Paraxial Analysis of Binary Polarization Gratings (bin-PGs)

We propose specially designed double anisotropic polarization diffraction gratings capable of producing a selective number of diffraction orders and with selective different states of polarization. Different polarization diffraction gratings are demonstrated, including linear polarization with horizontal, vertical and ± 45° orientations, and circular R and L polarization outputs. When illuminated with an arbitrary state of polarization, the system acts as a complete polarimeter where the intensities of the diffraction orders allow measurement of the Stokes parameters with a single shot. Experimental proof-of-concept is presented using a parallel-aligned liquid crystal display operating in a double pass architecture.

In the context of automated analysis of eye fundus images, it is an important common fallacy that prior works achieve very high scores in segmentation of lesions, and that fallacy is fueled by some reviews reporting very high scores, and perhaps some confusion with terms. A simple analysis of the detail of the few prior works that really do segmentation reveals scores between 7% and 70% in sensitivity for 1 FPI. That is clearly sub-par with medical doctors trained to detect signs of Diabetic Retinopathy, since they can distinguish well the contours of lesions in Eye Fundus Images (EFI). Still, a full segmentation of lesions could be an important step for both visualization and further automated analysis using rigorous quantification or areas and numbers of lesions to better diagnose. I discuss what prior work really does, using evidence-based analysis, and confront with segmentation networks, comparing on the terms used by prior work to show that the best performing segmentation network outperforms those prior works. I also compare architectures to understand how the network architecture influences the results. I conclude that, with the correct architecture and tuning, the semantic segmentation network improves up to 20 percentage points over prior work in the real task of segmentation of lesions. I also conclude that the network architecture and optimizations are important factors and that there are still important limitations in current work.

We found that the surface relief grating(SRG)made the nematic liquid crystal(LC)align along the grating direction. The SRGs were photo-fabricated on a liquid crystalline azobenzene side-chain polymer film by the conventional holographic method. The twisted nematic liquid crystal(TNLC)cell was prepared using two SRG substrates or two rubbed polyimide substrates as alignment layers, and the LC alignment was investigated by measuring the transmittance of a 90°-TNLC cell located between crossed polarizers. The LC aligning process was studied in-situ using the hybrid LC cell with the rubbed polyimide substrate and the SRG substrate as the reference and the test alignment layers, respectively. The in-situ transmission data show that the topological effect by the SRG formation and the intermolecular interaction effect by the azo chromophore alignment compete with each other.

Polarization gratings have been recorded in a combined liquid crystal (LC) cell made of a substrate covered with a photosensitive chalcogenide orientation layer and a reference substrate covered with a rubbed polyimide film. The gratings are formed due to the spatially modulated light-induced easy orientation axis on the chalcogenide surface recorded by two beams with opposite circular polarizations. The gratings are permanent, but they can be erased by one of the recording beams and re-recorded. The diffraction intensity of the circularly polarized light is achromatic and does not depend on the birefringence of the LC. The diffraction efficiency of the grating is of the order of a few percents. Application of an ac field causes a strong increase of the diffraction efficiency up to 45%.

Fast switching beam steering based on ferroelectric liquid crystal phase shutter and polarisation grating

A longest repeat query on a string, motivated by its applications in many subfields including computational biology, asks for the longest repetitive substring(s) covering a particular string position (point query). In this paper, we extend the longest repeat query from point query to \emph{interval query}, allowing the search for longest repeat(s) covering any position interval, and thus significantly improve the usability of the solution. Our method for interval query takes a different approach using the insight from a recent work on \emph{shortest unique substrings} [1], as the prior work's approach for point query becomes infeasible in the setting of interval query. Using the critical insight from [1], we propose an indexing structure, which can be constructed in the optimal $O(n)$ time and space for a string of size $n$, such that any future interval query can be answered in $O(1)$ time. Further, our solution can find \emph{all} longest repeats covering any given interval using optimal $O(occ)$ time, where $occ$ is the number of longest repeats covering that given interval, whereas the prior $O(n)$-time and space work can find only one candidate for each point query. Experiments with real-world biological data show that our proposal is competitive with prior works, both time and space wise, while providing with the new functionality of interval queries as opposed to point queries provided by prior works.

