Magnetic hot-spot generation at optical frequencies: from plasmonic metamolecules to all-dielectric nanoclusters

AbstractThe weakness of magnetic effects at optical frequencies is directly related to the lack of symmetry between electric and magnetic charges. Natural materials cease to exhibit appreciable magnetic phenomena at rather low frequencies and become unemployable for practical applications in optics. For this reason, historically important efforts were spent in the development of artificial materials. The first evidence in this direction was provided by split-ring resonators in the microwave range. However, the efficient scaling of these devices towards the optical frequencies has been prevented by the strong ohmic losses suffered by circulating currents. With all of these considerations, artificial optical magnetism has become an active topic of research, and particular attention has been devoted to tailor plasmonic metamolecules generating magnetic hot spots. Several routes have been proposed in these directions, leading, for example, to plasmon hybridization in 3D complex structures or Fano-like magnetic resonances. Concurrently, with the aim of electromagnetic manipulation at the nanoscale and in order to overcome the critical issue of heat dissipation, alternative strategies have been introduced and investigated. All-dielectric nanoparticles made of high-index semiconducting materials have been proposed, as they can support both magnetic and electric Mie resonances. Aside from their important role in fundamental physics, magnetic resonances also provide a new degree of freedom for nanostructured systems, which can trigger unconventional nanophotonic processes, such as nonlinear effects or electromagnetic field localization for enhanced spectroscopy and optical trapping

Plasmonic Nanoplatforms for Biochemical Sensing and Medical Applications

Plasmonics, the science of the excitation of surface plasmon polaritons (SPP) at the metal-dielectric interface under intense beam radiation, has been studied for its immense potential for developing numerous nanophotonic devices, optical circuits and lab-on-a-chip devices. The key feature, which makes the plasmonic structures promising is the ability to support strong resonances with different behaviors and tunable localized hotspots, excitable in a wide spectral range. Therefore, the fundamental understanding of light-matter interactions at subwavelength nanostructures and use of this understanding to tailor plasmonic nanostructures with the ability to sustain high-quality tunable resonant modes are essential toward the realization of highly functional devices with a wide range of applications from sensing to switching. We investigated the excitation of various plasmonic resonance modes (i.e. Fano resonances, and toroidal moments) using both optical and terahertz (THz) plasmonic metamolecules. By designing and fabricating various nanostructures, we successfully predicted, demonstrated and analyzed the excitation of plasmonic resonances, numerically and experimentally. A simple comparison between the sensitivity and lineshape quality of various optically driven resonances reveals that nonradiative toroidal moments are exotic plasmonic modes with strong sensitivity to environmental perturbations. Employing toroidal plasmonic metasurfaces, we demonstrated ultrafast plasmonic switches and highly sensitive sensors. Focusing on the biomedical applications of toroidal moments, we developed plasmonic metamaterials for fast and cost-effective infection diagnosis using the THz range of the spectrum. We used the exotic behavior of toroidal moments for the identification of Zika-virus (ZIKV) envelope proteins as the infectious nano-agents through two protocols: 1) direct biding of targeted biomarkers to the plasmonic metasurfaces, and 2) attaching gold nanoparticles to the plasmonic metasurfaces and binding the proteins to the particles to enhance the sensitivity. This led to developing ultrasensitive THz plasmonic metasensors for detection of nanoscale and low-molecular-weight biomarkers at the picomolar range of concentration. In summary, by using high-quality and pronounced toroidal moments as sensitive resonances, we have successfully designed, fabricated and characterized novel plasmonic toroidal metamaterials for the detection of infectious biomarkers using different methods. The proposed approach allowed us to compare and analyze the binding properties, sensitivity, repeatability, and limit of detection of the metasensing device

Engineering Plasmonic Nanocrystal Coupling Through Template-Assisted Self-Assembly

