High-efficiency broadband achromatic metalenses for visible full-stokes polarization imaging.

Polarization-imaging technology has important applications in target detection, communication, biomedicine, and other fields. A polarization imaging system based on metalenses, which provides new possibilities for the realization of highly integrated full-Stokes polarization imaging systems, can solve the problems of traditional polarization imaging systems, such as complex structures, large volumes, and the inability to simultaneously obtain linear and circular polarization states. However, currently designed metalens arrays that can achieve real-time full-Stokes polarization imaging can generally only be used for monochromatic detection, which significantly limits the amount of measured information of the object. Broad-spectrum polarization color imaging allows more image degrees of freedom, enabling more accurate characterization of polarization for multi-target object scenes in complex environments. To achieve broad-spectrum polarization imaging, we propose and design a metalens array that can achieve full-Stokes polarization imaging in the broadband visible range, in which the design process of metalenses for splitting and focusing broadband orthogonal circularly polarized light is emphasized. To design metalenses that can achieve polarization splitting and efficient focusing, we simulate and optimize the height and period of the nano-units and show that smaller periods and larger heights do not always result in higher-performance devices when designing multifunctional metalenses. The designed metalens array can split and diffraction-limited focus the orthogonal polarized incident light to the designated position with average focusing efficiencies of 59.2% under 460-680 nm TM linearly polarized light, 53.1% under TE linearly polarized light, 58.8% under left-handed circularly polarized light, and 52.7% under right-handed circularly polarized light. The designed metalenses can be applied to imaging systems, such as polarization imaging and polarization light-field imaging systems.

Photonic thermal switch via metamaterials made of vanadium dioxide-coated nanoparticles.

In this work, a photonic thermal switch is proposed based on the phase-change material vanadium dioxide (VO2) within the framework of near-field radiative heat transfer (NFRHT). The switch consists of two metamaterials filled with core-shell nanoparticles, with the shell made of VO2. Compared to traditional VO2 slabs, the proposed switch exhibits a more than two times increase in the switching ratio, reaching as high as 90.29% with a 100 nm vacuum gap. The improved switching effect is attributed to the capability of the VO2 shell to couple with the core, greatly enhancing heat transfer with the insulating VO2, while blocking the motivation of the core in the metallic state of VO2. The proposed switch opens pathways for active control of NFRHT and holds practical significance for developing thermal photon-based logic circuits.

Performance improvement of three-body radiative diodes driven by graphene surface plasmon polaritons.

As an analogue to an electrical diode, a radiative thermal diode allows radiation to transfer more efficiently in one direction than in the opposite direction by operating in a contactless mode. In this study, we demonstrated that within the framework of three-body photon thermal tunneling, the rectification performance of a three-body radiative diode can be greatly improved by bringing graphene into the system. The system is composed of three parallel slabs, with the hot and cold terminals of the diode coated with graphene films and the intermediate body made of vanadium dioxide (VO2). The rectification factor of the proposed radiative thermal diode reaches 300% with a 350 nm separation distance between the hot and cold terminals of the diode. With the help of graphene, the rectification performance of the radiative thermal diode can be improved by over 11 times. By analyzing the spectral heat flux and energy transmission coefficients, it was found that the improved performance is primarily attributed to the surface plasmon polaritons (SPPs) of graphene. They excite the modes of insulating VO2 in the forward-biased scenario by forming strongly coupled modes between graphene and VO2 and thus dramatically enhance the heat flux. However, for the reverse-biased scenario, the VO2 is at its metallic state, and thus, graphene SPPs cannot work by three-body photon thermal tunneling. Furthermore, the improvement was also investigated for different chemical potentials of graphene and geometric parameters of the three-body system. Our findings demonstrate the feasibility of using thermal-photon-based logical circuits, creating radiation-based communication technology and implementing thermal management approaches at the nanoscale.

Mechanism of polaritons coupling from perspective of equivalent MLC circuits model in slit arrays.

Magnetic polariton is a significant mode in tailoring thermal radiative properties with micro/nanostructures metamaterials and can be explained by equivalent inductor-capacitor circuit model. However, the equivalent inductor-capacitor circuit model is out of operation when the magnetic polariton resonance frequency is close to the surface plasmon polariton excitation frequency and cases of oblique incidence. In this work, we present a mutual inductor-inductor-capacitor circuit model to describe magnetic polariton resonance conditions. The mechanism of coupling between the surface plasmon polariton and magnetic polariton is explained from the perspective of equivalent circuits. The interaction between the surface plasmon polariton and magnetic polariton is studied and considered as a mutual inductance in the MLC circuit model. This model is still applicable in the case of oblique incidence. Slit arrays with different geometric parameters and incident angles are calculated to verify the rationality of the mutual inductor-inductor-capacitor circuit model. This study may allow us to predict features and parameters and achieve tailoring of the thermal radiative properties of the micro/nano-structures metamaterials.