Highly efficient and functionalized polarization gratings have been recorded in azobenzene-containing mesogenic composites with twisted nematic cell configurations. The polarization gratings formed in azobenzene-containing mesogenic composites show a high diffraction efficiency of more than 45% and convert the polarization state of light at the same time. The polarization direction of the diffracted laser beams can be controlled by the twisted angle of the nematic cell. These characteristics of the polarization gratings are well explained by means of Jones calculus.

Thin circular polarization gratings, characterized by high diffraction efficiency and large, up to 42°, diffraction angles were created by polarization holography for the first time. The high efficiency of the gratings is the result of the specific properties of a photo-crosslinkable liquid crystalline polymer and a two-step photochemical/thermal processing procedure. A diffraction efficiency of up to 98% at 532 nm has been achieved for gratings with periods of 700 nm. In contrast to polarization gratings with larger periods these gratings exhibit Bragg properties. So one beam is either transmitted or diffracted depending on the direction of the circular polarization of the incident light, whereas the maximal diffraction efficiency is achieved only at the proper incident angle. The fabrication procedure consists of holographic exposure of the film at room temperature which provides the photo-selective cycloaddition of cinnamic ester groups. Upon subsequent thermal annealing above Tg bulk photo-alignment of the LC polymer film occurs enhancing the optical anisotropy within the grating. The holographic patterning provides high spatial resolution, the arbitrary orientation of the LC director as well as high optical quality, thermal and chemical stability of the final gratings. Highly efficient symmetric and slanted circular polarization gratings were fabricated with the proposed technique.

Polarization holographic characteristics of TI/PMMA polymers by linearly polarized exposure

A series of spacer-free ionic azobenzene-containing stoichiometric complexes was prepared from monosulfonated azo dyes and cationic polyelectrolytes [methylated poly(4-vinylpyridine) (PVP) and poly(dimethylaminoethyl methacrylate) (PDM)]. Their thermal and structural characteristics and optical responses, particularly photoinduced birefringence (PIB) and surface relief grating (SRG) inscription, were investigated as a function of selected molecular parameters. All of the complexes have high apparent Tg values, 180−210 °C, and show liquid crystal (LC) order of the single-layer SmA type from ambient to very high temperature, usually to degradation. The range of LC order is greater in the complexes of chromophores with longer alkyl tails and shorter in the complex of the chromophore with an OH end group and the complex of methyl orange (MO) with PDM. PIB, SRG quality, and diffraction efficiency were all shown to depend in a similar way on molecular structural features: the more rigid the molecular structure,...

We demonstrate a broadband, thin-film, polarizing beam splitter based on an anisotropic diffraction grating composed of reactive mesogens (polymerizable liquid crystals). This achromatic polarization grating (PG) manifests high diffraction efficiency (approximately 100%) and high extinction ratio (> or = 1000:1) in both theory and experiment. We show an operational bandwidth Deltalambda/lambda0 approximately 56% (roughly spanning visible wavelength range) that represents more than a fourfold increase of bandwidth over conventional PGs (and significantly larger than any other grating). The diffraction angle and operational region (visible, near-infrared, midwave infrared, and ultraviolet wavelengths) may be easily tailored during fabrication. The essence of the achromatic design is a stack of two chiral PGs with an opposite twist sense and employs the principle of retardation compensation. We fully characterize its optical properties and derive the theoretical diffraction behavior.

Surface-relief gratings made with anisotropic materials are finding more applications. An example is grooved magneto-optic disks as data storage media. The present work is a reformulation of the couple-wave method, for solving the anisotropic grating problem, that is described in Refs. 1-3. [Since the method essentially is a modal method relying on expanding both the electromagnetic fields and the permittivity function into Fourier series, here it is referred to as the Fourier modal method (FMM).] It originated from the work documented in Refs. 4-7. Recently Lalanne and Morris4, and Granet and Guizal5 simultaneously reformulated the conventional FMM for isotropic gratings in TM polarization. As a result, the convergence of the method for highly conducting metallic gratings was greatly improved. Auslender and Hava6 also reported the same reformulation. The findings of these authors were mathematically justified and summarized in the form of three Fourier factorization rules7. The use of these factorization rules has led to improvement of convergence in two other cases: the C method for gratings with sharp edges8 and the FMM for crossed gratings.9 This conference paper briefly reports yet another successful application of the factorization rules. A detailed exposition will soon appear elsewhere.10

