Liquid Crystal Retarder Research Articles

Tunable sensitivity phase detection of transmitted-type dual-channel guided-mode resonance sensor based on phase-shift interferometry.

This paper reports on a transmitted-type dual-channel guided-mode resonance (GMR) sensor system that uses phase-shifting interferometry (PSI) to achieve tunable phase detection sensitivity. Five interference images are captured for the PSI phase calculation within ∼15 s by using a liquid crystal retarder and a USB web camera. The GMR sensor structure is formed by a nanoimprinting process, and the dual-channel sensor device structure for molding is fabricated using a 3D printer. By changing the rotation angle of the analyzer in front of the camera in the PSI system, the sensor detection sensitivity can be tuned. The proposed system may achieve high throughput as well as high sensitivity. The experimental results show that an optimal detection sensitivity of 6.82×10(-4) RIU can be achieved.

Evaluation of a linear birefringence measurement method with increased sensitivity.

The evaluation of a measurement method of linear birefringence with increased sensitivity is presented. The examined method is based on a substantial change of the geometrical phase caused by a small change of an examined medium's birefringence. The measuring setup consists of a linear polarizer, Wollaston compensator, and circular analyzer. The measurement is performed by tracking the phase shift of a fringe pattern. Specific orientation of the elements modifies the setup's response-the sensitivity of the setup can be controlled in a limited measuring range. The present paper concentrates on the factors affecting the setup's sensitivity as well as the accuracy of obtained results. The validity of the proposed approach has been demonstrated by measuring a phase retardance introduced by the liquid crystal retarder. A thousandfold increase in sensitivity has been obtained in the presented experiments, which allows the measurement of retardance introduced by the linear birefringent medium with an accuracy of 0.003° within the limited measuring range.

Analysis of a segmented q-plate tunable retarder for the generation of first-order vector beams.

In this work we study a prototype q-plate segmented tunable liquid crystal retarder device. It shows a large modulation range (5π rad for a wavelength of 633 nm and near 2π for 1550 nm) and a large clear aperture of one inch diameter. We analyze the operation of the q-plate in terms of Jones matrices and provide different matrix decompositions useful for its analysis, including the polarization transformations, the effect of the tunable phase shift, and the effect of quantization levels (the device is segmented in 12 angular sectors). We also show a very simple and robust optical system capable of generating all polarization states on the first-order Poincaré sphere. An optical polarization rotator and a linear retarder are used in a geometry that allows the generation of all states in the zero-order Poincaré sphere simply by tuning two retardance parameters. We then use this system with the q-plate device to directly map an input arbitrary state of polarization to a corresponding first-order vectorial beam. This optical system would be more practical for high speed and programmable generation of vector beams than other systems reported so far. Experimental results are presented.

Simple spectral technique to identify the ordinary and extraordinary axes of a liquid crystal retarder

We present a very simple method to distinguish between ordinary and extraordinary axes in an optical retarder. The method is based on inserting the retarder in between two crossed linear polarizers, oriented at 45° to the neutral axes. By tilting the retarder to obtain oblique-incidence illumination, a different behavior is observed depending on the orientation of the ordinary/extraordinary axes relative to the tilt direction. Simply using white light illumination from a tungsten lamp and spectral analysis by means of a portable spectrometer, it is possible to differentiate between ordinary and extraordinary axes. Theoretical analysis is provided, as well as the experimental verification with a liquid crystal variable retarder (LCR). A significant difference of the LCR retardance variation is obtained for different orientation of the LC director relative to the tilt direction.

New technique for the reduction of helicity-correlated instrumental asymmetries in photoemitted beams of spin-polarized electrons.

We present a new optical system that significantly reduces helicity-dependent instrumental intensity asymmetries. It is an extension of a previous scheme [Appl. Opt.47, 2465 (2008)], where one laser beam is split using a polarizing beam splitter into two with orthogonal linear polarizations. The beams are sent through a chopper, allowing only one to pass at a time. The two temporally separated beams are then spatially recombined using a second beam splitter. A liquid crystal retarder preceding the first beam splitter controls the relative intensity of the two oppositely polarized beams, allowing reduction of instrumental asymmetries. This system has been modified to include a spatial filter and a Pockels cell placed after the second beam splitter to act as a second active polarization element. Using this method, we can control instrumental asymmetries to ∼5×10(-7) in 1 h of data taking, which is comparable to the precision achieved in "second-generation" high energy electron-nuclear scattering parity violation experiments.