The construction of materials from nanocrystal building blocks represents a powerful new paradigm for materials design. Just as nature’s materials orchestrate intricate combinations of atoms from the library of the periodic table, nanocrystal “metamaterials” integrate individual nanocrystals into larger architectures with emergent collective properties. The individual nanocrystal “meta-atoms” that make up these materials are themselves each a nanoscale atomic system with tailorable size, shape, and elemental composition, enabling the creation of hierarchical materials with predesigned structure at multiple length scales. However, an improved fundamental understanding of the interactions among individual nanocrystals is needed in order to translate this structural control into enhanced functionality. The ability to form precise arrangements of nanocrystals and measure their collective properties is therefore essential for the continued development of nanocrystal metamaterials. In this dissertation, we utilize template-assisted self-assembly and spatially-resolved spectroscopy to form and characterize individual nanocrystal oligomers. At the intersection of “top-down” and “bottom-up” nanoscale patterning schemes, template-assisted self-assembly combines the design freedom of lithography with the chemical control of colloidal synthesis to achieve unique nanocrystal configurations. Here, we employ shape-selective templates to assemble new plasmonic structures, including heterodimers of Au nanorods and upconversion phosphors, a series of hexagonally-packed Au nanocrystal oligomers, and triangular formations of Au nanorods. Through experimental analysis and numerical simulation, we elucidate the means through which inter-nanocrystal coupling imparts collective optical properties to the plasmonic assemblies. Our self-assembly and measurement strategy offers a versatile platform for exploring optical interactions in a wide range of material systems and application areas

The Physics and Applications of a 3D Plasmonic Nanostructure

In this work, the dynamics of electromagnetic field interactions with free electrons in a 3D metallic nanostructure is evaluated theoretically. This dissertation starts by reviewing the relevant fundamentals of plasmonics and modern applications of plasmonic systems. Then, motivated by the need to have a simpler way of understanding the surface charge dynamics on complex plasmonic nanostructures, a new plasmon hybridization tree method is introduced. This method provides the plasmonicist with an intuitive way to determine the response of free electrons to incident light in complex nanostructures within the electrostatic regime. Next, a novel 3D plasmonic nanostructure utilizing reflective plasmonic coupling is designed to perform biosensing and plasmonic tweezing applications. By applying analytical and numerical methods, the effectiveness of this nanostructure at performing these applications is determined from the plasmonic response of the nanostructure to an excitation beam of coherent light. During this analysis, it was discovered that under certain conditions, this 3D nanostructure exhibits a plasmonic Fano resonance resulting from the interference of an in-plane dark mode and an out-of-plane bright mode. In evaluating this nanostructure for sensing changes in the local dielectric environment, a figure of merit of 68 is calculated, which is competitive with current localized surface plasmon resonance refractometric sensors. By evaluating the Maxwell stress tensor on a test particle in the vicinity of the nanostructure, it was found that under the right conditions, this plasmonic nanostructure design is capable of imparting forces greater than 10.5 nN on dielectric objects of nanoscale dimensions. The results obtained in these studies provides new routes to the design and engineering of 3D plasmonic nanostructures and Fano resonances in these systems. In addition, the nanostructure presented in this work and the design principles it utilizes have shown performance metrics which make it an important contribution to the fields of LSPR biosensing and plasmonic trapping and force transduction.Ph.D., Electrical Engineering -- Drexel University, 201

