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Examines a photopyroelectric method for measuring the thermal diffusivity of ambient gases with optimal precision. Sensor design; Improvement in the signal-to-noise ratio; Cavity-length scanning employed; Comparisons of measurement precision.

We report on an ultra-broadband metasurface perfect absorber in the visible and NIR region based on the triple Mie resonances generated from the three sets of nanopillars grown on SiO 2 substrate. The polarization-independent and wide-angular-insensitive (up to 70°) high absorption is realized within a surface layer that is only 284 nm thick. Double-beam UV interference lithography and followed sputter coating depositions are utilized to efficiently fabricate the proposed metasurface structures. Experimental results demonstrate that the average absorption is 0.961 (0.970 in simulation) at the wavelength band ranging from 400 nm to 1800 nm, and works well even the incident angle is large as 70°. The demonstrated device suggests an efficient way of designing and fabricating ultra-broadband perfect absorber with metasurface structures, which has huge application prospects in advanced nanophotonic devices and integrated photonic systems. • A polarization-independent, angular insensitive, ultra-broadband metasurface perfect absorber (MSPA) is demonstrated. • An average absorption of 0.970 at the wavelengths from 400 nm to 1800 nm is realized in a 284 nm thick structure. • The proposed MSPA is efficiently fabricated in large area. • Experimental results show that the average absorption at the wavelength band is 0.961. • The proposed MSPA works well even the incident angle is 70). [ABSTRACT FROM AUTHOR]

We report on an all normal dispersion harmonically mode-locked ytterbium-doped femtosecond fiber laser. The mode-locking mechanism of the laser is based on nonlinear polarization evolution with strong pulse shaping. Bandpass filtering is achieved with a cascaded long-period fiber grating, forming highly chirped pulses. The laser generates stable 2.5-ps duration pulses at a repetition rate of 125.39 MHz that could be dechirped externally to 310 fs with a maximum pulse energy of 0.36 nJ. [ABSTRACT FROM PUBLISHER]

We report on an all‐dielectric cut‐off perfect absorber and an optical long‐pass filter using a gallium arsenide multi‐nanocylinder array grown on SiO2 in the wavelength from ultraviolet to infrared. A high average absorption of 0.985 is realized in the absorption band and there is a sharp cut‐off between the absorption band and the non‐absorption band, where the extinction ratio is 12.17 dB, the extinction difference is 0.88, and the cut‐off slope is 0.0047 nm−1. Based on the cut‐off absorption mechanism, a transmissive long‐pass cut‐off filter is proposed with extinction ratio of 21.56 dB, extinction difference of 0.86 and cutoff slope of 0.0049 nm−1. The average transmission of only 0.006 in the stop band and 0.967 in the pass band is realized. The demonstrated devices suggest an efficient way of designing cut‐off perfect absorbers and optical filters with metasurface, which is of important applications in advanced nanophotonic devices and integrated photonic systems. [ABSTRACT FROM AUTHOR]

A chiral metalens of circular polarization dichroism (CPD) in the mid‐infrared region of 3–5 µm is demonstrated both theoretically and experimentally, in which one of the circularly polarized light beams (either left‐handed or right‐handed) is transmissively focused at a designed focal length, while the other beam is reflected. The metalens consists of subwavelength helical surface arrays covered by a gold thin film that gives rise to CPD with the handedness of the metallic helical surface, while the focusing is realized with a phase distribution that is found to be determined by the rotation angle of each individual helical surface along the radial direction of the lens. A chiral metalens of circular dichroism (CMCD) operating in the 3–5 µm region is designed and experimentally fabricated by the 3D laser direct writing technique followed by electron beam evaporation. Experimental focusing of the circularly polarized lights is demonstrated successfully with the lock‐in thermography technique to eliminate the severe background infrared radiation in the 3–5 µm wavelength region, and it is found that the results are consistent with the theoretical simulation. The demonstrated CMCD provides a new idea of implementing integrated functions of both imaging and circular dichroism with a single element. [ABSTRACT FROM AUTHOR]

