Theses

• Apertureless SNOM : a new tool for nano-optics.

   

  Research Area: 1. Nano-optics (experimental)

   

  Abstract

  In this thesis a new scanning near field optical microscope based on an apertureless scattering technique is introduced for resolving optical properties of surfaces with lateral resolution reaching 10 nm and better.

  The construction of the instrument is based on a dynamic mode operating atomic force microscope (AFM) which is coupled with a sophisticated heterodyne interferometric optical detection system. A continuous wave (cw) laser beam is focused onto the apex of the metallic or dielectric AFM tip. The backscattered light is collected and interfered with a reference beam which is slightly shifted in frequency with respect to the scattered beam. The interfering signals are detected by a fast avalanche photodiode. The result is a temporal beat modulation at the shift frequency. The scattered light consists of two parts of different spatial origin. One of them is the near field that contains information belonging to a very small vicinity of tip apex interacting with surface. The second part is the far field part which comes from parasitic scattering along the illuminated tip body and the sample surface. By demodulating the beat signal at higher harmonics of the tip vibration, the far field part can be suppressed effectively, leaving only the near field information of the surface-tip interaction. By raster scanning the sample under the AFM tip, information about the amplitude and phase of the near field belonging to the surface is obtained simultaneously with topography.

  This new apertureless scanning near field optical microscope (a-SNOM) features several advantages over the well-known aperture SNOM: High resolution limited essentially only by the tip apex dimension, and effective background suppression. Particular care has been taken in the operation settings of the AFM, since they are shown to be one of the sources of artifacts in the detected signal due mechanical nature of the AFM. When proper conditions are met, these mechanical interaction artifacts are minimal and the a-SNOM produces essentially only optical information. The demonstration of the a-SNOM operation on Au pattern on glass surface and Ag colloid on Si surface systems show that a high sensitivity to material contrast as well as a high spatial resolution is achieved.

• Implementation of an Apertureless Scanning Near-Field Optical Microscope for the Infrared Spectrum

   

  Research Area: 1. Nano-optics (experimental)

   

  In this thesis, a new apertureless scanning near-field optical microscope (aSNOM) with the ability to resolve optical properties of surfaces with a lateral resolution better than 50 nm in a broadband infrared (IR) spectrum is described.
  The high resolution beyond the diffraction limit is achieved by using the sharp tip of an atomic force microscope (AFM) to probe the local electromagnetic field of a sample situated at nanometer distance. A continuous wave (cw) laser beam is focused onto the apex of a metalized or dielectric AFM tip with an apex radius of about 10 nm. The backscattered light is mixed with a reference beam of the same wavelength and detected with a nitrogen cooled InSb-photovoltaic diode. This interferometric scheme allows to enhance the signal level and to detect both the amplitude and phase of the scattered light. The obtained signal is influenced by two components: the light scattered in a small vicinity of the tip apex containing near-field information from the sample, and a background field scattered from the body of the tip and from the sample. To suppress the background component, the AFM is used in non-contact mode with a small vibration amplitude, and the signal is demodulated at higher harmonics of the tip vibration frequency with a lock-in amplifier. By raster scanning the sample under the AFM tip, near-field information from the sample is obtained simultaneously with the topography.
  Imaging in a broad infrared spectrum is achieved by using a cw optical parametric oscillator (cw-OPO) as light source. The OPO has been carefully characterized and gives the instrument the capability to image in a spectral region from 1.5 to 3.8 µm with a small gap between 2.0 and 2.3 µm. By using spherical mirrors instead of lenses, the setup is nearly achromatic.
  As applications of the infrared aSNOM (IR-aSNOM), images of two different sub-wavelength gold structures on glass surfaces were taken. The first is a test sample that shows the capability of the instrument to image material contrast with a lateral resolution better than 50 nm, corresponding to λ/50. The second application shown is the investigation of split-ring resonators, which are of interest for the study of negative index materials. Maps of optical near-fields at the resonantly excited plasmonic structures are shown — with similar resolution.
• Apertureless SNOM : realistic modeling of the imaging process and measurements of resonant plasmonic nanostructures.

   

  Research Area: 2. Nano-optics (simulations)

   

  Abstract

  This thesis studies apertureless Scanning Near Field Optical Microscopy, a technique that uses the apex of a very sharp tip to obtain local optical information with lateral resolution much beyond the diffraction limit. Both theoretical and experimental results are discussed.

