Josep Nicolas

Developments in optics and performance at BL13-XALOC, the macromolecular crystallography beamline at the Alba Synchrotron

The design, performance and first results of the new macromolecular crystallography beamline BL13-XALOC at the ALBA Synchrotron are described, including new developments in optics and control.

Phase and amplitude modulation of elliptic polarization states by nonabsorbing anisotropic elements: application to liquid-crystal devices

We study the modulation of programmable birefringent devices when they are illuminated by elliptically polarized light. We apply the theory to liquid-crystal display panels (LCDs). We consider the setups at the input and the output sides of the LCD as polarization-state generators (PSGs) or detectors (PSDs). We demonstrate that once the programmable birefringent device is described by a physical model, the amplitude and phase modulation depend only on the polarization state at the input of the device and on the output state detected behind it. This permits optimization of the modulation response only in terms of the input and the output states and the physical model of the device. The procedure to find the PSG and PSD configurations is detailed by using a geometrical interpretation of the states and the plates on the Poincaré sphere.

A soft X-ray beamline for transmission X-ray microscopy at ALBA.

The MISTRAL beamline is one of the seven phase-I beamlines at the ALBA synchrotron light source (Barcelona, Spain) that will be opened to users at the end of 2010. MISTRAL will be devoted to cryotomography in the water window and multi-keV spectral regions for biological applications. The optics design consists of a plane-grating monochromator that has been implemented using variable-line-spacing gratings to fulfil the requirements of X-ray microscopy using a reflective condenser. For instance, a fixed-focus condition independent of the included angle, constant magnification as well as coma and spherical aberration corrections are achieved with this system. The reported design is of wider use.

MISTRAL: a transmission soft X-ray microscopy beamline for cryo nano-tomography of biological samples and magnetic domains imaging.

The performance of MISTRAL is reported, the soft X-ray transmission microscopy beamline at the ALBA light source (Barcelona, Spain) which is primarily dedicated to cryo soft X-ray tomography (cryo-SXT) for three-dimensional visualization of whole unstained cells at spatial resolutions down to 30 nm (half pitch). Short acquisition times allowing for high-throughput and correlative microscopy studies have promoted cryo-SXT as an emerging cellular imaging tool for structural cell biologists bridging the gap between optical and electron microscopy. In addition, the beamline offers the possibility of imaging magnetic domains in thin magnetic films that are illustrated here with an example.

The ALBA spectroscopic LEEM-PEEM experimental station: layout and performance

The performance of the spectroscopic LEEM-PEEM experimental station at the ALBA Synchrotron is presented: XPEEM lateral resolution down to better than 20 nm, electron energy resolution below 0.15 eV, and excellent long-term stability.

Optical encoding of color three-dimensional correlation

Three-dimensional (3D) correlation of color images, considering the color distribution as the third dimension, has been shown to be useful for color pattern recognition tasks. Nevertheless, 3D correlation cannot be directly performed on an optical correlator, that can only process two-dimensional (2D) signals. We propose a method to encode 3D functions onto 2D ones in such a way that the Fourier transform and correlation of these signals, that can be optically performed, encode the 3D Fourier transform and correlation of the 3D signals. The theory for the encoding is given and experimental results obtained in an optical correlator are shown.

Colour pattern recognition by three-dimensional correlation

We introduce the colour as a third dimension of images and extend the definition of Fourier transform to these three variable light distributions. Firstly we study the properties of the three-dimensional (3D) Fourier transform of these images, paying special attention to the properties of the colour variable transformation. Then, 3D correlation is introduced for these images and it is applied to pattern recognition of colour images. Some properties of correlation are extracted from the special characteristics of the third variable (the colour), like the special interpretation of the shift invariance along the colour dimension.

Design and performance of BOREAS, the beamline for resonant X-ray absorption and scattering experiments at the ALBA synchrotron light source

The optical design of the BOREAS beamline operating at the ALBA synchrotron radiation facility is described. BOREAS is dedicated to resonant X-ray absorption and scattering experiments using soft X-rays, in an unusually extended photon energy range from 80 to above 4000 eV, and with full polarization control. Its optical scheme includes a fixed-included-angle, variable-line-spacing grating monochromator and a pair of refocusing mirrors, equipped with benders, in a Kirkpatrick-Baez arrangement. It is equipped with two end-stations, one for X-ray magnetic circular dichroism and the other for resonant magnetic scattering. The commissioning results show that the expected beamline performance is achieved both in terms of energy resolution and of photon flux at the sample position.

Phasor analysis of eigenvectors generated in liquid-crystal displays

We analyze optical systems for creating phase-only operation of a liquid-crystal display (LCD). We introduce a new formalism in which any input or output polarization state can be described in terms of a linear combination of the eigenvectors of the LCD. We use a phasor analysis to examine the linear combination of the eigenvectors and show how improved performance can be obtained with a new configuration. We experimentally compare the intensity and phase operation for two configurations with the same spatial light modulator and show the improved performance with the new configuration.

Convergent optical correlator alignment based on frequency filtering

The convergent correlator is widely used but it presents the drawback of the alignment requirements: fine focusing of the input scene Fourier transform on the filter plane, filter centering, scaling the scene Fourier transform to match the filter size, and azimuth matching of the filter with the input scene. We propose a set of tests to obtain a precise alignment of the convergent correlator. These methods are based on frequency filtering properties and they are applicable either for amplitude input or for phase-encoded input. The tests we present allow us to fulfill all the alignment requirements. The theory on which these tests are based is explained. The experimental results obtained during the alignment procedure are presented. We show some additional verifications of the correct alignment of the convergent correlator.