Optical Retardation Research Articles

Minimize flow-induced uncertainty in polarization sensitive optical coherence tomography imaging using eigen decomposition.

Blood flow alters the scattering behavior of penetration light, causing instability in the polarization state to emerge at the underlying tissue during polarization-sensitive optical coherence tomography (PSOCT). We propose an eigen decomposition method to meet this challenge, where the static and dynamic scattering signals are separated for PSOCT to provide the polarization measurements of the tissue of interest that is located beneath the blood flow. Using flow phantoms made by Intralipid solution and 3D-printed birefringent material, we show the flow-induced effects on the measurements of sample birefringent properties of optical axis, phase retardation, and degree of polarization uniformity. We demonstrate the usefulness of the proposed method through in vivo imaging of the human nail fold.

Investigating the effect of SiO2 thin films with sculptured topographies on the alignment properties of liquid crystals

In the present work, sculptured thin films (STF) based on silicon dioxide (SiO2) induced on the indium tin oxide (ITO) coated substrates via glancing angle deposition (GLAD) technique using the physical vapor deposition (PVD) process. Our main motive to perform this work is to address the issues we have faced with the traditional alignment method and give a try to overcome those problems. Keeping these challenges in our mind, we are giving here an alternative to traditional method and explored this different alignment technique with GLAD process. We have fabricated a liquid crystal device (LCD) in a way of electrically-controlled birefringence (ECB) and placed the engineered substrates in anti-parallel alignment. Morphological, optical and electro-optic behaviour of resulting device has been studied in the ∼5 μm thick device. Effect of applied voltage on the morphology and electro-optic behaviour of liquid crystal has been established. Interestingly, LC material demonstrates electrically switchable birefringence colours viz. brown, green, pink, purple and mauve colour, under applied electric field in room temperature at 10X magnification under polarizing optical microscope. A very nice extinction and contrast has been obtained with fabricated ECB device, which elucidate the device quality with impeccable alignment without any disclination defects. Besides, the electro-optic characteristics it also shows a significant impact on the liquid crystal properties. The obtained results demonstrate nice optical retardation, a lower Frederick's threshold voltage i.e. 1.5 V and 19.03 dB higher contrast respectively. An impressive switching response time τon of 5.4 ms and τoff = 600 µs at 20 V has attained with respective device as well. Such change in parameters may correspond to the higher electrical torque experienced by the LC molecules on the application of applied field. Anchoring energy and pretilt angle of respective system has been calculated and correlated with observed behaviour. Thus, engineered device with this superior alignment method show remarkable electro-optic results which can open a new way for the development of fast LCD for advanced optical device applications.

Widefield quantitative polarized light microscopy using spectrally encoded null polarimetry.

Quantitative polarized light microscopy enables determination of optical retardation and azimuth of birefringent specimens and is a powerful tool for label-free imaging in the fields of biology and pathology. We have recently proposed a device for fast laser-scanning birefringence microscopy based on a near-infrared wavelength-swept laser and spectral encoding of polarization, resulting in a channeled spectrum generated during the wavelength-sweep of the laser and highly sensitive to optical retardation [Opt. Lett.49, 387 (2024)10.1364/OL.507576]. In this Letter, we propose its transposition to visible widefield imaging using a white light source and a high-order retarder for spectral encoding and a hyperspectral camera to record the channeled spectrum at each point of the image in parallel. The method proposed here allows for straightforward conversion of any widefield microscope into a highly sensitive and quantitative polarized light microscope.

Spectropolarimetric properties of vortex retarders

Vortex retarders are optical retarders with a uniform retardance, but with a fast axis that varies azimuthally over the area of the optic. This affects the radial and azimuthal polarization components of the incoming beam. The vortex retarders discussed here generate TEM10 and TEM20 Laguerre-Gaussian beams. While several varieties of these vortex retarders exist, designed to function at specific wavelengths, and a few studies have reported their use at those wavelengths, almost no data exists detailing their polarimetric properties outside those wavelengths. Using a Mueller matrix spectropolarimeter with dual-rotating retarders, the polarization properties of four zero-order half-wave vortex retarders have been measured across a broad wavelength range of 300–1100 nm. This data contributes to a more complete description of the spectral properties of these popular retarders.

