Publications

Sanchayeeta Jana, Simon Durst, Markus Lippitz
Fluorescence-Detected Two-Dimensional Electronic Spectroscopy of a Single Molecule
Nano Letters, 24, 12576-12581 (2024) online , ERef, abstract: show

Single-molecule fluorescence spectroscopy is a powerful method that avoids ensemble averaging, but its temporal resolution is limited by the fluorescence lifetime to nanoseconds at most. At the ensemble level, two-dimensional spectroscopy provides insight into ultrafast femtosecond processes, such as energy transfer and line broadening, even beyond the Fourier limit, by correlating pump and probe spectra. Here, we combine these two techniques and demonstrate coherent 2D spectroscopy of individual dibenzoterrylene (DBT) molecules at room temperature. We excite the molecule in a confocal microscope with a phase-modulated train of femtosecond pulses and detect the emitted fluorescence with single-photon counting detectors. Using a phase-sensitive detection scheme, we were able to measure the nonlinear 2D spectra of most of the DBT molecules that we studied. Our method is applicable to a wide range of single emitters and opens new avenues for understanding energy transfer in single quantum objects on ultrafast time scales.

Andreas Karg, Volodymyr Kuznetsov, Nicolas Helfricht, Markus Lippitz, Georg Papastavrou
Electrochemical grippers based on the tuning of surface forces for applications in micro- and nanorobotics
Scientific Reports, 13, 7885 (2023) online , ERef, abstract: show

Existing approaches to robotic manipulation often rely on external mechanical devices, such as hydraulic and pneumatic devices or grippers. Both types of devices can be adapted to microrobots only with difficulties and for nanorobots not all. Here, we present a fundamentally different approach that is based on tuning the acting surface forces themselves rather than applying external forces by grippers. Tuning of forces is achieved by the electrochemical control of an electrode’s diffuse layer. Such electrochemical grippers can be integrated directly into an atomic force microscope, allowing for ‘pick and place’ procedures typically used in macroscopic robotics. Due to the low potentials involved, small autonomous robots could as well be equipped with these electrochemical grippers that will be particularly useful in soft robotics as well as nanorobotics. Moreover, these grippers have no moving parts and can be incorporated in new concepts for actuators. The concept can easily be scaled down and applied to a wide range of objects, such as colloids, proteins, and macromolecules.

Michael Seidel, Yuhui Yang, Thorsten Schumacher, Yongheng Huo, Saimon Filipe Covre da Silva, Sven Rodt, Armando Rastelli, Stephan Reitzenstein, Markus Lippitz
Intermediate Field Coupling of Single Epitaxial Quantum Dots to Plasmonic Waveguides
Nano Letters, 23, 10532-10537 (2023) online , ERef, abstract: show

Key requirements for quantum plasmonic nanocircuits are reliable single-photon sources, high coupling efficiency to the plasmonic structures, and low propagation losses. Self-assembled epitaxially grown GaAs quantum dots are close to ideal as stable, bright, and narrowband single-photon emitters. Likewise, wet-chemically grown monocrystalline silver nanowires are among the best plasmonic waveguides. However, large propagation losses of surface plasmons on the high-index GaAs substrate prevent their direct combination. Here, we show by experiment and simulation that the best overall performance of the quantum plasmonic nanocircuit based on these building blocks is achieved in the intermediate field regime with an additional spacer layer between the quantum dot and the plasmonic waveguide. High-resolution cathodoluminescence measurements allow a precise determination of the coupling distance and support a simple analytical model to explain the overall performance. The coupling efficiency is increased up to four times by standing wave interference near the end of the waveguide.

Johannes Schust, Florian Mangold, Florian Sterl, Niklas Metz, Thorsten Schumacher, Markus Lippitz, Mario Hentschel, Harald Giessen
Spatially Resolved Nonlinear Plasmonics
Nano Letters, 23, 5141-5147 (2023) online , ERef, abstract: show

Nonlinear optical plasmonics investigates the emission of plasmonic nanoantennas with the aid of nonlinear spectroscopy. Here we introduce nonlinear spatially resolved spectroscopy (NSRS) which is capable of imaging the k-space as well as spatially resolving the THG signal of gold nanoantennas and investigating the emission of individual antennas by wide-field illumination of entire arrays. Hand in hand with theoretical simulations, we demonstrate our ability of imaging various oscillation modes inside the nanostructures and therefore spatial emission hotspots. Upon increasing intensity of the femtosecond excitation, an individual destruction threshold can be observed. We find certain antennas becoming exceptionally bright. By investigating those samples taking structural SEM images of the nanoantenna arrays afterward, our spatially resolved nonlinear image can be correlated with this data proving that antennas had deformed into a peanut-like shape. Thus, our NSRS setup enables the investigation of a nonlinear self-enhancement process of nanoantennas under critical laser excitation.

Christoph Schnupfhagn, Thorsten Schumacher, Paul Markus, Georg Papastavrou, Olha Aftenieva, Tobias A. F. König, Volodymyr Dudko, Marian Matejdes, Josef Breu, Markus Lippitz
Disentangling the Orientations of Spectrally Overlapping Transition Dipoles in Dense Dye Layers
Nano Letters, 22, 7499-7505 (2022) online , ERef, abstract: show

The transition dipole orientations of dye assemblies in heterostructures have a crucial impact on the efficiency of novel optoelectronic devices such as organic thin-film transistors and light-emitting diodes. These devices are frequently based on heterojunctions and tandem structures featuring multiple optical transitions. Precise knowledge of preferred orientations, spatial order, and spatial variations is highly relevant. We present a fast and universal large-area screening method to determine the transition dipole orientations in dye assemblies with diffraction-limited spatial resolution. Moreover, our hyperspectral imaging approach disentangles the orientations of different chromophores. As a demonstration, we apply our technique to dye monolayers with two optical transitions sandwiched between two ultrathin silicate nanosheets. A comprehensive model for dipole orientation distributions in monolayers reveals a long-range orientational order and a strong correlation between the two transitions.

Christian Schörner, Markus Lippitz
High-Q plasmonic nanowire-on-mirror resonators by atomically smooth single-crystalline silver flakes
The Journal of Chemical Physics, 155, 234202 (2021) online , ERef, abstract: show

Plasmonic nanoparticles in close vicinity to a metal surface confine light to nanoscale volumes within the insulating gap. With gap sizes in the range of a few nanometers or below, atomic-scale dynamical phenomena within the nanogap come into reach. However, at these tiny scales, an ultra-smooth material is a crucial requirement. Here, we demonstrate large-scale (50 μm) single-crystalline silver flakes with a truly atomically smooth surface, which are an ideal platform for vertically assembled silver plasmonic nanoresonators. We investigate crystalline silver nanowires in a sub-2 nm separation to the silver surface and observe narrow plasmonic resonances with a quality factor Q of about 20. We propose a concept toward the observation of the spectral diffusion of the lowest-frequency cavity plasmon resonance and present first measurements. Our study demonstrates the benefit of using purely crystalline silver for plasmonic nanoparticle-on-mirror resonators and further paves the way toward the observation of dynamic phenomena within a nanoscale gap.

