Polarization-resolved Imaging Research Articles

Super-oscillatory spots with different inhomogeneous linear polarized states

We present the formation of super-oscillatory (SO) spots by tightly focusing the inhomogeneous linear polarized beam of different polarization states. At the entrance pupil of the focusing lens, a suitable phase manipulation in the incident beam results in a small super-oscillatory spot. Our numerical study based on the vectorial diffraction theory shows that SO spots of controllable size and various polarization combinations are possible. We also discuss the effect of the different polarization patterns of the incident beam on the size and energy distribution of the generated SO spots, which are potentially valuable for the orientation determination of single molecules and polarization-resolved imaging. This study reveals more influence of polarization states on the different components of the focused beam under the utilization of the proposed method rather than the usual tight focusing conditions.

Snapshot Mueller spectropolarimeter imager

We introduce an imaging system that can simultaneously record complete Mueller polarization responses for a set of wavelength channels in a single image capture. The division-of-focal-plane concept combines a multiplexed illumination scheme based on Fourier optics together with an integrated telescopic light-field imaging system. Polarization-resolved imaging is achieved using broadband nanostructured plasmonic polarizers as functional pinhole apertures. The recording of polarization and wavelength information on the image sensor is highly interpretable. We also develop a calibration approach based on a customized neural network architecture that can produce calibrated measurements in real-time. As a proof-of-concept demonstration, we use our calibrated system to accurately reconstruct a thin film thickness map from a four-inch wafer. We anticipate that our concept will have utility in metrology, machine vision, computational imaging, and optical computing platforms.

Polarization-Resolved THz Imaging With Orthogonal Heterostructure Backward Diode Detectors

A fully integrated lens-coupled dual-polarization detector for imaging was designed, fabricated, and characterized in the terahertz (THz) region. Two zero-bias heterostructure backward diodes (HBDs) were monolithically integrated into a planar dual-polarization annular-slot antenna. An impedance matching network consisting of an interdigitated DC block and shorted stubs was implemented for maximum power transfer. The fabricated chip was then mounted on an extended hemispherical silicon lens for high antenna efficiency. The performance of the integrated detector has been characterized at 200 GHz. The detector module can simultaneously measure the two orthogonal linear polarized components of the incident THz waves, and hence obtain the polarization direction. On the basis of this polarimetric detection, polarization-resolved imaging of a Gaussian beam, as well as birefringent samples, was performed using the integrated detectors. The angular resolution of the polarization detection has been demonstrated to be as small as 3°. The single-pixel detector demonstrated here is amenable to array applications for THz focal-plane arrays that will have a significant impact on a wide range of remote sensing, though-barrier imaging, and detection/identification applications.

Monolithic Metamaterial-Integrated Graphene Terahertz Photodetector with Wavelength and Polarization Selectivity.

The frequency spectra and polarization states of terahertz waves can convey significant information about physical interactions and material properties. Compact and miniaturized on-chip platforms for effective capturing of these quantities are being extensively investigated because of their promising potential for paramount applications of terahertz technology such as in situ sensing and characterization. Here, we present a metamaterial-graphene hybrid device that integrates the functions of photodetection, wavelength, and polarization selectivity into a monolithic architecture. Leveraging the ultrahigh design freedom of metamaterial optical properties and the electronically controllable hot-carrier-assisted photothermoelectric effect in graphene, our detector shows resonantly enhanced photoresponse at two specific target wavelengths with orthogonal polarizations. We demonstrate its versatile capabilities for spectrally selective and polarization resolved imaging on a single-chip platform that is free from advanced optical components. Our strategy is beneficial to the future development of multifunctional, compact, and low-cost terahertz sensors.

Polarization-based smoke removal method for surgical images.

Smoke generated during surgery affects tissue visibility and degrades image quality, affecting surgical decisions and limiting further image processing and analysis. Polarization is a fundamental property of light and polarization-resolved imaging has been studied and applied to general visibility restoration scenarios such as for smog or mist removal or in underwater environments. However, there is no related research or application for surgical smoke removal. Due to differences between surgical smoke and general haze scenarios, we propose an alternative imaging degradation model by redefining the form of the transmission parameters. The analysis of the propagation of polarized light interacting with the mixed medium of smoke and tissue is proposed to realize polarization-based smoke removal (visibility restoration). Theoretical analysis and observation of experimental data shows that the cross-polarized channel data generated by multiple scattering is less affected by smoke compared to the co-polarized channel. The polarization difference calculation for different color channels can estimate the model transmission parameters and reconstruct the image with restored visibility. Qualitative and quantitative comparison with alternative methods show that the polarization-based image smoke-removal method can effectively reduce the degradation of biomedical images caused by surgical smoke and partially restore the original degree of polarization of the samples.

