Liquid Crystal Retarder Research Articles

Tunable Photoinduced Liquid Crystal Retarders for All-Optical Diffractive Deep Neural Networks

Tunable Photoinduced Liquid Crystal Retarders for All-Optical Diffractive Deep Neural Networks

Monte carlo simulations for tuning optical properties in a two-stage liquid crystal retarder system via Mueller matrix method

Monte carlo simulations for tuning optical properties in a two-stage liquid crystal retarder system via Mueller matrix method

Quantitative phase imaging by gradient retardance optical microscopy

Quantitative phase imaging (QPI) has become a vital tool in bioimaging, offering precise measurements of wavefront distortion and, thus, of key cellular metabolism metrics, such as dry mass and density. However, only a few QPI applications have been demonstrated in optically thick specimens, where scattering increases background and reduces contrast. Building upon the concept of structured illumination interferometry, we introduce Gradient Retardance Optical Microscopy (GROM) for QPI of both thin and thick samples. GROM transforms any standard Differential Interference Contrast (DIC) microscope into a QPI platform by incorporating a liquid crystal retarder into the illumination path, enabling independent phase-shifting of the DIC microscope's sheared beams. GROM greatly simplifies related configurations, reduces costs, and eradicates energy losses in parallel imaging modalities, such as fluorescence. We successfully tested GROM on a diverse range of specimens, from microbes and red blood cells to optically thick (~ 300 μm) plant roots without fixation or clearing.

Design and fabrication of a liquid crystal retarder for the mid- and far-infrared wavelength ranges

We designed and fabricated a liquid crystal (LC) retarder that operates within mid-infrared (MIR) and far-infrared (FIR) wavelength regions. Fluorine-based mixed LCs and BaF2 substrates were employed to assemble the LC retarder on the basis of their absorption spectra. A thin NiCr film, which served as a semitransparent electrode in the infrared region, was also formed on the BaF2 substrate. The fabricated LC retarder showed transmittances of 40% and 42% at wavelengths of 3.88µm and 9.5µm, respectively. Here, we demonstrated phase shift that can be electrically controlled to exceed π/2. Our LC retarder can be applied to the polarization modulation in the MIR and FIR wavelength regions.

Mueller matrix imaging polarimeter with polarization camera self-calibration applied to structured light components

This work presents a complete Mueller matrix imaging polarimeter that uses three liquid-crystal retarders and a pixelated polarization camera. The polarimeter is characterized and optimized with a standard correction procedure here adapted to be performed fully in-situ, without any additional element, based on considering the polarization camera as the reference. The accuracy limit caused by the extinction ratio in the camera micro-polarizers is analyzed. Finally, the imaging polarimeter is tested experimentally by analyzing well-known samples for structured light applications such as patterned retarders, a patterned polarizer, and a liquid-crystal depolarizer. The work is presented in a tutorial style useful to reproduce the procedure by non-experts in polarimetry.

Numerical quantization of interfering photons using liquid crystal retarder

Numerical quantization of interfering small photons using a liquid crystal (LC) retarder is investigated. A beam splitter can be considered a four-port device with the input-output relations in the Heisenberg picture using the creation and annihilation operator. We introduce the phase shift in one of the annihilation operators using the LC device, where, the amplitude of these operations is ultimately reduced. The alignment of an LC director is homogeneous and the director profile is calculated by solving the Euler–Lagrange equation. A total number of retardations can be calculated using the effective refractive index of the LC molecule integrating through cell thickness direction. As a result, a detail of the quantization operation is clarified.

Optimization of the Polarized Imaging Nephelometer (PI-Neph) for continuous monitoring of multiwavelength aerosol phase functions in support of space polarimetry missions

