Electromagnetic Forces and Torques: From Dielectrophoresis to Optical Tweezers.

Electromagnetic forces and torques enable many key technologies, including optical tweezers or dielectrophoresis. Interestingly, both techniques rely on the same physical process: the interaction of an oscillating electric field with a particle of matter. This work provides a unified framework to understand this interaction both when considering fields oscillating at low frequencies─dielectrophoresis─and high frequencies─optical tweezers. We draw useful parallels between these two techniques, discuss the different and often unstated assumptions they are based upon, and illustrate key applications in the fields of physical and analytical chemistry, biosensing, and colloidal science.

Planar 16-band metasurface-enhanced spectral filter for integrated image sensing.

We study theoretically and demonstrate experimentally a 16-band narrow band wavelength selective filter in the near-infrared range. The combination of a pair of distributed Bragg reflectors with a sub-wavelength grating metasurface embedded in the intra-cavity provides a narrow response which can be tuned by adjusting the geometry of the sub-wavelength grating metasurface. The key advantage of this approach is its ease of fabrication, where the spectral response is tuned by merely changing the grating period, resulting in a perfectly planar geometry that can be easily integrated with a broad variety of photodetectors, thus enabling attractive applications such as bio-imaging, time-of-flight sensors and LiDAR. The experimental results are supported by numerical simulations and effective medium theory that unveil the mechanisms that lead to the optical response of the device. It is also shown how the polarization dependence of the structure can be used to determine very accurately the polarization of incoming light.

Demonstration of a Plasmonic Nonlinear Pseudodiode.

We demonstrate a nonlinear plasmonic metasurface that exhibits strongly asymmetric second-harmonic generation: nonlinear scattering is efficient upon excitation in one direction, and it is substantially suppressed when the excitation direction is reversed, thus enabling a diode-like functionality. A significant (approximately 10 dB) extinction ratio of SHG upon opposite excitations is measured experimentally, and those findings are substantiated with full-wave simulations. This effect is achieved by employing a combination of two commonly used metals─aluminum and silver─producing a material composition asymmetry that results in a bianisotropic response of the system, as confirmed by performing homogenization analysis and extracting an effective susceptibility tensor. Finally, we discuss the implications of our results from the more fundamental perspectives of reciprocity and time-reversal asymmetry.

Protein Dielectrophoresis with Gradient Array of Conductive Electrodes Sheds New Light on Empirical Theory.

Dielectrophoresis (DEP) is a versatile tool for the precise microscale manipulation of a broad range of substances. To unleash the full potential of DEP for the manipulation of complex molecular-sized particulates such as proteins requires the development of appropriate theoretical models and their comprehensive experimental verification. Here, we construct an original DEP platform and test the Hölzel-Pethig empirical model for protein DEP. Three different proteins are studied: lysozyme, BSA, and lactoferrin. Their molecular Clausius-Mossotti function is obtained by detecting their trapping event via the measurement of the fluorescence intensity to identify the minimum electric field gradient required to overcome dispersive forces. We observe a significant discrepancy with published theoretical data and, after a very careful analysis to rule out experimental errors, conclude that more sophisticated theoretical models are required for the response of molecular entities in DEP fields. The developed experimental platform, which includes arrays of sawtooth metal electrode pairs with varying gaps and produces variations of the electric field gradient, provides a versatile tool that can broaden the utilization of DEP for molecular entities.

Quadrupolar susceptibility modeling of substrated metasurfaces with application to the generalized Brewster effect.

We derive generalized sheet transition conditions (GSTCs) including dipoles and quadrupoles, using generalized functions (distributions). This derivation verifies that the GSTCs are valid for metasurfaces in non-homogeneous environments, such as for practical metasurfaces fabricated on a substrate. The inclusion of quadrupoles and modeling of spatial dispersion provides additional hyper-susceptibility components which serve as degrees of freedom for wave transformations. We leverage them to demonstrate a generalized Brewster effect with multiple angles of incidence at which reflection is suppressed, along with an "anti-Brewster" effect where transmission is suppressed.

Trap-and-Track for Characterizing Surfactants at Interfaces.

