined to higher values of in-plane wavevectors, whereas SERS signal was uniformly distributed along all the wavevectors. The optical energy-momentum spectra from the distal end of the nanowire revealed strong polarization dependence of this differentiation. All these observations were corroborated by full-wave three-dimensional numerical simulations, which further revealed an interesting connection between out-coupled wavevectors and parameters such as hybridized modes in the gap-plasmon cavity, and orientation and location of molecular dipoles in the geometry. Our results reveal a new prospect of discriminating electronic and vibrational transitions in resonant dye molecules using a subwavelength gap plasmonic cavity in the continuous-wave excitation limit, and can be further harnessed to engineer molecular radiative relaxation processes in momentum space.

Sculpting Fano Resonances To Control Photonic-Plasmonic Hybridization

Hybrid photonic-plasmonic systems have tremendous potential as versatile platforms for the study and control of nanoscale light-matter interactions since their respective components have either high-quality factors or low mode volumes. Individual metallic nanoparticles deposited on optical microresonators provide an excellent example where ultrahigh-quality optical whispering-gallery modes can be combined with nanoscopic plasmonic mode volumes to maximize the system's photonic performance. Such optimization, however, is difficult in practice because of the inability to easily measure and tune critical system parameters. In this Letter, we present a general and practical method to determine the coupling strength and tailor the degree of hybridization in composite optical microresonator-plasmonic nanoparticle systems based on experimentally measured absorption spectra. Specifically, we use thermal annealing to control the detuning between a metal nanoparticle's localized surface plasmon resonance and the whispering-gallery modes of an optical microresonator cavity. We demonstrate the ability to sculpt Fano resonance lineshapes in the absorption spectrum and infer system parameters critical to elucidating the underlying photonic-plasmonic hybridization. We show that including decoherence processes is necessary to capture the evolution of the lineshapes. As a result, thermal annealing allows us to directly tune the degree of hybridization and various hybrid mode quantities such as the quality factor and mode volume and ultimately maximize the Purcell factor to be 10⁴.

Strongly enhanced molecular fluorescence with ultra-thin optical magnetic mirror metasurfaces

As a kind of two-dimensional metamaterial, metasurfaces can modify the amplitude, phase, and polarization of the transmitted or reflected electromagnetic wave, and thereby can be used for enhancing the light-matter interactions. Based on this notion, an optical magnetic mirror metasurface featuring periodic nanoscale grooves is designed to confine the strong electric field near the metal surface by magnetic responses. As a result, fluorescence from an ultra-thin layer of fluorescent polymer blend (∼15  nm) on the mirror surface can be strongly enhanced (by 45-fold in experiment). The fluorescence emission can be controlled by the polarization of excitation light since the responses of the magnetic mirror are polarization sensitive. This kind of magnetic mirror metasurface is potentially useful in biological monitors, optical sources, and chemical sensors.

Plasmonic opals: observation of a collective molecular exciton mode beyond the strong coupling

Achieving and controlling strong light-matter interactions in many-body systems is of paramount importance both for fundamental understanding and potential applications. In this paper we demonstrate both experimentally and theoretically how to manipulate strong coupling between the Bragg-plasmon mode supported by an organo-metallic array and molecular excitons in the form of J-aggregates dispersed on the hybrid structure. We observe experimentally the transition from a conventional strong coupling regime exhibiting the usual upper and lower polaritonic branches to a more complex regime, where a third nondispersive mode is seen, as the concentration of J-aggregates is increased. The numerical simulations confirm the presence of the third resonance. We attribute its physical nature to collective molecule-molecule interactions leading to a collective electromagnetic response. A simple analytical model is proposed to explain the physics of the third mode. The nonlinear dependence on molecular parameters followed from the model are confirmed in a set of rigorous numerical studies. It is shown that at the energy of the collective mode molecules oscillate completely out of phase with the incident radiation acting as an effictive thin metal layer.