Van der Waals epitaxy (vdWE) of GaN films on two‐dimensional (2D) materials can overcome a lattice mismatch during crystalline growth. Moreover, this type of epitaxy allows the GaN films to be released from 2D materials because of the weak van der Waals force. However, it is difficult to directly grow a single‐crystalline film on graphene. Here, using metal‐organic chemical vapor deposition (MOCVD), transferable single‐crystalline GaN films are synthesized on multilayer graphene (MLG)/SiC. The films exhibit a flat and continuous surface morphology and c‐axis orientation. Moreover, the GaN films are released from the graphene and transferred to flexible substrates by direct mechanical exfoliation. Furthermore, flexible ultraviolet (UV) photosensitive devices are fabricated and show excellent photoelectric performance in the UV‐range. This research lays a foundation for the application of GaN films in foldable‐display and flexible‐devices. [ABSTRACT FROM AUTHOR]

• The proposed algorithm effectively reduces the hybrid influence of three error sources based on the derived theoretical model, offering a novel method and perspective on how to reduce more error sources using fewer phase-shifting fringe patterns. • An improved LEF algorithm assisted with a newly developed Gamma prediction is further put forward, which not only serves as an initial value estimation for the proposed method but also allows its Gamma prediction submodule to be separately applied to other Gamma corrections. • The proposed hybrid errors reduction process, incorporating regularization operation, error point determination and compensation tricks, not only enhances the stability of least-squares solution but also improves the accuracy of phase retrieval. In this paper, a novel variable-frequency phase-shifting algorithm is proposed to reduce hybrid errors under structured-light illumination, which combines a least-squares iteration with regularization processing. Theoretical formulas and related calculation techniques for the variable-frequency fringe model are respectively derived and proposed considering phase-shifting errors, temporal intensity fluctuations and Gamma distortion. From the perspective of theoretical model, it is successfully verified and implemented to establish a minimal and sufficient system of equations using a least number of fringe patterns. To tackle the challenge of solving numerous unknown parameters in the derived model caused by these sources, a grouped step-by-step strategy is adopted during the least-squares iteration process. Error point determination and compensation tricks are further developed to reduce the phase jump problem caused by underdetermined solution. A Gamma prediction technique based on bi-frequency single-frame fringe patterns is also put forward to enhance the accuracy of phase retrieval results. Corresponding simulation and experimental results have demonstrated that the proposed algorithm with attractive potentiality can achieve a significant enhancement in accuracy using the least number of phase-shifting fringe patterns (taking bi-frequency two-step as an example in this study) compared to other existing competitive techniques. [ABSTRACT FROM AUTHOR]

As an alternative electron transport material to the chemically unstable ZnO nanoparticles (NPs), SnO2 NPs exhibit a great potential to construct high‐performance quantum dot light‐emitting diodes (QLEDs). However, only moderate device performance has been obtained for SnO2‐based QLEDs due to the low electron mobility, unfavorable energy band, and massive defects of SnO2 NPs. Herein, a strategy of transition metal doping is reported to achieve high‐quality manganese‐doped SnO2 (Mn‐SnO2) NPs to address the above problems. Specifically, the large bond energy of Mn─O bonds reduces the oxygen vacancy defects, prompting an effective suppression of the interfacial exciton quenching for massive radiative recombination. Moreover, the favorable energy band and high electron mobility for Mn‐SnO2 promote efficient electron injection and transportation. The good optoelectronic properties for Mn‐SnO2 NPs contribute to a great enhancement in device efficiency from 8.2 to 11.4% and a remarkable improvement in lifetime (T95) from 565.3 to 1009.2 h at 1000 cd−2, among the best performing ZnO‐free QLEDs. Notably, the Mn‐SnO2 based QLEDs show a very superior shelf stability to the QLEDs based on SnO2 and ZnO analogs. Consequently, this work reports an effective approach to achieve high‐quality SnO2 NPs for efficient and stable QLEDs. [ABSTRACT FROM AUTHOR]