Marián Matejdes, Matthias Stöter, Rafał Czerwieniec, Markus Leitl, Sabine Rosenfeldt, Thorsten Schumacher, Jonas Albert, Markus Lippitz, Hartmut Yersin, Josef Breu
Sandwich-Like Encapsulation of a Highly Luminescent Copper(I) Complex
Advanced Optical Materials, 9, 2100516 (2021) online , ERef, abstract: show

A small molecular weight cationic copper(I) complex showing high luminescence quantum yield based on a thermally activated delayed fluorescence mechanism is immobilized between two 1 nm thin silicate layers. Partial ion exchange of the emitter into a synthetic layered silicate (fluorohectorite) yields an ordered heterostructure with two types of strictly alternating interlayers: a monolayer of the cationic emitter and a monolayer of hydrated Na+ cations. Osmotic swelling of the latter produces dispersions of double-stacks in which the emitter monolayer is encapsulated between two silicate layers. The electrostatic attraction of the emitter interlayer with the oppositely charged silicate layers exerts electrostatic pressure on the emitter. Compared to crystalline salt, rigid confinement for the encapsulated emitter provides improved thermal stability and increased emission quantum yield at ambient temperature. The suspension of delaminated, micrometer-sized double-stacks of 3.9 nm thickness allows for easy solution processing of low-cost optoelectronic devices, such as light-emitting electrochemical cells and organic light-emitting diodes.

Christian Schörner, Markus Lippitz
Single Molecule Nonlinearity in a Plasmonic Waveguide
Nano Letters, 20, 2152-2156 (2020) online , ERef, abstract: show

Plasmonic waveguides offer the unique possibility to confine light far below the diffraction limit. Past room temperature experiments focused on efficient generation of single waveguide plasmons by a quantum emitter. However, only the simultaneous interaction of the emitter with multiple plasmonic fields would lead to functionality in a plasmonic circuit. Here, we demonstrate the nonlinear optical interaction of a single molecule and propagating plasmons. An individual terrylene diimide (TDI) molecule is placed in the nanogap between two single-crystalline silver nanowires. A visible wavelength pump pulse and a red-shifted depletion pulse travel along the waveguide, leading to stimulated emission depletion (STED) in the observed fluorescence. The efficiency increases by up to a factor of 50 compared to far-field excitation. Our study thus demonstrates remote nonlinear four-wave mixing at a single molecule with propagating plasmons. It paves the way toward functional quantum plasmonic circuits and improved nonlinear single-molecule spectroscopy.

Andreas Mark, Nicolas Helfricht, Astrid Rauh, Jinqiao Xue, Patrick Knödler, Thorsten Schumacher, Matthias Karg, Binyang Du, Markus Lippitz, Georg Papastavrou
Electrokinetics in Micro-channeled Cantilevers : Extending the Toolbox for Reversible Colloidal Probes and AFM-Based Nanofluidics
Scientific Reports, 9, 20294 (2019) online , ERef, abstract: show

The combination of atomic force microscopy (AFM) with nanofluidics, also referred to as FluidFM, has facilitated new applications in scanning ion conductance microscopy, direct force measurements, lithography, or controlled nanoparticle deposition. An essential element of this new type of AFMs is its cantilever, which bears an internal micro-channel with a defined aperture at the end. Here, we present a new approach for in-situ characterization of the internal micro-channels, which is non-destructive and based on electrochemical methods. It allows for probing the internal environment of a micro-channeled cantilever and the corresponding aperture, respectively. Acquiring the streaming current in the micro-channel allows to determine not only the state of the aperture over a wide range of ionic strengths but also the surface chemistry of the cantilever’s internal channel. The high practical applicability of this method is demonstrated by detecting the aspiration of polymeric, inorganic and hydrogel particles with diameters ranging from several µm down to 300 nm. By verifying in-situ the state of the aperture, i.e. open versus closed, electrophysiological or nano-deposition experiments will be significantly facilitated. Moreover, our approach is of high significance for direct force measurements by the FluidFM-technique and sub-micron colloidal probes.

Tzu-Yu Chen, Julian Obermeier, Thorsten Schumacher, Fan-Cheng Lin, Jer-Shing Huang, Markus Lippitz, Chen-Bin Huang
Modal Symmetry Controlled Second-Harmonic Generation by Propagating Plasmons
Nano Letters, 19, 6424-6428 (2019) online , ERef, abstract: show

A new concept for second-harmonic generation (SHG) in an optical nanocircuit is proposed. We demonstrate both theoretically and experimentally that the symmetry of an optical mode alone is sufficient to allow SHG even in centro-symmetric structures made of centro-symmetric material. The concept is realized using a plasmonic two-wire transmission-line (TWTL), which simultaneously supports a symmetric and an antisymmetric mode. We first confirm that emission of second-harmonic light into the symmetric mode of the waveguide is symmetry-allowed when the fundamental excited waveguide modes are either purely symmetric or antisymmetric. We further switch the emission into the antisymmetric mode when a controlled mixture of the fundamental modes is excited simultaneously. Our results open up a new degree of freedom into the designs of nonlinear optical components and should pave a new avenue toward multifunctional nanophotonic circuitry.

Joachim Krauth, Thorsten Schumacher, Josselin Defrance, Bernd Metzger, Markus Lippitz, Thomas Weiss, Harald Giessen, Mario Hentschel
Nonlinear Spectroscopy on the Plasmonic Analog of Electromagnetically Induced Absorption : Revealing Minute Structural Asymmetries
ACS Photonics, 6, 2850-2859 (2019) online , ERef, abstract: show

Tailoring not only the linear but also the nonlinear properties of plasmonic structures has been a longstanding idea. The plasmonic dolmen structure with its many degrees of freedom in design has been of particular interest. We are investigating this system in detail in the retarded weak-coupling regime, the so-called plasmonic analog of electromagnetically induced absorption. While it is generally expected that the enhanced absorbance leads to an increased nonlinear generation, we find that the details are more complex. A thorough wavelength and polarization resolved study reveals two distinct nonlinear contributions. Our nonlinear spectroscopy method exhibits a surprisingly high sensitivity to minute structural asymmetries. Our experimental results are corroborated by finite-element simulation. We envision that our findings will stimulate further research into phase tuning, structural symmetries, and manipulation in nonlinear plasmonic systems in order to fully exploit the ability to tailor the linear and specifically the nonlinear optical properties of the nanostructured matter.

Christian Schörner, Subhasis Adhikari, Markus Lippitz
A Single-Crystalline Silver Plasmonic Circuit for Visible Quantum Emitters
Nano Letters, 19, 3238-3243 (2019) online , ERef, abstract: show

Plasmonic waveguides are key elements in nanophotonic devices, serving as optical interconnects between nanoscale light sources and detectors. Multimode operation in plasmonic two-wire transmission lines promises important degrees of freedom for near-field manipulation and information encoding. However, highly confined plasmon propagation along gold nanostructures is typically limited to the near-infrared region due to ohmic losses, excluding all visible quantum emitters from plasmonic circuitry. We report on the top-down fabrication of complex plasmonic nanostructures in single-crystalline silver plates. We demonstrate the controlled remote excitation of a small ensemble of fluorophores by a set of waveguide modes and the emission of the visible luminescence into the waveguide with high efficiency. This approach opens up the study of a nanoscale light–matter interaction between complex plasmonic waveguides and a large variety of quantum emitters available in the visible spectral range.

