Design method for engineering the initial structure of a spectrometer.

A well-considered initial structure plays a key role in the design of an exceptional spectrometer. Previously, the design method for the optical initial structure (MOIS) that has only focused on the optical properties based on simple imaging formulas and coma-free conditions has been extensively researched. However, as the shape and size of any optical component are not considered for the MOIS, the optical parameters before and after optimization are very different, which results in a loss of reference value of the initial structure. In order to address the aforementioned issues, a more efficient design method for engineering initial structure (MEIS) of the spectrometer is proposed, where not only the above optical properties are considered but also the relative position and size of any optical component in order to avoid the interference between the optical components. For the MEIS, three important anti-interference conditions between components are deduced through ray tracing, and the relevant imaging formulas are derived by geometric optics, which leads to the rapid calculation of component parameters and the acquisition of an initial structure satisfying the corresponding design requirements by setting reasonable spacing margins. To verify the validity of the MEIS, a wide-band high-resolution spectrometer system with a large CCD Toucan 216 is designed within a wavelength range of 700-1000 nm and a resolution of 0.5 nm. Compared with the MOIS, the positions of each component in the MEIS are more rationalized, which significantly eliminates the complex optimization processes. For the MEIS, changes only in the position of the image plane occur with minimal variations in the axial and vertical wheelbase (less than 0.5 mm) as well as the deflection angle (only 0.5°), with favorable evaluation indices. The MEIS has an important reference value for the rapid and efficient design of excellent spectrometers.

Polarization-modulated broadband achromatic bifunctional metasurface in the visible light.

Achromatic bifunctional metasurface is of great significance in optical path miniaturization among advanced integrated optical systems. However, the reported achromatic metalenses mostly utilize a phase compensate scheme, which uses geometric phase to realize the functionality and uses transmission phase to compensate the chromatic aberration. In the phase compensation scheme, all the modulation freedoms of a nanofin are driven at the same time. This makes most of the broadband achromatic metalenses restricted to realizing single function. Also, the phase compensate scheme is always addressed with circularly polarized (CP) incidence, leading to a limitation in efficiency and optical path miniaturization. Moreover, for a bifunctional or multifunctional achromatic metalens, not all the nanofins will work at the same time. Owing to this, achromatic metalenses using a phase compensate scheme are usually of low focusing efficiencies. To this end, based on the pure transmission phase in the x-/y- axis provided by the birefringent nanofins structure, we proposed an all-dielectric polarization-modulated broadband achromatic bifunctional metalens (BABM) in the visible light. Applying two independent phases on one metalens at the same time, the proposed BABM realizes achromatism in a bifunctional metasurface. Releasing the freedom of nanofin's angular orientation, the proposed BABM breaks the dependence on CP incidence. As an achromatic bifunctional metalens, all the nanofins on the proposed BABM can work at the same time. Simulation results show that the designed BABM is capable of achromatically focusing the incident beam to a single focal spot and an optical vortex (OV) under the illumination of x- and y-polarization, respectively. In the designed waveband 500 nm (green) to 630 nm (red), the focal planes stay unchanged at the sampled wavelengths. Simulation results prove that the proposed metalens not only realized bifunctional achromatically, but also breaks the dependence of CP incidence. The proposed metalens has a numerical aperture of 0.34 and efficiencies of 33.6% and 34.6%. The proposed metalens has advantages of being flexible, single layer, convenient in manufacturing, and optical path miniaturization friendly, and will open a new page in advanced integrated optical systems.

A study on the relationship between the crystallization characteristics of quenched droplets and the effect of cell cryopreservation with Raman spectroscopy.

The cryopreservation method of microdroplets has steadily become widely employed in the cryopreservation of microscale biological samples such as various types of cells due to its fast cooling rate, significant reduction of the concentration of cryoprotectants, and practical liquid handling method. However, it is still necessary to consider the corresponding relationship between droplet size and concentration and the impact of crystallization during the cooling process on cell viability. The key may be a misunderstanding of the influencing factors of crystallization and vitrification behavior with concentration during cooling on the ultimate cell viability, which may be attributable to the inability to analyze the freezing state inside the microdroplets. Therefore, in this work, an in situ Raman observation system for droplet quenching was assembled to obtain Raman spectra in the frozen state, and the spectral characteristics of the crystallization and vitrification processes of microdroplets with varied concentrations and volumes were investigated. Furthermore, the degree of crystallization inside the droplets was quantitatively analyzed, and it was found that the ratio of the crystalline peak to hydrogen bond shoulder could clearly distinguish the degree of crystallization and the vitrified state, and the Raman crystallization characteristic parameters gradually increased with the decrease of concentrations. By obtaining the cooling curve and the overall cooling rate of quenching droplets, the vitrification state of the microdroplets was confirmed by theoretical analysis of the cooling characteristics of a DMSO solution system. In addition, the effect of cell cryopreservation was investigated using the microdroplet quenching device, and it was found that the key to cell survival during the quenching process of low-concentration microdroplets was dominated by the cooling rate and the internal crystallization degree, while the main influencing factor on high concentration was the toxic effect of a protective agent. In general, this work introduces a new nondestructive evaluation and analysis method for the cryopreservation of quenching microdroplets.

