Complete polarization state generator with one variable retarder and its application for fast and sensitive measuring of two-dimensional birefringence distribution

Swept-wavelength null polarimetry for highly sensitive birefringence laser scanning microscopy

We have recently demonstrated a high-speed null polarimeter [Opt. Express30, 18889 (2022)10.1364/OE.454193OPEXFF1094-4087] based on passive polarization optics and using a fast swept-wavelength laser source. We report here its implementation in a laser-scanning microscope setup, enabling highly sensitive linear retardance imaging with a pixel dwell time of 10 μs. The instrument is also able to measure light depolarization induced by the sample. Images of biological samples, including cancerous tissue and cells, illustrate its performances.

Organic volume and permeability variations in the surface layer of artificial and natural enamel carious lesions

OBJECTIVES

Test the hypothesis the type of enamel caries (natural, artificial induced by gel, and artificial induced by acid solutions) affect the organic volume and the permeability of the surface layer in enamel caries lesions.

DESIGN

Artificial enamel caries, induced by either acidic solution (organic-poor; Group 1) or acidic gel (organic-rich; Group 2), and natural non-cavitated inactive approximal enamel caries lesions (NEC; Group 3) were obtained, from which longitudinal ground sections were prepared. Measurements of the mineral (V_min) (by microradiography), and water (α) and organic (β) volumes (by optical birefringence) were obtained at three points in the surface layer (n = 30/group).

RESULTS

The main outcomes were the ratio between experimental β by predicted β (β Ratio) and the ratio between experimental and predicted permeabilities (α_d Ratio). β Ratio in Group 1 was lower than in Groups 2 (Cohen's d: -1.81; 95% CI:-1.45,-2.32; p < 0.001) and 3 (Cohen's d: -0.71; 95% CI:-0.27,-1.18; p = 0.004), and Group 2 surpassed Group 3 (Cohen's d: 0.49; 95% CI:0.07,0.94; p = 0.03). α_d Ratio in Group 1 was higher than in Groups 2 (Cohen's d: 1.86; 95% CI:1.49,2.33; p < 0.001) and 3 (Cohen's d: 0.60; 95% CI:0.18,1.14; p = 0.01), and Group 3 surpassed Group 2 (Cohen's d: 0.61; 95% CI:0.23,1.07; p = 0.01).

CONCLUSIONS

The highest organic volume and the lowest permeability occurred at the surface layer of gel-induced artificial enamel caries lesions, which should be preferred in in vitro studies on de- and remineralization and resin infiltration.

Instantaneous mapping of liquid crystal orientation using a polychromatic polarizing microscope

Polarizing microscopy brought about many advancements in the science of liquid crystals and other soft materials, including those of biological origin. Recent developments in optics and computer-based analysis enabled a new generation of quantitative polarizing microscopy which produces spatial maps of the optic axis. Unfortunately, most of the available approaches require a long acquisition time of multiple images which are then analyzed to produce the map. We describe a polychromatic polarizing microscope, which allows one to map the patterns of the optical axis in a single-shot exposure, thus enabling a fast temporal resolution. We present a comparative analysis of the new microscope with alternative techniques such as a conventional polarizing optical microscope and MicroImager of Hinds Instruments.

Analysis of accuracy and ambiguities in spatial measurements of birefringence in uniaxial anisotropic media

Accuracy and ambiguities in retardance and optical axis orientation spatial measurements are analyzed in detail in the context of the birefringence imaging method introduced by Shribak and Oldenbourg [Appl. Opt.42, 3009 (2003)APOPAI0003-693510.1364/AO.42.003009]. An alternative formula was derived in order to determine the optical axis orientation more accurately, and without indetermination in the case of a quarter-wave plate sample. Following Shribak and Oldenbourg's experimental configuration using two variable retarders, a linear polarizer, and five polarization probes, we examined the effect of the swing angle χ, which selected the ellipticity of each polarization state, on the accuracy of retardance (Δ) and axis orientation (ϕ) measurements. Using a quarter-wave plate, excellent agreement between measured and expected values was obtained for both the retardance and the axis orientation, as demonstrated by the statistical analysis of Δ and ϕ spatial distributions. The intrinsic ambiguity in the determination of Δ and ϕ for superimposed layers of transparent anisotropic cello-tape is discussed in detail, and solutions are provided to remove this ambiguity. An example of application of the method on geological samples is also presented. We believe our analysis will guide researchers willing to exploit this long-standing method in their laboratories.

