We present a new type of stereo microscopy which can be used for tracking in 3D over an extended depth. The use of Spatial Light Modulators (SLMs) in the Fourier plane of a microscope sample is a common technique in Holographic Optical Tweezers (HOT). This set up is readily transferable from a tweezer system to an imaging system, where the tweezing laser is replaced with a camera. Just as a HOT system can diffract many traps of different types, in the imaging system many different imaging types can be diffracted with the SLM. The type of imaging we have developed is stereo imaging combined with lens correction. This approach has similarities with human vision where each eye has a lens, and it also extends the depth over which we can accurately track particles.
We illustrate the abilities of an advanced full-field optical coherence microscope (FF-OCM) setup for characterization of technical materials with internal micro-structures and present this technique also for dynamic process monitoring, as strain-stress tests. Additionally we briefly illustrate the potential of image processing in context of the chosen applications. Furthermore, contrast modification techniques based on Fourier plane filtering are discussed.
The principle of Fourier plane filtering in coherence probe microscopy (CPM) is demonstrated. It is shown that an edge
contrast enhancement can be obtained in an isotropic and anisotropic way for different technical materials containing
interfaces or internal micro-structures. Additionally image processing completes the CPM imaging and analysis.
We present a novel full-field low-coherence interference (LCI) microscope, which exhibits ultra-high axial resolution due
to a broadband super continuum light source and which is at the same time capable to generate different contrast modes
by using Fourier-plane filtering with a spatial light modulator. By changing the phase and spatial frequencies of the backreflected
wavefront of the specimen in the sample arm of the interferometer, we are able to change the contrast in the
depth-resolved LCI images. By displaying different filters on the SLM, as e.g. spiral phase, the resulting images provide
particular enhancement of edges and internal structures, and expose details within the specimen that are not visible in
normal bright-field mode.
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 report coherent anti-Stokes Raman scattering (CARS) microscopy with ns-pulses. The chosen wide-field geometry allows imaging of the whole field of view at once, without scanning of the sample. Tuning the difference of the two incident laser frequencies overlapping at the sample to a specific vibrational level, one can map the spatial distribution of selected Raman active molecules. Both the CARS signal of the surrounding solvent can be excited (negative contrast) as well as the signal of the structure embedded by the solvent (positive contrast). As a biological sample we used slices of a sunflower seed and tuned to the vibrational transition of its ingredient - linoleic acid - at 2870 cm-1 which corresponds to the strongest C-H stretching vibration. Even with a single pair of laser pulses of 3 ns duration it was possible to acquire a rough, but still meaningful image.
Optoacoustic binary holography is applied to gain complete spatio-temporal control over ultrasonic beams. For flexible temporal intensity modulation of the sound-inducing laser light, an electro-optic modulator is used. Furthermore, in order to generate the desired spatial intensity distributions of the ultrasound, a spatial light modulator impresses synthetic binary holograms on the modulated light beam. At a light absorbing surface the optoacoustic effect converts the modulated light wave into an ultrasonic beam that propagates into water in the holographically predetermined way. With this approach we have successfully generated amplitude distributions that are difficult to realize with traditional piezo-electric techniques.
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.
We use coherent anti-Stokes Raman scattering (CARS) for functional imaging in microscopy. In contrast to other methods, excitation and detection are performed in a wide-field (non-confocal) setup, similar to a combination of dark field and epi-fluorescence microscopy. Thus, imaging of the whole sample is performed at once, i.e. without scanning, which promises the possibility of fast microscopy with vibrational contrast. The use of a nanosecond laser system rather
than typically used pico- or femtosecond systems facilitates a high spectroscopic resolution for various organic substances.
A combination of laser-induced ultrasound generation and ultrasonic holography for spatial control of the generated ultrasonic pulse is presented. Ultrasound is produced by absorption of laser pulses at an absorbing layer in a water tank via the optoacoustic effect. In order to produce a defined ultrasonic frequency in the MHz range, the laser pulses are harmonically time-modulated using an acousto-optic modulator (AOM). Additionally, the laser intensity is spatially controlled. This is realized with a high resolution liquid crystal spatial light modulator (LCD). A computer generated pattern is displayed at the LCD and projected by the expanded laser beam to an absorptive layer in the water tank. As a result, the emitted ultrasonic wave emerges in a predetermined way, which is an acoustical analogue to the effect of a "diffractive optical element" in laser optics. The flexible method of optical ultrasound generation and diffractive steering promises new applications in medical and technical ultrasound diagnostics.
We present a useful tool for manipulating biological samples in microscopy. Cells and cell organelles can be trapped, moved and stretched with a combination of optical tweezers and an optical trap consisting of two opposing single mode optical fibers. Experiments demonstrate trapping and manipulation of micro-beads, and even mechanical deformation of blood cells with the system.
Atoms interacting with standing light waves are a model system for the propagation of waves in static and time varying periodic media. We present here experiments studying the coherent motion of atomic deBroglie waves in periodic potentials made from on and off resonant light. We observe anomalous transmission of atoms through resonant standing light waves and experimentally confirm that atoms fulfilling the Bragg condition form a standing matter wave pattern. We furthermore demonstrate how Bragg diffraction of atomic matter waves at a time-modulated thick standing light wave can be used to coherently shift the deBroglie frequency of the diffracted atoms. Our frequency shifter for atomic matter waves is similar to an acousto-optic frequency shifter for photons.
A novel technique of recording holograms in spectral hole-burning systems has been developed. During hologram exposure, the frequency of narrow band laser light is `swept' over a frequency range, corresponding to a few homogeneous linewidths of the spectrally selective recording material. Simultaneously the phase of the hologram is controlled as a function of the frequency--the so called phase sweep function. Due to the phase reconstructing properties of holograms, this recording method programs the sample as a spectral amplitude and phase filter. This leads to interesting diffraction phenomena: plane swept holograms show strongly asymmetric signals in conjugated diffraction orders.
Experiments verifying a new method of storing spectral hole burning holograms, which yields reduced crosstalk, have been conducted. Results demonstrating the reduced width of this type of hologram in both frequency and applied electric field are presented. Using this technique, 6000 holograms have been recorded in a single spectral hole burning sample.
The fidelity of direct image correlation depends on the properties of the recording material. Results of plane wave interference are shown for oriented samples as well as polymer films. Also diffraction properties of holograms stored in frequency selective recording media are considered.
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