  The theoretical work is a significant advance towards the quantitative convergence of experiments and theoretical predictions, and should be useful in aiding the interpretation of measured images. Extended tips and substrates are used, and the detector is also carefully modeled. A static tip in vacuum serves to study the influence of the tip and illumination geometry on the far fields and on the near fields in the proximity of the tip apex, the volume used to probe the sample. Including a gold substrate and the commonly used demodulation scheme allows to study the discrimination of the components carrying the local information. A very good discrimination is verified for silicon tips and small oscillation amplitudes, as far as the tip interacts closely with the substrate and the oscillation remains highly sinusoidal. The imaging process is studied by including patterned substrates. The obtained signal is mostly sensitive to a few nanometers of depth into the sample, and the influence of the scanning conditions on the level of signal, background suppression and lateral resolution is characterized. Further, a closer look into the behavior of the extended physical detector reveals the influence of the spatial inhomogeneities of the scattered fields and, for interferometric measurements, the large significance of the optical phase.

  Experimentally, different techniques are first described that can facilitate images with clear local information. A cross polarization scheme is introduced which is very useful for non-perturbative measurements. It is applied to the mapping of the the field distribution surrounding plasmonic structures, for both the phase and the amplitude. Beyond dipolar resonances, I also study coupled dipoles and quadrupole field distributions. When imaging artifacts are avoided, the obtained images closely resemble theoretical expectations.

• Near-field optical investigations of plasmonic eigenmodes in thin metal films with total internal reflection illumination

   

  Research Area: 1. Nano-optics (experimental)

   

  In this thesis I report on the implementation of a new illumination scheme for apertureless Scanning Near Field Microscopy (aSNOM). In contrast to the conventional implementations of this tool, here the sample is excited from bellow under total internal reflection configuration. The direct implication of this setup is the decoupling of illumination and signal collection in the setup. Among the advantages of having an independent illumination, the ability of studying propagating Surface Plasmons Polaritons (SPP) and the reduction of parasitic signal are the most remarkable. The difficulties encountered during implementation, setup description, and performance tests are described after a brief review in the topics covered in the thesis. The newly implemented tool was used to study the propagation of SPP on extended and nanostructured thin gold films, excited with the classical Kretschmann configuration. The ability of mapping near field intensity and phase – one of the uniqueness of the tool – with ultimate optical resolution is demonstrated. The wavevector of the bounded propagating mode can be thus measured locally on the sample surface.

• Optical Wire Antennas : Near-Field Imaging, Modeling and Emission Patterns

   

  Research Area: 1. Nano-optics (experimental)

   

  Abstract

  In this thesis we study the properties of optical wire antennas. As experimental method for our investigations we use apertureless near-field optical microscopy. This technique achieves high spatial resolution well beyond the diffraction limit by utilizing the field enhancement at the apex of sharp tips. An interferometric measurement scheme allows us to detect both near-field intensity and optical phase. By using s-polarized light for illumination and detecting the p-polarized component of the backscattered light we are able to map the z-component of the electrical near- field. Optimizing polarizer and analyzer angles of our cross-polarization scheme ensures a background free plasmonic eigenmode mapping. By comparison with simulation data not including the tip we show that the measurement has little to no influence on the eigenmode.

  The samples investigated in this thesis are arrays of gold nano-wires prepared by electron beam lithography. We observe plasmon resonances in our near-field images as patterns of lobes and explain them by regarding the wires as one dimensional Fabry-Pérot resonators. The number of nodes in between the lobes is the resonance order. From eigenmodes well beyond quadrupolar order we extract both, propagation constant and reflection phase of the guided surface plasmon polariton with superb accuracy. The combined symmetry breaking effects of oblique illumination and retardation allow the excitation of dipole forbidden even-order resonances. By systematically varying the azimuthal illumination angle we are able to map the directional receiving and emission patterns of the wire antennas.

  In order to understand these patterns we develop an analytical model. In contrast to radio frequency (RF) antenna theories we not only assume surface currents but also take volume currents into account. The model also allows us to spotlight the differences between plasmonic and RF antennas. The equations we derive describe both, the property of the wires as resonators as well as the antenna emission / reception patterns. With just four – physically motivated – parameters we are able to fit measured as well as simulated data astonishingly well. With this model predicting the relative intensity and phase of the light absorbed and scattered by nano-wire antennas it has great potential for future research.