Whole-field principal angle and optical path retardation measurements using white light OPD scanning polariscope with first moment algorithm

White-light Optical Path Difference (OPD) scanning polariscope is currently an accepted technique for analyzing photoelastic specimens. While it has proven effective in overcoming limitations associated with phase-stepping and fringe scanning techniques, it is constrained when evaluating optical path retardation smaller than the light source coherence length. To address this limitation, a white-light OPD scanning polariscope, equipped with a first moment algorithm, has been introduced for the comprehensive examination of whole-field principal angle and optical path retardation. This article demonstrates the design and operational principles of the proposed polariscope, details an experimental setup for its deployment, and presents findings from practical applications of this setup. The results confirm the effectiveness and practicality of the proposed polariscope. Furthermore, these results highlight its precision, with standard deviations of 0.176° for principal angle and 6.4 nm for optical path retardation measurements.

Patterned Liquid Crystal Polymer Thin Films Improved Energy Conversion Efficiency at High Incident Angles for Photovoltaic Cells.

In this report, micro-patterned silicon semiconductor photovoltaic cells have been proposed to improve the efficiency in various incident sunlight angles, using homeotropic liquid crystal polymers. The anisotropic liquid crystal precursor solution based on a reactive mesogen has good flowing characteristics. It can be evenly coated on the silicon solar cells' surface by a conventional spreading technique, such as spin coating. Once cured, the polymers exhibit asymmetric transmittance properties. The optical retardation characteristics of the coated polymer films can be eventually determined by the applicable coating and curing parameters during the processes. The birefringence of light then influences the optical path and the divergence of any encountered sunlight. This allows more photons to enter the active semiconductor layers for optical absorption, resulting in an increase in the photon-to-electron conversion, and thus improving the photovoltaic cell efficiency. This new design is straightforward and could allow various patterns to be created for scientific development. The experimental results have evidenced that the energy conversion efficiency could be improved by 2-3% for the silicon photovoltaic cells, under direct sunlight or at no inclination, when the liquid crystal polymer precursor solution is prepared at 5%. In addition, the efficiency could be much more significantly improved to 14-16% when the angle is inclined to 45°. The unique patterned liquid crystal polymer thin films provide enhanced energy conversion efficiency for silicon photovoltaic cells. The design could be further evaluated for other solar cell applications.

Hyperspectral optical orbital angular momentum modulation from tunable structured waveplates.

Shaping the orbital angular momentum of optical pulses in the spectral domain is a means of managing light in space and time that offers many possible applications. However, these are limited by the small number of techniques available, whose flexibility does not yet rival that of the continuous regime. We propose here to implement a tunable hyperspectral management of the orbital angular momentum of a polychromatic light field. The main idea is to exploit the dispersive nature of geometric phase optical elements by intentionally choosing to work in a regime of high anisotropic optical retardance. An experimental proof of principle is demonstrated in the visible range using a supercontinuum laser and an optically thick, electrically controllable, liquid crystal structured wave plate.

Crystalline / glass nanoscale chemical separation induced by femtosecond laser pulses in aluminosilicate glass

An aluminosilicate glass with a molar composition of 60%Al2O3–40%SiO2 fabricated by the aerodynamic levitation technique was irradiated in volume using an infrared femtosecond laser. Optical retardance was observed and measured upon a wide range of varying laser parameters including pulse energy, repetition rate, pulse duration, writing speed, and laser polarization. A similar analysis was performed in silica glass (Suprasil CG) taken as a reference. We measured retardance values as high as 100 nm in the aluminosilicate glass, which is approximately 2x higher than previously investigated aluminosilicate glasses. Raman spectroscopy and electron microscopy techniques revealed the presence of an orientable lamellar structure, commonly referred to as nanogratings, composed of alternate SiO2-enriched amorphous layers with Al2O3-enriched ones. The latter are essentially crystallized in a 3Al2O3:2SiO2 Mullite phase. This internal structuring, the first one in a congruent glass, brings additional insights into the mechanisms leading to nanogratings formation, and may be proved useful for future glass-based functional optical devices with very large thermal stability.