Ultrafast time-resolved single-shot birefringence microscopy for laser-induced anisotropy

The interaction between ultrashort laser pulses and materials in the ultrafast time domain, especially regarding the effect of laser polarization, has attracted much attention. In this study, ultrafast time-resolved single-shot birefringence microscopy is performed to observe laser-induced anisotropy. The birefringences of the optical Kerr effect and laser-induced anisotropic nanostructures by femtosecond laser pulses in silica glass are measured, and their slow axis is confirmed to correspond to the linear polarization angle of the pump light. We discuss the time variations of these birefringences in the picosecond time domain.

Spatially resolved birefringence measurements with a digital micro-mirror device

We demonstrate a novel technique to measure spatially resolved birefringence structures in an all-digital fashion with a digital micro-mirror device (DMD). The technique exploits the polarization independence of DMDs to apply holographic phase control to orthogonal polarization components and requires only a static linear polarizer as an analyzer for the resulting phase shift polarization measurements. We show the efficacy of this approach by spatially resolving complex polarization structures, including nano-structured metasurfaces, customized liquid crystal devices, as well as chiral L-Alanine and N-Acetyl-L-cystein crystals. Concentration dependent measurements of optical rotation in glucose and fructose solutions are also presented, demonstrating the technique's versatility. Unlike conventional approaches, our technique is calibration free and has no moving parts, offers high frame rates and wavelength independence, and is low cost, making it highly suitable to a range of applications, including pharmaceutical manufacturing, saccharimetry and stress imaging.

Segmentation, retardation and mass approximation of birefringent particles on a standard light microscope

This study presents a simple technique for the approximation of retardation, thickness and mass of birefringent particles with a retardation from 8 to 231 nm retardation. Tuning of the imaging system (standard light microscope equipped with a left and a right circular polarizer) to match grey values of polymer retarder films of known retardation with rendered grey values allows for a robust calibration and accurate approximation of retardation. In addition, a technique for accurate particle segmentation using a Canny-Deriche algorithm was used to minimize the bias on mass estimated from different thresholding techniques. The technique was tested using microscopic calcitic plates called coccoliths produced by the marine algal group coccolithophores, and the results compare well with published coccolith mass estimates obtained from volumetric analysis. LAY DESCRIPTION: Material with certain optical properties display interference colours when observed in a light microscope under circular polarized light. This study presents a simple technique for measuring the thickness and retardation of small particles within the 8 to 231 nm retardation range based on the grey values of their interference colours. Retardation is a measure of the distance between waves of two mutually perpendicular polarized light waves after passing through material. The technique involves the tuning of a standard light microscope system equipped with a left and a right circular polarizer and a digital camera to match grey values of polymer retarder films with a known retardation with grey values of a digitially rendered Michel-Lévy chart. A technique for accurate isolation of particles from the image background using a Canny-Deriche algorithm is also described, which avoids possible biased results from thresholding. The techniques were tested using microscopic calcitic plates called coccoliths produced by the marine algal group coccolithophores, and the results compare well with published estimates obtained from volumetric analysis.

Polarimeter for measuring the properties of birefringent media in reflective mode

This work presents a description of a polarimetric system for measuring the properties of birefringent media. In our reflection system the applied Stokes polarimeter acts both as a generator of the light's selected polarization states as well as a light analyzer leaving the examined medium. The method is based on six intensity distribution measurements realized in six different configurations of polarizers/analyzers: four linear and two circular ones. Thus, we have achieved parallel polariscope for linear polarizers and the crossed polariscope for circular polarizers. Such a setup can be easily applied for linearly birefringent media properties measurements including dichroic ones. This measurement setup and the measurement method were successfully tested in a homogeneous medium and a medium with variable phase difference.

Multi-wavelength quantitative polarization and phase microscope

We introduce a snapshot multi-wavelength quantitative polarization and phase microscope (MQPPM) for measuring spectral dependent quantitative polarization and phase information. The system uniquely integrates a polarized light microscope and a snap-shot quantitative phase microscope in a single system, utilizing a novel full-Stokes camera operating in the red, green, and blue (RGB) spectrum. The linear retardance and fast axis orientation of a birefringent sample can be measured simultaneously in the visible spectra. Both theoretical analysis and experiments have been performed to demonstrate the capability of the proposed microscope. Data from liquid crystal and different biological samples are presented. We believe that MQPPM will be a useful tool in measuring quantitative polarization and phase information of live cells.

