Advanced active polymer probe for near-field optics.

We report on a novel, to the best of our knowledge, active probe for scanning near-field optical microscopy (SNOM). A fluorescent nanosphere, acting as the secondary source, is grafted in an electrostatic manner at the apex of a polymer tip integrated into the extremity of an optical fiber. Thanks to the high photostability and sensitivity of the secondary source, the near-field interaction with a gold nanocube is investigated. It is shown that the spatial resolution is well defined by the size of the fluorescent nanosphere. The polarization-dependent near-field images, which are consistent with the simulation, are ascribed to the local excitation rate enhancement. Meanwhile, measurement of the distance-dependent fluorescence lifetime of the nanosphere provides strong evidence that the local density of states is modified so that extra information on nano-emitters can be extracted during near-field scanning. This advanced active probe can thus potentially broaden the range of applications to include nanoscale thermal imaging, biochemical sensors, and the manipulation of nanoparticles.

Evidence of the retardation effect on the plasmonic resonances of aluminum nanodisks in the symmetric/asymmetric environment.

A single metallic nanodisk is the simplest plasmonic nanostructure, but it is robust enough to generate a Fano resonance in the forward and backward scattering spectra by the increment of nanodisk height in the symmetric and asymmetric dielectric environment. Thanks to the phase retardation effect, the non-uniform distribution of electric field along the height of aluminum (Al) nanodisk generates the out-of-plane higher-order modes, which interfere with the dipolar mode and subsequently result in the Fano-lineshape scattering spectra. Meanwhile, the symmetry-breaking effect by the dielectric substrate and the increment of refractive index of the symmetric dielectric environment further accelerate the phase retardation effect and contribute to the appearance of out-of-plane modes. The experimental results on the periodic Al nanodisk arrays with different heights confirm the retardation-induced higher modes in the asymmetric and symmetric environment. The appearance of higher modes and blueshifted main dips in the transmission spectra prove the dominant role of out-of-plane higher modes on the plasmonic resonances of the taller Al nanodisk.

Local Optical Chirality Induced by Near-Field Mode Interference in Achiral Plasmonic Metamolecules.

When circularly polarized light interacts with a nanostructure, the optical response depends on the geometry of the structure. If the nanostructure is chiral (i.e., it cannot be superimposed on its mirror image), then its optical response, both in near-field and far-field, depends on the handedness of the incident light. In contrast, achiral structures exhibit identical far-field responses for left- and right-circular polarization. Here, we show that a perfectly achiral nanostructure, a plasmonic metamolecule with trigonal D3h symmetry, exhibits a near-field response that is sensitive to the handedness of light. This effect stems from the near-field interference between the different plasmonic modes sustained by the plasmonic metamolecule under circularly polarized light excitation. The local chirality in a plasmonic trimer is then experimentally evidenced with nanoscale resolution using a molecular probe. Our experiments demonstrate that the optical near-field chirality can be imprinted into the photosensitive polymer, turning an optical chirality into a geometrical chirality that can be imaged using atomic force microscopy. These results are of interest for the field of polarization-sensitive photochemistry.

Broadband scattering by an aluminum nanoparticle array as a white pixel in commercial color printing applications.

Plasmonic color using metallic nanostructures has attracted considerable interest because of its subwavelength resolution and long sustainability. Significant efforts have been devoted to expanding the gamut of plasmonic color generation by tuning the composition, shape, and components in the primary pixel. In this study, we develop a novel and straightforward strategy for aluminum plasmonic color printing aimed at practical commercial applications. An array of aluminum nanodisks is designed for the broadband scattering of white pixels instead of the three primary colors. Examples presented include trademark and QR codes, which are common in the market of consumer advertising and item identification, that are encoded and fabricated in experiments with aluminum white color pixels to demonstrate feasibility. This simple and efficient strategy is compatible with cost-effective industrial fabrication methods, such as photolithography and nanoimprinting, and requires relatively simpler manufacturing procedures. Therefore, a new path is opened for the future with the extensive use of plasmonic color printing.

Femtosecond Direct Laser-Induced Assembly of Monolayer of Gold Nanostructures with Tunable Surface Plasmon Resonance and High Performance Localized Surface Plasmon Resonance and Surface Enhanced Raman Scattering Sensing.

