Publication
Life sciences have a constantly growing need for novel methodological approaches suitable to investigate the intracellular environment with increased temporal and spatial resolutions. Recently, optical near-field probes, such as laser-irradiated pointed metal, uncoated/metal-coated tapered optical fibers, as well as nano-emitters, such as single molecules or nanoparticles, have attracted increasing attention as key components of high-resolution microscopes. In parallel, super-resolution fluorescence microscopy, operating in far-field regime and overcoming the light diffraction limit, has markedly improved imaging resolution. Although both near- and far-field optical approaches drastically improve image resolution, they still represent numerous drawbacks, such as, technical difficulties concerning probe preparation (near-field) or potential damaging through very high light intensities (far-field). The aforementioned shortcomings of high-resolution optical microscopy can be overcome to a great extent by photonic force microscopy (PFM). PFM employs a strongly focused near-infrared (NIR) laser light to hold a dielectric or metallic particle as a local probe. Such an optical trap enables one to ‘cage’ a mesoscopic particle and track its three-dimensional (3D) thermal fluctuations in the surrounding environment, e.g. a viscous liquid. Therefore, besides offering near-field 3D imaging, PFM reports also on other important quantities concerning local mechanical properties, including force and viscosity as well as dynamical properties of the surrounding medium. This additional information is derived from the careful analysis of the jittery motion (so-called Brownian motion) of the trapped single-particle probe that collides with thermally-activated surrounding molecules. In this thesis, we first study in detail the Brownian motion of a single spherical particle that is confined within the harmonic potential of the optical trap. To this end, we optimize the NIR light path and electronic noise floor of our custom-built PFM set-up for detecting and quantifying resonances in the Brownian motion. These resonances arise from the coupling between the hydrodynamic memory and strong strength of the optical trap. Due to the high sensitivity of the short-time dynamics, the size of the particle can be measured by simultaneous fitting of the velocity autocorrelation function and power spectral density of its thermal fluctuations. In order to choose the best-suitable spherical probe for a given experiment in PFM, computational modeling based on a Matlab toolbox is performed. The generalized Lorenz-Mie theory is computed using the T -matrix method for various experimental conditions, including changes in the size and refractive index of the sphere, as well as different laser polarization states. The axial equilibrium position is examined to predict its location compared with the position of the laser focus. Optical forces acting on the sphere are investigated in 3D to hi