• Wavelength dependent opto-electronic characterization of carbon nanotube based field effect transistors

   

  Research Area: 1. Nano-optics (experimental)

   

  The following bachelor thesis which is based on a six-week internship at the Max Planck Institute for Solid State Research in Stuttgart gives first results of a project that studies the E11-transition of carbon nanotubes by the use of PhotoElectronic Transport Imaging (PETI).
  
  "Herein we demonstrate photocurrent imaging as a fast and effective tool to locate such [charge transport] barriers within individual [...] nanotubes contacted by metal electrodes. The locally induced photocurrents directly reflect the existence of built-in electric fields associated with the presence of depletion areas at the contacts or structural defects along the tube." [4] This quotation is perhaps the best summary of the method, which is explored in more detail in this thesis.
  
  PhotoElectronic Transport Imaging was found to be an important tool for the examination and characterization of small objects like carbon nan- otubes. Up to now, PETI has always been realized in the visible light range. In an absorption process, the energy difference between final and initial state has the same value as the energy of the absorbed photon. As band gaps of semiconducting carbon nanotubes are in the range of 0.4 - 0.8eV, infrared light is needed to probe at the same time the highest occupied and the lowest unoccupied states of a nanotube. The transition with the lowest possible en- ergy is called the E11-transition. This low energy transition might be of great interest in the analysis of some of the processes involved in optical absorption in carbon nanotubes, i.e. of excitonic behavior of carbon nanotubes. Tuning the wavelength and analyzing the taken photocurrent data should allow di- rect conclusions about the density of states (DOS) of the nanotubes and its excitonic behavior.
  
  In the following bachelor thesis the separate components of PETI are first of all explained in a theoretical part. Single Wall Carbon Nanotubes (SWNTs) are part of field effect transistors that are illuminated locally with infrared laser light. After this theoretical part, setup and analysis method are explained. A description of the results and finally the analysis of the data taken so far and improvements of the setup are following and lead into the conclusion with an outlook to actual research in the domain of carbon nanotubes.

• Setup of an achromatic, pump-probe apertureless nearfield optical microscope

   

  Research Area: 1. Nano-optics (experimental)

   

  Abstract In this thesis we describe the setup of an apertureless scanning near-field optical microscope with the improvements for achromaticity and pulsed laser excitation. The aSNOM presented here is optimized for plasmonic eigenmode mapping. Compared to the other near-field optical setups in our group this aSNOM is completely achromatic and the interferometer is accurately set to zero path difference. The achromaticity is achieved by using off-axis parabolic mirrors for focusing and collimating. It is ready for spectroscopy with high spatial resolution. A spatial resolution in the nanometer regime far beyond the diffraction limit is achieved, limited only by the apex radius of the Atomic Force Microscope (AFM) tip, independent of the wavelength of the light source used. An interferometer is used to interferometrically amplify the signal beam and to enable to measurements of the optical phase. The signal is measured by a silicon pin-diode detector providing a high sensitivity in the visible and near infrared regime, with a wide linear dynamic response. To improve the quality of the image of plasmonic samples, background suppression based on cross polarization scheme, harmonic detection and a confocal setup are employed. Furthermore, zero path difference between the interferometer arms allows the use of low coherent light sources as excitation source. Together with pulsed laser systems as excitation a high temporal resolution can be achieved using pump-probe technique.
  To test the setup imaging quality a sample with gold nano structures on glass is investigated at wavelength close to the resonance wavelength of the structures. Further measurement with the aSNOM technique are performed on waveguides connected to feed gap antennas made from monocrystalline gold on an ITO layer. The optical behavior of such waveguides is of high interest for the development of plasmonic nano circuits.

   

• Multiple-Multipole simulations of plasmonic nanostructures

   

  Research Area: 2. Nano-optics (simulations)

   

  In this thesis, we want to simulate the electromagnetic field of varying nanostructures of interchangeable materials illuminated by different excitation beams in order to analyze absorption and scattering processes in the near- and far-field regime. Essentially, this means that we need to solve Maxwell's equations numerically.
  