Dense Convolutional Neural Network for Identification of Raman Spectra.

The rapid development of cloud computing and deep learning makes the intelligent modes of applications widespread in various fields. The identification of Raman spectra can be realized in the cloud, due to its powerful computing, abundant spectral databases and advanced algorithms. Thus, it can reduce the dependence on the performance of the terminal instruments. However, the complexity of the detection environment can cause great interferences, which might significantly decrease the identification accuracies of algorithms. In this paper, a deep learning algorithm based on the Dense network has been proposed to satisfy the realization of this vision. The proposed Dense convolutional neural network has a very deep structure of over 40 layers and plenty of parameters to adjust the weight of different wavebands. In the kernel Dense blocks part of the network, it has a feed-forward fashion of connection for each layer to every other layer. It can alleviate the gradient vanishing or explosion problems, strengthen feature propagations, encourage feature reuses and enhance training efficiency. The network's special architecture mitigates noise interferences and ensures precise identification. The Dense network shows more accuracy and robustness compared to other CNN-based algorithms. We set up a database of 1600 Raman spectra consisting of 32 different types of liquid chemicals. They are detected using different postures as examples of interfered Raman spectra. In the 50 repeated training and testing sets, the Dense network can achieve a weighted accuracy of 99.99%. We have also tested the RRUFF database and the Dense network has a good performance. The proposed approach advances cloud-enabled Raman spectra identification, offering improved accuracy and adaptability for diverse identification tasks.

Perfect vortex beam with polarization-rotated functionality based on single-layer geometric-phase metasurface.

Perfect vortex (PV) beam has seen significant advances in fields like particle manipulation, optical tweezers, and particle trapping, due to the fact that its ring radius is independent of the topological charge. Although geometric-phase metasurfaces have been proposed to generate PV beams, it always relies on circularly or elliptically polarized incident light, which hinders the miniaturization of compact optical devices. Here, using orthogonal decomposition of polarization vectors (ODPV), we proposed a geometric-phase metasurface, which breaks the dependence of circular polarization, to generate PV beam. In the design of the metasurface, we introduced PV phase profiles corresponding to the left-handed circularly polarized (LCP) component and the right-handed circularly polarized (RCP) component into the metasurface based on the principle of ODPV. We further determined the rotation angle of each nanostructure of the metasurface by calculating the argument of the composite vector of LCP and RCP in the transmission field. Simulation results show that the proposed geometric-phase metasurface can generate the PV beam upon the illumination of a linearly polarized incident. Moreover, the PV beam with polarization-rotated functionality is achieved by setting the polarization rotation angle. Furthermore, dual PV beams with orthogonal polarization states is realized at the same time by superimposing two sets of phase profiles on a single metasurface. It is also demonstrated that the PV beam parameters, such as ring radius and/or topological charge, can be set on demand in the metasurface design. The proposed metasurface has the exceptional advantage of high fabrication tolerance and is optical path miniaturization friendly, and will open a new avenue in advanced compact and integrated optical systems.

Broadband achromatic longitudinal bifocal metalens in the visible range based on a single nanofin unit cell.

Metasurfaces provide a remarkable platform to manipulate over phase, amplitude, and polarization flexibly and precisely. Bifocal metalens draws great research interest due to its ability of converging wavefronts to different focal positions horizontally and longitudinally. However, as wavelength of incident light changes, chromatic aberration will cause the focal lengths reliance on the incident wavelength, which will affect the performance of metasurface, especially for longitudinal bifocal metalens. In this work, a broadband achromatic longitudinal bifocal metalens (BALBM) based on single nanofin unit cell is demonstrated. Pancharatnam-Berry (PB) phase is used to converge the incident light. Cross commixed sequence distribution (CCSD) is introduced to control the positions of focal points FLand FRwhen left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) incident. Propagation phase is used to compensate the phase difference caused by chromatic aberration. Simulation results show that in the continuous wavelength range from 500 nm to 700 nm, the focal point shifts caused by chromatic dispersion are reduced 81% for FL and 83% for FR, respectively. The focal length variations are stabilized to 6.21% for FLand 4.8% for FRcomparing with the focal lengths at the initial wavelength 500 nm. The proposed BALBM brings advances to bifocal metasurfaces in versatile application areas including machine vision, optical computed tomography and microimaging.

Polarization-controllable perfect vortex beam by a dielectric metasurface.

A perfect vortex beam has been attracting tremendous attention due to the fact that its ring radius is independent of the topological charge. Taking advantage of the superposition principle of phase in Fourier space, we proposed to generate perfect vortex beam using propagation-phase-based dielectric metasurface, which exhibits production efficiency larger than 83.5%. Due to the sensitivity of propagation phase to the polarization of incident beam, two sets of phase profiles can be imposed on a single dielectric metasurface, enabling the simultaneous generation of dual perfect vortex beams. Based on this property, convenient control to the radius and/or topological charge of perfect vortex beam is achieved by switching the incident polarization between two orthogonal polarizations, without redesigning metasurface or changing optical path. What's more important, the crosstalk of these two channels is low, less than 4%. Thus, the propagation-phase method of producing perfect vortex beam will find significant applications in optical communication, particle trapping, particle manipulation and holographic display.

