Are tiny metal nanoparticles harder than bulk? How do defects and domain boundaries influence local elasticity? Mechanical properties on the nanoscale are not well understood. Until recently, the available methods dictated ensemble measurements, which necessarily provide only averaged answers. In this project, we aim at pushing the detectability limit in nanomechanics to the sub-particle level by combining ultrafast with ultramicroscopic optical techniques.
The elastic properties of individual nanoparticles are reflected in their acoustical mode spectra in the GHz to THz range. Such acoustical oscillations of the whole particle can be triggered by impulsive laser heating, leading to local shape and density oscillations. The near-instantaneous local variations of the dielectric constant thus convey mechanical information in optical scattering properties and the scattered nearfields will allow spatial resolution down to a few nanometers.
We will map acoustical modes of plasmonic nanostructures by a nonlinear optical pump-probe near field microscope. This will allow us to investigate elasticity at the nanoscale, at the limit of continuum-mechanical description. At the same time, we will learn how metallic nanostructures respond mechanically to ultrafast laser pulses, which is of great importance for nonlinear optics using plasmonic antennas and waveguides.
Publications
Nanoantenna-enhanced ultrafast nonlinear spectroscopy of a single gold nanoparticle
Thorsten Schumacher, Kai Kratzer, David Molnar, Mario Hentschel, Harald Giessen, Markus Lippitz
Nature Communications 2 (2011) 333
Optical nanoantennas, just like their radio-frequency equivalents, enhance the light-matter interaction in their feed gap. Antenna enhancement of small signals promises to open a new regime in linear and nonlinear spectroscopy on the nanoscale. Without antennas especially the nonlinear spectroscopy of single nanoobjects is very demanding. Here we present the first antenna-enhanced ultrafast nonlinear optical spectroscopy. In particular, we use the antenna to determine the nonlinear transient absorption signal of a single gold nanoparticle caused by mechanical breathing oscillations. We increase the signal amplitu-de by an order of magnitude, which is in good agreement with our analytical and numerical models. Our method will find applications in linear and nonlinear spectroscopy of single nanoobjects, especially in simplifying such challenging experiments as transient absorption or multiphoton excitation
Real-space imaging of nanoplasmonic resonances
Ralf Vogelgesang and Alexandre Dmitriev
The Analyst 135 (2010) 1175
Resonant nanoplasmonic structures have long been recognized for their unique applications in subwavelength control of light for enhanced transmission, focussing, field confinement, decay rate management, etc. Increasingly, they are also integrated in electro-optical analytical sensors, shrinking the active volume while at the same time improving sensitivity and specificity. The microscopic imaging of resonances in such structures and also their dynamic variations has seen dramatic advances in recent years. In this Minireview we outline the current status of this rapidly evolving field, discussing both optical and electron microscopy approaches, the limiting issues in spatial resolution and data interpretation, the quantities that can be recorded, as well as the growing importance of time-resolving methods.
Plasmonic Nanowire Antennas: Experiment, Simulation, and Theory
Jens Dorfmller, Ralf Vogelgesang, Worawut Khunsin, Carsten Rockstuhl, Christoph Etrich, and Klaus Kern
Nano Letters 10 (2010) 3596
Recent advances in nanolithography have allowed shifting of the resonance frequency of antennas into the optical and visible wavelength range with potential applications, for example, in single molecule spectroscopy by fluorescence and directionality enhancement of molecules. Despite such great promise, the analytical means to describe the properties of optical antennas is still lacking. As the phase velocity of currents at optical frequencies in metals is much below the speed of light, standard radio frequency (RF) antenna theory does not apply directly. For the fundamental linear wire antenna, we present an analytical description that overcomes this shortage and reveals profound differences between RF and plasmonic antennas. It is fully supported by apertureless scanning near-field optical microscope measurements and finite-difference time-domain simulations. This theory is a starting point for the development of analytical models of more complex antenna structures.
Full simulations of the apertureless scanning near field optical microscopy signal: achievable resolution and contrast
R. Esteban, R. Vogelgesang, K. Kern
Optics Express 17 (2009) 2518
We simulate apertureless near-field optical imaging and obtain phase and amplitude scans of structured substrates for elastic scattering. The solution of the three-dimensional Maxwell equations does not involve approximations and we include large tips and substrates, strong interaction, interferometric detection and demodulation at higher harmonics. Such modeling represents a significant step towards quantitative simulations and offers the attractive possibility to study the individual influence of each relevant experimental parameter. We typically obtain highly localized signatures of the interaction of the tip with gold inclusions, superposed on a slowly varying background signal. The relative importance of both contributions and the achievable lateral resolution are strongly dependent on the geometry and scanning conditions. The simulations show sensitivity mostly to the first nanometers of the sample and underline the importance of scanning near the sample and being careful with mechanical anharmonicities on the tip oscillation. They also help to determine the influence of oscillation amplitude and demodulation harmonic.
Detection of Acoustic Oscillations of Single Gold Nanospheres by Time-Resolved Interferometry
Meindert A. van Dijk, Markus Lippitz, and Michel Orrit
Physical Review Letters 95 (2005) 267406
We measure the transient absorption of single gold particles with a common-path interferometer. The prompt electronic part of the signal provides images for diameters as small as 10 nm. Mechanical vibrations of single particles appear on a longer time scale (period of 16 ps for 50 nm diameter). They reveal the full heterogeneity of the ensemble, and the intrinsic damping of the vibration. We also observe a lower-frequency mode involving shear. Ultrafast pump-probe spectroscopy of individual particles opens new insight into mechanical properties of nanometer-sized objects.