Lifespan prediction procedure of volume nanogratings imprinted by femtosecond laser in optical glasses

Volume nanogratings imprinted by infrared femtosecond laser in oxide glasses exhibit a characteristic birefringent signature, which translates into measurable retardance. Upon thermal annealing, such signature is progressively erased, typical of nanograting erasure. In this work, we propose a procedure to predict the lifespan of nanogratings by two main approaches: 1/ numerical modeling of optical retardance ageing using the Rayleigh-Plesset equation, and 2/exploiting VAREPA (VAriable REaction PAthways) framework fed by simulated ageing data. By considering experimental time – temperature annealing conditions, the modeled retardance is gathered as a function of demarcation energy to build a so-called Master Curve and then compared to accelerated ageing experiments. The erasure constant rate k0 can be determined for 8 commercial optical glasses. Based on a distributed Rayleigh-Plesset model, k0 and activation energy distribution are linked to glass viscosity and its temperature dependency. Finally, we discussed the restrictions on VAREPA application for an accurate lifetime prediction. This work provides guidelines for the future development of nanogratings based devices and applications, including optical data storage, birefringent devices, and optical sensors, through a judicious choice of glass composition and associated properties.

Sub‐Nominal Resolution Fourier Transform Spectrometry with Chip‐Based Combs

AbstractChip‐based optical frequency combs address the demand for compact, bright, coherent light sources of equidistant phase‐locked lines. Traditionally, the Fourier Transform Spectroscopy (FTS) technique is considered a suboptimal choice for resolving comb lines in chip‐based sensing applications due to the requirement of long optical delays, and spectral distortion from the instrumental line shape. Here, a sub‐nominal resolution FTS technique is developed that extracts the comb's offset frequency in any spectral region directly from the measured interferogram without resorting to nonlinear ‐to‐ interferometry. This in turn enables 10's of MHz resolution spectrometry with millimeter optical retardations currently limited by the emission linewidth and phase noise of the used lasers. Low‐pressure Doppler‐broadened absorption lines probed by widely‐tunable chip‐scale mid‐infrared optical frequen with electrical pumping are fully resolved over a span of tens of nanometers. This versatile technique paves the way for compact, electrostatically‐actuated, or even all‐on‐chip high‐fidelity FTS, and can be readily applied to boost the resolution of existing commercial instruments with compact interferometers several hundred times.

White light OPD scanning polariscope for whole-field optical path retardation and principal angle examinations

Fringe-scanning (or TFP/RGB) and phase-stepping are now the most acceptable techniques for examining stressed photoelastic samples; however the former requires a calibration table, which is sensitive to the geometrical and optical parameters of the corresponding polariscope, and concentrates only on phase retardation measurements; the latter suffer from three potential issues: 1) the principal angle is indeterminate when the corresponding phase retardation is close or equal to an integer multiple of π, 2) the obtained phase retardation and principal angle are wrapped, and 3) the phase retardation and principal angle may couple together in certain situations. Free from the disadvantages of the fringe-scanning and phase-stepping polariscopes, a white light OPD (optical path difference) scanning polariscope composed of a white light laser, an OPD modulator, and a dark-field circular polariscope was proposed for whole-field optical path retardation (proportional to the phase retardation) and principal angle determinations. This paper introduces the theory of the polariscope, demonstrates the experimental setup for implementing the polariscope, and presents the results from the uses of the setup. The results manifest a standard deviation of 8 nm when repeatedly measuring an optical path retardation and 1.21ο when repeatedly measuring a principal angle; moreover, the results confirm the polariscope's validity and feasibility, but showing a limitation while adopting the polariscope to evaluate fringe orders < 3.