Tunable field enhancement in plasmonic nanostructures

Metallic nanostructures that contain bound geometries will support localised surface plasmon (LSP) resonances if they are illuminated with light of appropriate frequency. These LSP resonances result in a concentration of the electric field of the incident light into a volume which is smaller than the photon wavelength. Certain geometries that support LSP resonances are sensitive to the polarisation of incident light, and the enhanced electromagnetic field can therefore be tuned in situ by adjusting this polarisation. We have investigated polarisation tunable LSP field enhancement by observing, in the linear regime, the interaction of an asymmetric cruciform aperture structure with a chemical bond and, in the non-linear regime, the second harmonic generation (SHG) produced by three metallic nanostructures. Numerical simulations implementing rigorous coupled-wave analysis (RCWA) were used to find asymmetric cruciform aperture dimensions that produced LSP resonances when illuminated with light of a wavelength between 2 μm and 8 μm. Arrays of these apertures were fabricated in a 35 nm thick gold film on a transparent calcium fluoride (CaF_{2}) substrate. The fabrication methods used to create the apertures were either focused ion beam (FIB) milling, or electron beam lithography (EBL) with argon ion milling, of the gold film. Fourier transform infrared spectroscopy (FTIR) was used to measure the transmission and reflection spectra of these plasmonic nanostructures. The apertures were coated with poly(methyl methacrylate) (PMMA), which has a local absorption maximum at 5.784 μm created by the stretching of its carbonyl bonds. The transmission and reflection spectra of the PMMA-coated apertures were measured using FTIR. The interaction of the LSP and molecular resonances was shown to form an asymmetric Fano resonance at the carbonyl bond wavelength. We found that this Fano resonance can be tuned in situ by rotating the polarisation of incident light. A classical mechanical oscillator model was developed to interpret the reflection and transmission spectrum in terms of the interference of the LSP and molecular resonances. A quantum mechanical model was also developed and used to predict the absorption spectrum of the system. This quantum mechanical model provides information on the physical interactions within the system, and predicts a near-field mediated interaction between the plasmon and molecular resonances. Nonlinear optical measurements were made using an SHG microscope, which allowed the location of near-field SHG hotspots to be determined. Three geometries were measured using this technique using fundamental wavelengths of 800 nm or 1 μm. The first geometry, a chiral star structure, was found to display dichroic SHG that was dependent on the handedness of the incident circularly-polarised fundamental light. The second, a `windmill' structure, was used to investigate the dependence of near-field SHG on the linear polarisation of fundamental light; the ablation of these metallic windmill structures by the fundamental demonstrates that laser ablation of patterned surfaces is dependent on the LSP resonance of the constituent structures. Finally, the spatial dependence of SHG produced by a cruciform aperture structure in a gold film illuminated by linearly polarised light was observed. SHG intensity was found to be greatest along the axis of the cruciform which was perpendicular to the incident E field polarisation

Gradient metasurfaces: a review of fundamentals and applications

In the wake of intense research on metamaterials the two-dimensional analogue, known as metasurfaces, has attracted progressively increasing attention in recent years due to the ease of fabrication and smaller insertion losses, while enabling an unprecedented control over spatial distributions of transmitted and reflected optical fields. Metasurfaces represent optically thin planar arrays of resonant subwavelength elements that can be arranged in a strictly or quasi periodic fashion, or even in an aperiodic manner, depending on targeted optical wavefronts to be molded with their help. This paper reviews a broad subclass of metasurfaces, viz. gradient metasurfaces, which are devised to exhibit spatially varying optical responses resulting in spatially varying amplitudes, phases and polarizations of scattered fields. Starting with introducing the concept of gradient metasurfaces, we present classification of different metasurfaces from the viewpoint of their responses, differentiating electrical-dipole, geometric, reflective and Huygens' metasurfaces. The fundamental building blocks essential for the realization of metasurfaces are then discussed in order to elucidate the underlying physics of various physical realizations of both plasmonic and purely dielectric metasurfaces. We then overview the main applications of gradient metasurfaces, including waveplates, flat lenses, spiral phase plates, broadband absorbers, color printing, holograms, polarimeters and surface wave couplers. The review is terminated with a short section on recently developed nonlinear metasurfaces, followed by the outlook presenting our view on possible future developments and perspectives for future applications.Comment: Accepted for publication in Reports on Progress in Physic