Understanding the behavior of surfactants at interfaces is crucial for many applications in materials science and chemistry. Optical tweezers combined with trajectory analysis can become a powerful tool for investigating surfactant characteristics. In this study, we perform trap-and-track analysis to compare the behavior of cetyltrimethylammonium bromide (CTAB) and cetyltrimethylammonium chloride (CTAC) at water-glass interfaces. We use optical tweezers to trap a gold nanoparticle and statistically analyze the particle's movement in response to various surfactant concentrations, evidencing the rearrangement of surfactants adsorbed on glass surfaces. Our results show that counterions have a significant effect on surfactant behavior at the interface. The greater binding affinity of bromide ions to CTA+ micelle surfaces reduces the repulsion among surfactant head groups and enhances the mobility of micelles adsorbed on the interface. Our study provides valuable insights into the behavior of surfactants at interfaces and highlights the potential of optical tweezers for surfactant research. The development of this trap-and-track approach can have important implications for various applications, including drug delivery and nanomaterials.

Electromagnetic forces in the time domain.

We look beyond the standard time-average approach and investigate optical forces in the time domain. The formalism is developed for both the Abraham and Minkowski momenta, which appear to converge in the time domain. We unveil an extremely rich - and by far unexplored - physics associated with the dynamics of the optical forces, which can even attain negative values over short time intervals or produce low frequency dynamics that can excite mechanical oscillations in macroscopic objects under polychromatic illumination. The magnitude of this beating force is tightly linked to the average one. Implications of this work for transient optomechanics are discussed.

Pitfalls in the spectral measurements of polarization-altering metasurfaces.

The optical characterization of metasurfaces and nanostructures that alter the polarization of light is tricky and can lead to unphysical results, such as reflectance beyond unity. We track the origin of such pitfalls to the response of some typical optical components used in a commercial microscope or a custom-made setup. In particular, the beam splitter and some mirrors have different responses for both polarizations and can produce wrong results. A simple procedure is described to correct these erroneous results, based on the optical characterization of the different components in the optical setup. With this procedure, the experimental results match the numerical simulations perfectly. The methodology described here is simple and will enable the accurate spectral measurements of nanostructures and metasurfaces that alter the polarization of the incoming light.

Multipolar scattering analysis of hybrid metal-dielectric nanostructures.

We perform a systematic study showing the evolution of the multipoles along with the spectra for a hybrid metal-dielectric nanoantenna, a Si cylinder and an Ag disk stacked one on top of another, as its dimensions are varied one by one. We broaden our analysis to demonstrate the "magnetic light" at energies above 1 eV by varying the height of the Ag on the Si cylinder and below 1 eV by introducing insulating spacing between them. We also explore the appearance of the anapole state along with some exceptionally narrow spectral features by varying the radius of the Ag disk.

Role of electric currents in the Fano resonances of connected plasmonic structures.

In this work, we use finite elements simulations to study the far field properties of two plasmonic structures, namely a dipole antenna and a cylinder dimer, connected to a pair of nanorods. We show that electrical, rather than near field, coupling between the modes of these structures results in a characteristic Fano lineshape in the far field spectra. This insight provides a way of tailoring the far field properties of such systems to fit specific applications, especially maintaining the optical properties of plasmonic antennas once they are connected to nanoelectrodes. This work extends the previous understanding of Fano resonances as generated by a simple near field coupling and provides a route to an efficient design of functional plasmonic electrodes.

Hot carrier-mediated avalanche multiphoton photoluminescence from coupled Au-Al nanoantennas.

Avalanche multiphoton photoluminescence (AMPL) is observed from coupled Au-Al nanoantennas under intense laser pumping, which shows more than one order of magnitude emission intensity enhancement and distinct spectral features compared with ordinary metallic photoluminescence. The experiments are conducted by altering the incident laser intensity and polarization using a home-built scanning confocal optical microscope. The results show that AMPL originates from the recombination of avalanche hot carriers that are seeded by multiphoton ionization. Notably, at the excitation stage, multiphoton ionization is shown to be assisted by the local electromagnetic field enhancement produced by coupled plasmonic modes. At the emission step, the giant AMPL intensity can be evaluated as a function of the local field environment and the thermal factor for hot carriers, in accordance with a linear relationship between the power law exponent coefficient and the emitted photon energy. The dramatic change in the spectral profile is explained by spectral linewidth broadening mechanisms. This study offers nanospectroscopic evidence of both the potential optical damages for plasmonic nanostructures and the underlying physical nature of light-matter interactions under a strong laser field; it illustrates the significance of the emerging topics of plasmonic-enhanced spectroscopy and laser-induced breakdown spectroscopy.

