Total Internal Reflection Microscopy (TIRM) is a relatively new powerful technique to optically measure the interaction forces between a colloidal particle and a surface using single particle evanescent light scattering. The distribution of separation distances sampled by Brownian motion of a particle is used to obtain the potential energy profile of interactions between a particle and a plane surface with femto Newton resolution. In our labs the technique has been employed to measure electrostatic interactions, depletion forces, magnetic interactions, critical Casimir forces and non-equilibrium dynamics of colloids near a wall. Besides the development of a new light scattering model that can take into account the exact geometry and parameters of TIRM experiments, the latest technical improvement of the TIRM-method was the development of a new technique to measure in situ the intensity-distance-relation which is central to TIRM data evaluation and had to be assumed a priori until now. This new method largely extends the range where TIRM can be applied and even allows measurements on highly reflecting gold surfaces and particles. Furthermore, this technique was paving the way to new high precision measurements of Brownian dynamics of particles close to a wall which offered the possibility to investigate the influence of noise on force measurements, i.e. effects of spurious drift and force.
Novel perspectives for the application of total internal reflection microscopy 
G. Volpe, T. Brettschneider, L. Helden, C. Bechinger, Opt. Express 17, 23975 (2009)
Direct measurement of critical Casimir forces 
C. Hertlein, L. Helden, A. Gambassi, S. Dietrich, C. Bechinger
Nature 451, 172 (2008) download
Experimental Verification of an Exact Evanescent Light Scattering Model for TIRM 
C. Hertlein, N. Riefler, E. Eremina, T. Wriedt, Y. Eremin, L. Helden, C. Bechinger
Langmuir 24, 1 (2008) download
Comparison of T-matrix method with discrete sources method applied for total internal reflection 
N. Riefler, E. Eremina, C. Hertlein, L. Helden, Y. Eremin, T. Wriedt, C. Bechinger
J. Quant. Spec. & Rad. Transfer 106, 464-474 (2007) download
Single particle evanescent light scattering simulations for total internal reflection microscopy 
L. Helden, E. Eremina, N. Riefler, C. Hertlein, C. Bechinger, Y. Eremin, T. Wriedt
Applied Optics 45, 7299-7308 (2006) download
Thermodynamics of a colloidal particle in a time-dependent non-harmonic potential 
V.Blickle, T. Speck, L. Helden, U. Seifert, C. Bechinger
Phys. Rev. Lett. 96, 070603 (2006) download
With acousto-optic deflectors (AODs) arbitrary dynamical optical landscapes can be created which allow quasi-simulataneous optical tweezing of up to several hundred particles. This is achieved by deflection of a single incident laser beam on the travelling acoustic wave inside a transparent crystal. Upon changing the frequency and amplitude of this wave, the incoming laser beam can be modulated with respect to its angle and intensity with frequencies up to 50 kHz.
AOD systems are employed in experiments where more than just one laser trap is used, i.e. where extended optical landscapes are created. Since every individual trap can be varied independently with time, this allows to realise complex motional patterns of many colloidal particles. Examples for AOD experiments are Stochastic Resonance, Microfluidics, Ratched Cellular Automata and Giant Diffusion. A simple example of the possibilities and the versatility of AODs is demonstrated in our colloidal soccer game [de] on our german website.
A monolayer of colloidal particles exposed to interfering laser beams experience a periodic optical potential due to optical gradient forces. Because the number of beams, their angle of incidence and their intensity can be easily controlled, this allows the creation of artificial substrate potentials with tunable geometry and substrate strength. With this approach one can study structural phase transitions of monolayers on crystalline and quasicrystalline substrates.
Archimedean-like colloidal tilings on substrates with decagonal and tetradecagonal symmetry
M. Schmiedeberg, J. Mikhael, S. Rausch, J. Roth, L. Helden, C. Bechinger, and H. Stark
Eur. Phys. J. E 32, 25 (2010) download
Archimedean-like tiling on decagonal quasicrystalline surfaces
J. Mikhael, J. Roth, L. Helden, C. Bechinger
Nature 454, 501 (2008) download