Image phasor analysis in polarization-resolved optical scanning microscopy of neuroblastoma cells

Polarization-resolved scanning microscopy is a robust label-free technique based on light polarization effect on anisotropic materials. In our previous work, we have presented a distinguishing way to acquire extra information at the molecular level by merging the polarization-resolved imaging microscopy technique under an image phasor analysis. Indeed, the intuitive information out of the reciprocal spatial frequency space offered in a graphical way, coupled with a polarization analysis of the cell in-situ, probes changes of compaction in the chromatin image pixel-by-pixel.

Recognition of Bio-Structural Anisotropy by Polarization Resolved Imaging

In this paper, we develop a simple technique to identify material texture from far, by using polarization-resolved imaging. Such a technique can be easily implemented into industrial environments, where fast and cheap sensors are required. The technique has been applied to both isotropic references (Teflon bar) and anisotropic samples (wood). By studying the radiance of the samples illuminated by linearly polarized light, different and specific behaviours are identified for both isotropic and anisotropic samples, in terms of multipolar emission and linear dichroism, from which fibre orientation can be resolved.

Structures and topological defects in pressure-driven lyotropic chromonic liquid crystals

Lyotropic chromonic liquid crystals are water-based materials composed of self-assembled cylindrical aggregates. Their behavior under flow is poorly understood, and quantitatively resolving the optical retardance of the flowing liquid crystal has so far been limited by the imaging speed of current polarization-resolved imaging techniques. Here, we employ a single-shot quantitative polarization imaging method, termed polarized shearing interference microscopy, to quantify the spatial distribution and the dynamics of the structures emerging in nematic disodium cromoglycate solutions in a microfluidic channel. We show that pure-twist disclination loops nucleate in the bulk flow over a range of shear rates. These loops are elongated in the flow direction and exhibit a constant aspect ratio that is governed by the nonnegligible splay-bend anisotropy at the loop boundary. The size of the loops is set by the balance between nucleation forces and annihilation forces acting on the disclination. The fluctuations of the pure-twist disclination loops reflect the tumbling character of nematic disodium cromoglycate. Our study, including experiment, simulation, and scaling analysis, provides a comprehensive understanding of the structure and dynamics of pressure-driven lyotropic chromonic liquid crystals and might open new routes for using these materials to control assembly and flow of biological systems or particles in microfluidic devices.

Direct Generation of Radially Polarized Vector Vortex Beam with an Exciton-Polariton Laser

Vector vortex beams are a class of optical beams with singularities in their space-variant polarization. Vector vortex beam lasers have applications in many areas including imaging and communication, where vertical-cavity lasers emitting Gaussian beams have been most widely used so far. Generation of vector vortex beams from vertical-cavity lasers has required external control or modulation. Here, by utilizing a polarization-selective subwavelength grating as one of the reflectors in a vertical semiconductor microcavity, we design the spin textures of the polariton mode and demonstrate polariton lasing in a single-mode, radially polarized vector vortex beam. Polarization and phase distributions of the emission are characterized by polarization-resolved imaging and interferometry. This method of vector vortex laser beam generation allows low threshold power, stable single-mode operation, scalability, and on-chip integration, all of which are important for applications in imaging and communication.

Multitask deep-learning-based design of chiral plasmonic metamaterials

The field of chiral plasmonics has registered considerable progress with machine-learning (ML)-mediated metamaterial prototyping, drawing from the success of ML frameworks in other applications such as pattern and image recognition. Here, we present an end-to-end functional bidirectional deep-learning (DL) model for three-dimensional chiral metamaterial design and optimization. This ML model utilizes multitask joint learning features to recognize, generalize, and explore in detail the nontrivial relationship between the metamaterials’ geometry and their chiroptical response, eliminating the need for auxiliary networks or equivalent approaches to stabilize the physically relevant output. Our model efficiently realizes both forward and inverse retrieval tasks with great precision, offering a promising tool for iterative computational design tasks in complex physical systems. Finally, we explore the behavior of a sample ML-optimized structure in a practical application, assisting the sensing of biomolecular enantiomers. Other potential applications of our metastructure include photodetectors, polarization-resolved imaging, and circular dichroism (CD) spectroscopy, with our ML framework being applicable to a wider range of physical problems.