Diode laser lidar wind velocity sensor using a liquid-crystal retarder for non-mechanical beam-steering.

We extend the functionality of a low-cost CW diode laser coherent lidar from radial wind speed (scalar) sensing to wind velocity (vector) measurements. Both speed and horizontal direction of the wind at ~80 m remote distance are derived from two successive radial speed estimates by alternately steering the lidar probe beam in two different lines-of-sight (LOS) with a 60° angular separation. Dual-LOS beam-steering is implemented optically with no moving parts by means of a controllable liquid-crystal retarder (LCR). The LCR switches the polarization between two orthogonal linear states of the lidar beam so it either transmits through or reflects off a polarization splitter. The room-temperature switching time between the two LOS is measured to be in the order of 100 μs in one switch direction but 16 ms in the opposite transition. Radial wind speed measurement (at 33 Hz rate) while the lidar beam is repeatedly steered from one LOS to the other every half a second is experimentally demonstrated - resulting in 1 Hz rate estimates of wind velocity magnitude and direction at better than 0.1 m/s and 1° resolution, respectively.

Tunable achromatic liquid crystal waveplates.

The wavelength dependence of phase retarders is considered as a challenge in different applications such as polarimetric systems. A novel achromatic waveplate based on two nematic liquid crystal retarders is designed and demonstrated tunable over a wide spectral range using different voltages applied to the two retarders. The achromatic behavior is achieved based on the fact that the dispersion of the retardation of liquid crystals is voltage dependent. Achromatic quarter and half-waveplates are demonstrated using the same device as well as the tunability of the achromatic operation range.

Flexible polarimeter architecture based on a birefringent grating.

A polarimeter architecture is presented based on a birefringent grating displayed onto a parallel-aligned liquid crystal (LC) on silicon display (PAL-LCoS). The system is compact and flexible, since the size of the image can be adjusted by means of the period of the grating. The LCoS grating permits simultaneously measuring two orthogonal states of polarization (SOPs). By adding a wave plate, different couples of orthogonal SOPs can be detected. First, a basic proof of concept is presented using one quarter-wave and one half-wave plate with fixed retardances, which permit measuring the six SOPs classically used in polarimetry (linear states at 0°, 45°, 90°, and 135°, and R and L circular states). Next, the system is made fully programmable by incorporating a variable LC retarder (LCR). The LCR orientation and retardance values are optimized by means of the condition number indicator, in order to provide equivalent optimal accuracy. Experimental results of calibration images and test images are presented, showing the potentials of this architecture.

Liquid crystal polarimetry for metastability exchange optical pumping of 3He

We detail the design and operation of a compact, discharge light polarimeter for metastability exchange optical pumping of 3He gas near 1torr under a low magnetic field. The nuclear polarization of 3He can be discerned from its electron polarization, measured via the circular polarization of 668nm discharge light from an RF excitation. This apparatus measures the circular polarization of this very dim discharge light using a nematic liquid crystal wave retarder (LCR) and a high-gain, transimpedance amplified Si photodiode. We outline corrections required in such a measurement and discuss contributions to its systematic error.

Highly accurate spectral retardance characterization of a liquid crystal retarder including Fabry-Perot interference effects

Multiple-beam Fabry-Perot (FP) interferences occur in liquid crystal retarders (LCR) devoid of an antireflective coating. In this work, a highly accurate method to obtain the spectral retardance of such devices is presented. On the basis of a simple model of the LCR that includes FP effects and by using a voltage transfer function, we show how the FP features in the transmission spectrum can be used to accurately retrieve the ordinary and extraordinary spectral phase delays, and the voltage dependence of the latter. As a consequence, the modulation characteristics of the device are fully determined with high accuracy by means of a few off-state physical parameters which are wavelength-dependent, and a single voltage transfer function that is valid within the spectral range of characterization.

