Stefano Palomba

Three-dimensional atomic-scale structure of size-selected gold nanoclusters

An unambiguous determination of the three-dimensional structure of nanoparticles is challenging. Electron tomography requires a series of images taken for many different specimen orientations. This approach is ideal for stable and stationary structures. But ultrasmall nanoparticles are intrinsically structurally unstable and may interact with the incident electron beam, constraining the electron beam density that can be used and the duration of the observation. Here we use aberration-corrected scanning transmission electron microscopy, coupled with simple imaging simulation, to determine with atomic resolution the size, three-dimensional shape, orientation and atomic arrangement of size-selected gold nanoclusters that are preformed in the gas phase and soft-landed on an amorphous carbon substrate. The structures of gold nanoclusters containing 309±6 atoms can be identified with either Ino-decahedral, cuboctahedral or icosahedral geometries. Comparison with theoretical modelling of the system suggests that the structures are consistent with energetic considerations. The discovery that nanoscale gold particles function as active and selective catalysts for a variety of important chemical reactions has provoked much research interest in recent years. We believe that the detailed structure information we provide will help to unravel the role of these nanoclusters in size- and structure-specific catalytic reactions. We note that the technique will be of use in investigations of other supported ultrasmall metal cluster systems.

Compact Magnetic Antennas for Directional Excitation of Surface Plasmons

Plasmonics is considered as one of the most promising candidates for implementing the next generation of ultrafast and ultracompact photonic circuits. Considerable effort has been made to scale down individual plasmonic components into the nanometer regime. However, a compact plasmonic source that can efficiently generate surface plasmon polaritons (SPPs) and deliver SPPs to the region of interest is yet to be realized. Here, bridging the optical antenna theory and the recently developed concept of metamaterials, we demonstrate a subwavelength, highly efficient plasmonic source for directional generation of SPPs. The designed device consists of two nanomagnetic resonators with detuned resonant frequencies. At the operating wavelength, incident photons can be efficiently channeled into SPP waves modulated by the electric field polarization. By tailoring the relative phase at resonance and the separation between the two nanoresonators, SPPs can be steered to predominantly propagate along one specific direction. This novel magnetic nanoantenna paves a new way to manipulate photons in the near-field, and also could be useful for SPP-based nonlinear applications, active modulations, and wireless optical communications.

Nonlinear plasmonics with gold nanoparticle antennas

We investigate the nonlinear optical properties of gold nanoparticle pairs. Two excitation beams of frequencies ω1 and ω2 are used to induce nonlinear polarizations at the junction of a particle dimer. Nonlinearities of the second and third order can be controllably induced as a function of the dimer geometry, leading predominantly to second-harmonic generation (SHG), sum frequency generation (SFG) and four-wave mixing (4WM). Due to their center symmetry, dimers with identical particle diameters give rise to a very weak second-order response, without affecting the third-order response. Therefore, a sharp probe functionalized with a symmetric metal dimer acts as a nanoscale photon source emitting narrow-band photons of frequency 2ω1 − ω2. We demonstrate that this source can be employed as a near-field optical probe for high-resolution fluorescence imaging.

Near-Field Imaging with a Localized Nonlinear Light Source

We demonstrate high-resolution near-field imaging and spectroscopy using the nonlinear optical response of a gold nanoparticle pair as an excitation photon source. Femtosecond pulses of frequencies omega(1) and omega(2) are used to induce a nonlinear polarization at the four wave mixing (4WM) frequency 2omega(1) - omega(2) in the junction of the nanoparticle dimer. The nonlinear response leads to localized photon emission, which is employed as an excitation source for fluorescence and extinction imaging. The principle of this imaging technique is demonstrated for samples of fluorescent nanospheres and tubular J-aggregates.

