Vòrtex òptics de polarització generats mitjançant una micropantalla de cristall líquid

La tecnologia actual de cristall líquid permet la fabricació de micropantallas (microdisplays) amb una grandària típica de 1 o 2 centimetres quadrats i amb resolucions en l'ordre de 1000x1000 píxels. Aquestes pantalles poden també emprar-se en el camp de l'òptica difractiva per a la generació de nous elements òptics programables. En concret, investigadors del Departament de Física de la UAB han col·laborat amb investigadors de les universitats Miguel Hernández d'Elx, de la de Buenos Aires (Argentina) i de La Frontera (Xile), en l'estudi d'un element difractiu de polarització simple que genera, en el plànol de difracció, una estructura periòdica de vòrtex de polarització, i que s'ha presentat en el nostre article.La tecnología actual de cristal líquido permite la fabricación de micropantallas (microdisplays) con un tamaño típico de 1 o 2 centímetros cuadrados y con resoluciones en el orden de 1000x1000 píxeles. Estas pantallas pueden también emplearse en el campo de la óptica difractiva para la generación de novedosos elementos ópticos programables. En concreto, investigadores del Departamento de Física de la UAB han colaborado con investigadores de las universidades Miguel Hernández de Elche, de la de Buenos Aires (Argentina) y de La Frontera (Chile) en el estudio de un elemento difractivo de polarización simple que genera, en el plano de difracción, una estructura periódica de vórtices de polarización, y que se ha presentado en nuestro artículo.Current liquid crystal technology allows the production of microdisplays (LCD, Liquid Crystal Displays), with a typical size of 1 or 2 square centimetres, and with resolutions in the order of 1000x1000 pixels. These displays can be used in the field of diffractive optics for the generation of novel programmable optical diffractive elements. Researchers from the UAB Department of Physics have collaborated with researchers from the University Miguel Hernández of Elche (Spain), University of Buenos Aires (Argentina) and University de La Frontera (Chile), in the study of a simple polarization diffractive element that , in the diffraction field, generates a periodic structure of polarization vortices

Measuring the spatial deformation of a liquid-crystal on silicon display with a self-interference effect

We present a simple technique to characterize the spatial non-uniformity of a liquid-crystal on silicon (LCOS) spatial light modulator (SLM). It is based on illuminating the display with a wavelength out of the operation range, so there is a significant reflection at the output surface. As a consequence, a Gires–Tournois interferometer is directly created, without any alignment requirement and insensitive to vibrations. The beam reflected at the output surface is the reference beam, while the beam reflected at the silicon backplane is modulated with the addressed gray level in order to quantitatively derive its deformation.We provide an experimental demonstration using a LCOS-SLM designed to operate in the near-infrared range but illuminated with visible light

Achromatic linear retarder with tunable retardance

We present a universal design and proof-of-concept of a tunable linear retarder of uniform wavelength response in a broad spectral range. It consists of two half-wave retarders (HWR) between two quarter-wave retarders (QWRs), where the uniform retardance can be tuned continuously by simply rotating one of the HWRs. A proof-of-concept of this design is built by using commercially available Fresnel rhomb retarders that provide retardation with almost wavelength uniformity in the visible and near infrared from 450 to 1550 nm. The design is universal, since other achromatic QWRs and HWRs could also be employed. The system is experimentally demonstrated to control the state of polarization of a supercontinuum laser

Spectral performance of a zero-order liquid-crystal polymer commercial q-plate for the generation of vector beams at different wavelengths

Liquid-crystal polymer q-plates are commercial devices for generating vector beams at the design wavelength where the device exhibits half-wave (HW) retardance. Since they are not voltage addressable, the operational wavelength remains fixed. In this work we perform a broadband spectral characterization of the q-plate retardance as a function of wavelength, /(k), and identify the wavelengths with retardance values relevant for vector beam generation (p, p/2, and 3p/2). The wavelength is then used as a tuning parameter to change the device performance from a HW q-plate to a positive-QW or a negative-QW q-plate. These performances are analyzed using the Jones matrix formalism. We present a simple procedure to derive the polarization distribution of the vector beams expected at these QW wavelengths, as a superposition of the input polarization state and the output state of a HW q-plate. Experimental results using the red and blue lines of an Ar-Kr laser and an IR laser diode of 980 nm confirm the theoretical predictions. We show that for input linearly polarized light of 980 nm and 488 nm the device generates hybrid vector beams (where the ellipticity varies with the azimuthal angle), while for 647 nm pure radial vector beams with constant ellipticity are obtained. These results could extend the use of commercial q-plates for multicolour vector beam applications