  One nanometer (nm) is one billionth meter or 1E-9 m. This is roughly the diameter of a DNA double helix or about the one hundred thousandth part of the thickness of a human hair. The size of atoms is in the 1 AA = 1E-10 m range. So, the nanometer scale marks the transition between the atomic and macroscopic regime.
  
  In this region surface effects become relevant because the surface of a three dimensional object grows quadratically whereas the volume grows cubically and so the ratio between surface and volume increases for decreasing length scales. Physical effects start to depend highly on the exact geometry and size of the object's surface. Even small changes in geometry, which leave the total volume unaffected, can have a considerable impact. In other words: the smaller length regime does not simply mean that the physical effects stay the same and just become smaller. Instead, novel physics emerges for structures well below typical optical wavelengths.
  
  These size effects are also apparent in everyday life. For example nanoparticles cause the colorisation of stained-glass windows or scattering phenomena as we see them in sunsets. Specifically designed nanostructures manipulate light in ways not previously possible and have vast potential applications as for example in optical [1] and electron microscopy [2]. Gold nanoparticles can be used for radiative cancer treatment [3] or surface-enhanced Raman spectroscopy (SERS) [4].
  
  Studying such small objects implies theoretical and experimental challenges: Maxwell's equations are a set of four partial differential vector equations and do usually not provide analytical solutions even for simple models. Without predictions from theory, however, it is demanding and costly to manufacture nanostructures experimentally and measurements often remain without explanation.
  
  Simulations bridge the gap between experiment and theory by employing numerical approximations for specific environmental parameters in order to provide solutions to otherwise difficult scenarios. The big challenge in numerical simulation is to find a model that produces accurate numerical results and at the same time includes enough realistic systems to allow for the interpretation or prediction of experimental results. In this thesis, we solve the Maxwell equations by the so-called multiple multipole method (MMP) implemented in a software package known as MaX-1 [5], which is a semi-analytical, boundary discretization simulation method. It operates under the assumption of linear, homogeneous and isotropic materials, which is not restraining us considerably because most experiments abide by these assumptions. For spherical or rounded structures MMP produces extremely accurate results in a relatively short time.
  
  The unifying theme of this thesis is to apply MMP to spherical or rounded structures under changing environmental parameters such as material of varying temperature, excitation illumination and substrate support.
  
  First of all we need to study the theory of cross sections of optical scattering phenomena. These quantities are very important to us because we can simulate them with MMP. Chapter 2 offers a derivation of the cross sections starting with the Maxwell equations.
  
  The next step is to model the dielectric constant of standard metals such as gold or silver. In chapter 3, we start with the well-known Drude-Lorentz model and extend it by the so-called critical point model. We modify the plasma frequency in this model in order to describe heating effects. With this we can extrapolate the measurement data of the dielectric constants to temperatures other than room temperature. This analysis is useful for experimental scenarios such as pump-probe experiments where pump lasers heat up metallic nanostructures.
  
  Chapter 4 introduces MMP and offers solutions to some basic geometries such as spheres, cylinders and planes.
  
  Also, in chapter 5 we discuss different types of electromagnetic excitations commonly used in experiments such as plane waves, dark field illumination and Gauss beams. Moreover, we outline radial and azimuthal polarized excitations.
  
  Chapter 6 is based on the previous chapters and should not be read without them. We use our knowledge of optical cross sections (chapter 2) and apply it to single and dimer spherical structures (chapter 4) of different materials of varying temperature (chapter 3) that are illuminated by plane waves or focused beams (chapter 5).
  
  Our starting point is the simulation of the cross section of a single gold sphere under plane wave illumination. Also, we give an estimate for the change of the electric field due to heating effects. We complicate matters by looking at different temperatures, changing the illumination to focused beams and replacing the sphere by a shell. Dimer structures are the next logical step: we look at two spherical structures under plane wave illumination and discover, depending on the plane wave polarization, coupling and anti-coupling effects. Also, we investigate the change of the cross section due to heating effects. Then, we model more complex cases involving substrates. Most intriguingly, we predict the occurrence of an optical tweezer for dark field illumination of spheres buried into a sapphire substrate. This is the most complex part of the thesis and to our knowledge, this is the only simulation of such a structure so far. These structures are studied rather extensively in current research and make our simulations very useful [6{10].