High-Accuracy Correction of a Microlens Array for Plenoptic Imaging Sensors.

Microlens array (MLA) errors in plenoptic cameras can cause the confusion or mismatching of 4D spatio-angular information in the image space, significantly affecting the accuracy and efficiency of target reconstruction. In this paper, we present a high-accuracy correction method for light fields distorted by MLA errors. Subpixel feature points are extracted from the microlens subimages of a raw image to obtain correction matrices and perform registration of the corresponding subimages at a subpixel level. The proposed method is applied for correcting MLA errors of two different categories in light-field images, namely form errors and orientation errors. Experimental results show that the proposed method can rectify the geometric and intensity distortions of raw images accurately and improve the quality of light-field refocusing. Qualitative and quantitative comparisons between images before and after correction verify the performance of our method in terms of accuracy, stability, and adaptability.

Tunable absorption as multi-wavelength at infrared on graphene/hBN/Al grating structure.

This work designs a graphene/hBN/Al grating anisotropic hybrid structure. Formed by strong coupling between plasmonic Magnetic polaritons (MPs) in the metal grating and phonon-plasmon polaritons, hybrid hyperbolic phonon-plasmon polaritons in the graphene/hBN film have been excited, resulting in three sharp, high absorption peaks, which are 0.75, 0.97 and 0.97, formed at 5.92 µm, 6.32 µm, and 7.64 µm respectively. The absorption mechanisms have been theoretically analyzed. Local electromagnetic field and power dissipation density are depicted for further elucidating the underlying mechanisms. The different structural parameters and chemical potential, which affect the absorption peak were discussed. These numerical results can provide potential application in the field of optical detection and optoelectronic.

Flame temperature estimation from light field image processing.

We propose a novel flame temperature estimation method based on a flame light field sectioned imaging model of complex temperature distribution in different media. The proposed method relies on multi-pixel reconstruction to improve the resolution of sub-aperture images. In addition, the wavelet transform denoises the flame refocused image, and then the Lucy-Richardson algorithm deconvolves the image. The temperature estimation accuracy using the proposed method is higher than that reported in a previous work, with a larger temperature estimation range from 1250 K to 1800 K. Moreover, we found that deconvolution plays an important role in determining the temperature estimation accuracy.

Simple and fast approach to exploit the spectral reflection properties of liquid media.

Bidirectional reflectance distribution functions (BRDFs) are of importance for their wide applications. In this study, we presented a simple and fast approach to measure the spectral BRDF of both solid and liquid samples. Based on this approach, we fabricated a prototype and measured the BRDF value of some liquid samples such as water and NaCl solution at different wavelengths. According to the experimental data, we discussed the trend of the BRDF value of the NaCl solution of different concentrations. Then, the experimental data of the different NaCl solution at 637 nm were used to invert the parameters of a five-parameter model. Additionally, we fitted the parameters as a polynomial.

Rectification of Images Distorted by Microlens Array Errors in Plenoptic Cameras.

A plenoptic cameras is a sensor that records the 4D light-field distribution of target scenes. The surface errors of a microlens array (MLA) can cause the degradation and distortion of the raw image captured by a plenoptic camera, resulting in the confusion or loss of light-field information. To address this issue, we propose a method for the local rectification of distorted images using white light-field images. The method consists of microlens center calibration, geometric rectification, and grayscale rectification. The scope of its application to different sized errors and the rectification accuracy of three basic surface errors, including the overall accuracy and the local accuracy, are analyzed through simulation of imaging experiments. The rectified images have a significant improvement in quality, demonstrating the provision of precise light-field data for reconstruction of real objects.

Assessment of column aerosol optical properties using ground-based sun-photometer at urban Harbin, Northeast China.