We show femtosecond direct laser-induced assembly of gold nanostructures with plasmon resonance band variable as a function of laser irradiation in a wide range of visible wavelengths. A system of 2-photon lithography is used to achieve site-selectively controlled dewetting of a thin gold film into nanostructures in which size and shape are highly dependent on the laser power. Simultaneous measurements of localized surface plasmon resonance (LSPR) and surface enhanced Raman scattering (SERS) in the presence of various concentrations of trans-1,2-bis(4-pyridyl) ethylene (BPE) as target molecule are performed in order to highlight the relationship between structural dimensions, plasmonic effect, and detection activity. The resulting gold NPs exhibit high sensitivity as both LSPR and SERS sensors and allow the detection of picomolar concentrations of BPE with a SERS enhancement factor (SEF) of 1.33 × 109 and a linear detection range between 10-3 and 10-12 M.

Two-Color Single Hybrid Plasmonic Nanoemitters with Real Time Switchable Dominant Emission Wavelength.

We demonstrate two-color nanoemitters that enable the selection of the dominant emitting wavelength by varying the polarization of excitation light. The nanoemitters were fabricated via surface plasmon-triggered two-photon polymerization. By using two polymerizable solutions with different quantum dots, emitters of different colors can be positioned selectively in different orientations in the close vicinity of the metal nanoparticles. The dominant emission wavelength of the metal/polymer anisotropic hybrid nanoemitter thus can be selected by altering the incident polarization.

High-resolution imaging and spectroscopy of multipolar plasmonic resonances in aluminum nanoantennas.

We report on the high resolution imaging of multipolar plasmonic resonances in aluminum nanoantennas using electron energy loss spectroscopy (EELS). Plasmonic resonances ranging from near-infrared to ultraviolet (UV) are measured. The spatial distributions of the multipolar resonant modes are mapped and their energy dispersion is retrieved. The losses in the aluminum antennas are studied through the full width at half-maximum of the resonances, unveiling the weight of both interband and radiative damping mechanisms of the different multipolar resonances. In the blue-UV spectral range, high order resonant modes present a quality factor up to 8, two times higher than low order resonant modes at the same energy. This study demonstrates that near-infrared to ultraviolet tunable multipolar plasmonic resonances in aluminum nanoantennas with relatively high quality factors can be engineered. Aluminum nanoantennas are thus an appealing alternative to gold or silver ones in the visible and can be efficiently used for UV plasmonics.

Single step synthesis and organization of gold colloids assisted by copolymer templates.

We report here an original single-step process for the synthesis and self-organization of gold colloids by simply incorporating gold salts into a solution prepared using polystyrene (PS)-polymethylmethacrylate copolymer and thiolated PS with propylene glycol methyl ether acetate as a solvent. The spin-coating and annealing of this solution then allows the formation of PS domains. Depending on the polymer concentration of the as-prepared solution, there can be either one or several gold nanoparticles (Au NPs) per PS domain. For high concentrations of Au NPs in PS domains, the coupling between plasmonic NPs leads to the observation of a second peak in the optical extinction spectrum. Such a collective effect could be relevant for the development of optical strain sensors in the near future.

Rapid fabrication of micro-nanometric tapered fiber lens and characterization by a novel scanning optical microscope with submicron resolution.

In numerous applications of optical scanning microscopy, a reference tapered fiber lens with high symmetry at sub-wavelength scale remains a challenge. Here, we demonstrate the ability to manufacture it with a wide range of geometry control, either for the length from several hundred nanometers to several hundred microns, or for the curvature radius from several tens of nanometers to several microns on the endface of a single mode fiber. On this basis, a scanning optical microscope has been developed, which allows for fast characterization of various sub-wavelength tapered fiber lenses. Focal position and depth of microlenses with different geometries have been determined to be ranged from several hundreds of nanometers to several microns. FDTD calculations are consistent with experimental results.

Plasmonic-Enhanced Tunable Near-Infrared Photoresponse for Narrowband Organic Photodetectors.