Mode Modification of Plasmonic Gap Resonances Induced by Strong Coupling with Molecular Excitons

Plasmonic cavities can be used to control the atom-photon coupling process at the nanoscale, since they provide an ultrahigh density of optical states in an exceptionally small mode volume. Here we demonstrate strong coupling between molecular excitons and plasmonic resonances (so-called plexcitonic coupling) in a film-coupled nanocube cavity, which can induce profound and significant spectral and spatial modifications to the plasmonic gap modes. Within the spectral span of a single gap mode in the nanocube-film cavity with a 3 nm wide gap, the introduction of narrow-band J-aggregate dye molecules not only enables an anticrossing behavior in the spectral response but also splits the single spatial mode into two distinct modes that are easily identified by their far-field scattering profiles. Simulation results confirm the experimental findings, and the sensitivity of the plexcitonic coupling is explored using digital control of the gap spacing. Our work opens up a new perspective to study the strong coupling process, greatly extending the functionality of nanophotonic systems, with the potential to be applied in cavity quantum electrodynamic systems.

Fluorescence Enhancement on Large Area Self-Assembled Plasmonic-3D Photonic Crystals

Discontinuous plasmonic-3D photonic crystal hybrid structures are fabricated in order to evaluate the coupling effect of surface plasmon resonance and the photonic stop band. The nanostructures are prepared by silver sputtering deposition on top of hydrophobic 3D photonic crystals. The localized surface plasmon resonance of the nanostructure has a symbiotic relationship with the 3D photonic stop band, leading to highly tunable characteristics. Fluorescence enhancements of conjugated polymer and quantum dot based on these hybrid structures are studied. The maximum fluorescence enhancement for the conjugated polymer of poly(5-methoxy-2-(3-sulfopropoxy)-1,4-phenylenevinylene) potassium salt by a factor of 87 is achieved as compared with that on a glass substrate due to the enhanced near-field from the discontinuous plasmonic structures, strong scattering effects from rough metal surface with photonic stop band, and accelerated decay rates from metal-coupled excited state of the fluorophore. It is demonstrated that the enhancement induced by the hybrid structures has a larger effective distance (optimum thickness ≈130 nm) than conventional plasmonic systems. It is expected that this approach has tremendous potential in the field of sensors, fluorescence-imaging, and optoelectronic applications.

Confined Etching within 2D and 3D Colloidal Crystals for Tunable Nanostructured Templates: Local Environment Matters

We report the isotropic etching of 2D and 3D polystyrene (PS) nanosphere hcp arrays using a benchtop O₂ radio frequency plasma cleaner. Unexpectedly, this slow isotropic etching allows tuning of both particle diameter and shape. Due to a suppressed etching rate at the point of contact between the PS particles originating from their arrangement in 2D and 3D crystals, the spherical PS templates are converted into polyhedral structures with well-defined hexagonal cross sections in directions parallel and normal to the crystal c-axis. Additionally, we found that particles located at the edge (surface) of the hcp 2D (3D) crystals showed increased etch rates compared to those of the particles within the crystals. This indicates that 2D and 3D order affect how nanostructures chemically interact with their surroundings. This work also shows that the morphology of nanostructures periodically arranged in 2D and 3D supercrystals can be modified via gas-phase etching and programmed by the superlattice symmetry. To show the potential applications of this approach, we demonstrate the lithographic transfer of the PS template hexagonal cross section into Si substrates to generate Si nanowires with well-defined hexagonal cross sections using a combination of nanosphere lithography and metal-assisted chemical etching.