2D chiral metasurface have been widely used for planar circular dichroism (CD) devices. However, the design of 2D metasurfaces usually means a time‐consuming search of effective structures, which are typically limited to regular geometries and compromised performances. Here, an efficient method for 2D chiral metasurfaces with pixelated topological structure based on machine learning (ML) is proposed and demonstrated from which simultaneous high CD and extinction ratio (ER) either over broadband or at specific wavelength can be efficiently achieved. The proposed ML method combines both advantages of high efficiency of neural network (NN) and superior goal evolution ability of microbial genetic algorithm (MGA). Unlike traditional empirical‐driven methods, the hybrid framework and pixelated topologically deformable structures can fully exploit the potential of design space and push design capability to its physical limit. An average CD of 94.7% and ER of 15 dB over wavelength from 1.45–1.65 µm and a CD of >90%/ER of >27 dB at freely‐selected wavelength of 1.54 and 1.616 µm are obtained. Experiments with fabricated topological structures validate the theoretical predictions. The proposed pixelated structures with ML provide a universal method for precise tailoring of optical properties of metasurfaces which is otherwise unattainable with conventional regular geometries. [ABSTRACT FROM AUTHOR]

We propose and experimentally demonstrate a high efficient circularly polarizing dichroism waveplate (CPDW) using a Si-based all-dielectric 2Dchiral metasurface. We demonstrate that the CPDW exhibits a unique dichroism in that it functions as a transmissive quarter waveplate for one of either left-or right-handed circularly polarized incident lightand a reflective mirror for the opposite polarization. The circular polarization dichroism (CPD = IRCP − ILCP) in transmission at wavelength ~1.5 μm reaches 97% and the extinction ratio (ER = IRCP/ILCP) is as high as 345:1. Experimental fabrications and measurements of the proposed all-dielectric metasurface are implemented and found to be in excellent agreement with the simulations. The proposed all-dielectric chiral metasurface is of advantages of high-dichroism, easy-fabrication and standard semiconductor fabrication techniques compatible, which could lead to enhanced security in fiber and free-space communications, as well as imaging and sensing applications for circularly polarized light with a highly integrated photonic platform. [ABSTRACT FROM AUTHOR]

The optical sparse aperture approach has been shown to have significant potential in enlarging the aperture size and enhancing the imaging resolution of metalenses. However, because of frequency loss caused by the sparsity and dispersion of sparse aperture arrays, the stitch method for conventional optical sparse aperture metalenses has a non-uniform imaging resolution at different azimuth angles and cannot achieve the same resolution as the corresponding full-aperture metalens. Here, we design and demonstrate the application of a donut-like optical sparse aperture metalens with a lossless imaging property using a shear design method. This metalens has a diffraction-limited resolution of 153 lp/mm for all azimuth angles, which allows us to circumvent the non-uniform imaging resolution of the conventional optical sparse aperture metalens at different azimuth angles and enables us to achieve full resolution. Furthermore, noise resistance in practical images is achieved because of the improved modulation transfer function in the high-frequency region. Such a metalens without frequency loss has significant application potential in the fields of integrated optical systems, lightweight imaging devices, and wearable optics. [ABSTRACT FROM AUTHOR]

In this study, we established a theoretical photothermal model and its experimental validation for an infinitely long solid with an inner corner of arbitrary opening angle, with the solid being irradiated photothermally by a modulated laser beam of arbitrary spatial intensity distribution directed to the corner. The thermal-wave field distribution on the flat surfaces of the solid centered at the corner was obtained using the Green function method. Experimental results based on quantitative thermographic imaging were obtained and used to validate the theoretical model in which thermal diffusivity of an inner cornered stainless steel was measured. The thermal-wave theory based lock-in thermography imaging technique provides a quantitative tool for thermal property measurement and/or non-destructive evaluation of non-flat structures. It also generates valuable physical insights into the spatial distribution of the thermal-wave field in the neighborhood of geometric discontinuities such as inner corners in solids. [ABSTRACT FROM AUTHOR]

A flat and compact metasurface‐based optical metalens with superior performance can be widely used for focusing and imaging. However, the realization of a large‐aperture metalens, which is composed of billions of nanoscale building blocks, poses significant challenges for both process technology and manufacturing cost. Herein, an unconventional optical sparse‐aperture (UOSA) metalens is developed with a Wiener filtering algorithm to obtain a high‐quality image. The proposed UOSA metalens consists of three identical and concentric annular sectors sub‐aperture metalens based on a gallium nitride nanobrick array, which can effectively focus the unpolarized visible light into the same point. An effective aperture of 0.72 mm with lower cost is realized by utilizing less than 1/2 processing area. The experimental imaging results show that this all‐dielectric UOSA metalens with a diffraction‐limited resolution of 11.36 µm under 632.8 nm illumination is consistent with the simulated results. Furthermore, a relatively high focusing efficiency (≈51.2%) can also be achieved under the incident light of any polarization state. The approach with less area process, time‐saving, high efficiency, and large‐aperture characteristics offers vast potential applications in the fields of integrated optical systems, near‐eye imaging devices, and wearable optics. [ABSTRACT FROM AUTHOR]