A Scanning Near-field Optical Microscope (SNOM) was used to inscribe locally limited Surface Relief Gratings (SRGs) on pDR1M, which is an azo-polymer film. The influence of parameters such as the line scanning time and the line spacing on the amplitude and the period of the gratings is discussed. SRGs have been used to orient Liquid Crystals (LCs), where the LC alignment takes place due to the elastic distortions of the nematic medium close to the surface. In addition, the photoalignment of the azo-chromophores was also used to induce LC alignment. In this work we investigate the use of local SGRs together with the photoalignment to get a local control of the LC alignment properties. The advantage of this method is that it is reversible and the gratings can easily be erased.

The high coherence of laser light sources is a key to the application of diffractive optics usable in holographic AR/VR displays. This can be combined with switchable diffractive elements, which are advantageous for several optical functions used in immersive holographic displays such as shutters, polarization filters, for rapid beam deflection and selection. We demonstrate a compact, effective and robust diffraction wide-angle switchable beam-deflecting device based on circular polarization gratings possessing Bragg-performances (Bragg-PG) and a polarization switch. Such grating/polarization switch pair may, for instance, be a discrete switchable deflection element or as a switching element for pre-deflection with field lenses for application in holographic AR/VR displays. Micrometer-thick circular polarization gratings characterized by high diffraction efficiency (DE > 95%), large diffraction angles ( 95% in the vis spectral range) and stable symmetric and slanted circular Bragg polarization gratings were fabricated using the developed material and processing technique. The high usable diffraction angles combined with high DE make the Bragg-PG attractive for HMD AR/VR applications because of the system inherent short focus lengths and large numerical apertures needed to meet the low space budget in HMD and other optical systems.

Existing machine learning algorithms for human information inference are typically data-driven models trained on carefully labeled datasets. Given the significant labeling effort, traditional pure data-driven approaches are challenging to implement for emerging smart applications requiring long-term finer-grained information. Taking activities of daily life (ADL) tracking for elders as an example, prior work mostly focused on context-level information learning such as cooking and cleaning. [8]. However, new applications such as evaluating elders' cognitive impairments progress by tracking their ADL engagement requires finer-grained, i.e., action-level information [7]. In practice, labeling the day-length data at such granularity can be very expensive and requires a lot of human efforts [9]. My research focuses on the inference problems in the scope of human physical condition monitoring and activity recognition with limited labeled data. To alleviate the effort of labeling large amounts of data, prior works on semi-supervised learning combine a small amount of labeled data with a large amount of unlabeled data to train the model. However, as the label granularity (number of classes) increasing, the difficulty to distinguish nuance distinctions between finer-grained classes escalates as well. This makes training a robust semi-supervised model for finer-grained classification with less labels difficult if not impossible. Fortunately, coarse-grained (context-level) labels is usually available or cheaper to obtain in practice. In this case, the multi-granularity hierarchy between finer and coarse labels follows the aggregation relation defined in [5]. This hierarchical relation can be leveraged in the tasks of inferring finer-grained information. In addition, it is illustrated by the previous study that co-located multi modality sensing systems capture complementary aspects of the same event [6]. The research question I focus on is how to infer finer-grained human information with coarse-grained labeled data leveraging complementary multi-modal sensing? I target three directions: 1) a cross-granularity semi-supervised setting: how to utilize coarse-grained labeled data with a small amount of finer-grained labeled data to infer finer-grained human information, 2) cross-granularity relationship learning: how to learn the multi-granularity class hierarchy from data and further help the finer-grained human information acquisition, 3) enhancing inference granularity by leveraging multi-model sensing: how to leverage the complimentary co-located multiple sensing modalities to accurately infer finer-grained human information?