Design of plasmonic directional antennas via evolutionary optimization.

We demonstrate inverse design of plasmonic nanoantennas for directional light scattering. Our method is based on a combination of full-field electrodynamical simulations via the Green dyadic method and evolutionary optimization (EO). Without any initial bias, we find that the geometries reproducibly found by EO work on the same principles as radio-frequency antennas. We demonstrate the versatility of our approach by designing various directional optical antennas for different scattering problems. EO-based nanoantenna design has tremendous potential for a multitude of applications like nano-scale information routing and processing or single-molecule spectroscopy. Furthermore, EO can help to derive general design rules and to identify inherent physical limitations for photonic nanoparticles and metasurfaces.

Enhancement of electric and magnetic dipole transition of rare-earth-doped thin films tailored by high-index dielectric nanostructures.

We propose a simple experimental technique to separately map the emission from electric and magnetic dipole transitions close to single dielectric nanostructures, using a few-nanometer thin film of rare-earth-ion-doped clusters. Rare-earth ions provide electric and magnetic dipole transitions of similar magnitude. By recording the photoluminescence from the deposited layer excited by a focused laser beam, we are able to simultaneously map the electric and magnetic emission enhancement on individual nanostructures. In spite of being a diffraction-limited far-field method with a spatial resolution of a few hundred nanometers, our approach appeals by its simplicity and high signal-to-noise ratio. We demonstrate our technique at the example of single silicon nanorods and dimers, in which we find a significant separation of electric and magnetic near-field contributions. Our method paves the way towards the efficient and rapid characterization of the electric and magnetic optical response of complex photonic nanostructures.

Molecular decay rate near nonlocal plasmonic particles.

When the size of metal nanoparticles is smaller than typically 10 nm, their optical response becomes sensitive to both spatial dispersion and quantum size effects associated with the confinement of the conduction electrons inside the particle. In this Letter, we propose a nonlocal scheme to compute molecular decay rates near spherical nanoparticles which includes the electron-electron interactions through a simple model of electronic polarizabilities. The plasmonic particle is schematized by a dynamic dipolar polarizability α(NL)(ω), and the quantum system is characterized by a two-level system. In this scheme, the light matter interaction is described in terms of classical field susceptibilities. This theoretical framework could be extended to address the influence of nonlocality on the dynamics of quantum systems placed in the vicinity of nano-objects of arbitrary morphologies.

Tailoring and imaging the plasmonic local density of states in crystalline nanoprisms.

Surface plasmon (SP) technologies exploit the spectral and spatial properties of collective electronic oscillations in noble metals placed in an incident optical field. Yet the SP local density of states (LDOS), which rule the energy transducing phenomena between the SP and the electromagnetic field, is much less exploited. Here, we use two-photon luminescence (TPL) microscopy to reveal the SP-LDOS in thin single-crystalline triangular gold nanoprisms produced by a quantitative one-pot synthesis at room temperature. Variations of the polarization and the wavelength of the incident light redistribute the TPL intensity into two-dimensional plasmonic resonator patterns that are faithfully reproduced by theoretical simulations. We demonstrate that experimental TPL maps can be considered as the convolution of the SP-LDOS with the diffraction-limited Gaussian light beam. Finally, the SP modal distribution is tuned by the spatial coupling of nanoprisms, thus allowing a new modal design of plasmonic information processing devices.

Plasphonics: local hybridization of plasmons and phonons.

We show that the interaction between localized surface plasmons sustained by a metallic nano-antenna and delocalized phonons lying at the surface of an heteropolar semiconductor can generate a new class of hybrid electromagnetic modes. These plasphonic modes are investigated using an analytical model completed by accurate Green dyadic numerical simulations. When surface plasmon and surface phonon frequencies match, the optical resonances exhibit a large Rabi splitting typical of strongly interacting two-level systems. Based on numerical simulations of the electric near-field maps, we investigate the nature of the plaphonic excitations. In particular, we point out a strong local field enhancement boosted by the phononic surface. This effect is interpreted in terms of light harvesting by the plasmonic antenna from the phononic surface. We thus introduce the concept of active phononic surfaces that may be exploited for far-infared optoelectronic devices and sensors.

Damping of the acoustic vibrations of individual gold nanoparticles.