Markus Pfeiffer, Paola Atkinson, Armando Rastelli, Oliver G. Schmidt, Harald Giessen, Markus Lippitz, Klas Lindfors
Coupling a single solid-state quantum emitter to an array of resonant plasmonic antennas
Scientific Reports, 8, 3415 (2018) online , ERef, abstract: show

Plasmon resonant arrays or meta-surfaces shape both the incoming optical field and the local density of states for emission processes. They provide large regions of enhanced emission from emitters and greater design flexibility than single nanoantennas. This makes them of great interest for engineering optical absorption and emission. Here we study the coupling of a single quantum emitter, a self-assembled semiconductor quantum dot, to a plasmonic meta-surface. We investigate the influence of the spectral properties of the nanoantennas and the position of the emitter in the unit cell of the structure. We observe a resonant enhancement due to emitter-array coupling in the far-field regime and find a clear difference from the interaction of an emitter with a single antenna.

Julian Obermeier, Thorsten Schumacher, Markus Lippitz
Nonlinear spectroscopy of plasmonic nanoparticles
Advances in Physics: X, 3, 496-517 (2018) online , ERef, abstract: show

The plasmon resonance of a metal nanoparticle increases the optical field amplitude in and around the particle with respect to the incoming wave. In consequence, optical effects that are nonlinear in their field amplitude profit from this increased field. In general, a plasmonic structure can react nonlinearly by itself and it can also enhance the effect of the nonlinearity in its environment, which we consider as plasmonic nanoantenna. In this paper, we review third-order nonlinear effects such as third-harmonic generation, pump-probe spectroscopy, coherent anti-Stokes Raman scattering and four-wave mixing of and near plasmonic nanostructures. All these processes are described by very similar equations for the nonlinear polarization, but the underling physics differs.

Max J. Schnepf, Yannic Brasse, Fabian R. Goßler, Anja M. Steiner, Julian Obermeier, Markus Lippitz, Andreas Fery, Tobias A. F. König
Single Particle Spectroscopy of Radiative Processes in Colloid-to-Film-Coupled Nanoantennas
Zeitschrift für Physikalische Chemie : International Journal of Research in Physical Chemistry and Chemical Physics, 232, 1593-1606 (2018) online , ERef, abstract: show

We present a fluorescent emitter (rhodamine B) coupled to a dielectric or metallic interface as well as a metallic cavity to study their radiative decay processes. Supported by finite-difference time-domain (FDTD) simulations, we correlate the non-radiative and radiative decay rates with the absorption and scattering cross section efficiencies, respectively. On a single particle level, we use atomic force microscopy (AFM), scanning electron microscopy (SEM), scattering spectroscopy, fluorescence life time imaging (FLIM) and time-correlated single photon counting (TCSPC) to evaluate the enhanced fluorescence decay at the same location. With this study, we show a colloidal gain material, which can be integrated into lattices using existing directed self-assembled methods to study their coherent energy transfer.

Daniela Wolf, Markus Lippitz, Dong-Hun Chae, Tobias Utikal, Patrick Herlinger, Jurgen Smet, Harald Giessen
Spectroscopy of Graphene at the Saddle Point
In: Binder, Rolf (ed.): Optical Properties of Graphene
Hackensack, NJ : World Scientific Publishing, 2017. - pp. 325-348
online , ERef, abstract: show

In contrast to the previous chapters of this book, in the following sections the optical properties of graphene far away from the Dirac points will be treated. As has been shown earlier, at frequencies above the far-infrared region, the optical response of graphene is dominated by interband transitions between the valence and conduction band. The band structure of graphene exhibits a saddle point between two neighboring Dirac cones, so that the optical absorption will deviate from its constant value of πα for photon energies in the visible and ultraviolet regime… Read More: http://www.worldscientific.com/doi/abs/10.1142/9789813148758_0010

Christian Schörner, Daniela Wolf, Thorsten Schumacher, Peter Bauer, Mukundan Thelakkat, Markus Lippitz
Nondestructive Probing of a Photoswitchable Dithienylethene Coupled to Plasmonic Nanostructures
The Journal of Physical Chemistry C, 121, 16528-16532 (2017) online , ERef, abstract: show

We demonstrate read-out of the conformational state of photoswitchable molecules, without modifying that state by the read-out process itself. The ring-opening and ring-closing reaction do not only change the absorption spectrum of a molecular layer, but also its index of refraction. A thin layer of a dithienylethene derivative was combined with well-designed gold nanostructures. The particle plasmon resonance of these nanostructures is extremely sensitive to the photoinduced change in the environment and can therefore be used to probe the state of the photochromic switch. The probing wavelength now interrogates the plasmonic structure, not the molecular film, and can thus be conveniently placed in a transparent spectral range, e.g., the near infrared. We find good agreement between experiments and numerical simulations with regard to spectral signatures of the plasmon resonance.

Xiaofei Wu, Ping Jiang, Gary Razinskas, Yongheng Huo, Hongyi Zhang, Martin Kamp, Armando Rastelli, Oliver G. Schmidt, Bert Hecht, Klas Lindfors, Markus Lippitz
On-Chip Single-Plasmon Nanocircuit Driven by a Self-Assembled Quantum Dot
Nano Letters, 17, 4291-4296 (2017) online , ERef, abstract: show

Quantum photonics holds great promise for future technologies such as secure communication, quantum computation, quantum simulation, and quantum metrology. An outstanding challenge for quantum photonics is to develop scalable miniature circuits that integrate single-photon sources, linear optical components, and detectors on a chip. Plasmonic nanocircuits will play essential roles in such developments. However, for quantum plasmonic circuits, integration of stable, bright, and narrow-band single photon sources in the structure has so far not been reported. Here we present a plasmonic nanocircuit driven by a self-assembled GaAs quantum dot. Through a planar dielectric-plasmonic hybrid waveguide, the quantum dot efficiently excites narrow-band single plasmons that are guided in a two-wire transmission line until they are converted into single photons by an optical antenna. Our work demonstrates the feasibility of fully on-chip plasmonic nanocircuits for quantum optical applications.

Klas Lindfors, Daniel Dregely, Markus Lippitz, Nader Engheta, Michael Totzeck, Harald Giessen
Imaging and Steering Unidirectional Emission from Nanoantenna Array Metasurfaces
ACS Photonics, 3, 286-292 (2016) online , ERef, abstract: show

In radiofrequency antenna engineering, the array factor made long-distance communication with steerable transmission and receiving possible. At optical frequencies, low-loss signal transmission via free space by using nanoantennas is still in its infancy. Here, we suggest applying the array factor to the optical frequency regime by shaping the radiation pattern of plasmonic metasurfaces featuring nanoantenna arrays. We arrange dipolar gold nanoantennas operating at 785 nm wavelength in wavelength-sized arrays and control the phase that drives the antenna elements. We obtain collimated and unidirectional radiation from this metasurface upon illumination with circularly polarized light, which is not prone to major losses as in common plasmonic waveguide structures. We furthermore demonstrate switching the unidirectional emission to opposite directions with additional beamsteering by modifying the array factor. Our experiment corroborates the evidence for spin–orbit coupling between the helicity of light and suitably designed plasmonic metasurfaces, which can exhibit the spin-Hall effect for light.