Hybrid Metal-Dielectric Metasurfaces for Refractive Index Sensing.

Hybrid metal-dielectric nanostructures have recently gained prominence because they combine strong field enhancement of plasmonic metals and the several low-loss radiation channels of dielectric resonators, which are qualities pertaining to the best of both worlds. In this work, an array of such hybrid nanoantennas is successfully fabricated over a large area and utilized for bulk refractive index sensing with a sensitivity of 208 nm/RIU. Each nanoantenna combines a Si cylinder with an Al disk, separated by a SiO2 spacer. Its optical response is analyzed in detail using the multipoles supported by its subparts and their mutual coupling. The nanoantenna is further modified experimentally with an undercut in the SiO2 region to increase the interaction of the electric field with the background medium, which augments the sensitivity to 245 nm/RIU. A detailed multipole analysis of the hybrid nanoantenna supports our experimental findings.

Multipole interplay controls optical forces and ultra-directional scattering.

We analyze the superposition of Cartesian multipoles to reveal the mechanisms underlying the origin of optical forces. We show that a multipolar decomposition approach significantly simplifies the analysis of this problem and leads to a very intuitive explanation of optical forces based on the interference between multipoles. We provide an in-depth analysis of the radiation coming from the object, starting from low-order multipole interactions up to quadrupolar terms. Interestingly, by varying the phase difference between multipoles, the optical force as well as the total radiation directivity can be well controlled. The theory developed in this paper may also serve as a reference for ultra-directional light steering applications.

Teaching optics to a machine learning network.

In this Letter, we demonstrate how harmonic oscillator equations can be integrated in a neural network to improve the spectral response prediction for an optical system. We use the optical properties of a one-dimensional nanoslit array for a practical implementation of the study. This method allows to build more generalizable relations between the input parameters of the array and its optical properties, showing a 20-fold improvement for parameters outside the range used for the training. We also show how this model generates the output spectrum from phenomenological relationships between the input parameters and the output spectrum, indicating how it grasps the physical mechanisms of the optical response of the structure.

Electronic Structure-Dependent Surface Plasmon Resonance in Single Au-Fe Nanoalloys.

The relationship between composition and plasmonic properties in noble metal nanoalloys is still largely unexplored. Yet, nanoalloys of noble metals, such as gold, with transition elements, such as iron, have unique properties and a number of potential applications, ranging from nanomedicine to magneto-plasmonics and plasmon-enhanced catalysis. Here, we investigate the localized surface plasmon resonance at the level of the single Au-Fe nanoparticle by applying a strategy that combines experimental measurements using near field electron energy loss spectroscopy with theoretical studies via a full wave numerical analysis and density functional theory calculations of electronic structure. We show that, as the iron fraction increases, the plasmon resonance is blue-shifted and significantly damped, as a consequence of the changes in the electronic band structure of the alloy. This allows the identification of three relevant phenomena to be considered in the design and realization of any plasmonic nanoalloy, specifically: the appearance of new states around the Fermi level; the change in the free electron density of the metal; and the blue shift of interband transitions. Overall, this study provides new opportunities for the control of the optical response in Au-Fe and other plasmonic nanoalloys, which are useful for the realization of magneto-plasmonic devices for molecular sensing, thermo-plasmonics, bioimaging, photocatalysis, and the amplification of spectroscopic signals by local field enhancement.

Studying the different coupling regimes for a plasmonic particle in a plasmonic trap.

Plasmonic antennas improve the stiffness and resolution of optical tweezers by producing a strong near-field. When the antenna traps metallic objects, the optically-resonant object affects the near-field trap, and this interaction should be examined to estimate the optical force accurately. We study this effect in detail by evaluating the force using both Maxwell's stress tensor and the dipole approximation. In spite of the strong optical interaction between the particle and the antenna, the results show that the dipole approximation remains accurate for calculating forces on Rayleigh particles. For particles whose sizes exceed the dipole limit, we observe different coupling regimes where the force becomes either attractive or repulsive. The distributions of field amplitudes and polarization charges explain such a behavior.

