Search

Physical Review Letters, 123 (15), ISSN:0031-9007, ISSN:1079-7114

The momentum transfer between a photon and an object defines a fundamental limit for the precision with which the object can be measured. If the object oscillates at a frequency $\Omega_0$, this measurement back-action adds quanta $\hbar\Omega_0$ to the oscillator's energy at a rate $\Gamma_{\rm recoil}$, a process called photon recoil heating, and sets bounds to quantum coherence times in cavity optomechanical systems. Here, we use an optically levitated nanoparticle in ultrahigh vacuum to directly measure $\Gamma_{\rm recoil}$. By means of a phase-sensitive feedback scheme, we cool the harmonic motion of the nanoparticle from ambient to micro-Kelvin temperatures and measure its reheating rate under the influence of the radiation field. The recoil heating rate is measured for different particle sizes and for different excitation powers, without the need for cavity optics or cryogenic environments. The measurements are in quantitative agreement with theoretical predictions and provide valuable guidance for the realization of quantum ground-state cooling protocols and the measurement of ultrasmall forces

We study the dynamics of a laser-trapped nanoparticle in high vacuum. Using parametric coupling to an external excitation source, the linewidth of the nanoparticle's oscillation can be reduced by three orders of magnitude. We show that the oscillation of the nanoparticle and the excitation source are synchronized, exhibiting a well-defined phase relationship. Furthermore, the external source can be used to controllably drive the nanoparticle into the nonlinear regime, thereby generating strong coupling between the different translational modes of the nanoparticle. Our work contributes to the understanding of the nonlinear dynamics of levitated nanoparticles in high vacuum and paves the way for studies of pattern formation, chaos, and stochastic resonance., 5 pages, 3 figures

We investigate the physics of photoinduced heat generation in plasmonic structures by using a novel thermal microscopy technique based on molecular fluorescence polarization anisotropy. This technique enables us to image the heat source distribution in light-absorbing systems such as plasmonic nanostructures. While the temperature distribution in plasmonic nanostructures is always fairly uniform because of the fast thermal diffusion in metals, we show that the heat source density is much more contrasted. Unexpectedly the heat origin (thermal hot spots) usually does not correspond to the optical hot spots of the plasmon mode. Numerical simulations based on the Green dyadic method confirm our observations and enable us to derive the general physical rules governing heat generation in plasmonic structures.

A unique feature of surface plasmon polaritons (SPPs) is that their in-plane momentum is larger than the momentum of free-propagating photons of the same energy. Therefore, it is believed that they can be excited only by evanescent fields created by total internal reflection or by local scattering. Here, we provide the first demonstration of free-space excitation of surface plasmons by means of nonlinear four-wave mixing. The process involves the vectorial addition of the momenta of three incident photons, making it possible to penetrate the light cone and directly couple to the SPP dispersion curve. Using this technique, surface plasmons can be launched on any metal surface by simply overlapping two beams of laser pulses incident from resonant directions. The excitation scheme is also applicable to other bound modes, such as waveguide modes, surface phonon polaritons, and excitations of 2D electron gases.

We report on a particularly strong third-order nonlinear response from nanostructured gold surfaces. Two incident laser beams with frequencies omega{1} and omega{2} give rise to four-wave mixing (4WM) fields with frequencies 2omega{1}-omega{2} and 2omega{2}-omega{1}. We demonstrate that the nonlinear response can be purely evanescent and that nanostructured surfaces convert the evanescent energy into propagating radiation, thereby increasing the efficiency of frequency conversion. The emitted 4WM radiation is found to be directional, polarized, coherent, and both frequency and angle tunable. The ability to perform efficient frequency conversion in reduced dimensions provides new opportunities for nanophotonics and active plasmonics.

We present spatially resolved spectral mode mapping of resonant plasmon gap antennas using two-photon luminescence microspectroscopy. The obtained maps are in good agreement with 3D calculations of the antenna modes. The evolution of the modal field with wavelength, both in the gap and along the two coupled gold nanowires forming the antenna, is directly visualized. At resonance, the luminescence for the gap area is enhanced at least 80 times and a comparison with the antenna extremities shows a dynamical charge redistribution due to the near-field coupling between the two arms.

We present a quantitative analysis of 2D surface plasmon based optical tweezers able to trap microcolloids at a patterned metal surface under low laser intensity. Photonic force microscopy is used to assess the properties of surface plasmon traps, such as confinement and stiffness, revealing stable trapping with forces in the range of a few tens of femtonewtons. We also investigate the specificities of surface plasmon tweezers with respect to conventional 3D tweezers responsible for their selectivity to the trapped specimen's size. The accurate engineering of the trapping properties through the adjustment of the illumination parameters opens new perspectives in the realization of future optically driven on-a-chip devices.

In a recent letter Chicanne {\em et al.} [1] reported the experimental observation of the electromagnetic local density of states LDOS established by gold nanostructures. The obtained images have been compared with combinations of partial LDOSs defined in terms of the imaginary part of the Green-tensor ${\bf G}^I = [{\bf G}-{\bf G}^\dag]/(2i)$ calculated at the tip position. Moreover just this comparison was the criterion for the choice of the optimum tip design. These results support the point of view that ${\cal G}_{\bf u} =-({2 \omega}/{\pi c^2}) {\bf u} \cdot {\bf G}^I({\bf r}, {\bf r}, \omega) \cdot {\bf u}$ (${\bf u}$ is the unit vector used to define the effective dipole associated to the illuminating tip) is the key quantity to interpret SNOM images in analogy with the electronic LDOS measured by the scanning tunneling microscope (STM). Rigorous Green-tensor analysis shows that ${\cal G}_{\bf u}$ (that is also the key quantity determining spontaneous decay rates of molecular transitions) is not the correct key quantity, and that measurements in Ref. [1] should have been compared with a different quantity. Moreover the identification of ${\cal G}_{\bf u}$ with the detected SNOM signal can lead to unphysical results.

International audience; 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.