Bernd Metzger, Mario Hentschel, Maxim Nesterov, Thorsten Schumacher, Markus Lippitz, Harald Giessen
Nonlinear optics of complex plasmonic structures : linear and third-order optical response of orthogonally coupled metallic nanoantennas
Applied Physics B, 122, 77 (2016) online , ERef, abstract: show

We investigate the polarization-resolved linear and third-order optical response of plasmonic nanostructure arrays that consist of orthogonally coupled gold nanoantennas. By rotating the incident light polarization direction, either one of the two eigenmodes of the coupled system or a superposition of the eigenmodes can be excited. We find that when an eigenmode is driven by the external light field, the generated third-harmonic signals exhibit the same polarization direction as the fundamental field. In contrast, when a superposition of the two eigenmodes is excited, third-harmonic can efficiently be radiated at the perpendicular polarization direction. Furthermore, the interference of the coherent third-harmonic signals radiated from both nanorods proves that the phase between the two plasmonic oscillators changes in the third-harmonic signal over 3 pi when the laser is spectrally tuned over the resonance, rather than over pi as in the case of the fundamental field. Finally, almost all details of the linear and the nonlinear spectra can be described by an anharmonic coupled oscillator model, which we discuss in detail and which provides deep insight into the linear and the nonlinear optical response of coupled plasmonic nanoantennas.

Thorsten Schumacher, Matthias Brandstetter, Daniela Wolf, Kai Kratzer, Mario Hentschel, Harald Giessen, Markus Lippitz
The optimal antenna for nonlinear spectroscopy of weakly and strongly scattering nanoobjects
Applied Physics B, 122, 91 (2016) online , ERef, abstract: show

Optical nanoantennas, i.e., arrangements of plasmonic nanostructures, promise to enhance the light–matter interaction on the nanoscale. In particular, nonlinear optical spectroscopy of single nanoobjects would profit from such an antenna, as nonlinear optical effects are already weak for bulk material, and become almost undetectable for single nanoobjects. We investigate the design of optical nanoantennas for transient absorption spectroscopy in two different cases: the mechanical breathing mode of a metal nanodisk and the quantum-confined carrier dynamics in a single CdSe nanowire. In the latter case, an antenna with a resonance at the desired wavelength optimally increases the light intensity at the nanoobject. In the first case, the perturbation of the antenna by the investigated nanosystem cannot be neglected and off-resonant antennas become most efficient.

Daniela Wolf, Thorsten Schumacher, Markus Lippitz
Shaping the nonlinear near field
Nature Communications, 7, 10361 (2016) online , ERef, abstract: show

Light scattering at plasmonic nanoparticles and their assemblies has led to a wealth of applications in metamaterials and nano-optics. Although shaping of fields around nanostructures is widely studied, the influence of the field inside the nanostructures is often overlooked. The linear field distribution inside the structure taken to the third power causes third-harmonic generation, a nonlinear optical response of matter. Here we demonstrate by a far field Fourier imaging method how this simple fact can be used to shape complex fields around a single particle alone. We employ this scheme to switch the third-harmonic emission from a single point source to two spatially separated but coherent sources, as in Young's double-slit assembly. We envision applications as diverse as coherently feeding antenna arrays and optical spectroscopy of spatially extended electronic states.

Hongyi Zhang, Yongheng Huo, Klas Lindfors, Yonghai Chen, Oliver G. Schmidt, Armando Rastelli, Markus Lippitz
Narrow-line self-assembled GaAs quantum dots for plasmonics
Applied Physics Letters, 106, 101110 (2015) online , ERef, abstract: show

We demonstrate efficient coupling of excitons in near-surface GaAs quantum dots (QDs) to surface-plasmon polaritons. We observe distinct changes in the photoluminescence of the emitters as the distance between the QDs and the gold interface decreases. Based on an electric point-dipole model, we identify the surface plasmon launching rates for different QD-surface distances. While in conventional far-field experiments only a few percent of the emitted photons can be collected due to the high refractive index semiconductor substrate, already for distances around 30 nm the plasmon launching-rate becomes comparable to the emission rate into bulk photon modes, thus much larger than the photon collection rate. For even smaller distances, the degrading optical properties of the emitter counterweight the increasing coupling efficiency to plasmonic modes.

Mario Hentschel, Tobias Utikal, Bernd Metzger, Harald Giessen, Markus Lippitz
Nonlinear Plasmon Optics
In: Sakabe, Shuji ; Lienau, Christoph ; Grunwald, Rüdiger (ed.): Progress in Nonlinear Nano-Optics
Cham : Springer, 2015. - pp. 155-181 . - (Nano-Optics and Nanophotonics)
online , ERef, abstract: show

We study nonlinear optics in plasmonic nanosystems, discuss the role of structural symmetries and the influence of linear optical properties. In particular, we investigate third harmonic generation from dimer nanoantennas and show that the nonlinear optical response, in contrast to common belief, is not governed by gap nonlinearities but fully described by the linear optical properties of the antenna. A simple nonlinear harmonic oscillator model is shown to reproduce all experimental features.

Bernd Metzger, Mario Hentschel, Thorsten Schumacher, Markus Lippitz, Xingchen Ye, Christopher B. Murray, Bastian Knabe, Karsten Buse, Harald Giessen
Doubling the Efficiency of Third Harmonic Generation by Positioning ITO Nanocrystals into the Hot-Spot of Plasmonic Gap-Antennas
Nano Letters, 14, 2867-2872 (2014) online , ERef, abstract: show

We incorporate dielectric indium tin oxide nanocrystals into the hot-spot of gold nanogap-antennas and perform third harmonic spectroscopy on these hybrid nanostructure arrays. The combined system shows a 2-fold increase of the radiated third harmonic intensity when compared to bare gold antennas. In order to identify the origin of the enhanced nonlinear response we perform finite element simulations of the nanostructures, which are in excellent agreement with our measurements. We find that the third harmonic signal enhancement is mainly related to changes in the linear optical properties of the plasmonic antenna resonances when the ITO nanocrystals are incorporated. Furthermore, the dominant source of the third harmonic is found to be located in the gold volume of the plasmonic antennas.