Anticorrelation of Photoluminescence from Gold Nanoparticle Dimers with Hot-Spot Intensity

Bulk gold shows photoluminescence (PL) with a negligible quantum yield of ∼10-10, which can be increased by orders of magnitude in the case of gold nanoparticles. This bears huge potential to use noble metal nanoparticles as fluorescent and unbleachable stains in bioimaging or for optical data storage. Commonly, the enhancement of the PL yield is attributed to nanoparticle plasmons, specifically to the enhancements of scattering or absorption cross sections. Tuning the shape or geometry of gold nanostructures (e.g., via reducing the distance between two nanoparticles) allows for redshifting both the scattering and the PL spectra. However, while the scattering cross section increases with a plasmonic redshift, the PL yield decreases, indicating that the common simple picture of a plasmonically boosted gold luminescence needs more detailed consideration. In particular, precise experiments as well as numerical simulations are required. Hence, we systematically varied the distance between the tips of two gold bipyramids on the nanometer scale using AFM manipulation and recorded the PL and the scattering spectra for each separation. We find that the PL intensity decreases as the interparticle coupling increases. This anticorrelation is explained by a theoretical model where both the gold-intrinsic d-band hole recombination probabilities as well as the field strength inside the nanostructure are considered. The scattering cross section or the field strength in the hot-spot between the tips of the bipyramids are not relevant for the PL intensity. Besides, we not only observe PL supported by dipolar plasmon resonances, but also measure and simulate PL supported by higher order plasmonic modes.

2D-ordered dielectric sub-micron bowls on a metal surface: a useful hybrid plasmonic-photonic structure

Recently, it has been demonstrated that the combination of periodic dielectric structures with metallic structures provides an efficient means to yield a synergetic optical response or functionality in the resultant hybrid plasmonic-photonic systems. In this work, a new hybrid plasmonic-photonic structure of 2D-ordered dielectric sub-micron bowls on a flat gold surface was proposed, prepared, and theoretically and experimentally characterized. This hybrid structure supports two types of modes: surface plasmon polaritons bound at the metallic surface and waveguided mode of light confined in the cavity of bowls. Optical responses of this hybrid structure as well as the spatial electric field distribution of each mode are found to be strongly dependent on the structural parameters of this system, and thus could be widely modified on demand. Importantly, compared to the widely studied hybrid systems, namely the flat metallic surface coated with a monolayer array of latex spheres, the waveguided mode with strong field enhancement appearing in the cavities of bowls is more facilely accessible and thus suitable for practical use. For demonstration, a 2D-ordered silica sub-micron bowl array deposited on a flat gold surface was fabricated and used as a regenerable platform for fluorescence enhancement by simply accommodating emitters in bowls. All the simulation and experiment results indicate that the 2D-ordered dielectric sub-micron bowls on a metal surface should be a useful hybrid plasmonic-photonic system with great potential for applications such as sensors or tunable emitting devices if appropriate periods and materials are employed.

Turning on plasmonic lattice modes in metallic nanoantenna arrays via silicon thin films

We study control of optical coupling of plasmon resonances in metallic nanoantenna arrays using ultrathin layers of silicon. This technique allows one to establish and tune plasmonic lattice modes of such arrays, demonstrating a controlled transformation from the localized surface plasmon resonances of individual nanoantennas to their optimized collective lattice modes. Depending on the polarization and incident angle of light, our results support two different types of the silicon-induced plasmonic lattice resonances. For s-polarization these resonances follow the Rayleigh anomaly, while for p-polarization an increase in the incident angle makes the lattice resonances significantly narrower and slightly blueshifted.

Local Field Modulation Induced Three-Order Upconversion Enhancement: Combining Surface Plasmon Effect and Photonic Crystal Effect

A 2D surface plasmon photonic crystal (SPPC) is achieved by implanting gold nanorods onto the periodic surface apertures of the poly(methyl methacrylate) (PMMA) opal photonic crystals. On the surface of the SPPC, the overall upconversion luminescence intensity of NaYF4 :Yb(3+) , Er(3+) under 980 nm excitation is improved more than 10(3) fold. The device is easily shifted to a transparent flexible substrate, applied to flexible displays.

The effect of fluorophore incorporation on fluorescence enhancement in colloidal photonic crystals

Diffusion-swelling dye incorporation method improves photonic structure-induced emission enhancement.