Compared with intensity imaging, more inherent information of objects can be obtained by polarization and multispectral imaging. Here, a broadband and highly integrated single‐chip full Stokes polarization‐spectral (FSPS) imaging is demonstrated using all‐dielectric spatially multiplexed metalens (SMM) in the wavelength band from 1400 to 1700 nm. The proposed FSPS‐SMM is composed of three sets of off‐axis sub‐metalens simultaneously working for pairs of 0°/90°, 45°/135° linear polarization (LP), and left‐handed/right‐handed circular polarization (CP), respectively. Experimental results show that the optical resolution of each sub‐metalens of the fabricated FSPS‐SMM reaches the diffraction limit of the full‐aperture metalens although the area of each sub‐metalens is only 1/3 of the entire metalens, the averaged polarization extinction ratio of both LP and CP reaches 32.8:1, and the energy efficiency reaches 81.8% at the design wavelength and 60% averagely in the whole 300 nm bandwidth, which breaks the 50% limit of conventional polarizer‐based methods. The proposed design provides a new idea for high‐efficiency and high‐resolution full Stokes polarization‐spectral imaging and multichannel information processing. [ABSTRACT FROM AUTHOR]

In this study, a theoretical model is established for a wedge-like solid with an open sector surrounded by walls of radius $$R$$ of a cylindrical rod illuminated by a modulated circular Gaussian incident beam by means of the Green's function method in cylindrical coordinates. An analytical expression for the thermal-wave field in such a sample is presented. The theory is validated by reducing the arbitrary geometrical structure of the wedge to simpler geometries. It is shown that the frequency dependence of the thermal-wave field near the edge exhibits a large phase lag compared with that at a location far from the edge. The theory provides a foundation for quantitatively characterizing wedge-shaped industrial samples, such as metals with sintered edges, using photothermal methods in a non-contact and non-destructive manner. [ABSTRACT FROM AUTHOR]

A theoretical model for evaluating solid multilayered spherical solids heated by a frequency-modulated light beam using the Green function method is presented. The specific thermal-wave Green function corresponding to the composite structure has been derived. The characteristics of the thermal-wave field with respect to the thermophysical, geometrical, and measurement parameters are presented. Unlike the quadrupole method, the Green function method is capable of evaluating thermal-wave fields at any point of multilayered structures with arbitrary intensity distributions of the incident laser beams. This study establishes applications of thermal-wave fields in both cylindrical and spherical samples using the Green function method and is of importance in characterizing radially inhomogeneous spherical solids. [ABSTRACT FROM AUTHOR]

Abstract: Highly polarized light from an InGaN/GaN light emitting diode is proposed using an embedded multi-layer metallic/dielectric sub-wavelength grating and a dielectric transition layer. Transmission of transverse magnetic mode (TTM), reflection of transverse electric mode, and polarization extinction ratio (ER) were calculated using commercial “GSOLVER” software, based on a full vector implementation of Rigorous Coupled-Wave Analysis algorithm. TTM and ER were found to be largely enhanced by the presence of the transition layer, made of MgF2 or SiO2, placed between GaN and the grating section. TTM >95% and ER >34dB for closely optimized Al/MgF2 gratings were predicted. These values are significantly higher than those obtained by single-layer metallic gratings. [Copyright &y& Elsevier]

Metalens consisting of arrays of subwavelength nanostructures with ultrathin, lightweight, and tailorable characteristics have been in the spotlight to develop next-generation optical elements beyond existing refractive or diffractive optics. However, billions of subwavelength nanostructures of the metalens pose a serious challenge for fabrication, especially for the large-area ones. Herein, we designed and fabricated an unconventional optical sparse aperture (UOSA) metalens that consists of four identical and concentric annular sectors sub-aperture metalens, and it has the larger nonzero domain of modulation transfer function compared with the OSA metalens. The numerical simulation and experiments jointly show that the UOSA metalens has a limited diffraction resolution (0.91 µm) as the conventional full aperture metalens. The UOSA method can not only enlarge the effective aperture of the metalens with lower cost and less processing time but also extend a new degree of freedom for the design of the OSA metalens. [ABSTRACT FROM AUTHOR]