In this letter, the ultrafast vibrational dynamics of individual gold nanorings has been investigated by femtosecond transient absorption spectroscopy. Two acoustic vibration modes have been detected and identified. The influence of the mechanical coupling at the nanoparticle/substrate interface on the acoustic vibrations of the nano-objects is discussed. Moreover, by changing the environment of the nanoring, we provide a clear evidence of the impact of the surrounding medium on the damping of the acoustic vibrations. Such results are reported here for the first time on individual nanoparticles. This work points out a new sensing method based on the sensitivity of the acoustic vibration damping to the surrounding medium.

Acousto-plasmonic and surface-enhanced Raman scattering properties of coupled gold nanospheres/nanodisk trimers.

This work is devoted to the fundamental understanding of the interaction between acoustic vibrations and surface plasmons in metallic nano-objects. The acoustoplasmonic properties of coupled spherical gold nanoparticles and nanodisk trimers are investigated experimentally by optical transmission measurements and resonant Raman scattering experiments. For excitation close to resonance with the localized surface plasmons of the nanodisk trimers, we are able to detect several intense Raman bands generated by the spherical gold nanoparticles. On the basis of both vibrational dynamics calculations and Raman selection rules, the measured Raman bands are assigned to fundamental and overtones of the quadrupolar and breathing vibration modes of the spherical gold nanoparticles. Simulations of the electric near-field intensity maps performed at the Raman probe wavelengths showed strong localization of the optical energy in the vicinity of the nanodisk trimers, thus corroborating the role of the interaction between the acoustic vibrations of the spherical nanoparticles and the surface plasmons of the nanodisk trimers. Acoustic phonons surface enhanced Raman scattering is here demonstrated for the first time for such coupled plasmonic systems. This work paves the way to surface plasmon engineering for sensing the vibrational properties of nanoparticles.

Challenges in nanofabrication for efficient optical metasurfaces.

Optical metasurfaces have raised immense expectations as cheaper and lighter alternatives to bulk optical components. In recent years, novel components combining multiple optical functions have been proposed pushing further the level of requirement on the manufacturing precision of these objects. In this work, we study in details the influence of the most common fabrication errors on the optical response of a metasurface and quantitatively assess the tolerance to fabrication errors based on extensive numerical simulations. We illustrate these results with the design, fabrication and characterization of a silicon nanoresonator-based metasurface that operates as a beam deflector in the near-infrared range.

Gold nanoring trimers: a versatile structure for infrared sensing.

In this work we report on the observation of surface plasmon properties of periodic arrays of gold nanoring trimers fabricated by electron beam lithography. It is shown that the localized surface plasmon resonances of such gold ring trimers occur in the infrared spectral region and are strongly influenced by the nanoring geometry and their relative positions. Based on numerical simulations of the optical extinction spectra and of the electric near-field intensity maps, the resonances are assigned to surface plasmon states arising from the strong intra-trimer electromagnetic interaction. We show that the nanoring trimer configuration allows for generating infrared surface plasmon resonances associated with strongly localized electromagnetic energy, thus providing plasmonic nanoresonators well-suited for sensing and surface enhanced near-infrared Raman spectroscopy.

Dual wavelength sensing based on interacting gold nanodisk trimers.

Fabrication and surface plasmon properties of gold nanostructures consisting of periodic arrays of disk trimers are reported. Using electron beam lithography, disk diameters as small as 96 nm and gaps between disks as narrow as 10 nm have been achieved with an unprecedented degree of control and reproducibility. The disk trimers exhibit intense visible and infrared surface plasmon resonances which are studied as a function of the disk diameter and of the pitch between trimers. Based on simulations of the optical extinction spectra and of the electric near-field intensity maps, the resonances are assigned to a single trimer response and to collective surface plasmon excitations involving electromagnetic interaction between the trimers. The sensing properties of the disk trimers are investigated using various coating media. The reported results demonstrate the possible use of gold disk trimers for dual wavelength chemical sensing.

Acousto-plasmonic hot spots in metallic nano-objects.

