Showing papers by "Ignacio Moreno published in 2020"

Mueller Matrix Polarimetric Imaging Analysis of Optical Components for the Generation of Cylindrical Vector Beams

Abstract: In this work, we performed a Mueller matrix imaging analysis of two commercial optical components usually employed to generate and manipulate vector beams—a radial polarizer and a liquid-crystal q-plate. These two elements generate vector beams by different polarization mechanisms—polarizance and retardance, respectively. The quality of the vector beams relies on the quality of the device that generates them. Therefore, it is of interest to apply the well-established polarimetric imaging techniques to evaluate these optical components by identifying their spatial homogeneity in diattenuation, polarizance, depolarization, and retardance, as well as the spatial variation of the angles of polarizance and retardance vectors. For this purpose, we applied a customized imaging Mueller matrix polarimeter based on liquid-crystal retarders and a polarization camera. Experimental results were compared to the numerical simulations, considering the theoretical Mueller matrix. This kind of polarimetric characterization could be very helpful to the manufacturers and users of these devices.

Analysis of Hybrid Vector Beams Generated with a Detuned Q-Plate

Abstract: We use a tunable commercial liquid-crystal device tuned to a quarter-wave retardance to study the generation and dynamics of different types of hybrid vector beams. The standard situation where the q-plate is illuminated by a Gaussian beam is compared with other cases where the input beam is a vortex or a pure vector beam. As a result, standard hybrid vector beams but also petal-like hybrid vector beams are generated. These beams are analyzed in the near field and compared with the far field distribution, where their hybrid nature is observed as a transformation of the intensity and polarization patterns. Analytical calculations and numerical results confirm the experiments. We include an approach that provides an intuitive physical explanation of the polarization patterns in terms of mode superpositions and their transformation upon propagation based on their different Gouy phase. The tunable q-plate device presents worthy advantages, since it allows a compact and efficient generation of pure and hybrid vector beams to study these effects.

Liquid-crystal polarization state generator

Abstract: An important problem in imaging polarimetry occurs when the optical axis of the system and the center of the camera sensor get misaligned. This situation typically occurs after rotating the polarizing element mounts in order to change the input state of polarization. This work presents a liquid-crystal polarization state generator, devoid of moving parts, which can generate any arbitrary state of polarization (SOP) on the Poincare sphere through the phase-shift manipulation of two voltage-controlled variable retarders. The proposed optical system consists of a linear polarizer, cascaded by two liquidcrystal retarders (LCR1 and LCR2) and a quarter-wave plate. We show that by varying the retardance of LCR1 but keeping the LCR2 retardance constant, the SOP moves along the corresponding meridian of the Poincare sphere. When the reverse is done, the SOP follows a trajectory along the given parallel. Experimental results are compared to numerical simulations where we calculate the Stokes parameters and represent the trajectories of the SOP on the Poincare sphere as we change the voltage addressed to the LCRs. Good agreement between theory and experiment is obtained if we take into account the Fabry-Perot interference effects on these variable retarders. This system can also be used as a polarization state analyzer. To verify the performance of this system as analyzer, the Mueller Matrix of a retarder plate is determined by imaging polarimetry.

Dynamic control of Bessel beams through high-phase diffractive axicons

Abstract: In this work, we present the realization of a high-phase diffractive axicon. For that purpose, we use a spatial light modulator that exhibits 10π phase modulation. We compare the results with standard diffractive axicons that exhibit 2π phase modulation. We show that high-phase modulation axicons generate Bessel beams with a shorter range and a smaller radius than standard axicons with the same period. We also find that the higher phase modulation regime provides improved diffraction efficiency since fringing effects are reduced. Therefore, dynamic control of Bessel beams is presented, controlled through the phase modulation dynamic range.

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

Abstract: 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.

Double-ring interference of binary diffractive axicons

Abstract: We report on the interference between the double rings generated by the Fourier transform of a binary diffractive axicon. These two rings have the same size and correspond to the ± 1 diffracted order beams. The interference condition between both rings can be easily changed by adding a constant phase bias, resulting in a central ring that is either dark or bright. Additionally, this interference condition can be changed along the ring and can be easily tuned, thus allowing greater flexibility. We present experimental results obtained with a binary π-phase liquid-crystal spatial light modulator. These patterns might find applications in optical trapping systems, where the bright or dark regions could trap particles whose refractive index is either higher or lower than the medium.

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

Abstract: 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.

In-phase sub-Nyquist lenslet arrays encoded onto spatial light modulators.