Markus Pfeiffer, Klas Lindfors, Hongyi Zhang, Bernhard Fenk, Fritz Phillipp, Paola Atkinson, Armando Rastelli, Oliver G. Schmidt, Harald Giessen, Markus Lippitz
Eleven Nanometer Alignment Precision of a Plasmonic Nanoantenna with a Self-Assembled GaAs Quantum Dot
Nano Letters, 14, 197-201 (2014) online , ERef, abstract: show

Plasmonics offers the opportunity of tailoring the interaction of light with single quantum emitters. However, the strong field localization of plasmons requires spatial fabrication accuracy far beyond what is required for other nanophotonic technologies. Furthermore, this accuracy has to be achieved across different fabrication processes to combine quantum emitters and plasmonics. We demonstrate a solution to this critical problem by controlled positioning of plasmonic nanoantennas with an accuracy of 11 nm next to single self-assembled GaAs semiconductor quantum dots, whose position can be determined with nanometer precision. These dots do not suffer from blinking or bleaching or from random orientation of the transition dipole moment as colloidal nanocrystals do. Our method introduces flexible fabrication of arbitrary nanostructures coupled to single-photon sources in a controllable and scalable fashion.

Daniel Dregely, Klas Lindfors, Markus Lippitz, Nader Engheta, Michael Totzeck, Harald Giessen
Imaging and steering an optical wireless nanoantenna link
Nature Communications, 5, 4354 (2014) online , ERef, abstract: show

Optical nanoantennas tailor the transmission and reception of optical signals. Owing to their capacity to control the direction and angular distribution of optical radiation over a broad spectral range, nanoantennas are promising components for optical communication in nanocircuits. Here we measure wireless optical power transfer between plasmonic nanoantennas in the far-field and demonstrate changeable signal routing to different nanoscopic receivers via beamsteering. We image the radiation pattern of single-optical nanoantennas using a photoluminescence technique, which allows mapping of the unperturbed intensity distribution around plasmonic structures. We quantify the distance dependence of the power transmission between transmitter and receiver by deterministically positioning nanoscopic fluorescent receivers around the transmitting nanoantenna. By adjusting the wavefront of the optical field incident on the transmitter, we achieve directional control of the transmitted radiation over a broad range of 29 degrees. This enables wireless power transfer from one transmitter to different receivers.

Bernd Metzger, Thorsten Schumacher, Mario Hentschel, Markus Lippitz, Harald Giessen
Third Harmonic Mechanism in Complex Plasmonic Fano Structures
ACS Photonics, 1, 471-476 (2014) online , ERef, abstract: show

We perform third harmonic spectroscopy of dolmen-type nanostructures, which exhibit plasmonic Fano resonances in the near-infrared. Strong third harmonic emission is predominantly radiated close to the low energy peak of the Fano resonance. Furthermore, we find that the third harmonic polarization of the subradiant mode interferes destructively and diminishes the nonlinear signal in the far-field. By comparing the experimental third harmonic spectra with finite element simulations and an anharmonic oscillator model, we find strong indications that the source of the third harmonic is the optical nonlinearity of the bare gold enhanced by the resonant plasmonic polarization.

Mikko J. Huttunen, Klas Lindfors, Domenico Andriano, Jouni Mäkitalo, Godofredo Bautista, Markus Lippitz, Martti Kauranen
Three-dimensional winged nanocone optical antennas
Optics Letters, 39, 3686-3689 (2014) online , ERef, abstract: show

We introduce 3D optical antennas based on winged nanocones. The antennas support particle plasmon oscillations with current distributions that facilitate transformation of transverse far-field radiation to strong longitudinal local fields near the cone apices. We characterize the optical responses of the antennas by their extinction spectra and by second-harmonic generation microscopy with cylindrical vector beams. The results demonstrate a new 3D polarization- controllable optical antenna for applications in apertureless near-field microscopy, spectroscopy, and plasmonic sensing.

Thorsten Schumacher, Harald Giessen, Markus Lippitz
Ultrafast Spectroscopy of Quantum Confined States in a Single CdSe Nanowire
Nano Letters, 13, 1706-1710 (2013) online , ERef, abstract: show

We measure for the first time transient absorption spectra of individual CdSe nanowires with about 10 nm diameter. Confinement of the carrier wave functions leads to discrete states which can be described by a six-band effective mass model. Combining transient absorption and luminescence spectroscopy allows us to track the excitation dynamics in the visible and near-infrared spectral range. About 10 of all absorbed photons lead to an excitation of the lowest energy state. Of these excitations, less than 1 lead to a photon in the optical far-field. Almost all emission is reabsorbed by other parts of the nanowire. These findings might explain the low overall quantum efficiency of CdSe nanowires.

Daniel Dregely, Klas Lindfors, Jens Dorfmüller, Mario Hentschel, Merle Becker, Jörg Wrachtrup, Markus Lippitz, Ralf Vogelgesang, Harald Giessen
Plasmonic antennas, positioning, and coupling of individual quantum systems
Physica Status Solidi B, 249, 666-677 (2012) online , ERef, abstract: show

Plasmonic nanoantennas can enhance the radiative decay rate of quantum emitters via the Purcell-effect. Similar to their radiofrequency equivalents, they can also direct the emitted light into preferential directions. In this paper we first investigate plasmonic Yagi-Uda antennas that are able to confine light to and direct light from subwavelength size volumes. Hence, enhanced transition rates and directed emission are expected when near-field coupling between quantum emitters and the antennas is achieved. Second, we present suitable techniques to couple different quantum systems to plasmonic antennas. We use top-down fabrication techniques to achieve positioning of individual quantum emitters relative to plasmonic nanostructures with an accuracy better than 10?nm. We assure a sufficiently small distance for an efficient near-field coupling of the transition dipole to the plasmonic nanoantenna, which is, however, large enough not to quench the transition. The hybrid system using quantum dots, molecules, or nitrogen-vacancy (NV)-centers in diamond can serve as an efficient single photon source. It is suitable for high-speed information transfer at optical frequencies on the nanoscale for future applications.

Markus Pfeiffer, Klas Lindfors, Paola Atkinson, Armando Rastelli, Oliver G. Schmidt, Harald Giessen, Markus Lippitz
Positioning plasmonic nanostructures on single quantum emitters
Physica Status Solidi B, 249, 678-686 (2012) online , ERef, abstract: show

The controlled positioning and fabrication of nanostructures on single quantum emitters allows tuning and engineering their properties. Here, we demonstrate two methods to modify the light emission properties of single semiconductor quantum dots using plasmonic nanostructures. First, we have used atomic force microscopy (AFM) based nanomanipulation to position plasmon resonant gold nanoparticles on single near-surface GaAs quantum dots. The particles act as optical antennas significantly enhancing the photoluminescence signal. The increase in luminescence is shown to originate from an increased excitation rate of the emitter. We have also developed a method to fabricate nanostructures aligned to near-surface semiconductor quantum dots using electron beam lithography (EBL). The accuracy of the alignment procedure achieved in this work is better than 30?nm. The flexibility of our approach allows single semiconductor quantum dots to be coupled to complex device structures in order to obtain new functionalities. As a first example we show how excitons in quantum dots can be coupled to plasmonic waveguides. Our results pave the way for realizing integrated nanoscale optical circuits for quantum optics research and applications.

