Focusing Surface Acoustic Waves with a Plasmonic Hypersonic Lens.

Plasmonic nanoantennas have proven to be efficient transducers of electromagnetic to mechanical energy and vice versa. The sudden thermal expansion of these structures after an ultrafast optical pulsed excitation leads to the emission of hypersonic acoustic waves to the supporting substrate, which can be detected by another antenna that acts as a high-sensitivity mechanical probe due to the strong modulation of its optical response. Here, we propose and experimentally demonstrate a nanoscale acoustic lens comprised of 11 gold nanodisks whose collective oscillation at gigahertz frequencies gives rise to an interference pattern that results in a diffraction-limited surface acoustic beam of about 340 nm width, with an amplitude contrast of 60%. Via spatially decoupled pump-probe experiments, we were able to map the radiated acoustic energy in the proximity of the focal area, obtaining a very good agreement with the continuum elastic theory.

Challenges on optical printing of colloidal nanoparticles.

While colloidal chemistry provides ways to obtain a great variety of nanoparticles with different shapes, sizes, material compositions, and surface functions, their controlled deposition and combination on arbitrary positions of substrates remain a considerable challenge. Over the last ten years, optical printing arose as a versatile method to achieve this purpose for different kinds of nanoparticles. In this article, we review the state of the art of optical printing of single nanoparticles and discuss its strengths, limitations, and future perspectives by focusing on four main challenges: printing accuracy, resolution, selectivity, and nanoparticle photostability.

Direct Detection of Optical Forces of Magnetic Nature in Dielectric Nanoantennas.

Optical forces on nanostructures are usually characterized by their interaction with the electric field component of the light wave, given that most materials present negligible magnetic response at optical frequencies. This is not the case however of a high-refractive-index dielectric nanoantenna, which has been recently shown to efficiently support both electric and magnetic optical modes. In this work, we use a photoinduced force microscopy configuration to measure optically induced forces produced by a germanium nanoantenna on a surrounding silicon near-field probe. We reveal the spatial distribution, character, and magnitude of the generated forces when exciting the nanoantenna at its anapole state condition. We retrieve optical force maps showing values of up to 20 pN, which are found to be mainly magnetic in nature, according to our numerical simulations. The results of this investigation open new pathways for the study, detection, and generation of magnetic light forces at the nanometer scale.

Acoustic Far-Field Hypersonic Surface Wave Detection with Single Plasmonic Nanoantennas.

The optical properties of small metallic particles allow us to bridge the gap between the myriad of subdiffraction local phenomena and macroscopic optical elements. The optomechanical coupling between mechanical vibrations of Au nanoparticles and their optical response due to collective electronic oscillations leads to the emission and the detection of surface acoustic waves (SAWs) by single metallic nanoantennas. We take two Au nanoparticles, one acting as a source and the other as a receptor of SAWs and, even though these antennas are separated by distances orders of magnitude larger than the characteristic subnanometric displacements of vibrations, we probe the frequency content, wave speed, and amplitude decay of SAWs originating from the damping of coherent mechanical modes of the source. Two-color pump-probe experiments and numerical methods reveal the characteristic Rayleigh wave behavior of emitted SAWs, and show that the SAW-induced optical modulation of the receptor antenna allows us to accurately probe the frequency of the source, even when the eigenmodes of source and receptor are detuned.

Monitoring in real-time focal adhesion protein dynamics in response to a discrete mechanical stimulus.

The adhesion of cells to the extracellular matrix is a hierarchical, force-dependent, multistage process that evolves at several temporal scales. An understanding of this complex process requires a precise measurement of forces and its correlation with protein responses in living cells. We present a method to quantitatively assess live cell responses to a local and specific mechanical stimulus. Our approach combines atomic force microscopy with fluorescence imaging. Using this approach, we evaluated the recruitment of adhesion proteins such as vinculin, focal adhesion kinase, paxillin, and zyxin triggered by applying forces in the nN regime to live cells. We observed in real time the development of nascent adhesion sites, evident from the accumulation of early adhesion proteins at the position where the force was applied. We show that the method can be used to quantify the recruitment characteristic times for adhesion proteins in the formation of focal complexes. We also found a spatial remodeling of the mature focal adhesion protein zyxin as a function of the applied force. Our approach allows the study of a variety of complex biological processes involved in cellular mechanotransduction.

Tailored Hypersound Generation in Single Plasmonic Nanoantennas.

Ultrashort laser pulses impinging on a plasmonic nanostructure trigger a highly dynamic scenario in the interplay of electronic relaxation with lattice vibrations, which can be experimentally probed via the generation of coherent phonons. In this Letter, we present studies of hypersound generation in the range of a few to tens of gigahertz on single gold plasmonic nanoantennas, which have additionally been subjected to predesigned mechanical constraints via silica bridges. Using these hybrid gold/silica nanoantennas, we demonstrate experimentally and via numerical simulations how mechanical constraints allow control over their vibrational mode spectrum. Degenerate pump-probe techniques with double modulation are performed in order to detect the small changes produced in the probe transmission by the mechanical oscillations of these single nanoantennas.

Non-plasmonic nanoantennas for surface enhanced spectroscopies with ultra-low heat conversion.