Enhanced Solar Cell Conversion Efficiency Using Birefringent Liquid Crystal Polymer Homeotropic Films from Reactive Mesogens

Novel birefringent liquid crystal polymer homeotropic films have been coated on semiconductor solar cells to improve the effective incident sunlight angles. The liquid crystal polymer precursor, based on reactive mesogens, is fluidic and flows like liquid. It would distribute uniformly on the solar cell sample surface by any traditional coating technique. The birefringence for light, due to the liquid crystal retardation properties, manipulated the optical length and the deflection of incident light, thus allowed an increase in the energy conversion efficiency. The expensive sunlight tracking systems could be avoided. The processing parameters can be tuned such as different mesogen concentrations and plate speeds of spin-coating. The results showed that the solar cell conversion efficiency was improved from 14.56% to 14.85% at an incident sunlight angle of 15°. It was further improved from 13.40% to 13.81% when the angle was 30°. The interesting angular dependency on solar cell efficiency enhancement has been evaluated.

Liquid crystal retarder spectral retardance characterization based on a Cauchy dispersion relation and a voltage transfer function

We present a methodology for the spectral characterization of the optical modulation properties of a liquid crystal retarder (LCR). The method includes its complete description with a single Cauchy dispersion relation and a single voltage transfer function. As a result, an accurate description of the LCR retardance is achieved, both versus applied voltage and versus wavelength, with very few parameters. Finally, an imaging polarimetric system has also been developed to characterize the spatial variations in the device.

Reflective Liquid Crystal Polarization Rotator with Ultraviolet-Polymerizable Liquid Crystal Retardation Film

We demonstrate a reflective liquid crystal (LC) polarization rotator with a UV-polymerizable LC retardation film inside an LC cell. The LC polarization rotator consists of two birefringence layers; one layer is a UV-polymerizable LC film that acts as a quarter-wave plate and the other is a nematic LC layer that acts as an electrically tunable retardation film. The polarization direction of the linearly polarized incident light is found to rotate in a sufficient angular range of 2π, depending on the retardation of the electrically tunable LC layer. Additionally, good polarization rotation properties are achieved for an arbitrary polarization direction of the incident light. The influence of the error of the retardation from the designed value on the polarization rotation properties is investigated in detail. The polarization rotation function is found to slightly deteriorate with an increase in the retardation error of the UV-polymerizable LC film, whereas the ellipticity of the output light is sensitively enhanced with an increase in the retardation error of the quarter-wavelength plate.

Novel liquid crystal multistage circular polarization switch for three-dimensional laser pico projectors

Crosstalk is one of the most important factors affecting the image quality of polarization switching three-dimensional (3D) projectors. The variation in the phase difference of a polarization switch owing to dispersion degrades crosstalk performance. We propose a novel multistage circular polarization switch comprising liquid crystal devices using a ferroelectric liquid crystal (FLC) in-plane fast switch and a thick nematic liquid crystal (NLC) retarder. In this paper, we present a proof-of-concept model of a 3D laser pico projector using a scanning MEMS head in combination with the polarization switch. We also evaluate the crosstalk and 3D performance of the projector.

Ultrahigh-resolution full-field optical coherence tomography using spatial coherence gating and quasi-monochromatic illumination

We developed an ultrahigh-resolution full-field optical coherence tomography (FF-OCT) microscope that is based on the spatial, rather than the temporal, coherence gating. The microscope is capable of observing three-dimensional microbiological structures as small as 0.4 μm × 0.4 μm × 1.0 μm (xyz) using quasi-monochromatic light and a liquid crystal retarder. Unlike traditional FF-OCT systems, this microscope can be operated in high resolution for any preferable wavelength with minimized defocusing and dispersion effects. High-resolution images of an onion cell are presented.