Simple multi-wavelength imaging of birefringence:case study of silk

Polarised light imaging microscopy, with the addition of a liquid crystal (LC) phase retarder, was used to determine the birefringence of silk fibres with high (∼1 μm) spatial resolution. The measurement was carried out with the silk fibres (the optical slow axis) and the slow axis of the LC-retarder set at parallel angles. The direct fit of the transmission data allowed for high fidelity determination of the birefringence Δn ≈ 1.63 × 10^-2 (with ∼2% uncertainty) of the brown silk fibre, (Antheraea pernyi) averaged over the wavelength range λ = (425-625) nm. By measuring retardance at four separate wavelengths, it was possible to determine the true value of the birefringence of a thicker sample when an optical path may include a large number of wavelengths. The numerical procedures and required hardware are described for the do-it-yourself assembly of the imaging polariscope at a fractional budget compared to commercial units.

A laser scanning microscope executing intraframe polarization switching of the illumination beam

The polarization of the illumination beam in a beam scanning microscope such as the confocal microscope plays an important role in extracting the orientational information of the molecules in the specimen. In this paper, we present the development of a beam scanning microscope comprising a custom designed optical arrangement to obtain images of the same target with different polarizations of the illumination beam. The optical arrangement, based on a ferroelectric liquid crystal spatial light modulator (FELCSLM), can generate homogeneous as well as non-homogeneous user defined polarization profiles over the cross-sectional area of the illumination beam. Here, we employ a computer generated holography technique and exploit the programmability of the FELCSLM display to considerably reduce the time gap between two successive illuminations of each location of the specimen with two different polarizations. We demonstrate the working of the beam scanning microscope where the polarization profile of the illumination beam is switched at the end of every line scanned, in contrast to a conventional beam scanning microscope where the polarization can be switched at the end of every frame scanned. Preliminary experimental results obtained using a polarization sensitive target confirm the feasibility of the proposed scheme.

Mapping optical path length and image enhancement using quantitative orientation-independent differential interference contrast microscopy

We describe the principles of using orientation-independent differential interference contrast (OI-DIC) microscopy for mapping optical path length (OPL). Computation of the scalar two-dimensional OPL map is based on an experimentally received map of the OPL gradient vector field. Two methods of contrast enhancement for the OPL image, which reveal hardly visible structures and organelles, are presented. The results obtained can be used for reconstruction of a volume image. We have confirmed that a standard research grade light microscope equipped with the OI-DIC and 100 × / 1.3 NA objective lens, which was not specially selected for minimum wavefront and polarization aberrations, provides OPL noise level of ? 0.5 ?? nm and lateral resolution if ? 300 ?? nm at a wavelength of 546 nm. The new technology is the next step in the development of the DIC microscopy. It can replace standard DIC prisms on existing commercial microscope systems without modification. This will allow biological researchers that already have microscopy setups to expand the performance of their systems.

Living Cells and Dynamic Molecules Observed with the Polarized Light Microscope: the Legacy of Shinya Inoué

In 1948, Shinya Inoué arrived in the United States for graduate studies at Princeton. A year later he came to Woods Hole, starting a long tradition of summer research at the Marine Biological Laboratory (MBL), which quickly became Inoué's scientific home. Primed by his Japanese mentor, Katsuma Dan, Inoué followed Dan's mantra to work with healthy, living cells, on a fundamental problem (mitosis), with a unique tool set that he refined for precise and quantitative observations (polarized light microscopy), and a fresh and brilliant mind that was unafraid of challenging current dogma. Building on this potent combination, Inoué contributed landmark observations and concepts in cell biology, including the notion that there are dynamic, fine structures inside living cells, in which molecular assemblies such as mitotic spindle fibers exist in delicate equilibrium with their molecular building blocks suspended in the cytoplasm. In the late 1970s and 1980s, Inoué and others at the MBL were instrumental in conceiving video microscopy, a groundbreaking technique which married light microscopy and electronic imaging, ushering in a revolution in how we know and what we know about living cells and the molecular mechanisms of life. Here, we recount some of Inoué's accomplishments and describe how his legacy has shaped current activities in polarized light imaging at the MBL.

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.

