Cluster of Excellence –
University of Freiburg

Prof. Dr. Alexander Rohrbach

Prof. Dr. Alexander Rohrbach

Bio- and Nanophotonics, Department of Microsystems Engineering, University of Freiburg

+49 761 203 7536


Our goal is to understand the structure, dynamics, and mechanics of cells and biomaterials at the scale of the wavelength of light and below. Therefore we investigate measuring and manipulating techniques, such as


  • New laser scanning microscopy methods to optimize the interaction between light and matter
  • Optical tweezers including 3-D particle tracking at microseconds-speed and nanometer precision
  • Computer controled holographic illumination systems

We use these methods to investigate

  • Thermally fluctuating systems and phenomena in soft materials (cells and complex liquids)
  • The nanomechanics of molecular motors and the cytoskeleton

Our basic physical research serves as a platform for developing new technologies for the following fields of application. High-resolution microscopy for researching versatile cell, polymer, and surface structures: We improve optical resolution and contrast of 3-D images by intelligent combinations of diffraction-limited illumination of smallest structures (0.1 – 1 ?m) and light scattered at the object to be studied. Here one distinguishes between light carrying relevant object information and light carrying no or wrong information.

Measuring and structuring nanotechnology: How do you build something that is so small that you cannot even see it under a microscope? By using laser optical tweezers (optical traps) we try to assemble the smallest structures of spherical or rod-shaped building blocks. These are smaller than 0.1 ?m and are typically metals or semi-conductors, meaning they have multiple functional properties. We utilize new measuring techniques, such as optically trapped and guided probes or holographic interference, to optically measure and “see” the assembled systems quickly and precisely.

Biophysics and biotechnology: Processes occurring in cells or at their membranes are not only determined by biochemical laws, but also by purely physical laws which regulate reaction kinetics or cell mechanics by diffusion, fluctuation, and molecular motors. This affects the generation and propagation of cellular signals and is one of the subjects of research in the Biological Signalling Studies (bioss) Cluster of Excellence. The fluctuation-controlled absorption of bacteria, viruses, and drugs through still unknown diffusion properties nearby cell membranes plays an important role in medicine and pharmaceutical research. We strive to understand these physical laws by investigating isolated cellular subsystems to find out how they react to specific changes in their environmental conditions.


10 selected publications:

  • Fast, label-free super-resolution live-cell imaging using rotating coherent scattering (ROCS) microscopy
    Jünger F, Olshausen P, Rohrbach A (2016).
    Sci Rep. 28;6:30393
  • Separation of ballistic and diffusive fluorescence photons in confocal Light-Sheet Microscopy of Arabidopsis roots.
    Meinert T, Tietz O, Palme KJ, Rohrbach A (2016).
    Sci Rep. 6:30378.
  • Surface imaging beyond the diffraction limit with optically trapped spheres.
    Friedrich L, Rohrbach A (2015).
    Nat Nanotechnol. 10(12):1064-9.
  • Measuring local viscosities near plasma membranes of living cells with photonic force microscopy.
    Jünger F, Kohler F, Meinel A, Meyer T, Nitschke R, Erhard B, Rohrbach A (2015).
    Biophys J. 109(5):869-82.
  • Superresolution Imaging of Dynamic MreB Filaments in B. subtilis - A Multiple-Motor-Driven Transport?
    v Olshausen P, Soufo HJD, Graumann P, Wicker K, Heintzmann R, Rohrbach A (2013).
    Biophys J. 105(5):1171 – 1181
  • Object adapted optical trapping and shape tracking of energy switching helical bacteria.
    Koch M. Rohrbach A (2012).
    Nat Photonics, 6, 680 - 686
  • Microfluidic sorting of arbitrary cells with dynamic optical tweezers.
    Landenberger B, Höfemann H, Wadle S, Rohrbach A (2012).
    Lab Chip, 12, 3177-3188.
  • Propagation stability of self-reconstructing Bessel beams enables contrast-enhanced imaging in thick media.
    Fahrbach FO, Rohrbach A (2012)
    Nature Communications 3: p. 632.
  • Microscopy with self-reconstructing beams.
    Fahrbach, FOP, Simon P, Rohrbach A (2010). 
    Nature Photonics. 4(11):780-785
  • Filopodia act as phagocytic tentacles and pull with discrete steps and a load-dependent velocity.
    Kress H, Stelzer EHK, Holzer D, Buss F, Griffiths G, Rohrbach A (2007). 
    Proc Nat Acad Sci.104, 11633–11638