We investigate the acousto-plasmonic dynamics of metallic nano-objects by means of resonant Raman scattering and time-resolved femtosecond transient absorption. We observe an unexpectedly strong acoustic vibration band in the Raman scattering of silver nanocolumns, usually not found in isolated nano-objects. The frequency and the polarization of this unexpected Raman band allow us to assign it to breathing-like acoustic vibration modes. On the basis of full electromagnetic near-field calculations coupled to the elasticity theory, we introduce a new concept of "acousto-plasmonic hot spots" which arise here because of the indented shape of the nanocolumns. These hot spots combine both highly localized surface plasmons and strong shape deformation by the acoustic vibrations at specific sites of the nano-objects. We show that the coupling between breathing-like acoustic vibrations and surface plasmons at the "acousto-plasmonic hot spots" is strongly enhanced, turning almost silent vibration modes into efficient Raman scatterers.

Sculpting nanometer-sized light landscape with plasmonic nanocolumns.

Plasmonic structures are commonly used to both confine and enhance surface electromagnetic fields. In the past ten years, their peculiar optical properties have given rise to many promising applications ranging from high density data storage to surface optical trapping. In this context, we investigated both far-field and near-field optical response of a collection of densely packed silver nanocolumns embedded in amorphous aluminum oxide using the discrete dipole approximation. In the far field, a good fit of the calculated to the experimental absorption spectra can only be achieved when in addition to interaction between neighboring nanocolumns, a nanorod shape with periodic shrinks mimicking the experimental morphology of the nanocolumns is used. In the near field, modulated field intensities following the nanocolumns distribution and tunable with the incident wavelength are predicted outside the region occupied by the nanocolumns. This plasmonic image transfer has a resolution of approximately 1.8D where D is the diameter of the nanocolumns that in our case is 2.4 nm.

Polarization conversion in plasmonic nanoantennas for metasurfaces using structural asymmetry and mode hybridization.

Polarization control using single plasmonic nanoantennas is of interest for subwavelength optical components in nano-optical circuits and metasurfaces. Here, we investigate the role of two mechanisms for polarization conversion by plasmonic antennas: Structural asymmetry and plasmon hybridization through strong coupling. As a model system we investigate L-shaped antennas consisting of two orthogonal nanorods which lengths and coupling strength can be independently controlled. An analytical model based on field susceptibilities is developed to extract key parameters and to address the influence of antenna morphology and excitation wavelength on polarization conversion efficiency and scattering intensities. Optical spectroscopy experiments performed on individual antennas, further supported by electrodynamical simulations based on the Green Dyadic Method, confirm the trends extracted from the analytical model. Mode hybridization and structural asymmetry allow address-ing different input polarizations and wavelengths, providing additional degrees of freedom for agile polarization conversion in nanophotonic devices.

Evolutionary multi-objective optimization of colour pixels based on dielectric nanoantennas.

The rational design of photonic nanostructures consists of anticipating their optical response from systematic variations of simple models. This strategy, however, has limited success when multiple objectives are simultaneously targeted, because it requires demanding computational schemes. To this end, evolutionary algorithms can drive the morphology of a nano-object towards an optimum through several cycles of selection, mutation and cross-over, mimicking the process of natural selection. Here, we present a numerical technique that can allow the design of photonic nanostructures with optical properties optimized along several arbitrary objectives. In particular, we combine evolutionary multi-objective algorithms with frequency-domain electrodynamical simulations to optimize the design of colour pixels based on silicon nanostructures that resonate at two user-defined, polarization-dependent wavelengths. The scattering spectra of optimized pixels fabricated by electron-beam lithography show excellent agreement with the targeted objectives. The method is self-adaptive to arbitrary constraints and therefore particularly apt for the design of complex structures within predefined technological limits.

Modal engineering of Surface Plasmons in apertured Au Nanoprisms.

Crystalline gold nanoprisms of sub-micrometric size sustain high order plasmon modes in the visible and near infrared range that open a new realm for plasmon modal design, integrated coplanar devices and logic gates. In this article, we explore the tailoring of the surface plasmon local density of states (SP-LDOS) by embedding a single defect, namely a small hole, carved in the platelet by focused ion beam (FIB). The change in the SP-LDOS of the hybrid structure is monitored by two-photon luminescence (TPL) microscopy. The dependency of the two-dimensional optical field intensity maps on the linear polarization of the tightly focused femtosecond laser beam reveals the conditions for which the hole defect significantly affects the initial modes. A detailed numerical analysis of the spectral characteristics of the SP-LDOS based on the Green dyadic method clearly indicates that the hole size and location can be exploited to tune or remove selected SP modes.

Plasmonic pumping of excitonic photoluminescence in hybrid MoS2-Au nanostructures.