Non-display bio-optic applications of liquid crystals

Liquid crystal (LC) devices have been largely developed for LC displays and in the last decade for optical telecommunications; however, their application in the field of optical imaging is just starting to emerge. They can be miniaturised and thus have a great potential to be used with miniature optical imaging systems for biomedical applications. LC devices specifically designed for integration into biomedical optical imaging systems are reviewed. Using a combination of a few LC retarders, spectropolarimetric imaging of tissue can be obtained. LC tunable filters with high dynamic range and large throughput are designed for hyperspectral imaging and for spectral domain optical coherence tomography. The designs are based on several concepts, both using the classical stack of retarders and using more modern designs based on single layer in a Fabry–Perot cavity, in a resonant waveguide structure or in a photonic crystal.

Orientation-Independent DIC Microscopy with Fast Switching Shear Direction and its Applications

A conventional DIC (differential interference contrast) microscope shows the two-dimensional distribution of refractive index gradient encountered along the shear direction. Thus contrast of DIC images varies proportionally with the cosine of the angle made by the azimuth of the refractive index gradient and the direction of wavefront shear. It is therefore necessary to examine unknown objects at several azimuth orientations. To overcome the limitations of available systems, we have built an assembly, which allows the bias and shear directions to be switched rapidly without mechanically rotating the specimen or the prisms. The assembly consists of two standard Nomarski prisms with liquid crystal polarization rotator in between. When the polarization rotator is in OFF state, the total shear direction of the assembly is at +45-deg to the shear direction of the first prism. If the polarization rotator is in ON state, the total shear direction is at −45-deg to the shear direction of the first prism. Thus, when one would switch the polarization, the shear direction would be rotated by 90-deg. We added one assembly to the illumination path and another one to the imaging path of the standard Olympus BX-61 microscope. Also one variable liquid crystal retarder was installed in order to change a bias. Four raw DIC images at two orthogonal shear directions and two inverse biases are captured and processed within a second. Then the quantitative image of refractive index distribution within a thin optical section is displayed on a computer screen. The orientation-independent DIC data obtained can also be used to compute the quantitative distribution of refractive index gradient or to generate enhanced, regular DIC images with any desired shear direction. The OI-DIC system can be combined with other techniques, such as fluorescence and polarization microscopy.

Multiple-stage liquid crystal tuned filter

A voltage controlled multiple-stage tunable filter with broad continuous tuning range was studied in theory and experiment. Four liquid crystal retarders, two quartz plates with certain integral ratio phase retardations and five polarization films were assembled into a traditional Lyot filter. The tunable function was achieved by using the electro-birefringence effect of liquid crystal. The result of numerical simulation showed that the spectral resolution was ∼10 nm around bands of 500–700 nm; side forms rejection ratio and FWHM changed in contrast to considering and not considering the birefringence dispersion effect of liquid crystal and quartz; the shape of filtered light displayed by spectral photometer would be affected by spectrum bands of light source in spectral photometer. A four-stage tunable filter was made and tested, experiment results agreed with the numerical simulation results very well, the research results could be applied as reference for the study of broad spectrum tunable filters.

Spectroscopic sensitive polarimeter for biomedical applications

We present the design and calibration of a spectroscopic sensitive polarimeter. The polarimeter can measure the full Stokes vector in the wavelength range 550 to 750 nm with 1-nm resolution and consists of a fiber-based spectrophotometer, a white light emitting diode light source, two liquid crystal retarders, and one polarizer. Calibration of the system is achieved with a scheme that does not require knowledge of the polarizing elements' orientation or retardation. Six intensity spectra are required to calculate the full spectrum Stokes vector. Error in the polarimeter is less than 5%. We report the Stokes vectors for light transmitted through nonscattering polarizing elements as well as a measurement of the depolarizing properties of chicken muscle at several wavelengths.

Design of a scanning polarimetric scatterometer for rough surface scattering measurements

This paper presents the mechanical design of a goniometric scatterometer to be used to measure the light scattered in rough surfaces. Advances in the instrumentation of a polarization measurement system are also presented. It has been found that the linear part of the retardance vs. voltage curves for a liquid crystal retarder corresponds to retardance between λ and λ/2 and that this range of retardance leads to variations in the frequency components of the time-varying detected signal for sinusoidal voltage. Preliminary results for a Stokes polarimeter are presented.