Spatial-Polarization-Independent Parametric Up-Conversion of Vectorially Structured Light

Spatial polarization independent (SPI) parametric conversion is the basis of many optical applications, such as SPI frequency interface for communication channels carried by vector modes and upconversion detection for polarization-resolved imaging. However, realizing such conversion remains a challenge. In this proof-of-principle work, we demonstrated SPI parametric upconversion using a polarization Sagnac nonlinear interferometer based on type-II second-harmonic generation (SHG). Our results show that the vector (including both polarization and intensity) profile and associated SOC state of the vector signal beam could be transferred to the SHG beam with a high fidelity. The principle lays a foundation of SPI frequency interface for quantum/classical channels based on vector modes and also paves the way for upconversion detection of polarization-resolved imaging in Mid-/far-infrared region.

Fluorescence polarization filtering for accurate single molecule localization

Single-Molecule Localization Microscopy (SMLM) has become one of the most important methods of super-resolution fluorescence microscopy. It is based on the precise localization of single molecules in wide-field microscopy images. It is well known that the localization accuracy can show a significant bias if the imaged molecules have a fixed orientation and are located either close to an interface or not exactly within the focal plane of the microscope. In this Letter, we propose a simple solution to this problem, which is based on polarization-resolved imaging. This method can be easily implemented into any existing SMLM setup, and we demonstrate its performance by imaging single dye molecules embedded into a polymer film, which fixes their orientation in space.

Identification, tracking, and sizing of nano-sized particles using dual-view polarization-resolved digital holography and T-matrix modeling

In this paper, we demonstrate how polarization-resolved holography can be used to determine if a particle is spherical or not and to estimate the size information of nanoparticles. The T-matrix method is used to model the scattered light from both spheres and spheroids. A dual-view polarization-resolved imaging system is used in order to obtain polarization ratio angles (β1,β2). From the obtained β1 and β2, it is possible to estimate whether or not a particle is spherical. It is found that nonsphericity only has a minor effect up to around sizes of 120 nm, and for that range, a spherical approximation is valid. For larger particles, the orientation influences the polarization response greatly. The size of a nonspherical particle can be estimated from the polarization ratio angles. The upper limit we can estimate unambiguously is around 200 nm. Finally, the model is applied to experimental measurements of naturally occurring particles in purified water. From the measurements, it is possible to separate spherical from nonspherical particles and also give a rough estimate of the size.

Development of a confocal scanning microscope for fluorescence imaging and spectroscopy at variable temperatures.

We developed and tested a confocal scanning optical microscope that fits into a thermally controlled, commercial research cryostat designed for operation from ambient temperature down to below 4 K. The home-built microscope is a fiber-coupled, self-contained instrument based on readily available mechanical and optical components. Its sample module is sealed in a protective stainless steel tube that minimizes vibrations caused by the flow of cryogenic gas. A high numerical aperture microscope objective specifically designed for cryogenic and high-vacuum applications focuses the excitation light onto the sample, while the core of an optical fiber attached to an avalanche photodiode acts as the confocal detection pinhole. The sample is displaced using a piezotube scanner mounted on top of a three-axis, low-temperature nanopositioner assembly for coarse sample positioning. A broadband polarizing cube beam splitter in the emission path allows for polarization-resolved imaging and spectroscopy. Fluorescence excitation scans are acquired with custom-written software that correlates fluorescence photon counts with the output from a high precision wavelength meter, which is part of a narrow-band, tunable dye laser setup. The imaging and spectral data acquisition capabilities of the microscope were confirmed using a variety of samples and excitation wavelengths at temperatures ranging from 5 K to room temperature.

Polarization Split Lensing via Polarization and Phase Control with Metasurfaces at Visible Frequencies

Under the trend of miniaturization and reduction of system complexity, conventional bulky photonic elements are expected to be replaced by new compact and multi-functional photonic elements. In this letter, we propose a birefringent dielectric metasurface (DM) that could provide both phase and polarization control at visible wavelength. Within this platform, we successfully realize a multi-functional polarization split metalens. Such a single metalens can perform the multiple functions of a polarizing resolve imaging system without additional optical elements. Polarization properties of light are resolved, an arbitrarily polarized incident beam is split into two orthogonally polarized beams, in which the transverse magnetic (TM) component and the transverse electric (TE) component are focused on the right and left side of the central axis, respectively. The metalens maintains diffraction-limited focal spots with high focusing efficiencies (above 80%). This multi-functional metalens may found potential applications in polarization-resolved imaging, spectroscopy, and on-chip optoelectronic devices.