Polychromatic polarization microscope: bringing colors to a colorless world

Interference of two combined white light beams produces Newton colors if one of the beams is retarded relative to the other by from 400 nm to 2000 nm. In this case the corresponding interfering spectral components are added as two scalars at the beam combination. If the retardance is below 400 nm the two-beam interference produces grey shades only. The interference colors are widely used for analyzing birefringent samples in mineralogy. However, many of biological structures have retardance <100 nm. Therefore, cells and tissues under a regular polarization microscope are seen as grey image, which contrast disappears at certain orientations. Here we are proposing for the first time using vector interference of polarized light in which the full spectrum colors are created at retardance of several nanometers, with the hue determined by orientation of the birefringent structure. The previously colorless birefringent images of organelles, cells, and tissues become vividly colored. This approach can open up new possibilities for the study of biological specimens with weak birefringent structures, diagnosing various diseases, imaging low birefringent crystals, and creating new methods for controlling colors of the light beam.

Polarized light imaging of birefringence and diattenuation at high resolution and high sensitivity

Polarized light microscopy provides unique opportunities for analyzing the molecular order in man-made and natural materials, including biological structures inside living cells, tissues, and whole organisms. 20 years ago, the LC-PolScope was introduced as a modern version of the traditional polarizing microscope enhanced by liquid crystal devices for the control of polarization, and by electronic imaging and digital image processing for fast and comprehensive image acquisition and analysis. The LCPolScope is commonly used for birefringence imaging, analyzing the spatial and temporal variations of the differential phase delay in ordered and transparent materials. Here we describe an alternative use of the LC-PolScope for imaging the polarization dependent transmittance of dichroic materials. We explain the minor changes needed to convert the instrument between the two imaging modes, discuss the relationship between the quantities measured with either instrument, and touch on the physical connection between refractive index, birefringence, transmittance, diattenuation, and dichroism.

Polarized light microscopy in reproductive and developmental biology

The polarized light microscope reveals orientational order in native molecular structures inside living cells, tissues, and whole organisms. It is a powerful tool used to monitor and analyze the early developmental stages of organisms that lend themselves to microscopic observations. In this article, we briefly discuss the components specific to a traditional polarizing microscope and some historically important observations on: chromosome packing in the sperm head, the first zygote division of the sea urchin, and differentiation initiated by the first asymmetric cell division in the sand dollar. We then introduce the LC-PolScope and describe its use for measuring birefringence and polarized fluorescence in living cells and tissues. Applications range from the enucleation of mouse oocytes to analyzing the polarized fluorescence of the water strider acrosome. We end with new results on the birefringence of the developing chick brain, which we analyzed between developmental stages of days 12-20.

Quantitative orientation-independent differential interference contrast microscope with fast switching shear direction and bias modulation

We describe a quantitative orientation-independent differential interference contrast (DIC) microscope, which allows bias retardation to be modulated and shear directions to be switched rapidly without any mechanical movement. The shear direction is switched by a regular liquid-crystal cell sandwiched between two standard DIC prisms. Another liquid-crystal cell modulates the bias. Techniques for measuring parameters of DIC prisms and calibrating the bias are shown. Two sets of raw DIC images with the orthogonal shear directions are captured within 1 s. Then the quantitative image of optical path gradient distribution within a thin optical section is computed. The gradient data are used to obtain a quantitative distribution of the optical path, which represents the refractive index gradient or height distribution. Computing enhanced regular DIC images with any desired shear direction is also possible.

Fully tunable active polarization imager for contrast enhancement and partial polarimetry

We present the design and the practical implementation of a polarimetric imaging system based on liquid-crystal modulators that allows generation and analysis of any polarization state on the Poincaré sphere. This system is more versatile than standard Mueller imagers that are based on optimized, but limited, sets of illumination and analysis states. Examples of benefits brought by these extra degrees of freedom are illustrated on two different applications: contrast enhancement and extraction of partial polarimetric properties of a scene.

On the influence of noise statistics on polarimetric contrast optimization

In active scalar polarimetric imaging systems, the illumination and analysis polarization states are degrees of freedom that can be used to maximize the performance. These optimal states depend on the statistics of the noise that perturbs image acquisition. We investigate the problem of optimization of discrimination ability (contrast) of such imagers in the presence of three different types of noise statistics frequently encountered in optical images (Gaussian, Poisson, and Gamma). To compare these different situations within a common theoretical framework, we use the Bhattacharyya distance and the Fisher ratio as measures of contrast. We show that the optimal states depend on a trade-off between the target/background intensity difference and the average intensity in the acquired image, and that this trade-off depends on the noise statistics. On a few examples, we show that the gain in contrast obtained by implementing the states adapted to the noise statistics actually present in the image can be significant.