Aerosol plays a key role in determining radiative balance, regional climate and human health. Severe air pollution over Northeast China in recent years urges more comprehensive studies to figure out the adverse effects caused by excessive aerosols. In this study, column aerosol measurements over urban Harbin, a metropolis located at the highest latitude in Northeast China, during May 2016 to March 2017 were conducted using a CIMEL sun-photometer to analyze local aerosol properties and its variation from different aspects. According to the observations, aerosol optical depth at 440nm (AOD440) ranges from 0.07 up to 1.54, and the large variability in both AOD440 and Angstrom Exponent (AE440-870) indicates the frequent change of aerosol types due of different emission sources. Coarse mode particles dominated Harbin during the studying period because of the long-range transported dust and probably the suspended snow crystals in winter. As the wavelength increases, relatively consistent decrease trends of single scattering albedo (SSA) and asymmetry factor (ASY) were observed in spring, autumn, and winter, indicating the presence of absorbing polluted aerosols. Mixed type (MIX) aerosol dominated the study region with a total percentage of 34%, and biomass burning and urban industry (BB/UI), clean continental (CC), and desert dust (DD) aerosols were found to be 31%, 27%, and 8%, respectively. The current work fills up the optical characteristics of aerosols in Harbin, and will contribute to the in-depth understanding of local aerosol variation and regional climate change over Northeast China.

Transient polarized radiative transfer analysis in a scattering medium by a discontinuous finite element method.

Transient (time-dependent) polarized radiative transfer in a scattering medium exposed to an external collimated beam illumination is conducted based on the time-dependent polarized radiative transfer theory. The transient term, which persists the nanosecond order time and cannot be ignored for the time-dependent radiative transfer problems induced by a short-pulsed beam, is considered as well as the polarization effect of the radiation. A discontinuous finite element method (DFEM) is developed for the transient vector radiative transfer problem and the derivation of the discrete form of the governing equation is presented. The correctness of the developed DFEM is first verified by comparing the DFEM solutions with the results from the literature. The DFEM is then applied to study the transient polarized radiative transfer induced by a pulsed beam. The time-dependent Stokes vector components are calculated, plotted and analyzed as functions of the axis coordinate and discrete direction. Effects of the diffuse/specular boundary and the incident beam polarization state with respect to the Stokes vector components are further analyzed for cases of different boundary reflection modes and incident beam illuminations.

Nanoparticle-crystal towards an absorbing meta-coating.

In this paper, a double layer nanoparticle-crystal has been proposed, which shown incident and polarization angle, substrate independences for spectral absorptivity. Such phenomenon originates from the near-field light redistribution and excitation of internal collective oscillating. This kind of nanoparticle-crystal can be made of various types of metal with similar optical responses. A three oscillators mode has been proposed in this paper to understand the shift between global and internal collective oscillating, and verify the physical picture demonstrated. That kind of near-field redistribution result in a prototype of novel meta-coating, and facilitates the large scale application of metamaterial.

Graphene plasmonics for surface enhancement near-infrared absorptivity.

Monolayer graphene has poor absorption in the near-infrared region. Its layer is only as thick as a single atom so it cannot have a high absorptivity. In this paper, in order to form a hybrid system, the absorption characteristics of monolayer graphene covering a metal/dielectric/metal substrate has been theoretically analyzed. The magnetic polaritons in the metal/dielectric couple with the plasmonic resonance in the graphene to dramatically enhance the graphene absorptivity. This study analyzes the factors that enhance the absorptivity, including the geometric parameters and the relative positions of the graphene. The local electromagnetic field and the power dissipation density are illustrated to explain the underlying mechanisms further. These numerical results can provide potential application in the field of optical detection and optoelectronic devices.

Responses transition in a monolayer Al-Al2O3 nanoparticle-crystal due to oxidation.

Nanoparticle is a promising candidate for large scale fabrication of metamaterial. However, optical responses for metamaterial made of abound metal like Al can be thoroughly changed due to oxidization. Especially for nanoparticle whose aspect ratio is extremely high, oxidation usually occurs. So to understand how the responses shift in a nanoparticle system due to oxidization is essential for large scale application of metamaterial. In this paper, we have concluded and quantified two general principles describing this transition in a monolayer Al-Al2O3 nanoparticle-crystal, which can be used in a thermophotovoltaic system. Square pattern, in which the unit of changing crystal is a square cell made up of Al and Al2O3 particles, is firstly demonstrated. A double oscillators model has been proposed to understand the interference between different absorption modes and their coupling. Using near-field distribution, equivalent inductor-capacitor model and dispersion relationship of surface Plasmon polariton, we have distinguished the resonance modes, concluded the transition principles in a simple case. Then the two principles are applied in a larger cell to verify its university. After detailed demonstration of symmetric square pattern, models and principles are extrapolated to more complex non-symmetric systems. The basic understanding gained here will help the design of robust large-scale metamaterial.

Transient/time-dependent radiative transfer in a two-dimensional scattering medium considering the polarization effect.