Near-infrared (NIR) narrowband organic photodetectors (OPDs) can be essential building blocks for emerging applications including wireless optical communication and light detection, but further improvement of their performances remains to be a great challenge. Herein, a light manipulation strategy combining solution-processable gold nanorings (AuNRs)-based hole transporting layer (HTL) and an optical microcavity is proposed to achieve high-performance NIR narrowband OPDs. Optical microcavities with a Fabry-Pérot resonator structure, guided by theoretical simulation, are coupled with PM6:BTP-eC9-based OPDs to exhibit highly tunable NIR selectivity. The further integration of AuNRs array with NIR-customized localized surface plasmon resonance in the HTL of the NIR narrowband OPDs enables evident NIR absorption enhancement, yielding a specific detectivity exceeding 1013 Jones (1.5 × 1012 Jones, calculated from noise spectral density) at 820 nm, along with a finely selective photoresponse (full width at half-maximum of 80 nm) and a 3-fold increase in photocurrent intensity. Finally, the practical application of our OPDs is demonstrated in an NIR communication system. These results reveal the great potential of an appropriate optical design for developing highly performing NIR narrowband OPDs.

Intense Bessel-like beams arising from pyramid-shaped microtips.

We show both numerically and experimentally that intense, narrow, and low-divergence beams of light are produced at the apex of dielectric pyramid-shaped microtips. These beams exhibit a Bessel transverse profile but are narrower than the usual Bessel beam, allowing for a significant enhancement of the light intensity inside the beam. They are generated by axicon-like structures with submicrometric height imprinted in glass by combining optical lithography and chemical etching. The resulting beams are experimentally imaged using fluorescence microscopy, in remarkable agreement with numerical computations.

Plasmon-based free-radical photopolymerization: effect of diffusion on nanolithography processes.

This Article interrogates the mechanisms responsible for nanoscale photopolymerization induced by confined and enhanced electromagnetic fields. Surface plasmon dipolar resonance of individual Ag nanoparticles was used as an optical near-field source to locally trigger the reaction of a photopolymerizable formulation. Laser excitation of the nanoparticles embedded in the formulation reproducibly generates polymer features with typical dimensions ranging from 2 nm to a few tens of nanometer. We have determined the physicochemical parameters and mechanisms controlling the spatial extent of the photopolymerization process. We found that the diffusion of the dye is the main process limiting the polymerization reaction, as opposed to what is observed at the microscale with an equivalent chemical system. This approach demonstrates that plasmon-based polymerization can achieve true nanometer scale resolution and also provides a unique opportunity to investigate photochemistry at this length scale.

Integration of polymer microlens array at fiber bundle extremity by photopolymerization.

We present a novel route to directly integrate an array of microlenses at the extremity of an optical fiber bundle. The method is based on photopolymerization at the end of the fiber. The method is based on the control of exposure dose and volume of the deposited droplet of photopolymerizable formulation. Optical properties of the integrated microlenses are discussed on the basis of FDTD calculations.

Surface roughness and substrate induced symmetry-breaking: influence on the plasmonic properties of aluminum nanostructure arrays.

The surface topography is known to play an important role on the near- and far-field optical properties of metallic nanoparticles. In particular, aluminum (Al) nanoparticles are commonly fabricated through evaporation techniques, therefore exhibiting elevated surface roughness additionally to their native oxide layer. In this study, the mode-dependent influence of surface roughness on the plasmonic properties sustained by Al nanodisks (NDs) is first numerically investigated using a realistic model taking into account the thin native oxide layer. Due to the symmetry-breaking induced by the supporting dielectric substrate to Al ND, it appears that the roughness affects differently the substrate-induced out-of-plane quadrupolar mode (below 300 nm) and the in-plane dipolar mode sustained by the Al ND. By increasing the top surface roughness of the Al ND, the substrate-induced quadrupolar mode is significantly damped especially in the ultraviolet regime, while the dipolar resonance is broadened and redshifted. The explanation of these effects relies in the decoherence and dissipation of the collective electronic oscillations as a result of the top surface roughness to the different near-field distribution of the out-of-plane quadrupolar mode and in-plane dipolar mode. Moreover, the influences of the diameter of Al ND, dielectric substrate with different refractive index, and the oxidation of Al ND on these two modes are also investigated. Particularly, the quadrupolar mode disappears with surface roughness and oxidation, explaining why this mode is very weak and sometimes barely visible on evaporated Al nanostructures reported in the literature. Finally, these results are experimentally confirmed by characterizing the optical properties of periodic Al ND arrays.

Quantitative Temperature Measurements in Gold Nanorods Using Digital Holography.

Temperature characterization and quantification at the nanoscale remain core challenges in applications based on photoinduced heating of nanoparticles. Here, we propose a new approach to obtain quantitative temperature measurements on individual nanoparticles by combining modulated photothermal stimulation and heterodyne digital holography. From full-field reconstructed holograms, the temperature is determined with a precision of 0.3 K via a simple approach without requiring any calibration or fitting parameters. As an application, the dependence of temperature on the aspect ratio of gold nanoparticles is investigated. A good agreement with numerical simulation is observed.