Active graphene metasurfaces for optoelectronic applications

Plasmonic metasurfaces are optical components that enhance the light-matter interaction and can control the flow of light. The scalability and universality of the metasurface design enables their deployment across the entire electromagnetic spectrum. Especially attractive are the metasurfaces designed to operate in the mid-infrared part of the optical spectrum. That is due to two factors: the variety of technological applications and the limited choice of conventional optical components in the mid-IR spectral region. Graphene has emerged as a promising optoelectronic material because its optical properties can be rapidly and dramatically changed using electric gating. In particular in the mid-IR regime, graphene has plasmonic properties and can be utilized as a tunable inductor for active modulation of the light with an ultra-fast rate. However graphene’s weak optical response, especially in the infrared part of the spectrum, remains the key challenge to developing practical graphene-based optical devices such as modulators, infrared detectors, and tunable reflect-arrays. In this thesis, we take advantage of plasmonic metasurfaces to enhance the light interaction with graphene for crucial optoelectronic applications. The ability to modulate light at a high-speed is an important part of modern communication systems. We use the plasmonic properties of graphene in the mid-IR range to spectrally shift a narrow-width Fano resonance. Using this approach, we achieve strong modulation of amplitude and phase using plasmonic Fano-resonant metasurfaces integrated with graphene. We also demonstrate that the strong spectral shift of the plasmonic resonance can be used to extract one of the key optical parameters of graphene: the free carrier scattering rate. Another interesting branch of optics is wave-front engineering. The ability to dynamically manipulate the phase, inspires interesting applications such as beam steering and holograms. We use a Michelson interferometry to measure the graphene-induced phase modulation. In particular, we show that it is possible to modulate the phase of electromagnetic wave while the amplitude is constant. We demonstrate proof of concept application of active phase modulation in motion sensing and polarization conversion. An emerging area of optoelectronic is ultra-fast photodetectors based on graphene and other 2-D materials. We employ Full-wave and electrostatic simulations to study the performance of a metasurface-based photodetector. Circuit analysis is utilized to provide a mathematical relation for the responsivity of the metasurface-based graphene photodetector. It is shown that electrically-connected metasurfaces can dramatically enhance the collection efficiency of graphene photodetectors.Physic

直交偏光二重共鳴を有するAuメタ表面の第二高調波生成

Tohoku University石原照也課

Design of an environment-indipendent, tunable 3D DNA-origami plasmonic sensor

DNA origami nanotechnology engineers DNA as the building blocks of newly conceived self-assembled materials and devices. Due to its high degree of customization and its precise spatial addressability, DNA origami provides an unmatched platform for nanoscale structures and devices design. Gold nanoparticles (AuNP) have been largely investigated because of their peculiar optical properties and in particular their localized surface plasmon resonance (LSPR) that modifies significantly the electromagnetic environment in a thin shell around them, and provides a tool with unrivalled potential to tune the local optical properties. The combination of DNA origami frameworks and AuNP into DNA based-plasmonic nanostructures offers a concrete approach for optical properties engineering. It has been successfully applied to design biosensor and to enhance Raman scattering or fluorescence emission. Moreover, it has been exploited to design molecular ruler in which the inter-particle gap is controlled with nanometric precision through the transduction of the conformational changes into univocally detectable optical signals. In this thesis I present my PhD work which aims at the design of an environment-independent AuNP decorated-DNA origami. A tetrahedral DNA shape structure has been selected for its three dimensional robustness and thus a DNA origami prototype has been assembled, characterized with SEM, TEM and AFM to verify the proper folding of the structure. The origami was equipped with an actuator probe which recognizes a specific target oligonucleotide inducing a structural reconfiguration of the tetrahedron. To detect the conformational change triggered by the hybridization event, I functionalized the origami with two gold nanoparticles placed in two opposite facets at a known distance of 10 nm: the change of the interparticle gap is effectively transduced in a LSPR shift. This working principle has been verified with optical extinction measurements and the interparticle distance reduction has been confirmed by SEM imaging and SAXS analysis performed in the SAXS beamline of Elettra Synchrotron, thus confirming that the operation of the device and its transduction mechanism are the same no matter of the external conditions, being them dry, liquid or solid