Directing fluorescence with plasmonic and photonic structures

Fluorescence technology pervades all areas of chemical and biological sciences. In recent years, it is being realized that traditional fluorescence can be enriched in many ways by harnessing the power of plasmonic or photonic structures that have remarkable abilities to mold the flow of optical energy. Conventional fluorescence is omnidirectional in nature, which makes it difficult to capture the entire emission. Suitably designed emission directivity can improve collection efficiency and is desirable for many fluorescence-based applications like sensing, imaging, single molecule spectroscopy, and optical communication. By incorporating fluorophores in plasmonic or photonic substrates, it is possible to tailor the optical environment surrounding the fluorophores and to modify the spatial distribution of emission. This promising approach works on the principle of near-field interaction of fluorescence with spectrally overlapping optical modes present in the substrates. In this Account, we present our studies on directional emission with different kinds of planar metallic, dielectric, and hybrid structures. In metal-dielectric substrates, the coupling of fluorescence with surface plasmons leads to directional surface-plasmon-coupled emission with characteristic dispersion and polarization properties. In one-dimensional photonic crystals (1DPC), fluorophores can interact with Bloch surface waves, giving rise to sharply directional Bloch surface wave-coupled emission. The interaction of fluorescence with Fabry-Pérot-like modes in metal-dielectric-metal substrates and with Tamm states in plasmonic-photonic hybrid substrates provides beaming emission normal to the substrate surface. These interesting features are explained in the context of reflectivity dispersion diagrams, which provide a complete picture of the mode profiles and the corresponding coupled emission patterns. Other than planar substrates, specially fabricated plasmonic nanoantennas also have tremendous potential in controlling and steering fluorescence beams. Some representative studies by other research groups with various nanoantenna structures are described. While there are complexities to near-field interactions of fluorescence with plasmonic and photonic structures, there are also many exciting possibilities. The routing of each emission wavelength along a specific direction with a given angular width and polarization will allow spatial and spectral multiplexing. Directional emission close to surface normal will be particularly useful for microscopy and array-based studies. Application-specific angular emission patterns can be obtained by varying the design parameters of the plasmonic/photonic substrates in a flexible manner. We anticipate that the ability to control the flow of emitted light in the nanoscale will lead to the development of a new generation of fluorescence-based assays, instrumentation, portable diagnostics, and emissive devices.

Plasmonic absorption activated trapping and assembling of colloidal crystals with non-resonant continuous gold films

Here we report the realization of trapping and assembly of colloidal crystals on continuous gold thin films based on the combined effect of thermophoresis and thermal convection associated with plasmonic optical heating.

Shaping plasmon beams via the controlled illumination of finite-size plasmonic crystals

Plasmonic crystals provide many passive and active optical functionalities, including enhanced sensing, optical nonlinearities, light extraction from LEDs and coupling to and from subwavelength waveguides. Here we study, both experimentally and numerically, the coherent control of SPP beam excitation in finite size plasmonic crystals under focussed illumination. The correct combination of the illuminating spot size, its position relative to the plasmonic crystal, wavelength and polarisation enables the efficient shaping and directionality of SPP beam launching. We show that under strongly focussed illumination, the illuminated part of the crystal acts as an antenna, launching surface plasmon waves which are subsequently filtered by the surrounding periodic lattice. Changing the illumination conditions provides rich opportunities to engineer the SPP emission pattern. This offers an alternative technique to actively modulate and control plasmonic signals, either via micro- and nano-electromechanical switches or with electro- and all-optical beam steering which have direct implications for the development of new integrated nanophotonic devices, such as plasmonic couplers and switches and on-chip signal demultiplexing. This approach can be generalised to all kinds of surface waves, either for the coupling and discrimination of light in planar dielectric waveguides or the generation and control of non-diffractive SPP beams.