Synthesis and characterization of low retardation colorless polyimides containing m-phenylenediamine with different pendant groups

In this study, 4-biphenyl-3,5-diaminobenzoate (E- m-PDA) was synthesized by three steps using 3,5-dinitrobenzoic acid as a starting material. Two series of colorless polyimides (CPIs) PI-(1a-1d) and PI-(2a-2d) were prepared by traditional two-step polycondensation of m-phenylenediamine ( m-PDA) based diamines with different pendant groups (-H, CF3, COOH and biphenyl ester) with aromatic dianhydrides BPADA and 6FDA, respectively. All of the prepared CPIs exhibited high thermal stability with 5% weight loss at temperatures ranging from 428 to 539°C in nitrogen, and glass transition temperatures ( Tgs) ranging from 228 to 332°C. These PI films displayed a UV-vis absorption cut-off wavelength at 338–376 nm and transparency of 76%–89% at a wavelength of 450 nm. Both series of colorless PI films also showed low birefringence values of 0.0021–0.0100. In addition, the optical retardations of these CPIs were extremely low (48–220 nm), indicating low optical anisotropy of these CPI films. Colorless PI film PI-2d prepared based on E- m-PDA and 6FDA had good comprehensive properties of low birefringence ([Formula: see text] n of 0.0064), high Tg (319°C) and excellent transparency ( T450 of 88%).

Theoretical and experimental investigations in thermo-mechanical properties of fused silica with pulsed CO2 laser ablation.

Laser ablation is widely used as a flexible and non-contact processing technology for the fabrication of fused silica. However, the introduction of thermal stress inevitably leads to crack growth and reduces the lifetime of fused silica. Due to the complicated coupling interaction and properties of fused silica, the unclear thermal stress formation is the bottleneck restricting further development of laser ablation. In this article, a three-dimensional multi-physics thermo-mechanical model was developed to reveal the evolution mechanism, and experiments were performed to validate the simulated results. The surface morphology evolution was elaborated during process cycles, with recoil pressure identified as the key factor in determining surface morphology. Moreover, thermal stress was quantified utilizing optical retardance and stress birefringence, effectively distinguishing between non-thermal and thermal stress induced by laser ablation. The theoretical simulations fit well with experimental measurements. Meanwhile, stress distribution and evolution behaviors were revealed under different processing parameters by this model. This work not only contributes to a profound understanding of the laser ablation process but also establishes a theoretical foundation for achieving high surface quality and non-thermal stress laser ablation.

Synthesis and characterization of semi‐alicyclic transparent polyimide films containing benzanilide and fluorene units with improved high‐temperature dimensional stability and low‐optical retardations

AbstractTwo solution‐processable semi‐alicyclic transparent polyimide (PI) films with improved high‐temperature dimensional stability and low‐optical retardations were prepared from the stereo‐isomeric alicyclic dianhydrides, 1S,2R,4S,5R‐hydrogenated pyromellitic dianhydride (ccHPMDA) and 1R,2S,4S,5R‐hydrogenated pyromellitic dianhydride (ctHPMDA) and an aromatic diamine, 9,9‐bis[4‐(4‐aminobenzamide)phenyl]fluorene (FDAADA), respectively. The alicyclic cyclohexane units in the dianhydride moiety endowed the derived PI‐1 (ccHPMDA‐FDAADA) and PI‐2 (ctHPMDA‐FDAADA) films good optical transparency, the rigid‐rod benzanilide units provided the PI films low coefficients of thermal expansion (CTE), and the fluorene units resulted in good solution processability and low‐optical retardations (Rth) for the PI films. The PI films exhibited the glass transition temperatures over 400°C and the CTE value of 33.2 × 10−6/K for PI‐1 and 29.0 × 10−6/K for PI‐2 in the temperature range of 50 ~ 250°C, respectively. In addition, the PI films showed good optical transparency in the ultraviolet–visible light region with the optical transmittances higher than 80% for PI‐1 film at the wavelength of 500 nm. The PI films exhibited the average refractive indices of 1.6505 for PI‐1 and 1.6485 for PI‐2 film, respectively. The birefringence values of the PI films were all lower than 0.01, resulting in the low‐optical anisotropies with the optical retardation (Rth) of 77 nm for PI‐1 film and 84 nm for PI‐2 film.

