Paolo Maioli

Coherent Control of an Atomic Collision in a Cavity

Following a recent proposal by S. B. Zheng and G. C. Guo [Phys. Rev. Lett. 85, 2392 (2000)], we report an experiment in which two Rydberg atoms crossing a nonresonant cavity are entangled by coherent energy exchange. The process, mediated by the virtual emission and absorption of a microwave photon, is characterized by a collision mixing angle 4 orders of magnitude larger than for atoms colliding in free space with the same impact parameter. The final entangled state is controlled by adjusting the atom-cavity detuning. This procedure, essentially insensitive to thermal fields and to photon decay, opens promising perspectives for complex entanglement manipulations.

Size dependent surface plasmon resonance broadening in non-spherical nanoparticles: Single gold nanorods

The spectral response of metallic nanoparticles is dominated by the Localized Surface Plasmon Resonance (LSPR). While its spectral position is well understood, measuring and interpreting the LSPR linewidth evolution with shape and size is still an important issue for both fundamental investigations and applications. It needs the use of single particle experiments on nanoparticles with a well controlled environment, to avoid inhomogeneous broadening and/or spurious effects. The dominant effect related to size reduction is an increase of the surface contribution to the linewidth of the LSPR, due to the quantum confinement of the electrons.

Ultrafast Photoinduced Charge Separation in Metal–Semiconductor Nanohybrids

Hybrid nano-objects formed by two or more disparate materials are among the most promising and versatile nanosystems. A key parameter in their properties is interaction between their components. In this context we have investigated ultrafast charge separation in semiconductor-metal nanohybrids using a model system of gold-tipped CdS nanorods in a matchstick architecture. Experiments are performed using an optical time-resolved pump-probe technique, exciting either the semiconductor or the metal component of the particles, and probing the light-induced change of their optical response. Electron-hole pairs photoexcited in the semiconductor part of the nanohybrids are shown to undergo rapid charge separation with the electron transferred to the metal part on a sub-20 fs time scale. This ultrafast gold charging leads to a transient red-shift and broadening of the metal surface plasmon resonance, in agreement with results for free clusters but in contrast to observation for static charging of gold nanoparticles in liquid environments. Quantitative comparison with a theoretical model is in excellent agreement with the experimental results, confirming photoexcitation of one electron-hole pair per nanohybrid followed by ultrafast charge separation. The results also point to the utilization of such metal-semiconductor nanohybrids in light-harvesting applications and in photocatalysis.

Entanglement of a mesoscopic field with an atom induced by photon graininess in a cavity

We observe that a mesoscopic field made of several tens of microwave photons exhibits quantum features when interacting with a single Rydberg atom in a high-Q cavity. The field is split into two components whose phases differ by an angle inversely proportional to the square root of the average photon number. The field and the atomic dipole are phase entangled. These manifestations of photon graininess vanish at the classical limit. This experiment opens the way to studies of large quantum state superpositions at the quantum-classical boundary.

Probing Elasticity at the Nanoscale: Terahertz Acoustic Vibration of Small Metal Nanoparticles

The acoustic response of surface-controlled metal (Pt) nanoparticles is investigated in the small size range, between 1.3 and 3 nm (i.e., 75-950 atoms), using time-resolved spectroscopy. Acoustic vibration of the nanoparticles is demonstrated, with frequencies ranging from 1.1 to 2.6 THz, opening the way to the development of THz acoustic resonators. The frequencies, measured with a noncontact optical method, are in excellent agreement with the prediction of a macroscopic approach based on the continuous elastic model, together with the bulk material elastic constants. This demonstrates the validity of this model at the nanoscale and the weak impact of size reduction on the elastic properties of a material, even for nanoparticles formed by less than 100 atoms.

Optical Response of Individual Au–Ag@SiO2 Heterodimers

The optical extinction response of individual Au-Ag@SiO2 heterodimers whose individual morphologies are determined by transmission electron microscopy (TEM) is investigated using spatial modulation spectroscopy. The extinction spectra show two resonances spectrally close to the surface plasmon resonances of the constituting Au and Ag@SiO2 core-shell particles. The interparticle electromagnetic coupling is demonstrated to induce a large increase of the optical extinction of the dimer around its Au-like surface plasmon resonance for light polarized along its axis, as compared to that for perpendicular polarization and to that of an isolated Au nanoparticle. For spherical particles, this interaction also leads to comparable shifts with light polarization of the two dimer resonances, an effect masked or even reversed for particles significantly deviating from sphericity. Both amplitude and spectral effects are found to be in excellent quantitative agreement with numerical simulations when using the TEM-measured dimer morphology (i.e., size, shape, and orientation of the individual dimers), stressing the importance of individual morphology characterization for interpreting heterodimer optical response.

Surface Plasmon Resonance Properties of Single Elongated Nano-objects: Gold Nanobipyramids and Nanorods

The spectral characteristics (wavelength and line width) and the optical extinction cross-section of the longitudinal localized surface plasmon resonance (LSPR) of individual gold nanobipyramids have been quantitatively measured using the spatial modulation spectroscopy technique. The morphology of the same individual nanoparticles has been determined by transmission electron microscopy (TEM). The experimental results are thus interpreted with a numerical model using the TEM measured sizes of the particles as an input, and either including the substrate or assuming a mean homogeneous environment. Results are compared to those obtained for individual nanorods and also show the importance of the local environment of the particle on the detailed description of its spectral position and extinction amplitude.

Anisotropy effects on the time-resolved spectroscopy of the acoustic vibrations of nanoobjects

The impact of spherical symmetry breaking and crystallinity on the response of the vibrational acoustic modes of a nanoobject in frequency- and time-domain experiments is investigated using the finite-element analysis method. The results show that introduction of shape anisotropy, i.e., evolution from a nanosphere to a nanorod, modifies the periods of the fundamental radial and quadrupolar modes and leads to activation of a quadrupolar-like mode in time-domain studies. In contrast, crystallinity is shown to have a negligible impact on the breathing mode frequency of a nanosphere formed by a cubic crystal and does not activate any quadrupolar mode. The effect of an anisotropic excitation in time-resolved measurements of a large nanosphere is also discussed.

Acoustic Vibrations in Bimetallic Au@Pd Core-Shell Nanorods.

The acoustic vibrations of gold nanorods coated with palladium were investigated as a function of Pd amount using ultrafast pump-probe spectroscopy. Both the extensional and breathing vibrational modes of the nanorods were coherently excited and detected. This permits precise determination of their periods, which were found to decrease and increase with Pd deposition, for the extensional and vibrational modes, respectively. These opposite behaviors reflect changes of the nanoparticle size and mechanical properties, in agreement with numerical simulations. Comparison of experimental and computed periods yields information on the amount of deposited Pd, providing a novel tool to characterize bicomponent nano-objects for small fractions of one of the components (Pd/Au atomic fraction down to 5%).

Acoustic Vibrations of Metal-Dielectric Core–Shell Nanoparticles

The acoustic vibrations of metal nanoparticles encapsulated in a dielectric shell (Ag@SiO(2)) were investigated using a time-resolved femtosecond technique. The measured vibration periods significantly differ from those predicted for the bare metal cores and, depending on the relative core and shell sizes, were found to be either larger or smaller than them. These results show that the vibration of the whole core-shell particle is excited and detected. Moreover, vibrational periods are in excellent agreement with the predictions of a model based on continuum thermoelasticity. However, such agreement is obtained only if a good mechanical contact of the metal and dielectric parts of the core-shell particle is assumed, providing a unique way to probe this contact in multimaterial or hybrid nano-objects.