Transient/time-dependent radiative transfer in a two-dimensional scattering medium is numerically solved by the discontinuous finite element method (DFEM). The time-dependent term of the transient vector radiative transfer equation is discretized by the second-order central difference scheme and the space domain is discretized into non-overlapping quadrilateral elements by using the discontinuous finite element approach. The accuracy of the transient DFEM model for the radiative transfer equation considering the polarization effect is verified by comparing the time-resolved Stokes vector component distributions against the steady solutions for a polarized radiative transfer problem in a two-dimensional rectangular enclosure filled with a scattering medium. The transient polarized radiative transfer problems in a scattering medium exposed to an external beam and in an irregular emitting medium are solved. The distributions of the time-resolved Stokes vector components are presented and discussed.

Multi-focused microlens array optimization and light field imaging study based on Monte Carlo method.

Plenoptic cameras are used for capturing flames in studies of high-temperature phenomena. However, simulations of plenoptic camera models can be used prior to the experiment improve experimental efficiency and reduce cost. In this work, microlens arrays, which are based on the established light field camera model, are optimized into a hexagonal structure with three types of microlenses. With this improved plenoptic camera model, light field imaging of static objects and flame are simulated using the calibrated parameters of the Raytrix camera (R29). The optimized models improve the image resolution, imaging screen utilization, and shooting range of depth of field.

Lattice Boltzmann method for one-dimensional vector radiative transfer.

A one-dimensional vector radiative transfer (VRT) model based on lattice Boltzmann method (LBM) that considers polarization using four Stokes parameters is developed. The angular space is discretized by the discrete-ordinates approach, and the spatial discretization is conducted by LBM. LBM has such attractive properties as simple calculation procedure, straightforward and efficient handing of boundary conditions, and capability of stable and accurate simulation. To validate the performance of LBM for vector radiative transfer, four various test problems are examined. The first case investigates the non-scattering thermal-emitting atmosphere with no external collimated solar. For the other three cases, the external collimated solar and three different scattering types are considered. Particularly, the LBM is extended to solve VRT in the atmospheric aerosol system where the scattering function contains singularities and the hemisphere space distributions for the Stokes vector are presented and discussed. The accuracy and computational efficiency of this algorithm are discussed. Numerical results show that the LBM is accurate, flexible and effective to solve one-dimensional polarized radiative transfer problems.

Correction model for microlens array assembly error in light field camera.

In a light field camera, a microlens array (MLA) assembly error can affect the quality of the image. In this study, aiming to ensure corrective imaging using a light field camera, we accurately evaluate and eliminate the assembly error. We used an error image and a standard image to confirm the MLA assembly error, and we developed an assembly error correction model combined with an image quality evaluation index to correct the error. The proposed error correction model can be employed for various assembly errors and different error ranges. Quantitative analyses are performed for these different scenarios. The proposed model can be applied in accurate imaging using a light field camera, four-dimensional optical radiation field information reconstitution, MLA manufacturing and assembly processes, etc.

Three-dimensional temperature field measurement of flame using a single light field camera.

Compared with conventional camera, the light field camera takes the advantage of being capable of recording the direction and intensity information of each ray projected onto the CCD (charge couple device) sensor simultaneously. In this paper, a novel method is proposed for reconstructing three-dimensional (3-D) temperature field of a flame based on a single light field camera. A radiative imaging of a single light field camera is also modeled for the flame. In this model, the principal ray represents the beam projected onto the pixel of the CCD sensor. The radiation direction of the ray from the flame outside the camera is obtained according to thin lens equation based on geometrical optics. The intensities of the principal rays recorded by the pixels on the CCD sensor are mathematically modeled based on radiative transfer equation. The temperature distribution of the flame is then reconstructed by solving the mathematical model through the use of least square QR-factorization algorithm (LSQR). The numerical simulations and experiments are carried out to investigate the validity of the proposed method. The results presented in this study show that the proposed method is capable of reconstructing the 3-D temperature field of a flame.

Direct wavefront manipulating for a transverse electric wave microlens.

Due to the polarization nature of the transverse electric electromagnetic wave, manipulating it has been a difficult task and can be even more challenging for integrated on-chip optics. In this Letter, a transverse electric wave manipulating method based on direct wavefront bending and its physical picture have been proposed. Even with only five cells, the microlens can exhibit a focusing pattern and retrieve sub-wavelength spatial features. An analytical mode has been proposed to help understand the physical picture and verify the result. This Letter facilitates the basic understanding for transverse electric wave manipulating and the design of integrated optical elements.