Manipulating light absorption in dye-doped dielectric films on reflecting surfaces

We experimentally and numerically developed a tunable absorbing nanoscale thin-film system, comprising of dye molecules doped dielectric coatings on reflecting surfaces, the absorption behaviors of which can be flexibly tuned by adjusting the system parameters, i.e. the coating thickness and the doping concentration of dye molecules. Specifically, with appropriate system parameters, our absorbing thin-film system exhibits very directional and polarization dependent absorption properties, which can be significantly altered if applied with different parameters. Calculations demonstrate the unique absorption behaviors are a result of coupling between molecular absorption and Fabry-Perot resonances in the thin-film cavity. In addition, we theoretically show that both the spectral and directional range of the absorption in the thin-film system can be intentionally regulated by doping dyes with different absorption band and setting proper excitation conditions of Fabry-Perot resonances.

Coherent fluorescence emission by using hybrid photonic-plasmonic crystals

The spatial and temporal coherence of the fluorescence emission controlled by a quasi-two-dimensional hybrid photonic-plasmonic crystal structure covered with a thin fluorescent-molecular-doped dielectric film is investigated experimentally. A simple theoretical model to describe how a confined quasi-two-dimensional optical mode may induce coherent fluorescence emission is also presented. Concerning the spatial coherence, it is experimentally observed that the coherence area in the plane of the light source is in excess of 49 μm², which results in enhanced directional fluorescence emission. Concerning temporal coherence, the obtained coherence time is 4 times longer than that of the normal fluorescence emission in vacuum. Moreover, a Young's double-slit interference experiment is performed to directly confirm the spatially coherent emission. This smoking gun proof of spatial coherence is reported here for the first time for the optical-mode-modified emission.

Parallel collective resonances in arrays of gold nanorods

In this work we discuss the excitation of parallel collective resonances in arrays of gold nanoparticles. Parallel collective resonances result from the coupling of the nanoparticles localized surface plasmons with diffraction orders traveling in the direction parallel to the polarization vector. While they provide field enhancement and delocalization as the standard collective resonances, our results suggest that parallel resonances could exhibit greater tolerance to index asymmetry in the environment surrounding the arrays. The near- and far-field properties of these resonances are analyzed, both experimentally and numerically.

Dye-doped spheres with plasmonic semi-shells: Lasing modes and scattering at realistic gain levels

We numerically simulate the compensation of absorption, the near-field enhancement as well as the differential far-field scattering cross section for dye-doped polystyrene spheres (radius 195 nm), which are half-covered by a silver layer of 10-40 nm thickness. Such silver capped spheres are interesting candidates for nanoplasmonic lasers, so-called spasers. We find that spasing requires gain levels less than 3.7 times higher than those in commercially available dye-doped spheres. However, commercially available concentrations are already apt to achieve negative absorption, and to narrow and enhance scattering by higher order modes. Narrowing of the plasmonic modes by gain also makes visible higher order modes, which are normally obscured by the broad spectral features of the lower order modes. We further show that the angular distribution of the far-field scattering of the spasing modes is by no means dipole-like and is very sensitive to the geometry of the structure.

Steering Fluorescence Emission with Metal-Dielectric-Metal Structures of Au, Ag and Al

Directional control over fluorescence emission is important for improving the sensitivity of fluorescence based techniques. In recent years, plasmonic and photonic structures have shown great promise in shaping the spectral and spatial distribution of fluorescence, which otherwise is typically isotropic in nature and independent of the observation direction. In this work we have explored the potential of metal-dielectric-metal (MDM) structures composed of Au, Ag or Al in steering the fluorescence emission from various probes emitting in the NIR, Visible or UV/blue region. We show that depending on the optical properties of the metal and the thickness of the dielectric layer, the emission from randomly oriented fluorophores embedded within the MDM substrate is transformed into beaming emission normal to the substrate. Agreement of the observed angular emission patterns with reflectivity calculations reveals that the directional emission is due to the coupling of the fluorescence with the electromagnetic modes supported by the MDM structure.