Sławomir Drobczyński

Micro-step localization using double charge optical vortex interferometer

We investigate the diffraction effects of focused Gaussian beams yielding a double optical vortex by a nano-step structure fabricated in a transparent media. When approaching such a step the double vortex splits into single ones which move in a characteristic way. By observing this movement we can determine the position of the step with high resolution. Our theoretical predictions were verified experimentally.

New scanning technique for the optical vortex microscope

In the optical vortex microscopy the focused Gaussian beam with optical vortex scans a sample. An optical vortex can be introduced into a laser beam with the use of a special optical element--a vortex lens. When moving the vortex lens, the optical vortex changes its position inside the spot formed by a focused laser beam. This effect can be used as a new precise scanning technique. In this paper, we study the optical vortex behavior at the sample plane. We also estimate if the new scanning technique results in observable effects that could be used for a phase object detection.

Cluster formation in ferrofluids induced by holographic optical tweezers

Holographic optical tweezers were used to show the interaction between a strongly focused laser beam and magnetic nanoparticles in ferrofluid. When the light intensity was high enough, magnetic nanoparticles were removed from the beam center and formed a dark ring. The same behavior was observed when focusing vortex or Bessel beams. The interactions between two or more separated rings of magnetic nanoparticles created by independent optical traps were also observed.

Adjustment method of an imaging Stokes polarimeter based on liquid crystal variable retarders

The description of adjustment of an imaging Stokes polarimeter constructed and tested in our laboratory is presented. Our polarimeters operation is based on six fast intensity distribution measurements realized in six different configurations of linear and circular analyzers. Using liquid crystal variable retarders (LCVRs) makes this construction compact and mechanically simple. However, new problems arise with proper azimuthal alignment as well as with proper LCVR voltage adjustment. Three basic steps of the adjustment procedure adapted to the specific construction of our polarimeter are described in detail. Some remarks concerning the critical parameters of the used CCD cameras parameters are also presented, as well as experimental verifications of the setups accuracy acquired due to the proper adjustment process.

Dynamic polarization states and birefringence distributions measurements in spatial elliptical polariscope using Fourier analysis method

A new method to measure the light polarization state and the birefringent media parameters is proposed. We have used the setup described previously, consisting of two pairs of the linear Wollaston and circular compensators which form a set of two spatially modulated elliptical compensators. We have modified this setup introducing some carrier frequencies in all compensators and assuming that the second linear one would introduce the frequency which is a multiplicity of the basis frequency of the first linear compensator. Both of these modifications allow calculating all polarization parameters of polarized light or birefringent medium from only one measured intensity distribution of the light outcoming the described setup. They allow measuring not only the parameters of homogeneous beams/mediums but also x,y-distributions of all desired parameters, like azimuth and ellipticity angles of the light or first medium eigenvector and the phase difference introduced by this medium. The proposed calculation method comprises of Fourier analysis of obtained intensity distribution with some manipulation of coordinate system and filtration of obtained data. This method is claimed to be simple and fast enough to be treated as a real-time method.

Imaging polarimeter for linear birefringence measurements using a liquid crystal modulator

We present the imaging polarimetry method with phase shifting. The use of a liquid crystal modulator makes this method equally as quick as the one of imaging polarimetry with carrier frequency. This work presents a simple optical arrangement, together with the liquid crystal (LC) calibration procedure and experimental results confirming the correctness of the method and its accuracy.

Application of space periodic variation of light polarization in imaging polarimetry

The application of space periodic variation of light polarization for measurement and calculation of the distribution of the phase retardation between two eigenwaves propagating inside a linearly birefringent media and the distribution of the azimuth angle of the first eigenvector is described. The measuring method proposed does not require any mechanical movements or rotations of any optical elements. Application of a liquid crystal (LC) modulator instead of a quarter-wave plate gives an opportunity to introduce the required phase shift. The space periodic modulation of the polarization of light is achieved by the use of a Wollaston prism placed inside the path of the light beam. Then a fast Fourier transform is used for further calculations. The number of measurements of the light intensity at the output of the system is minimized to two. These assumptions make the proposed method very fast, which is especially important in measurements of the objects with optical anisotropy that is changing in time.

Transmission imaging polarimetry for a linear birefringent medium using a carrier fringe method

We present an imaging polarimeter in transmission mode that is based on a carrier frequency method and allows a spatially resolved polarimetric description of nondichroic linear birefringent media. The apparatus incorporates a generator of polarization states in the incoming pathway and a Wollaston prism and a linear polarizer as the analyzer unit. A series of two fringe pattern images of the birefringent sample under study, corresponding to two independent polarization states of the generator unit, were recorded. From these images and by using Fourier analysis, the 2D distribution of azimuth angle and retardation were calculated. Two alternative generator units were used: (i) a linear polarizer combined with a rotatory quarter-wave plate and (ii) a liquid-crystal variable retarder. A uniform quarter-wave plate at different orientations was measured with both generator units to demonstrate the effectiveness and the accuracy of the method. The mean absolute deviations were 1.8 degrees and 4.1 degrees for the azimuth and the retardation, respectively, with the first generator unit, and 2.9 degrees and 4.4 degrees for the second one. Moreover, some nonuniform birefringent samples presenting wider ranges of azimuth and retardation were also tested.

Moving average process underlying the holographic–optical–tweezers experiments

We study the statistical properties of recordings that contain time-dependent positions of a bead trapped in optical tweezers. Analysis of such a time series indicates that the commonly accepted model, i.e., the autoregressive process of first-order, is not sufficient to fit the data. We show the presence of a first-order moving average part in the dynamical model of the system. We explain the origin of this part as an influence of the high-frequency CCD camera on the measurements. We show that this influence evidently depends on the applied exposure time. The proposed autoregressive moving average model appears to reflect perfectly all statistical features of the high-frequency recording data.

Modeling of influence of liquid crystal modulator adjustment on reconstruction of birefringence and azimuth angle in imaging polarimetry with carrier frequency

The imaging polarimetry with carrier frequency uses periodically space variation of light polarization obtained with a birefringent wedge, inserted into an optical setup. Implementation of required phase shifting in measuring optical system is performed by use of Liquid Crystal Modulator (LCM). Application of LCM instead of rotational phase plates significantly accelerates measurements and enables recording fast, dynamical variations of both distributions of birefringence and azimuth angle of anisotropic objects. Accuracy of calculation of birefringence and azimuth angle distributions of measured object depends on orientation of LCM in relation to the other optical elements of the system and on accuracy of phase shift given by the LCM. This paper presents the model calculations, which enable to evaluate the influence of the adjustment parameters of LCM on reconstruction of the maps of birefringence and azimuth angle of the measured object.