Markus Lippitz, P. Michler
Preface
Physica Status Solidi B, 249, 643 (2012) online , ERef

Mario Hentschel, Tobias Utikal, Harald Giessen, Markus Lippitz
Quantitative Modeling of the Third Harmonic Emission Spectrum of Plasmonic Nanoantennas
Nano Letters, 12, 3778-3782 (2012) online , ERef, abstract: show

Plasmonic dimer nanoantennas are characterized by a strong enhancement of the optical field, leading to large nonlinear effects. The third harmonic emission spectrum thus depends strongly on the antenna shape and size as well as on its gap size. Despite the complex shape of the nanostructure, we find that for a large range of different geometries the nonlinear spectral properties are fully determined by the linear response of the antenna. We find excellent agreement between the measured spectra and predictions from a simple nonlinear oscillator model. We extract the oscillator parameters from the linear spectrum and use the amplitude of the nonlinear perturbation only as scaling parameter of the third harmonic spectra. Deviations from the model only occur for gap sizes below 20 nm, indicating that only for these small distances the antenna hot spot contributes noticeable to the third harmonic generation. Because of its simplicity and intuitiveness, our model allows for the rational design of efficient plasmonic nonlinear light sources and is thus crucial for the design of future plasmonic devices that give substantial enhancement of nonlinear processes such as higher harmonics generation as well as difference frequency mixing for plasmonically enhanced terahertz generation.

Bernd Metzger, Mario Hentschel, Markus Lippitz, Harald Giessen
Third-harmonic spectroscopy and modeling of the nonlinear response of plasmonic nanoantennas
Optics Letters, 37, 4741-4743 (2012) online , ERef, abstract: show

We perform third-harmonic (TH) spectroscopy on rod-type gold nanoantenna arrays using widely tunable sub-30 fs laser pulses. We find the peak of the TH generation efficiency of the antenna arrays always slightly redshifted with respect to the peak of their linear extinction spectrum. We model the wavelength-dependent TH response quantitatively using a nonlinear harmonic oscillator model.

Christian Wolpert, Lijuan Wang, Armando Rastelli, Oliver G. Schmidt, Harald Giessen, Markus Lippitz
Transient absorption spectroscopy of a single lateral InGaAs quantum dot molecule
Physica Status Solidi B, 249, 731-736 (2012) online , ERef, abstract: show

Lateral quantum dot molecules promise a scalable approach to quantum registers. The individual neutral exciton states can be controlled by a lateral bias voltage. In this way radiative emission of localized excitons can be switched between the two quantum dots in the molecule. Here, we report on coherent absorption spectroscopy of a single quantum dot molecule. By transient reflection spectroscopy we demonstrate that not only the emission but also the absorption switches from one dot to the other when varying the applied electric field. This result is consistent with Rabi flopping showing a radically suppressed dipole moment for the non-emitting exciton transition.

Christian Wolpert, Christian Dicken, Paola Atkinson, Lijuan Wang, Armando Rastelli, Oliver G. Schmidt, Harald Giessen, Markus Lippitz
Transient Reflection : A Versatile Technique for Ultrafast Spectroscopy of a Single Quantum Dot in Complex Environments
Nano Letters, 12, 453-457 (2012) online , ERef, abstract: show

Increasingly complex structures such as optical antennas or cavities are coupled to self-assembled quantum dots to harvest their quantum-optical properties. In many cases, these structures pose a problem for common methods of ultrafast spectroscopy used to write and read out the state of the quantum dot. We present a pure far-field method that only requires optical access to the quantum dot and does not impose further restrictions on sample design. We demonstrate Rabi oscillations and perturbed free induction decay of single GaAs quantum dots that have a dipole moment as small as 18 D. Our method will greatly facilitate ultrafast spectroscopy of complex quantum-optical circuits.

Christian Wolpert, Christian Dicken, Lijuan Wang, Paola Atkinson, Armando Rastelli, Oliver G. Schmidt, Harald Giessen, Markus Lippitz
Ultrafast coherent spectroscopy of a single self-assembled quantum dot
Physica Status Solidi B, 249, 721-730 (2012) online , ERef, abstract: show

The controlled interaction of several quantum dots (QDs) mediated by plasmonic or photonic nanostructures promises interesting new functionality in the fields of quantum computing and telecommunication. Ultrafast laser pulses can be used to write and read out the state of the QD. We review ultrafast coherent spectroscopy of single QDs. The focus of this article is on the technique of transient reflection spectroscopy which can be applied to a broad range of samples and devices. It only requires optical access to a single quantum system next to a reflecting surface. We demonstrate the versatility of our approach by presenting several quantum optical studies such as Rabi oscillations, perturbed free induction decay, and quantum beats from an entangled excitonic state in weakly absorbing GaAs QDs. We expect this experimental method to make coherent experiments possible in elaborate devices where quantum emitters are interacting with a complex environment such as plasmonic waveguides or antennas.

Dong-Hun Chae, Tobias Utikal, Siegfried Weisenburger, Harald Giessen, Klaus von Klitzing, Markus Lippitz, Jurgen Smet
Excitonic Fano Resonance in Free-Standing Graphene
Nano Letters, 11, 1379-1382 (2011) online , ERef, abstract: show

We investigate the role of electron-hole correlations in the absorption of free-standing monolayer and bilayer graphene using optical transmission spectroscopy from 1.5 to 5.5 eV. Line shape analysis demonstrates that the ultraviolet region is dominated by an asymmetric Fano resonance. We attribute this to an excitonic resonance that forms near the van Hove singularity at the saddle point of the band structure and couples to the Dirac continuum. The Fano model quantitatively describes the experimental data all the way down to the infrared. In contrast, the common noninteracting particle picture cannot describe our data. These results suggest a profound connection between the absorption properties and the topology of the graphene band structure.

Thorsten Schumacher, Kai Kratzer, David Molnar, Mario Hentschel, Harald Giessen, Markus Lippitz
Nanoantenna-enhanced ultrafast nonlinear spectroscopy of a single gold nanoparticle
Nature Communications, 2, 333 (2011) online , ERef, abstract: show

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.

Tobias Utikal, Thomas Zentgraf, Sergei G. Tikhodeev, Markus Lippitz, Harald Giessen
Tailoring the photonic band splitting in metallodielectric photonic crystal superlattices
Physical Review B, 84, 075101 (2011) online , ERef, abstract: show

We experimentally and theoretically investigate the influence of a structured supercell on the band splitting of one-dimensional metallodielectric photonic crystal superlattices. We show that the splitting of the photonic bands can be modified by periodic structuring of the elementary unit cell of the photonic crystal. For our investigation we constructed metallic photonic crystal superlattices by creating supercells from standard photonic crystal building blocks and arranged them at certain distances apart. The optical properties were obtained by conventional angle-resolved white-light transmission measurements.

M. J. Winterhalder, A. Zumbusch, Markus Lippitz, Michel Orrit
Toward Far-field Vibrational Spectroscopy of Single Molecules at Room Temperature
The Journal of Physical Chemistry B, 115, 5425-5430 (2011) online , ERef, abstract: show

We propose a new scheme for the extraction of chemically sensitive vibrational information from a single fluorescent molecule at room temperature. Our approach is based on a three-photon fluorescence excitation scheme, with selectivity in the production of a vibrational population of the ground state. We estimate the expected signal in perturbation theory for a standard dye molecule, compare its magnitude qualitatively to noise and various background sources, and discuss the experimental realization of this scheme.

