By performing experiments at an air-water interface, we operate Holographic Optical Tweezers in a qualitatively
new environment. In this regime, trapping and moving of micro particles may allow access to parameters like
local viscosity and surface tension. Polystyrene micro beads are naturally stabilized in the interface due to a
minimum in surface energy. For this reason, they can also be manipulated by light patterns with small axial field
gradients, without causing the particles to escape due to scattering forces. In this manner, the interface provides
a true two-dimensional "working environment", where particles can be manipulated with high effciency. For
example, we demonstrate different optical "micro tools", which utilize scattering and gradient forces to enable
controlled transport of matter within the surface.
Established phase contrast methods in microscopy use the phase-shifted zeroth order Fourier component of an image-carrying light wave as a reference wave for interferometric superposition with the remaining part of the image wave. Our method consists of a spatial Fourier filtering of the image wave with a spiral phase element which leads to an edge enhancement of both amplitude and phase objects. The spiral phase element is realized by displaying a high resolution phase hologram on a computer-controlled reflective spatial light modulator. The edge enhancement is isotropic which means that all edges are highlighted simultaneously. Controlling the phase of the central area of the hologram leads to an interference image that has a 3-dimensional appearance of the object. In order to allow for white light imaging, the dispersion is compensated by a special double-diffraction setup.
We use a high resolution liquid crystal spatial light modulator (SLM) as phase modulator to generate different kinds of filters for light microscopy, placing it in a Fourier plane of the optical pathway. Manipulating light with a so called phase vortex filter can lead to an interesting kind of phase contrast imaging with remarkable properties. Using spatial coherent illumination from a laser diode, we observe strong edge enhancement within both phase and amplitude objects. The amplification is comparable to the Nomarski method, but with a higher degree of isotropy. The assembly can also provide an informative "shadow effect" (known as pseudo-relief) for shallow structures whose variations in thickness are smaller than the light wavelength. Furthermore, the method may be useful for an alternative kind of interferometric measurements, which solves an existing problem in conventional interferometry.
We present a flexible setup for steering of laser tweezers using a high resolution spatial light modulator (SLM). Moving of e.g. trapped cells in the focal plane of the microscope objective is possible without the need for time consuming re-calculation of holograms. Numerous light spots or other modes like the so called "doughnut modes", which carry angular momentum, can be created and controlled independently by "mouse-dragging" the hologram window at the SLM display. In addition, undesired diffraction orders are suppressed using adequately calculated fresnel holograms.