The International Day of Light in Spain: a three years perspective

In this article we report on the activities in Spain related to the celebration of the International Day of Light (IDL). The Spanish Committee for the IDL was constituted in 2017, as a continuation of the previous Committee for the International Year of Light (2015). This Committee involves different academic, scientific and industrial partners in Spain related to light science and light technologies, and develops actions to promote and disseminate the IDL activities. After the success of the 2018 and 2019 editions, the 2020 IDL celebration has been totally affected by the Covid-19 pandemic, and most of the programmed events were cancelled. Nevertheless, some on-line activities were developed. The aim of this work is to review the IDL impact in Spain in these last three years

The first International Day of Light in Spain

In this work, we review the main activities related to the International Day of Light (IDL) in Spain. We describe the actions of the Spanish Committee for IDL that has been created in Spain, reactivating the Spanish Committee for the International Year of Light created in 2015. The Spanish Committee of the IDL has been very active in promoting the realization of events, and providing tools for the IDL related information dissemination, as the specific related webpage for the IDL in Spain. The Spanish Committee also was the Spanish contact with the International Secretariat of the IDL. We describe the flagship IDL event in Spain that was organized in the Faculty of Physics Sciences of Universidad Complutense de Madrid on May 16th 2018. And we report on many other activities that have been developed in different places in Spain. These activities of the IDL in Spain are intended to make the society aware of the importance of the light and the light based technologies for the progress and well-being of the humankind. They have also been oriented to attract young talent to the studies of science and engineering

Gouy phase effects on propagation of pure and hybrid vector beams

The robustness of the polarization spatial distribution of vector beams upon propagation is crucial for a number of applications, including optical communications and materials processing. This study has been commonly centered on Gouy phase effects on focused vector beams. In this work, we present a theoretical and experimental analysis of the Gouy phase’s effects on the propagation of pure and hybrid vector beams. Experimental results at various axial planes, before and past the focus, are obtained by using a simplified liquid-crystal spatial light modulator-based optical system that allows the easy generation of these beams. Furthermore, a new alternative optical set-up that is devoid of moving elements is demonstrated, which simplifies this study. We experimentally verify the differences between pure and hybrid vector beams upon propagation. While the first ones remain stable, hybrid vector beams show Gouy phase effects that demonstrate an optical activity where the local polarization states rotate by an angle that depends on the propagation distance. Experimental results agree with the theory

Efficient on-axis SLM engineering of optical vector modes

This work presents a method for the efficient experimental generation of arbitrary polarized vector beam modes. The optical system employs two liquid-crystal on silicon (LCOS) spatial light modulators (SLM) in a common path architecture, avoiding the use of beam-splitters. Each SLM displays a different phase-only mask, each one encoding a different pattern onto two orthogonal linear polarization components of the input beam. These phase-only masks are designed using a recently proposed random technique to encode complex amplitude values. This encoding technique reconstructs the complex function on-axis, thus avoiding incorporating carrier phases. By addressing such properly designed phase-only holograms we demonstrate arbitrary scalar modes on each polarization component, whose superposition results in a vector beam mode. Different superpositions of Laguerre–Gaussian and Hermite–Gaussian modes are obtained and the generated vector beam modes are analyzed. Moreover, the addition of a phase-bias proves itself useful to perform a phase-shifting technique in order to evaluate the correct phase of the generated vector beam

Implementación de un interferómetro de difracción puntual con un monopíxel de cristal líquido

En este trabajo se presenta la implementación de un interferómetro por difracción puntual construido con un monopíxel de cristal líquido. En primer lugar se ha fabricado un monopíxel de cristal líquido con alineamiento paralelo de unas dimensiones de 3x3 cm2, en el que el electrodo cubre toda la superficie excepto en un orificio central de unas 50 ?m. Este orificio es el que actuará como punto difractor. Aplicando diferentes tensiones se puede cambiar la fase de la onda que llega al píxel en relación al punto central. Se ha construido un interferómetro con este elemento. Se captan 4 interferogramas con lo que se puede obtener la distribución de amplitud y fase de la onda. Se aplica este sistema para obtener un holograma digital y enfocar digitalmente diferentes planos de un objeto tridimensional

Compact LCOS–SLM Based Polarization Pattern Beam Generator

In this paper, a compact optical system for generating arbitrary spatial light polarization patterns is demonstrated. The system uses a single high-resolution liquid crystal (LC) on silicon (LCOS) spatial light modulator. A specialized optical mount is designed and fabricated using a 3D printer, in order to build a compact dual optical architecture, where two different phase patterns are encoded on two adjacent halves of the LCOS screen, with a polarization transformation in between. The final polarization state is controlled via two rotations of the Poincaré sphere. In addition, a relative phase term is added, which is calculated based on spherical trigonometry on the Poincaré sphere. Experimental results are presented that show the effectiveness of the system to produce polarization patterns