Tobias Utikal, Thomas Zentgraf, Thomas Paul, Carsten Rockstuhl, Falk Lederer, Markus Lippitz, Harald Giessen
Towards the Origin of the Nonlinear Response in Hybrid Plasmonic Systems
Physical Review Letters, 106, 133901 (2011) online , ERef, abstract: show

Plasmonic systems are known for their distinct nonlinear optical properties when compared to purely dielectric materials. Although it is well accepted that the enhanced nonlinear processes in plasmonic-dielectric compounds are related to the excitation of localized plasmon resonances, their exact origin is concealed by the local field enhancement in the surrounding material and the nonlinearity in the metal. Here, we show that the origin of third-harmonic generation in hybrid plasmonic-dielectric compounds can be unambiguously identified from the shape of the nonlinear spectrum.

Tobias Utikal, Mark I. Stockman, Albert P. Heberle, Markus Lippitz, Harald Giessen
All-Optical Control of the Ultrafast Dynamics of a Hybrid Plasmonic System
Physical Review Letters, 104, 113903 (2010) online , ERef, abstract: show

We demonstrate complete all-optical and phase-stable control of the linear optical polarization and the nonlinear coherent response (third-harmonic generation) of a hybrid nanoplasmonic-photonic system. A few tens of femtoseconds after the excitation, we turn the response on and off at any given point in time and probe its temporal evolution throughout the control process with a three-pulse nonlinear optical technique. After being switched off, the polarization and the nonlinear radiation remain off permanently. All experiments agree well with numerical simulations based on a damped harmonic oscillator model.

Harald Giessen, Markus Lippitz
Directing Light Emission from Quantum Dots
Science, 329, 910-911 (2010) online , ERef

Markus Pfeiffer, Klas Lindfors, Christian Wolpert, Paola Atkinson, Mohamed Benyoucef, Armando Rastelli, Oliver G. Schmidt, Harald Giessen, Markus Lippitz
Enhancing the Optical Excitation Efficiency of a Single Self-Assembled Quantum Dot with a Plasmonic Nanoantenna
Nano Letters, 10, 4555-4558 (2010) online , ERef, abstract: show

We demonstrate how the controlled positioning of a plasmonic nanoparticle modifies the photoluminescence of a single epitaxial GaAs quantum dot. The antenna particle leads to an increase of the luminescence intensity by about a factor of 8. Spectrally and temporally resolved photoluminescence measurements prove an increase of the quantum dot's excitation rate.

M. Geiselmann, Tobias Utikal, Markus Lippitz, Harald Giessen
Tailoring the ultrafast dynamics of the magnetic mode in magnetic photonic crystals
Physical Review B, 81, 235101 (2010) online , ERef, abstract: show

We investigate the ultrafast time dynamics of magnetic waveguide-particle-plasmon polaritons in a magnetic photonic crystal. This magnetic mode consists of the antisymmetric localized charge oscillation in metallic cut-wire pairs. At the appropriate periodicity, strong coupling and polaritonic hybridization take place between the antisymmetric plasmon resonance and the excited photonic mode of the underlying slab waveguide. By varying the lattice period and the wire cross section of the structure, tailoring of the temporal dynamics of the magnetic polariton is possible. Simulations are in good agreement with third-order nonlinear autocorrelation function measurements and confirm extremely long dephasing times of the polaritonic system. Future applications could include all-optical control of optical magnetism.

Anna L. Tchebotareva, Meindert A. van Dijk, Paul V. Ruijgrok, Vincent Fokkema, Marcel H. S. Hesselberth, Markus Lippitz, Michel Orrit
Acoustic and Optical Modes of Single Dumbbells of Gold Nanoparticles
ChemPhysChem, 10, 111-114 (2009) online , ERef

T. Ergin, T. Benkert, Harald Giessen, Markus Lippitz
Ultrafast time-resolved spectroscopy of one-dimensional metal-dielectric photonic crystals
Physical Review B, 79, 245134 (2009) online , ERef, abstract: show

We study the all-optical switching behavior of one-dimensional metal-dielectric photonic crystals due to the nonlinearity induced by a hot electron gas. A polychromatic pump-probe setup is used to determine the wavelength and pump intensity dependence of the ultrafast transmission suppression as well as the dynamics of the process on a subpicosecond time scale. We find ultrafast (subpicosecond) as well as a slow (millisecond) behavior. We present a model of the ultrafast dynamics and nonlinear response, which can fit the measured data well and allows us to separate the thermal and the electronic response of the system.

Meindert A. van Dijk, Markus Lippitz, Daniel Stolwijk, Michel Orrit
A common-path interferometer for time-resolved and shot-noise-limited detection of single nanoparticles
Optics Express, 15, 2273-2287 (2007) online , ERef, abstract: show

We give a detailed description of a novel method for timeresolved experiments on single non-luminescent nanoparticles. The method is based on the combination of pump-probe spectroscopy and a common-path interferometer. In our interferometer, probe and reference arms are separated in time and polarization by a birefringent crystal. The interferometer, fully described by an analytical model, allows us to separately detect the real and imaginary contributions to the signal. We demonstrate the possibilities of the setup by time-resolved detection of single gold nanoparticles as small as 10 nm in diameter, and of acoustic oscillations of particles larger than 40 nm in diameter. (c) 2007 Optical Society of America.

Meindert A. van Dijk, Anna L. Tchebotareva, Michel Orrit, Markus Lippitz, S. Berciaud, D. Lasne, L. Cognet, B. Lounis
Absorption and scattering microscopy of single metal nanoparticles
Physical Chemistry Chemical Physics, 8, 3486-3495 (2006) online , ERef, abstract: show

Several recently developed detection techniques opened studies of individual metal nanoparticles (1-100 nm in diameter) in the optical far field. Eliminating averaging over the broad size and shape distributions produced by even the best of current synthesis methods, these studies hold great promise for gaining a deeper insight into many of the properties of metal nanoparticles, notably electronic and vibrational relaxation. All methods are based on detection of a scattered wave emitted either by the particle itself, or by its close environment. Direct absorption and interference techniques rely on the particle's scattering and have similar limits in signal-to-noise ratio. The photothermal method uses a photo-induced change in the refractive index of the environment as an additional step to scatter a wave with a different wavelength. This leads to a considerable improvement in signal-to-background ratio, and thus to a much higher sensitivity. We briefly discuss and compare these various techniques, review the new results they generated so far, and conclude on their great potential for nanoscience and for single-molecule labelling in biological assays and live cells.

Meindert A. van Dijk, Markus Lippitz, Michel Orrit
Detection of acoustic oscillations of single gold nanospheres by time-resolved interferometry
Physical Review Letters, 95, 267406 (2005) online , ERef, abstract: show

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.