Birefringent polyvinyl alcohol layers as retardation components for display devices

AbstractOptically birefringent polymers have found utility in numerous applications, especially in liquid crystal displays. This work is focused on inducing molecular ordering in polyvinyl alcohol films in different conditions. The samples were modified by new mixed approaches: drawing, rubbing before drying, rubbing before drying followed by drawing, and combined rubbing, drawing and then parallel scratching. The level of chain alignment in the samples was quantified by birefringence measurements using several experimental techniques, namely interferometry, polarization ellipse approach, channeled spectra and polarized light refractometry. The produced anisotropy was correlated with the sample morphology examined via optical microscopy under crossed polarizers. Optical retardation was found to be larger as the generated birefringence of mechanically deformed polyvinyl alcohol foils was higher. It was found the new proposed approach of ordering polymer sample by combined rubbing, drawing and then scratching is most suitable as retardation layer for compensating light leakage in display devices.

Residual stress modeling of mitigated fused silica damage sites with CO2 laser annealing

A numerical model based on measured fictive temperature distributions is explored to evaluate the residual stress fields of CO2 laser-annealed mitigated fused silica damage sites. The proposed model extracts the residual strain from the differences in thermoelastic contraction of fused silica with different fictive temperatures from the initial frozen-in temperatures to ambient temperature. The residual stress fields of mitigated damage sites for the CO2 laser-annealed case are obtained by a finite element analysis of equilibrium equations and constitutive equations. The simulated results indicate that the proposed model can accurately evaluate the residual stress fields of laser-annealed mitigated damage sites with a complex thermal history. The calculated maximum hoop stress is in good agreement with the reported experimental result. The estimated optical retardance profiles from the calculated radial and hoop stress fields are consistent with the photoelastic measurements. These results provide sufficient evidence to demonstrate the suitability of the proposed model for describing the residual stresses of mitigated fused silica damage sites after CO2 laser annealing.

Synthesis and Characterization of Organo-Soluble Polyimides Based on Polycondensation Chemistry of Fluorene-Containing Dianhydride and Amide-Bridged Diamines with Good Optical Transparency and Glass Transition Temperatures over 400 °C.

Polymeric optical films with light colors, good optical transparency and high thermal resistance have gained increasing attention in advanced optoelectronic areas in recent years. However, it is somewhat inter-conflicting for achieving the good optical properties to the conventional thermal resistant polymers, such as the standard aromatic polyimide (PI) films, which are well known for the excellent combined properties and also the deep colors. In this work, a series of wholly aromatic PI films were prepared via the polycondensation chemistry of one fluorene-containing dianhydride, 9,9-bis(3,4-dicarboxyphenyl)fluorene dianhydride (FDAn) and several aromatic diamines with amide linkages in the main chain, including 9,9-bis [4-(4-aminobenzamide)phenyl]fluorene (FDAADA), 2,2'-bis(trifluoromethyl)-4,4'-bis[4-(4-aminobenzamide)] biphenyl (ABTFMB), and 2,2'-bis(trifluoromethyl)-4,4'-bis[4-(4-amino-3-methyl)benzamide] biphenyl (MABTFMB). The derived FLPI-1 (FDAn-FDAADA), FLPI-2 (FDAn-ABTFMB) and FLPI-3 (FDAn-MABTFMB) resins showed good solubility in the polar aprotic solvents, such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAc) and dimethyl sulfoxide (DMSO). The solution-processing FDAn-PI films exhibited good optical transmittance over 80.0% at a wavelength of 500 nm (T500), yellow indices (b*) in the range of 1.01-5.20, and haze values lower than 1.0%. In addition, the FDAn-PI films showed low optical retardance with optical retardation (Rth) values in the range of 31.7-390.6 nm. At the same time, the FDAn-PI films exhibited extremely high glass transition temperatures (Tg) over 420 °C according to dynamic mechanical analysis (DMA) tests. The FDAn-PI films showed good dimensional stability at elevated temperatures with linear coefficients of thermal expansion (CTE) in the range of (31.8-45.8) × 10-6/K.