We report on the fabrication of monolayer MoS2-coated gold nanoantennas combining chemical vapor deposition, e-beam lithography surface patterning, and a soft lift-off/transfer technique. The optical properties of these hybrid plasmonic-excitonic nanostructures are investigated using spatially resolved photoluminescence spectroscopy. Off- and in-resonance plasmonic pumping of the MoS2 excitonic luminescence showed distinct behaviors. For plasmonically mediated pumping, we found a significant enhancement (∼65%) of the photoluminescence intensity, clear evidence that the optical properties of the MoS2 monolayer are strongly influenced by the nanoantenna surface plasmons. In addition, a systematic photoluminescence broadening and red-shift in nanoantenna locations is observed which is interpreted in terms of plasmonic enhanced optical absorption and subsequent heating of the MoS2 monolayers. Using a temperature calibration procedure based on photoluminescence spectral characteristics, we were able to estimate the local temperature changes. We found that the plasmonically induced MoS2 temperature increase is nearly four times larger than in the MoS2 reference temperatures. This study shines light on the plasmonic-excitonic interaction in these hybrid metal/semiconductor nanostructures and provides a unique approach for the engineering of optoelectronic devices based on the light-to-current conversion.

Optimal polarization conversion in coupled dimer plasmonic nanoantennas for metasurfaces.

We demonstrate that polarization conversion in coupled dimer antennas, used in phase discontinuity metasurfaces, can be tuned by careful design. By controlling the gap width, a strong variation of the coupling strength and polarization conversion is found between capacitively and conductively coupled antennas. A theoretical two-oscillator model is proposed, which shows a universal scaling of the degree of polarization conversion with the energy splitting of the symmetric and antisymmetric modes supported by the antennas. Using single antenna spectroscopy, we find good agreement for the scaling of mode splitting and polarization conversion with gap width over the range from capacitive to conductive coupling. Next to linear polarization conversion, we demonstrate single-antenna linear to circular polarization conversion. Our results provide strategies for phase-discontinuity metasurfaces and ultracompact polarization optics.

Surface plasmon damping quantified with an electron nanoprobe.

Fabrication and synthesis of plasmonic structures is rapidly moving towards sub-nanometer accuracy in control over shape and inter-particle distance. This holds the promise for developing device components based on novel, non-classical electro-optical effects. Monochromated electron energy-loss spectroscopy (EELS) has in recent years demonstrated its value as a qualitative experimental technique in nano-optics and plasmonic due to its unprecedented spatial resolution. Here, we demonstrate that EELS can also be used quantitatively, to probe surface plasmon kinetics and damping in single nanostructures. Using this approach, we present from a large (>50) series of individual gold nanoparticles the plasmon Quality factors and the plasmon Dephasing times, as a function of energy/frequency. It is shown that the measured general trend applies to regular particle shapes (rods, spheres) as well as irregular shapes (dendritic, branched morphologies). The combination of direct sub-nanometer imaging with EELS-based plasmon damping analysis launches quantitative nanoplasmonics research into the sub-nanometer realm.

Plasmonic nanoparticle networks for light and heat concentration.

Self-assembled plasmonic nanoparticle networks (PNN) composed of chains of 12 nm diameter crystalline gold nanoparticles exhibit a longitudinally coupled plasmon mode centered at 700 nm. We have exploited this longitudinal absorption band to efficiently confine light fields and concentrate heat sources in the close vicinity of these plasmonic chain networks. The mapping of the two phenomena on the same superstructures was performed by combining two-photon luminescence and fluorescence polarization anisotropy imaging techniques. Besides the light and heat concentration, we show experimentally that the planar spatial distribution of optical field intensity can be simply modulated by controlling the linear polarization of the incident optical excitation. On the contrary, the heat production, which is obtained here by exciting the structures within the optically transparent window of biological tissues, is evenly spread over the entire PNN. This contrasts with the usual case of localized heating in continuous nanowires, thus opening opportunities for these networks in light-induced hyperthermia applications. Furthermore, we propose a unified theoretical framework to account for both the nonlinear optical and thermal near-fields around PNN. The associated numerical simulations, based on a Green's function formalism, are in excellent agreement with the experimental images. This formalism therefore provides a versatile tool for the accurate engineering of optical and thermodynamical properties of complex plasmonic colloidal architectures.