This work deals with the set up and optimization of a commercial multiwavelength polarized imaging nephelometer (PI-Neph, Airphoton PIN100) designed to measure F11 (phase function) and –F12/F11 (polarized phase function) from ambient aerosol samples. Such measurements are critical for the success of upcoming satellite missions based on polarimetry measurements because they will support and validate retrieval techniques. The main novelty of the instrument with respect to previous versions consists of using only one beam instead of a mirror system to fold it, avoiding internal reflections of light and loss of energy within the laser beam. The instrument also includes the automatization of the imaging technique that makes possible continuous measurements in near real time of the aerosol phase functions at different states of linear polarization directly from ambient air. Laser emits at three different wavelengths (405, 515 and 660 nm) and the beam goes through a wire grid polarizer for linearly polarizing the light, followed by two liquid crystal retarders that control the state of linear polarization. The detector is a CMOS camera perpendicular to the beam which allows to obtain final phase functions in the range 5°–175° (losses of some angles by physical limitations and stray light). The calibration of the instrument is discussed and includes geometric calibration (to account for different light paths to the camera) plus absolute calibration (to obtain physical units). Temporal stability of calibrations is observed, and different data quality tests are presented in order to obtain reliable values of F11 and –F12/F11. For an automatic operation of the instrument that permits real time phase matrix measurements, a data quality check algorithm that avoids non-physical measurements and filters outliers has been developed. Validation of the scattering matrix elements based on monodisperse aerosol measurements is also done with polystyrene latex spheres (PSL), showing good agreement with Mie theory. Finally, first results of the instrument operating at the University of Granada (UGR) station from the Andalusian Global ObseRvatory of the Atmosphere (AGORA) since April 2022 are presented. Very good agreements of the integrated scattering coefficients with an integrating nephelometer (TSI, model 3563) were obtained with high correlation coefficients for the three wavelengths (R2 > 0.81). The calculated root-mean-square error (RMSE) reveals that the performance of the PI-Neph in the measurement of the scattering coefficient is better in the 660 nm and 515 nm wavelengths than in the 405 nm one, although differences between datasets are still low. The observed differences in phase functions and especially polarized phase functions (both in shape and spectral differences) under changing atmospheric scenarios (Saharan dust, pollution and background aerosol conditions) indicated that these variables are sensitive and reliable to classify the type of ambient aerosol. Particularly, the combined measurements of F11 and –F12/F11 shows a unique potential of the PI-Neph for aerosol typology.

Portable PIMI polarization imaging device based on automatic polarization recognition.

This paper proposes a new portable polarization parametric indirect microscopy imaging without a liquid crystal (LC) retarder. The polarization was modulated by a polarizer automatically rotating when the camera took raw images sequentially. A specific mark tagged the polarization states of each camera's snapshot in the optical illumination path. A computer vision portable polarization parametric indirect microscopy imagrecognition algorithm was developed to retrieve the unknown polarization states from each raw camera image to ensure that the right polarization modulation states were used in the PIMI processing algorithm. The system's performance was verified by obtaining PIMI parametric images of human facial skin. The proposed method avoids the error problem caused by the LC modulator and significantly reduces the whole system's cost.

Comparative study of liquid crystal variable retarder with rubbed polyimide and SiO2 thin films by glancing angle deposition

A comparative study on the optical, morphological and laser-induced damage characteristics of rubbed polyimide, SiO2 thin films by glancing angle deposition, and the corresponding liquid crystal (LC) retarders, is presented in this paper. SiO2 inorganic thin films were prepared with a deposition angle of 45[Formula: see text], and traditional polyimide (PI) films were prepared by spin coating and rubbing. The experimental results indicate that the SiO2 thin films have a good transmittance and resistance to laser damage. The transmittance of the SiO2 thin films was above 92% in the wavelength range 400–1200 nm, and that of the corresponding LC retarder was approximately 90% in the wavelength range 800–1200 nm. The laser-induced damage threshold of the SiO2 thin films reached 32.6 J/cm2, and that of the corresponding LC retarder was 1.84 J/cm2. These results are better than those of the PI alignment films. SiO2 thin films have potential for use in the LC optical devices for laser applications.

Fast tunable metamaterial liquid crystal achromatic waveplate

Abstract Photonic metamaterials combined with liquid crystals (LCs) for tunability is a great niche for building miniature devices with high performance such as fast flat tunable lenses, tunable filters, and waveplates. Sub-wavelength or nano-grating surfaces are homogenized to uniaxial waveplates with negative birefringence of unique dispersion when the period is less than the wavelength by at least a few times. This uniaxial metasurface, combined with the LC layer, is shown to act as a tunable retardation achromatic waveplate with 8 μm thick LC layer operating over wide spectral and angular ranges, as compared to using two nematic liquid crystal (NLC) retarders of thicknesses on the order of 30–60 μm, when no metasurface is used. Hence the device becomes miniature and 50× faster due to the thinner liquid crystal layer. The silicon nano-grating of 351 nm pitch and 0.282 fill factor is designed and fabricated to operate in the short-wave infrared range (SWIR). Switching between three achromatic retardation levels: full-, half-, and quarter-waveplates is accomplished by changing the applied voltages on the NLC cell with a switching time of a few milliseconds. This device has applications in fast broadband shutters, low coherence phase shift interferometry, ellipso-polarimetry, dynamic control of light intensity, and smart windows.