Abstract: When encoding diffractive lenses onto a spatial light modulator (SLM), there is a Nyquist limit to the smallest focal length that can be formed. When this limit is surpassed, a two-dimensional array of lenslets is formed. There have been very few discussions on the performance of these lenslets. In this work, we focus on the phase distribution of these lenses in the array. We show that, for certain values of the focal length, the lenslets are all in perfect phase. We show that this situation happens for a total number of N/4 different discrete equidistant sub-Nyquist focal lengths, where N×N is the number of pixels in the SLM. We find other distances in between where the array is composed of two sets of lenslets with a relative π phase among them. Finally, we illustrate these phase distributions in the application to generate an array of vortex producing lenses. We expect that these results might be useful for high-accuracy interferometric or multiple imaging where this phase must be exactly the same for each replica.

Heterogeneity in the Linear Shiny White Structures in Melanomas Seen With Polarized Light According to Histopathological Association: Cross-sectional Observational Study in 118 Cutaneous Melanomas

Abstract: Polarized dermoscopy enables visualization of linear shiny white structures in melanomas, thought to be due to the existence of fibrosis in the dermis. Our objective was to establish the existence of two types of linear shiny white structures and assess their association with different histological structures. We performed a cross-sectional study including all non-acral, non-facial melanomas from our hospital with linear shiny white structures. The outcome variable was the type of linear shiny white structures: shiny white streaks and white strands. We evaluated their association with explanatory variables that may affect the reflectance of melanomas and Breslow index. We used χ2 statistics and also calculated the sensitivity and specificity of each linear shiny white structure to predict those variables. We detected linear shiny white structures in 118 melanomas. Regarding shiny white streaks, we only found a statistically significant positive relationship with fibrosis in the papillary dermis. Regarding white strands, we found statistically significant and positive relationships with hyperkeratosis, Breslow index of 0.8 mm or more and acanthosis. Sensitivity and specificity study revealed that the presence of shiny white streaks was the most sensitive (81.7%) and specific (72.3%) for fibrosis in the papillary dermis, and presence of white strands was the most sensitive (91.1%) and specific (85.7%) for hyperkeratosis.

Geometric-phase grating as an optical vortex generator and detector

Abstract: Optical vortices are promising for increasing the bandwidth of optical communications by encoding information in the topological charge. For this purpose, it is essential to develop efficient and compact systems for generating and detecting vortex beams of different topological charges. We present the design and the theoretical and experimental analysis of a 2D liquid-crystal geometric-phase diffraction grating based on the optimal triplicator profile. The grating generates an array of 3x3 equi-energetic optical vortices of different integer topological charges and with the maximum theoretical diffraction efficiency. The performance of this grating as a vortex detector is demonstrated by illuminating it with vortex beams of different topological charges. In a recent work we mathematically proved a π/2 phase shift between the zero and the two lateral first diffraction orders generated by the optimal triplicator design. Here, we show the implications that this factor has in the polarization of the diffraction orders when the optimal triplicator is encoded as a geometric-phase element.

Spatial light modulators with large phase-modulation: application to encode lenses with very short focal lengths

Abstract: In standard phase spatial light modulators (SLM), showing a phase modulation of 2π radians, there is a limit to the shortest focal length of an encoded lens. This Nyquist focal length limit is caused by the aliasing originated by the SLM pixelated structure. In this work we show that SLMs with very large phase modulation (in our case about five cycles of 2π radians) can be used to surpass this effect and efficiently encode lenses with focal lengths much lower than the Nyquist limit. We include experimental results showing that this limit is reduced by a factor of about 1/10.

Vector modes generation using efficient on-axis system

Abstract: Arbitrary polarized vector beam modes are efficiently generated. We use two flicker-free liquid-crystal on silicon (LCOS) spatial light modulators (SLM) in an on-axis common path architecture. Different superpositions of vector modes are obtained with high light efficiency.

Efficient generation of vector beams

Abstract: A variety of methods have been developed in the last years to generate vector beams. In this work we will present some techniques that we have developed for the efficient and compact generation of arbitrary polarized vector beams. They are based on the use of geometric-phase elements combined with liquid-crystal on silicon (LCOS) spatial-light modulators (SLM) in a common-path architecture. LCOS-SLMs were used to encode a phase-only diffractive mask that encodes complex functions with high diffraction efficiency. We demonstrate the generation of arbitrary scalar modes and vector beam modes.

Measuring the non-uniformity of a liquid-crystal on silicon spatial light modulator with light out of the operation range

Abstract: A technique to derive the spatial deformation of a liquid-crystal on silicon spatial light modulator based on a self-interference effect, generated when it is illuminated with a wavelength far from the operation range, is presented.