Design, Synthesis and Properties of Semi-Alicyclic Colorless and Transparent Polyimide Films with High Glass Transition Temperatures and Low Retardation for Potential Applications in Flexible Electronics.

Polyimide (PI) optical films with high glass transition temperatures (high-Tg), high optical transparency, and low optical retardations (low-Rth) are highly desired in advanced optoelectronic applications. However, the standard PI films usually suffer from deep colors, high optical anisotropies and limited Tg values. In the current work, a series of semi-alicyclic colorless and transparent PI (CPI) films were developed from hydrogenated pyromellitic dianhydride stereoisomers, 1S,2R,4S,5R-hydrogenated pyromellitic dianhydride and 1R,2S,4S,5R-hydrogenated pyromellitic dianhydride, and fluorene-containing diamines, including 9,9-bis(4-aminophenyl)fluorene and 9,9-bis(3-fluoro-4-aminophenyl)fluorene, respectively. The derived CPI films showed Tg values higher than 420 °C according to differential scanning calorimetry measurements. In addition, the fluorene-based CPI film showed optical transmittances higher than 80% at the wavelength of 400 nm, with yellow indices in the range of 0.60~1.01 and haze values below 3.0%. The CPI films showed average refractive indices from 1.5407 to 1.6309, extremely low birefringence at the level of minus fourth power of ten, and further exhibited quite low optical retardations below 10 nm.

Negative biaxial retarder made from reactive molecules with X-shaped core structure

Although biaxial retarder films have merits of optical applications, limited numbers of the biaxial retarder have been developed using reactive mesogen (RM) materials. New X-shaped RM (XRM) molecules were synthesized and optical retarder films were fabricated using mixtures of a rodlike RM and the XRM. The in-plane retardation, the out-of-plane retardation and the NZ coefficient of the retarder films were investigated. The retarder films represented negative biaxial property or z-plate property depending on the chemical structure and the concentration of XRM.

Birefringence analysis of collagen supraorganization in cat corneas with tropical keratopathy.

To evaluate the birefringent properties of the cornea and examine the supraorganizational aspects of collagen fibers in cats with tropical keratopathy. In this study, 10-micrometer-thick sections of corneal tissue from cats with tropical keratopathy were examined, both in the opaque and transparent areas of the anterior stroma. Control samples were obtained from healthy cat corneas. Polarized light microscopy was employed to evaluate the birefringent properties using two distinct methods. The first method involved measuring the optical retardation associated with corneal birefringence, while the second method assessed the alignment/waviness of the birefringent collagen fibers. Differences were significant when p < .05. Tropical keratopathy resulted in a significant rise (p < .05) in optical retardation in both opaque and transparent regions of the cat cornea. In the anterior stroma, both the opaque zones and transparent tissue exhibited a higher degree of collagen fiber packing than the control corneas. However, no significant differences (p > .05) in alignment were observed between the transparent tissue of the diseased cornea and the healthy corneas. Supraorganizational changes in collagen fiber packing are not restricted to lesion zones in cat corneas affected by tropical keratopathy. Such alterations also occur in the corneal tissue of the anterior stroma adjoining the lesions. Therefore, it is plausible that the transparent tissue of the anterior stroma in corneas affected by the disease may have functional abnormalities, despite its macroscopic healthy appearance. Additional investigations are required to clarify the implications of these potential defects and their conceivable contribution to tropical keratopathy.