Tuning of an optical dimer nanoantenna by electrically controlling its load impedance.

Optical antennas are elementary units used to direct optical radiation to the nanoscale. Here we demonstrate an active control over individual antenna performances by an external electrical trigger. We find that by an in-plane command of an anisotropic load medium, the electromagnetic interaction between individual elements constituting an optical antenna can be controlled, resulting in a strong polarization and tuning response. An active command of the antenna is a prerequisite for directing light wave through the utilization of such a device.

Influence of order-to-disorder transitions on the optical properties of the aluminum plasmonic metasurface.

To mimic the optical influence of disorder in condensed matter, the effect of uniform disorder on plasmonic resonances were investigated numerically and experimentally on aluminum (Al) nanoparticle arrays. Resorting to the analogue of a plasmonic periodic array to a crystal on the sharp optical spectrum and its anisotropy, the disorder in the transition from crystal to glass (with broadened spectrum and isotropy) is imitated by three kinds of Al plasmonic metasurfaces: varying the displacement, size and rotation of each Al nanoparticle in the periodic array. The random variation on the location or size of each Al nanodisk in the plasmonic crystal induces broadening and reduction of their plasmonic resonances without significantly shifting its wavelength. Moreover, by rotating each Al nanorod in the plasmonic crystal by a random angle, the polarization dependence of plasmonic resonances is progressively decreased by increasing the rotation disorder. Thanks to these three kinds of Al metasurfaces, an enlightened understanding of the random physics in the solid state and the influence of manufacturing deviation in nanophotonics is supported.

Hybrid plasmonic nano-emitters with controlled single quantum emitter positioning on the local excitation field.

Hybrid plasmonic nano-emitters based on the combination of quantum dot emitters (QD) and plasmonic nanoantennas open up new perspectives in the control of light. However, precise positioning of any active medium at the nanoscale constitutes a challenge. Here, we report on the optimal overlap of antenna's near-field and active medium whose spatial distribution is controlled via a plasmon-triggered 2-photon polymerization of a photosensitive formulation containing QDs. Au nanoparticles of various geometries are considered. The response of these hybrid nano-emitters is shown to be highly sensitive to the light polarization. Different light emission states are evidenced by photoluminescence measurements. These states correspond to polarization-sensitive nanoscale overlap between the exciting local field and the active medium distribution. The decrease of the QD concentration within the monomer formulation allows trapping of a single quantum dot in the vicinity of the Au particle. The latter objects show polarization-dependent switching in the single-photon regime.

Plasmonic electromagnetic hot spots temporally addressed by photoinduced molecular displacement.

We report the observation of temporally varying electromagnetic hot spots in plasmonic nanostructures. Changes in the field amplitude, position, and spatial features are induced by embedding plasmonic silver nanorods in the photoresponsive azo-polymer. This polymer undergoes cis-trans isomerization and wormlike transport within resonant optical fields, producing a time-varying local dielectric environment that alters the locations where electromagnetic hot spots are produced. Finite-difference time-domain and Monte Carlo simulations that model the induced field and corresponding material response are presented to aid in the interpretation of the experimental results. Evidence for propagating plasmons induced at the ends of the rods is also presented.

Detecting a Zeptogram of Pyridine with a Hybrid Plasmonic-Photonic Nanosensor.

Thanks to their small sensing volume, nanosensors based on localized surface plasmon resonances (LSPR) allow the detection of minute amounts of analytes, down to the single-molecule limit. However, the detected analytes are often large molecules, such as proteins. The detection of small molecules remains largely unexplored. Here, we use a hybrid photonic-plasmonic nanosensor to detect a small target molecule (pyridine). The sensor's design is based on a dielectric photonic microstructure acting as an antenna, which efficiently funnels light toward a plasmonic transducer and enhances the detection efficiency. This sensor exhibits a limit of detection as small as 10-14 mol L-1. Using a calibration procedure based on electrodynamical numerical simulations, we compute the number of detected molecules. This yields a limit of detection in mass of 4 zeptograms (1 zg = 10-21 g), a record value for plasmonic molecular sensors. Our system can hence be seen as an optical molecular weighing scale, enabling room temperature detection of mass at the zeptogram scale.