Detector bandwidth and polarization switching rates: spectrophotometric observations of the Sun by the Birmingham Solar Oscillations Network

Abstract The Birmingham Solar Oscillations Network (BiSON) observes acoustic oscillations of the Sun. The dominant noise source is caused by fluctuations of Earth’s atmosphere, and BiSON seeks to mitigate this effect by combining multiple rapid observations in alternating polarization states. Current instrumentation uses bespoke Pockels-effect cells to select the polarization state. Here, we investigate an alternative off-the-shelf solution, a liquid crystal (LC) retarder, and discuss the potential impact of differences in performance. We show through electrical simulation of the photodiode-based detectors, and assessment of both types of polarization device, that although the switching rate is slower the off-the-shelf LC retarder is a viable replacement for a bespoke Pockels-effect cell. The simplifications arising from the use of off-the-shelf components allow easier and quicker instrumentation deployment.

Complete Mueller matrix imaging polarimeter for evaluating optical components for structured light

We present a complete Mueller matrix (MM) imaging polarimeter based on liquid-crystal retarders and a pixelated polarization camera. The polarimeter instrument is first calibrated and optimized, and then applied for the precise characterization of optical components used for the generation of structured light, like patterned retarders and patterned polarizers.

Effects of typical liquid-crystal retarder errors on optimized Stokes polarimeters.

Liquid crystal variable retarders (LCVRs) are often used in Stokes polarimeters as they allow the measurement of different polarization components by applying an electric field that manipulates the induced retardance. However, the optical retardance introduced by these devices is in general not homogenous across the aperture. Another problem with this type of devices is that the fast-axis orientation is not homogenous, and it changes with the applied voltage. For the optimization of polarimeters, in terms of the noise amplification from the intensity measurements to the polarimetric data, the condition number (CN) is often used, but the effects of LCVR spatial variations are not considered. This paper analyzes the impact of errors in LCVRs in a set of optimized Stokes polarimeters simulated by adding errors in the induced retardance and fast-axis orientation. Then, the CN is calculated to observe the effect of these errors on the optimization. We show how errors in the LCVRs lead to different impacts in the polarimetric measurements for different optimized polarimeters, depending on their experimental parameters. Furthermore, we present the propagation error theory to choose the best experimental parameters to reduce the nonideal effects in optimized polarimeters.

Polarization-probe polarization-imaging system in near-infrared regime using a polarization grating

A polarization-probe polarization-imaging (PPPI) system was developed for the near-infrared (NIR) regime. This system comprises two components operating as a polarization generator and a polarization analyzer to enable polarization image capture under polarized light illumination. The captured polarization images contain considerable object information because the illuminating polarized light beams are affected by many of the Mueller matrix elements. By assembling the polarization camera using two liquid crystal retarders and a polarization grating, the PPPI system offers the potential to measure the Stokes parameters fully with a high extinction ratio, even in the NIR region. The PPPI system’s feasibility was demonstrated experimentally. Its dependence on the state of polarization (SoP) of the illuminating polarized light was discussed. The polarization image acquired by the PPPI system is strongly dependent on the illuminating light’s SoP, so the appropriate SoP must be selected for each object to enhance the polarization image contrast. This PPPI system should expand the range of polarization imaging applications, including LiDAR, product inspection, and bio-imaging.

Phase distributions accompanying the conical diffraction through a Pasteur acentric biaxial crystal, displayed with Bi2ZnOB2O6

The conical diffraction of an optical wave through a biaxial crystal leads to an electric vector field with a specific polarization and phases distributions assessable from a unitary operator. The optical activity, if the crystal is a Pasteur acentric one, modifies deeply the phenomenon and two phases are required instead of one for pure birefringence. These modifications in a Bi2ZnOB2O6 crystal were characterized using a polarimetric method and an interferential conoscope equipped with a liquid crystal retarder allowing the measurement of the Pancharatman phase difference between the electric field under study and a vertically or horizontally polarized reference beam. The orbital angular momentum is linked with this phase. All the experimental data and their theoretical description are in reasonably good agreement without adjusting any optical parameters.

