Levitodynamics: Levitation and control of microscopic objects in vacuum.

The control of levitated nano- and micro-objects in vacuum­which capitalizes on scientific achievements in the fields of atomic physics, control theory, and optomechanics­is of considerable interest. The ability to couple the motion of levitated systems to internal degrees of freedom, as well as to external forces and systems, provides opportunities for science and technology. Attractive research directions, ranging from fundamental quantum physics to commercial sensors, have been unlocked by the many recent experimental achievements, including motional ground-state cooling of an optically levitated nanoparticle. Here we review the status, challenges, and prospects of levitodynamics, the multidisciplinary research area devoted to understanding, controlling, and using levitated nano- and micro-objects in vacuum.

Long-range optofluidic control with plasmon heating.

Using light to manipulate fluids has been a long-sought-after goal for lab-on-a-chip applications to address the size mismatch between bulky external fluid controllers and microfluidic devices. Yet, this goal has remained elusive due to the complexity of thermally driven fluid dynamic phenomena, and the lack of approaches that allow comprehensive multiscale and multiparameter studies. Here, we report an innovative optofluidic platform that fulfills this need by combining digital holographic microscopy with state-of-the-art thermoplasmonics, allowing us to identify the different contributions from thermophoresis, thermo-osmosis, convection, and radiation pressure. In our experiments, we demonstrate that a local thermal perturbation at the microscale can lead to mm-scale changes in both the particle and fluid dynamics, thus achieving long-range transport. Furthermore, thanks to a comprehensive parameter study involving sample geometry, temperature increase, light fluence, and size of the heat source, we showcase an integrated and reconfigurable all-optical control strategy for microfluidic devices, thereby opening new frontiers in fluid actuation technology.

Accurate Mass Measurement of a Levitated Nanomechanical Resonator for Precision Force-Sensing.

Nanomechanical resonators are widely operated as force and mass sensors with sensitivities in the zepto-Newton (10-21) and yocto-gram (10-24) regime, respectively. Their accuracy, however, is usually undermined by high uncertainties in the effective mass of the system, whose estimation is a nontrivial task. This critical issue can be addressed in levitodynamics, where the nanoresonator typically consists of a single silica nanoparticle of well-defined mass. Yet, current methods assess the mass of the levitated nanoparticles with uncertainties up to a few tens of percent, therefore preventing to achieve unprecedented sensing performances. Here, we present a novel measurement protocol that uses the electric field from a surrounding plate capacitor to directly drive a charged optically levitated particle in moderate vacuum. The developed technique estimates the mass within a statistical error below 1% and a systematic error of ∼2%, and paves the way toward more reliable sensing and metrology applications of levitodynamics systems.

Optically levitated nanoparticle as a model system for stochastic bistable dynamics.

Nano-mechanical resonators have gained an increasing importance in nanotechnology owing to their contributions to both fundamental and applied science. Yet, their small dimensions and mass raises some challenges as their dynamics gets dominated by nonlinearities that degrade their performance, for instance in sensing applications. Here, we report on the precise control of the nonlinear and stochastic bistable dynamics of a levitated nanoparticle in high vacuum. We demonstrate how it can lead to efficient signal amplification schemes, including stochastic resonance. This work contributes to showing the use of levitated nanoparticles as a model system for stochastic bistable dynamics, with applications to a wide variety of fields.

Three-dimensional manipulation with scanning near-field optical nanotweezers.

Recent advances in nanotechnologies have prompted the need for tools to accurately and non-invasively manipulate individual nano-objects. Among the possible strategies, optical forces have been predicted to provide researchers with nano-optical tweezers capable of trapping a specimen and moving it in three dimensions. In practice, however, the combination of weak optical forces and photothermal issues has thus far prevented their experimental realization. Here, we demonstrate the first three-dimensional optical manipulation of single 50 nm dielectric objects with near-field nanotweezers. The nano-optical trap is built by engineering a bowtie plasmonic aperture at the extremity of a tapered metal-coated optical fibre. Both the trapping operation and monitoring are performed through the optical fibre, making these nanotweezers totally autonomous and free of bulky optical elements. The achieved trapping performances allow for the trapped specimen to be moved over tens of micrometres over a period of several minutes with very low in-trap intensities. This non-invasive approach is foreseen to open new horizons in nanosciences by offering an unprecedented level of control of nanosized objects, including heat-sensitive biospecimens.

Enhancement of high harmonic generation by confining electron motion in plasmonic nanostrutures.

We study high-order harmonic generation (HHG) resulting from the illumination of plasmonic nanostructures with a short laser pulse of long wavelength. We demonstrate that both the confinement of the electron motion and the inhomogeneous character of the laser electric field play an important role in the HHG process and lead to a significant increase of the harmonic cutoff. In particular, in bow-tie nanostructures with small gaps, electron trajectories with large excursion amplitudes experience significant confinement and their contribution is essentially suppressed. In order to understand and characterize this feature, we combine the numerical solution of the time-dependent Schrödinger equation (TDSE) with the electric fields obtained from 3D finite element simulations. We employ time-frequency analysis to extract more detailed information from the TDSE results and classical tools to explain the extended harmonic spectra. The spatial inhomogeneity of the laser electric field modifies substantially the electron trajectories and contributes also to cutoff increase.

Charge distribution induced inside complex plasmonic nanoparticles.