Polarized super-resolution structural imaging inside amyloid fibrils using Thioflavine T

Thioflavin T (ThT) is standardly used as a fluorescent marker to detect aggregation of amyloid fibrils by conventional fluorescence microscopy, including polarization resolved imaging that brings information on the orientational order of the fibrils. These techniques are however diffraction limited and cannot provide fine structural details at the fibrils scales of 10–100 nm, which lie beyond the diffraction limit. In this work, we evaluate the capacity of ThT to photoswitch when bound to insulin amyloids by adjusting the redox properties of its environment. We demonstrate that on-off duty cycles, intensity and photostability of the ThT fluorescence emission under adequate buffer conditions permit stochastic super-resolution imaging with a localization precision close to 20 nm. We show moreover that signal to noise conditions allow polarized orientational imaging of single ThT molecules, which reveals ultra-structure signatures related to protofilaments twisting within amyloid fibrils.

Broadband ultrathin circular polarizer at visible and near-infrared wavelengths using a non-resonant characteristic in helically stacked nano-gratings.

Modern imaging and spectroscopy systems require to implement diverse functionalities with thin thickness and wide wavelength ranges. In order to meet this demand, polarization-resolved imaging has been widely investigated with integrated circular polarizers. However, the circular polarizers which operate at the entire visible wavelengths and have a thickness of several tens of nanometers have not been developed yet. Here, a circular polarizer, operating at the entire visible wavelength range, is demonstrated using helically stacked aluminum nano-grating layers. High extinction ratio and broad operation bandwidth are simultaneously achieved by using non-resonant anisotropic characteristics of the nano-grating. It is theoretically verified that the averaged extinction ratio becomes up to 8 over the entire visible wavelength range while having a thickness of 390 nm. Also, the feasibility of the proposed structure and circular polarization selectivity at the visible wavelength range are experimentally verified. It is expected that the proposed structure will lead to extreme miniaturization of a circular polarizer and contribute greatly to the development of mobile/wearable imaging systems such as virtual reality and augmented reality displays.

Hybrid Helix Metamaterials for Giant and Ultrawide Circular Dichroism

Circular polarization-resolved imaging and spectroscopy have versatile applications ranging from astronomy and biological and biomedical studies to remote sensing. To achieve real-time imagery with high contrast and rich information, integratable circular polarizers with high extinction ratios, broad operation bandwidths, and low insertion loss are urgently desired, while to our knowledge no structures proposed previously can balance these three vital parameters. In this paper, a concept of a hybrid helix metamaterial has been proposed to achieve ultrawide and giant circular dichroism with low insertion loss simultaneously. By coupling the localized and delocalized resonant modes, a broad and strong blocking band for unselected polarization can be realized with a pencil-like configuration. A giant extinction ratio of 5.9 × 105 can be obtained with a six-pitch hybrid single-helix array, which is almost 7 times larger than the best result reported by Behera et al. in 2015. A very high average extinction rat...

Polarization-Resolved Imaging for Both Photoelastic and Photoluminescence Characterization of Photovoltaic Silicon Wafers

The solar industry uses low-cost solidification processing methods to produce silicon wafer-based solar cells. The solidification processing can introduce crystalline defects and residual stresses in the wafers, which may impact the electrical performance and mechanical reliability of a finished solar cell. This paper presents a polarized infrared imaging system that achieves both infrared photoelastic analysis and polarized photoluminescence imaging. A polarization video-processing algorithm is used to resolve the polarization state of the detected photoelastic and photoluminescence signals and simultaneously reduce the noise. Defects in multi-crystalline silicon photovoltaic wafers are investigated using the polarized photoluminescence imaging setup, which can capture both the band-to-band and defect-band photoluminescence emission and spatially resolve the defect structures. The photoluminescence imaging results are qualitatively compared to the infrared photoelastic images to investigate the coupled electrical and mechanical properties of the defect structures. The technology described here creates a pathway to rapid full-field wafer quality inspection in a manufacturing setting and will help to improve wafer material processing.

Polarization-resolved terahertz imaging of intra- and inter-laminar damages in hybrid fiber-reinforced composite laminate subject to low-velocity impact

Terahertz imaging is applied to characterize a hybrid fiber-reinforced composite laminate subject to low-velocity impact in this study. The hybrid fiber-reinforced composite laminate comprises unidirectional glass/epoxy and carbon/epoxy plies with a cross-ply stack pattern. Both impact-induced intra- and inter-laminar damages are successfully detected, and the damage evolution throughout the thickness is also evaluated. The interaction between the terahertz polarization and carbon-fiber orientation is investigated in detail. Inter-laminar damage at the interface and the intra-laminar damage close to the same interface can be differentiated via polarization-resolved imaging. With a parameter fitting method based on multiple regression analysis, delamination is characterized quantitatively. Terahertz C- and B-scan images clearly exhibit the propagation of the damage from the top to the bottom surface in three dimensions.