Nanoplasmonics has recently revolutionized our ability to control light on the nanoscale. Using metallic nanostructures with tailored shapes, it is possible to efficiently focus light into nanoscale field 'hot spots'. High field enhancement factors have been achieved in such optical nanoantennas, enabling transformative science in the areas of single molecule interactions, highly enhanced nonlinearities and nanoscale waveguiding. Unfortunately, these large enhancements come at the price of high optical losses due to absorption in the metal, severely limiting real-world applications. Via the realization of a novel nanophotonic platform based on dielectric nanostructures to form efficient nanoantennas with ultra-low light-into-heat conversion, here we demonstrate an approach that overcomes these limitations. We show that dimer-like silicon-based single nanoantennas produce both high surface enhanced fluorescence and surface enhanced Raman scattering, while at the same time generating a negligible temperature increase in their hot spots and surrounding environments.

High-efficiency second harmonic generation from a single hybrid ZnO nanowire/Au plasmonic nano-oligomer.

We introduce a plasmonic-semiconductor hybrid nanosystem, consisting of a ZnO nanowire coupled to a gold pentamer oligomer by crossing the hot-spot. It is demonstrated that the hybrid system exhibits a second harmonic (SH) conversion efficiency of ∼3 × 10(-5)%, which is among the highest values for a nanoscale object at optical frequencies reported so far. The SH intensity was found to be ∼1700 times larger than that from the same nanowire excited outside the hot-spot. Placing high nonlinear susceptibility materials precisely in plasmonic confined-field regions to enhance SH generation opens new perspectives for highly efficient light frequency up-conversion on the nanoscale.

Metal nanoparticle ensembles: tunable laser pulses distinguish monomer from dimer vibrations.

Resonant interaction of laser pulses with plasmons is used to identify vibrations associated with isolated spheres and pairs of contacting spheres in a system of gold nanoparticles. The optical pulses generate coherent mechanical oscillations of both monomers and dimers in the 5-150 GHz range, the amplitudes of which exhibit a strong enhancement when the laser central wavelength is tuned to resonate with the corresponding plasmon. Because of the resonant selection in the excitation process, the widths of the acoustic modes are significantly smaller than broadening caused by the spread in radii in the ensemble.

Coverage and aggregation of gold nanoparticles on silanized glasses.

We study the dynamics of the coverage and aggregation of gold nanoparticles over organosilanized glass substrates for different sizes of nanoparticles. We present measurements of extinction spectra and nanoparticle counting statistics and demonstrate that both methods are equivalent describing those processes. We introduce models that describe the mentioned dynamics, which are characterized by an exponential-like function with two relevant parameters: a saturation value and a characteristic time. The electrostatic repulsion plays a significant role in both processes. The aggregation is dominated by the mobility of the isolated nanoparticles, which first join in dimers and, further in time, in clusters of higher number of nanoparticles.

Photothermal measurement of absorption and scattering losses in thin films excited by surface plasmons.

We present a novel noncontact, photothermal technique, based on the focus error signal of a commercial CD pickup head that allows direct determination of absorption in thin films. Combined with extinction methods, this technique yields the scattering contribution to the losses. Surface plasmon polaritons are excited using the Kretschmann configuration in thin Au films of varying thickness. By measuring the extinction and absorption simultaneously, it is shown that dielectric constants and thickness retrieval leads to inconsistencies if the model does not account for scattering.

Molecular orientation in self-assembled multilayers measured by second harmonic generation using femtosecond pulses.

We present measurements of the optical second-harmonic generation in self assembled multilayer films of PAZO/PAH polymers with the aim to investigate molecular order in the layer-by-layer architecture. The experiments are performed in transmission, using a femtosecond Ti:Sa pulsed laser, which allows a more accurate determination of the amplitude of the second harmonic signal, without interference fringes usually present in nanosecond experiments. We found that the first bilayer, in contact with the substrate, presents a broad distribution of the orientation of the molecules, while the addition of successive bilayers (up to 12) produces ordering of the molecules with a small tilt angle respect to the surface normal. This result, together with the monotonic increment of the second harmonic signal with the number of layers indicates that the molecules grow orderly assembled in these films.

Femtosecond soliton source with fast and broad spectral tunability.

We present a complete set of measurements and numerical simulations of a femtosecond soliton source with fast and broad spectral tunability and nearly constant pulse width and average power. Solitons generated in a photonic crystal fiber, at the low-power coupling regime, can be tuned in a broad range of wavelengths, from 850 to 1200 nm using the input power as the control parameter. These solitons keep almost constant time duration (approximately 40 fs) and spectral widths (approximately 20 nm) over the entire measured spectra regardless of input power. Our numerical simulations agree well with measurements and predict a wide working wavelength range and robustness to input parameters.

Optically induced multispin entanglement in a semiconductor quantum well.

According to quantum mechanics, a many-particle system is allowed to exhibit non-local behaviour, in that measurements performed on one of the particles can affect a second one that is far away. These so-called entangled states are crucial for the implementation of most quantum information protocols and, in particular, gates for quantum computation. Here we use ultrafast optical pulses and coherent techniques to create and control spin-entangled states in an ensemble of non-interacting electrons bound to donors (at least three) and at least two Mn2+ ions in a CdTe quantum well. Our method, relying on the exchange interaction between localized excitons and paramagnetic impurities, can in principle be applied to entangle an arbitrarily large number of spins.