Modes interplay and dynamics in the second harmonic generation of plasmonic nanostructures.

The full wave surface integral equation computation of the second harmonic generation (SHG) dynamics for metal spheres and nanorods - presented as multimedia files - is performed to reveal the dynamics of the modes supported by the nanostructure. We demonstrate that the interplay between different modes controls the nonlinear response and that the size-induced redshift of the eigenmodes can be manipulated by adjusting the nanostructure geometry, so that the SHG signal can be boosted at specified frequencies. We show that the SHG radiation is not necessarily quadrupolar in spherical nanoparticles, as it is often assumed. Finally, we introduce an efficient way to reduce the SHG calculation time.

High-quality single crystal diamond diffraction gratings fabricated by crystallographic etching.

We demonstrate a novel method for fabricating single crystal diamond diffraction gratings based on crystallographic etching that yields high-quality diffraction gratings from commercially available <100> diamond plates. Both V-groove and rectangular gratings were fabricated and characterised using scanning electron microscopy and atomic force microscopy, revealing angles of 57° and 87° depending on the crystal orientation, with mean roughness below Ra = 5 nm on the sidewalls. The gratings were also optically characterised, showing good agreement with simulated results. The fabrication method demonstrated in this contribution shows the way for manufacturing high-quality diamond diffractive components that surpass existing devices both in quality and manufacturability.

Photocatalytic ammonia production enhanced by a plasmonic near-field and hot electrons originating from aluminium nanostructures.

Ammonia production at room temperature and atmospheric pressure is in high demand to assist in energy saving and the protection of the environment worldwide, as well as to help reduce CO2 emissions. Recently, plasmonic nanomaterials have been frequently used for solar to chemical energy conversion, which has the potential to replace existing energy-intensive industrial processes. In our approach, plasmonic aluminium nanotriangles (AlNTs) were used to investigate the impact of plasmonic effects on photocatalytic ammonia production. Plasmonic near-field coupling to a semiconductor and hot electron generation from AlNTs were studied in detail through the use of electrochemical photocurrent measurements. A narrowband LED beam with a central wavelength at 365 nm was used to illuminate the AlNTs and their hot electron generation efficiency was estimated to be 2 × 10-4%, resulting in an ammonia production rate of 4 × 10-5 µM h-1 mW-1 cm-2, which corresponds to a quantum efficiency of 2.5 × 10-5%. In the case of plasmonic near-field coupling, AlNTs-embedded TiO2 demonstrates a charge-carrier generation efficiency of 2.7%, which is ∼2.3 times higher than that of bare TiO2. The ammonia production rate of AlNTs-TiO2 is 0.1 µM h-1 mW-1 cm-2 with a quantum efficiency of ∼0.06%, which corresponds to ∼2.4 times that of the rate demonstrated by bare TiO2 (0.04 µM h-1 mW-1 cm-2, quantum efficiency ∼ 0.025%). The obtained results confirm successful ammonia production through nitrogen splitting at room temperature and under atmospheric pressure. Moreover, according to the presented results, the use of plasmonic aluminium structures remarkably improves the ammonia production rate.

Less Is More: Enhancement of Second-Harmonic Generation from Metasurfaces by Reduced Nanoparticle Density.

We investigate optical second-harmonic generation (SHG) from metasurfaces where noncentrosymmetric V-shaped gold nanoparticles are ordered into regular array configurations. In contrast to expectations, a substantial enhancement of the SHG signal is observed when the number density of the particles in the array is reduced. More specifically, by halving the number density, we obtain over 5-fold enhancement in SHG intensity. This striking result is attributed to favorable interparticle interactions mediated by the lattice, where surface-lattice resonances lead to spectral narrowing of the plasmon resonances. Importantly, however, the results cannot be explained by the improved quality of the plasmon resonance alone. Instead, the lattice interactions also lead to further enhancement of the local fields at the particles. The experimental observations agree very well with results obtained from numerical simulations including lattice interactions.