Meindert A. van Dijk, Markus Lippitz, Michel Orrit
Far-field optical microscopy of single metal nanoparticles
Accounts of Chemical Research, 38, 594-601 (2005) online , ERef, abstract: show

Individual noble-metal particles, with sizes ranging from a few tenths to some hundreds of nanometers, can now be detected by far-field optics. Single-particle microscopy gives access to inhomogeneity, distributions, and fluctuations, which were previously hidden in ensemble experiments. Scattering methods rely on dark-field illumination, spectral signatures of the metal particles, or both. More advanced techniques provide high sensitivity and improved selectivity with respect to other scatterers by isolating metal-specific signals, for example the refractive index change due to heating of the environment by a pump beam or the time-resolved optical response of the particle to a short pump pulse. We review and compare linear and nonlinear methods in far-field optical microscopy that have reached the single-particle regime by means of scattered light, thermal effects, photoluminescence, or nonlinear frequency generation.

Markus Lippitz, Florian Kulzer, Michel Orrit
Statistical evaluation of single nano-object fluorescence
ChemPhysChem, 6, 770-789 (2005) online , ERef, abstract: show

Single nano-objects display strong fluctuations of their fluorescence signals. These random and irreproducible variations must be subject to statistical analysis to provide microscopic information. We review the main evaluation methods used so for by experimentalists in the field of single-molecule spectroscopy: time traces, correlation functions, distributions of "on" and "off" times, higher-order correlations. We compare their advantages and weaknesses from a theoretical point of view, illustrating our main conclusions with simple numerical simulations. We then review experiments on different types of single nano-objects, the phenomena which are observed and the statistical analyses applied to them.

Markus Lippitz, Meindert A. van Dijk, Michel Orrit
Third-harmonic generation from single gold nanoparticles
Nano Letters, 5, 799-802 (2005) online , ERef, abstract: show

We report the first observation of third-harmonic signals from individual gold colloids down to 40 nm diameter. Excited with 1-ps pulses at 1500 nm, the colloids generate 500-nm light, close to the plasmon resonance. The third-harmonic intensity varies as the square of the colloid surface area. Although weak, the third-harmonic signals of gold labels as small as 15 nm in diameter are expected to be accessible with 100-fs pulses. They could be used in microscopy for single-biomolecule tracking.

Markus Lippitz, Christian G. Hübner, Thomas Christ, Holger Eichner, Patrice Bordat, Andreas Herrmann, Klaus Müllen, Thomas Basche
Coherent electronic coupling versus localization in individual molecular dimers
Physical Review Letters, 92, 103001 (2004) online , ERef, abstract: show

We have investigated electronic excitation transfer in individual molecular dimers by time and spectrally resolved confocal fluorescence microscopy. The single molecule measurements allow for directly probing the distribution of the electronic coupling strengths due to static disorder in the polymer host. We find dimers where the excitation is delocalized (superradiant emission) while for others emission originates from a localized state. Transitions between delocalized and localized states as observed for a given dimer are attributed to structural fluctuations of the guest-host system.

Wolfgang Erker, Markus Lippitz, Thomas Basche, Heinz Decker
Toward oxygen binding curves of single respiratory proteins
Micron, 35, 111-113 (2004) online , ERef, abstract: show

Oxygen binding curves of single molecules promise to discriminate between different models describing cooperativity because load distributions are accessible. Individual tarantula hemocyanins could be detected by fluorescence correlation spectroscopy using intrinsic tryptophan fluorescence as sensor of bound oxygen. However, imaging of immobilized proteins was not possible due to fast photo-bleaching. It is shown that tetra-methyl-carboxy-rhodamine (TAMRA), commonly used as a fluorescence label in single-molecule spectroscopy, can also be applied to monitor bound oxygen. The dye's fluorescence is quenched due to Forster energy transfer to the oxygenated active sites of hemocyanin. (C) 2003 Elsevier Ltd. All rights reserved.

Markus Lippitz, Wolfgang Erker, Heinz Decker, K. E. van Holde, Thomas Basche
Two-photon excitation microscopy of tryptophan-containing proteins
Proceedings of the National Academy of Sciences of the United States of America, 99, 2772-2777 (2002) online , ERef, abstract: show

We have examined the feasibility of observing single protein molecules by means of their intrinsic tryptophan emission after two-photon excitation. A respiratory protein from spiders, the 24-meric hemocyanin, containing 148 tryptophans, was studied in its native state under almost in vivo conditions. In this specific case, the intensity of the tryptophan emission signals the oxygen load, allowing one to investigate molecular cooperativity. As a system with even higher tryptophan content, we also investigated latex spheres covered with the protein avidin, resulting in 340 tryptophans per sphere. The ratio of the fluorescence quantum efficiency to the bleaching efficiency was found to vary between 2 and 180 after two-photon excitation for tryptophan free in buffer solution, in hemocyanin, and in avidin-coated spheres. In the case of hemocyanin, this ratio leads to about four photons detected before photobleaching. Although this number is quite small, the diffusion of individual protein molecules could be detected by fluorescence correlation spectroscopy. In avidin-coated spheres, the tryptophans exhibit a higher photostability, so that even imaging of single spheres becomes possible. As an unexpected result of the measurements, it was discovered that the population of the oxygenated state of hemocyanin can be changed by means of a one-photon process with the same laser source that monitors this population in a two-photon process.

Martin Schüttler, Mirko Leuschner, Markus Lippitz, Wolfgang W. Rühle, Harald Giessen
Towards the origin of the shear force in near-field microscopy
Japanese Journal of Applied Physics, 40, 813-818 (2001) online , ERef, abstract: show

The shear force from a gold or a graphite sample acting on an approaching near-field optical probe is studied in detail. The adiabatic and dissipative contributions to the force are clearly distinguished by monitoring the amplitude as well as the phase of the tip vibration when the tip approaches the surfaces. We also take into account that not only the damping and the resonance frequency but also the mass of the system changes when the tip approaches the surface. The relative strength of the contributions to the force varies differently but characteristically with the distance of the two samples, starting at a much larger distance in the case of graphite. The adiabatic contribution is larger in the case of the gold sample. Measurements at various temperatures are performed using the gold sample, showing a dependence of the shear force on the varying conditions.

Markus Lippitz, Martin Schüttler, Harald Giessen, M. Born, Wolfgang W. Rühle
Bandwidth enhancement of a shear-force-controlled distance regulation in near-field microscopy
Journal of Applied Physics, 86, 100-106 (1999) online , ERef, abstract: show

The distance between sample and probe in a scanning near-field optical microscope is regulated via tracing the shear-force on the tip which is glued to a tuning fork piezo. A lock-in technique is used. We demonstrate that the bandwidth of the control loop is increased if not only amplitude or phase, but a favorable combination of both is used as feedback signal. The enhancement of bandwidth is connected with a reduction of signal-to-noise ratio. The optimum combination of both, bandwidth and signal-to-noise ratio, can be adjusted purely electronically to the specific needs of an experiment. A theoretical model is developed that discloses the relation between the mechanical and electrical properties of the combination of tuning fork and fiber tip. The frequency response of the shear-force detection system is calculated with a numerical simulation based on this model. Experimental frequency response curves are well fitted by these simulations. Our results are especially important for low-temperature scanning microscopy, where the bandwidth enhancement is essential for obtaining a reasonable scanning speed. (C) 1999 American Institute of Physics. S0021-8979(99)04713-1.