We developed a versatile numerical technique to compute the three-dimensional charge distribution inside plasmonic nanoparticles. This method can be easily applied to investigate the charge distribution inside arbitrarily complex plasmonic nanostructures and to identify the nature of the multipolar plasmon modes involved at plasmonic resonances. Its ability to unravel the physical origin of plasmonic spectral features is demonstrated in the case of a single gold nanotriangle and of a gold nano-antenna. Finally, we show how the volume charge distribution can be used to define and compute the first terms of the multipolar expansion.

Temperature mapping near plasmonic nanostructures using fluorescence polarization anisotropy.

We report on a thermal imaging technique based on fluorescence polarization anisotropy measurements, which enables mapping the local temperature near nanometer-sized heat sources with 300 nm spatial resolution and a typical accuracy of 0.1 degrees C. The principle is demonstrated by mapping the temperature landscape around plasmonic nano-structures heated by near-infrared light. By assessing directly the molecules' Brownian dynamics, it is shown that fluorescence polarization anisotropy is a robust and reliable method which overcomes the limitations of previous thermal imaging techniques. It opens new perspectives in medicine, nanoelectronics and nanofluidics where a control of temperature of a few degrees at the nanoscale is required.

Nano-optical trapping of Rayleigh particles and Escherichia coli bacteria with resonant optical antennas.

Immobilizing individual living microorganisms at designated positions in space is important to study their metabolism and to initiate an in situ scrutiny of the complexity of life at the nanoscale. While optical tweezers enable the trapping of large cells at the focus of a laser beam, they face difficulties in maintaining them steady and can become invasive and produce substantial damage that prevents preserving the organisms intact for sufficient time to be studied. Here we demonstrate a novel optical trapping scheme that allows us to hold living Escherichia coli bacteria for several hours using moderate light intensities. We pattern metallic nanoantennas on a glass substrate to produce strong light intensity gradients responsible for the trapping mechanism. Several individual bacteria are trapped simultaneously with their orientation fixed by the asymmetry of the antennas. This unprecedented immobilization of bacteria opens an avenue toward observing nanoscopic processes associated with cell metabolism, as well as the response of individual live microorganisms to external stimuli, much in the same way as pluricellular organisms are studied in biology.

Individual gold dimers investigated by far- and near-field imaging.

Plasmon resonances in 3D nanoparticle arrangements can produce strong localized optical fields, which are of importance for any application involving interaction of light with subwavelength volumes of matter down to the molecular level. In particular, remarkable field enhancement and confinement occur in a dimer geometry formed by two identical closely spaced particles. Although, recent advances in nanofabrication have rendered the fabrication of complex plasmon architectures more accessible, addressing their local fields in a nonperturbative fashion remains not straightforward, because metallic nanostructures are rather sensitive to their local environment. Here we study gold dimers fabricated by e-beam lithography. Individual dimers are imaged both by far- and near-field methods. First, the near-field electromagnetic interaction in an ensemble of dimers is investigated by scattering spectroscopy, using dark field microscopy. Next, to probe their local field, we explore the luminescence of individual gold dimers utilizing a confocal microscope with single molecule detection sensitivity. We provide a statistical analysis of the dimer luminescence for different incident polarizations, with direct comparison to single particles (monomers). Finally, the near-field transmission of the resonant dimers is mapped with a subwavelength resolution using polarized controlled near-field scanning optical microscopy. Surprisingly, no clear evidence of the high mode density in the dimer gap is observed. This result may be attributed to the limited coupling of the field emitted by the aperture probe to the dimer mode.

Spatially resolved photonic transfer through mesoscopic heterowires.

We report spatially resolved observations of light wave propagation along high refraction index dielectric heterowires lying on a transparent substrate. The heterowires are made of linear chains of closely packed mesoscopic particles. The optical excitation of these heterowires is performed through channel waveguides featuring submicrometer transverse cross sections. Both numerical simulations and near-field optical images, recorded with a photon scanning tunneling microscope, agree to show that, at visible frequencies, tuning the periodicity of the heterowires controls the propagation length within a range of several micrometers.

Imaging the local density of states of optical corrals.

This paper reports the experimental observation, at optical frequencies, of the electromagnetic local density of states established by nanostructures corresponding to the recently introduced concept of optical corral [G. Colas des Francs et al., Phys. Rev. Lett. 86, 4950 (2001)]. The images obtained by a scanning near-field optical microscope under specific operational conditions are found in agreement with the theoretical maps of the optical local density of states. A clear functionality of detection by the scanning near-field optical microscope is thereby identified since the theoretical maps are computed without including any specific tip model.

Addressing and imaging high optical index dielectric ridges in the optical near field.

Experimental observation of light coupling between TiO(2) integrated waveguides of subwavelength cross section and pure three-dimensional evanescent light fields is reported. This near-field optical phenomenon is produced by controlling the location of the focusing of a laser beam totally reflected at the surface of the sample. The phenomenon is observed in direct space with a photon scanning tunneling microscope. Dielectric ridges several tens of micrometers long have been efficiently excited with this technique. Upon excitation, the extremities of the linear dielectric wires display intense light spots localized both inside and around the ridge. For ridge lengths up to 30 microm, the observed phenomenon has been reproduced numerically with a parallel implementation based on the three-dimensional Green dyadic method.