Michelson interferometer for phase shifting interferometry with a liquid crystal retarder

We propose an interferometric system to measure phase objects using electronic phase shifting techniques. The experimental setup is based on a polarization Michelson type interferometer, where a liquid crystal retarder is placed at the entrance pupil as a phase shifter. First, the interferometric setup is used for the calibration of a phase-shifting device. Then, the interferometer is used to measure a phase object employing generalized phase-shifting algorithms to retrieve the phase solving the nonlinear response of the liquid crystal retarder. Experimental results are presented, and the advantages and limitations of the proposed technique are also discussed.

Liquid crystal polymer phase retarder aligned with the inorganic layer by glancing angle deposition

We investigated the design and fabrication of liquid crystal polymer phase retarder. The liquid crystal polymer was aligned with the inorganic dielectric layer by glancing angle deposition. The retardation was measured by the Soleil–Babinet compensator. The average retardation of liquid crystal polymer retarder was 71 nm with the layer thickness of 385 nm. And the retardation tolerance was 1.4 nm with the nine-point uniformity test. The average transmittance of the phase retarder was 93.2% between 400 and 1200 nm wavelength without antireflective coating on its front and rear surfaces. This study paves a new way to fabricate the larger aperture phase retarders.

Helical Nanostructures of Ferroelectric Liquid Crystals as Fast Phase Retarders for Spectral Information Extraction Devices: A Comparison with the Nematic Liquid Crystal Phase Retarders.

Extraction of spectral information using liquid crystal (LC) retarders has recently become a topic of great interest because of its importance for creating hyper- and multispectral images in a compact and inexpensive way. However, this method of hyperspectral imaging requires thick LC-layer retarders (50 µm–100 µm and above) to obtain spectral modulation signals for reliable signal reconstruction. This makes the device extremely slow in the case of nematic LCs (NLCs), since the response time of NLCs increases proportionally to the square of the LC-layer thickness, which excludes fast dynamic processes monitoring. In this paper, we explore two approaches for solving the speed problem: the first is based on the use of faster nanospiral ferroelectric liquid crystals as an alternative to NLCs, and the second is based on using a passive multiband filter and focuses on multispectral extraction rather than hyperspectral. A detailed comparative study of nematic and ferroelectric devices is presented. The study is carried out using a 9-spectral bands passive spectral filter, covering the visible and near-infrared ranges. We propose the concept of multispectral rather than hyperspectral extraction, where a small number of wavelengths are sufficient for specific applications.

Measure by interferential conoscopy of the phase distribution acquired from conical diffraction

We have measured the Pancharatman phase difference between the electrical field emerging from the conical diffraction (CD) phenomenon in the KGd(WO2)4 (KGW) biaxial crystal and a vertical polarized reference, each field traveling in one arm of an interferentiel conoscope. This phase is obtained as the retardation introduced by a liquid crystal retarder in the reference beam which maximizes the interference intensity for each camera pixel. The full characterization of the Uˆkx,ky operator describing the CD was obtained including polarimetric measurements. We have found that the azimuthal variation of the eigen-values of the dielectric tensor was responsible for an azimuthal variation of the phase of the Uˆ operator. The link with the orbital angular momentum of the beam is provided in the cases of a uniformly polarized field and in a vector field.

Vector-apodizing phase plate coronagraph: design, current performance, and future development [Invited

Over the last decade, the vector-apodizing phase plate (vAPP) coronagraph has been developed from concept to on-sky application in many high-contrast imaging systems on 8 m class telescopes. The vAPP is a geometric-phase patterned coronagraph that is inherently broadband, and its manufacturing is enabled only by direct-write technology for liquid-crystal patterns. The vAPP generates two coronagraphic point spread functions (PSFs) that cancel starlight on opposite sides of the PSF and have opposite circular polarization states. The efficiency, that is, the amount of light in these PSFs, depends on the retardance offset from a half-wave of the liquid-crystal retarder. Using different liquid-crystal recipes to tune the retardance, different vAPPs operate with high efficiencies (${\gt}96\%$) in the visible and thermal infrared (0.55 µm to 5 µm). Since 2015, seven vAPPs have been installed in a total of six different instruments, including Magellan/MagAO, Magellan/MagAO-X, Subaru/SCExAO, and LBT/LMIRcam. Using two integral field spectrographs installed on the latter two instruments, these vAPPs can provide low-resolution spectra (${\rm{R}} \sim 30$) between 1 µm and 5 µm. We review the design process, development, commissioning, on-sky performance, and first scientific results of all commissioned vAPPs. We report on the lessons learned and conclude with perspectives for future developments and applications.