Imaging the vibrations of nanomechanical resonators means measuring their flexural mode shapes from the dependence of their frequency response on in-plane position. Applied to two-dimensional resonators, this technique provides a wealth of information on the mechanical properties of atomically-thin membranes. We present a simple and robust system to image the vibrations of few layer graphene (FLG) resonators at room temperature and in vacuum with an in-plane displacement precision of ≈0.20 μ m. It consists of a sturdy vacuum enclosure mounted on a three-axis micropositioning stage and designed for free space optical measurements of vibrations. The system is equipped with ultraflexible radio frequency transmission lines to electrically actuate resonators. With it we characterize the lowest frequency mode of FLG resonators by measuring its frequency response as a function of position on the membrane and by extracting its effective mass. We use the background noise of the undriven vibrational spectrum to calibrate in-plane displacement. Finally, we measure the first three vibration modes of a resonator whose membrane is partially folded and find that folds locally suppress vibrations. • Flexural vibrations of few layer graphene resonators are imaged by moving a large vacuum enclosure. • The effective mass of the fundamental mode is measured. • Background noise of the undriven vibrational spectrum is used to calibrate in-plane displacement. • Ultraflexible transmission lines are developed to drive resonators at radio frequencies. [ABSTRACT FROM AUTHOR]

We report on a non-sharp-corner quarter wave plate (NCQW) within the single layer of only 8 nm thickness structured by the Ag hollow elliptical ring array, where the strong localized surface plasmons (LSP) resonances are excited. By manipulating the parameters of the hollow elliptical ring, the transmitted amplitude and phase of the two orthogonal components are well controlled. The phase difference of π/2 and amplitude ratio of 1 is realized simultaneously at the wavelength of 834 nm with the transmission of 0.46. The proposed NCQW also works well in an ultrawide wavelength band of 110 nm, which suggests an efficient way of exciting LSP resonances and designing wave plates, and provides a great potential for advanced nanophotonic devices and integrated photonic systems. [ABSTRACT FROM AUTHOR]

In this work the surface thermal-wave field (ac temperature) of a solid cylinder illuminated by a modulated light beam is calculated first in two cases: a multilayered cylinder and a cylinder the radial thermal conductivity of which varies continuously. It is demonstrated numerically that, using a few layers of different thicknesses, the surface thermal-wave field of a cylindrical sample with continuously varying radial thermal conductivity can be calculated with high accuracy. Next, an inverse procedure based on the multilayered model is used to reconstruct the radial thermal conductivity profile of hardened C1018 steel rods, the surface temperature of which was measured by photothermal radiometry. The reconstructed thermal conductivity depth profile has a similar shape to those found for flat samples of this material and shows a qualitative anticorrelation with the hardness depth profile. [ABSTRACT FROM AUTHOR]

Due to the unique properties of two-dimensional (2D) materials, van der Waals epitaxy or the remote epitaxy of nonplanar and planar materials on 2D materials plays an important role in various applications; however, it is often limited by low crystal quality due to misorientation in nonplanar materials or grain boundaries and high threading dislocation densities in planar materials. Selective area growth (SAG), in which orientations are concordant, grain boundaries can be avoided, and threading dislocations can be annihilated, is an effective way to overcome this limitation. It is believed that microscale materials are the predominant masks for SAG, and the nanoselective area growth (NSAG) technology is superior to SAG as it can allow the mechanical exfoliation of the epilayer from the substrate. Herein, we demonstrated the fabrication of architectural GaN nanostructures by the self-assembly NSAG (SNSAG) technology using multilayer graphene (MLG) as a nanomask. The microstructure and optical properties were characterized by scanning electron microscopy, transmission electron microscopy, cathodoluminescence, and micro-Raman and micro-photoluminescence spectroscopy techniques for evidencing the high-quality GaN nanostructures; moreover, high-quality, stress-relaxation GaN films were grown by SNSAG on MLG/SiC and mechanically exfoliated from the substrate for obtaining free-standing GaN. This technique enables the integration of any nonplanar and planar semiconductor material with graphene for extensible applications. [ABSTRACT FROM AUTHOR]