Optical negative refraction by four-wave mixing in thin metallic nanostructures

The law of refraction first derived by Snellius and later introduced as the Huygens-Fermat principle, states that the incidence and refracted angles of a light wave at the interface of two different materials are related to the ratio of the refractive indices in each medium. Whereas all natural materials have a positive refractive index and therefore exhibit refraction in the positive direction, artificially engineered negative index metamaterials have been shown capable of bending light waves negatively. Such a negative refractive index is the key to achieving a perfect lens that is capable of imaging well below the diffraction limit. However, negative index metamaterials are typically lossy, narrow band, and require complicated fabrication processes. Recently, an alternative approach to obtain negative refraction from a very thin nonlinear film has been proposed and experimentally demonstrated in the microwave region. However, such approaches use phase conjugation, which makes optical implementations difficult. Here, we report a simple but different scheme to demonstrate experimentally nonlinear negative refraction at optical frequencies using four-wave mixing in nanostructured metal films. The refractive index can be designed at will by simply tuning the wavelengths of the interacting waves, which could have potential impact on many important applications, such as superlens imaging.

Nonlinear Dark-Field Microscopy

Dark-field microscopy is a background-free imaging method that provides high sensitivity and a large signal-to-noise ratio. It finds application in nanoscale detection, biophysics and biosensing, particle tracking, single molecule spectroscopy, X-ray imaging, and failure analysis of materials. In dark-field microscopy, the unscattered light path is typically excluded from the angular range of signal detection. This restriction reduces the numerical aperture and affects the resolution. Here we introduce a nonlinear dark-field scheme that overcomes this restriction. Two laser beams of frequencies ω1 and ω2 are used to illuminate a sample surface and to generate a purely evanescent field at the four-wave mixing (4WM) frequency ω4wm = 2ω1 - ω2. The evanescent 4WM field scatters at sample features and generates radiation that is detected by standard far-field optics. This nonlinear dark-field scheme works with samples of any material and is compatible with applications ranging from biological imaging to failure analysis.

Pinning of size-selected Pd nanoclusters on graphite

The production of stable cluster arrays on smooth surfaces has several potential technological applications. We report a study of the pinning of size-selected palladium nanoclusters on the graphite surface. The clusters formed during gas aggregation in vacuum are projected with sufficient kinetic energy to create a defect in the graphite surface. The energy necessary to create such an immobilizing defect is investigated as a function of the palladium cluster size. The palladium pinning energy is found to deviate from the simple binary collision model as appropriate to previously reported silver and gold results. This finding is in agreement with the deviation of nickel clusters and points to the influence of the interatomic cluster bonding on the mechanics of the collision.

Nonlinear plasmonics at planar metal surfaces

We investigate the nonlinear optical response of a noble metal surface. We derive the components of the third-order nonlinear susceptibility and determine an absolute value of χ(3)≈0.2 nm2 V−2, a value that is more than two orders of magnitude larger than the values found for typical nonlinear laser crystals. Using nonlinear four-wave mixing (4WM) with incident laser pulses of frequencies ω1 and ω2, we generate fields oscillating at the nonlinear frequency ω4WM=2ω1−ω2. We identify and discuss three distinct regimes: (i) a regime where the 4WM field is propagating, (ii) a regime where it is evanescent, and (iii) a regime where the nonlinear response couples to surface plasmon polaritons.

New avenues for phase matching in nonlinear hyperbolic metamaterials

Nonlinear optical processes, which are of paramount importance in science and technology, involve the generation of new frequencies. This requires phase matching to avoid that light generated at different positions interferes destructively. Of the two original approaches to achieve this, one relies on birefringence in optical crystals, and is therefore limited by the dispersion of naturally occurring materials, whereas the other, quasi-phase-matching, requires direct modulation of material properties, which is not universally possible. To overcome these limitations, we propose to exploit the unique dispersion afforded by hyperbolic metamaterials, where the refractive index can be arbitrarily large. We systematically analyse the ensuing opportunities and demonstrate that hyperbolic phase matching can be achieved with a wide range of material parameters, offering access to the use of nonlinear media for which phase matching cannot be achieved by other means. With the rapid development in the fabrication of hyperbolic metamaterials, our approach is destined to bring significant advantages over conventional techniques for the phase matching of a variety of nonlinear processes.

Tuning the cavity modes of a Fabry-Perot resonator using gold nanoparticles.

We study the interaction between a planar Fabry-Perot microcavity and a single embedded gold nanoparticle. We record spatially resolved white-light transmission spectra and show that the interaction between microcavity and nanoparticle gives rise to a redshift of the cavity modes, in accordance with the Bethe-Schwinger cavity perturbation formula. The spectral tuning of cavity modes by discrete nanoparticles can be exploited for applications such as optical and micromechanical sensing.