Orbit-induced localized spin angular momentum of vector circular Airy vortex beam in the paraxial regime.

We theoretically study the propagation properties of the vector circular Airy vortex beam in detail. The results show that the orbital angular momentum can induce a localized spin angular momentum after autofocusing in the paraxial regime, which leads to an abrupt polarization transition just before the focal plane. However, there is no angular momentum conversion from orbital angular momentum to spin angular momentum during the whole propagation process. We provide an intuitive explanation for the appearance of such spin angular momentum localization. This investigation is expected to advance our understanding of the vector properties of circular Airy beam and optical spin-orbit coupling.

Occlusion disparity refinement for stereo matching through the geometric prior-based adaptive label search.

In stereo matching, occlusion disparity refinement is one of the challenges when attempting to improve disparity accuracy. In order to refine the disparity in occluded regions, a geometric prior guided adaptive label search method and sequential disparity filling strategy are proposed. In our method, considering the scene structural correlation between pixels, the geometric prior information such as image patch similarity, matching distance, and disparity constraint is used in the proposed label search energy function and the disparity labels are searched by superpixel matching. Thus, the reliable disparity labels are adaptively searched and propagated for occlusion filling. In order to improve the accuracy in large occluded regions, by using the proposed sequential filling strategy, occluded regions are decomposed into multiple blocks and filled in multiple steps from the periphery; thus, reliable labels are iteratively propagated to the interior of occluded regions without violating the smooth disparity assumption. Experimental results on the Middlebury V3 benchmark show that, compared with other state-of-the-art algorithms, the proposed method achieves better disparity results under multiple criteria. The proposed method can provide better disparity refinement for typical stereo matching algorithms.

Automated material identification with a Raman spectrometer based on the contribution enhancement of small differences and the adaptive target Raman peak subtraction.

There is only a small difference in Raman peaks between two materials, but they also represent different molecular materials. Therefore, the accurate identification ability for similar materials with small differences among their Raman peaks plays a key role in Raman spectrometers for material identification. However, the noises, the baseline (i.e., fluorescence backgrounds), and the requirements, such as fast and automated detection, of excellent user experiences cause many difficulties. In this paper, the target Raman peak is directly subtracted from the detected Raman spectrum by the adaptive minimum root mean square error (RMSE) estimation for a residual spectrum. Unlike the usual methods in which the detected Raman peak needs to be first recovered by removing the baseline from its Raman spectrum and then to be compared with the target Raman peak, our method can effectively enhance the contribution of small differences between the detected and the target Raman peak on the residual spectrum so as to make the RMSE of the residual spectrum more sensitive with increasing differences. On the other hand, the obtained RMSE of the residual spectrum only has a small change for the detected Raman spectrum with various baselines. So the common criteria (i.e., the third-order polynomials describing RMSE) to identify the detected Raman spectrum with various baselines and the target Raman spectrum is presented. Simulation results show that the small difference, where there is only an additional small Raman peak as low as 1/25 of the maximum peak height, can also be accurately identified. Experiments also demonstrate that similar materials can be accurately identified, whereas some commercial Raman spectrometers fail to identify them. Our method effectively deals with the problem in which the error of the complex baseline correction causes erroneous judgement in Raman spectrometers for material identification.

Snell-like and Fresnel-like formulas of the dual-phase-gradient metasurface.

By patterning the metasurface of two phase gradients that are both space-orthogonal and polarization-orthogonal, we derived the three-dimensional (3D) Snell-like formula and the Fresnel-like formula of the proposed metasurface. Theoretically, the dual-phase-gradient metasurface resembles biaxial-like birefringence, i.e., decomposing any homogeneously polarized incident beam into two anomalously refracted beams whose polarizations vary as the incident beam. According to the Fresnel-like formula, the relative intensity between the two anomalously refracted beams not only depends on the incidence angle and the polarization ellipticity of the incident beam being similar to the biaxial crystals, but it also depends on the polarization ellipticity orientation even for a given incident polarization, which is an unique property absent in the biaxial crystals. All the theoretical analyses were numerically demonstrated. The 3D Snell-like and Fresnel-like formulas will make the design of functional devices based on the dual-phase-gradient metasurface much easier.

Dielectric longitudinal bifocal metalens with adjustable intensity and high focusing efficiency: erratum.

A number of erratums are presented to correct the inadvertent typing mistakes in our paper.

Dielectric longitudinal bifocal metalens with adjustable intensity and high focusing efficiency.

Metalens recently attracts enormous attention due to its microscale figure and versatile functionalities. With the combination of geometric phase and propagation phase, we first wrote the phase equation of bifocal metalens that can high efficiently focus incidence into one or two foci in tandem along longitudinal direction, depending on the polarization of incidence. More importantly, the relative intensity of the two foci can be modulated conveniently by changing the ellipticity of incidence, which is different from previous bifocal metalenses need to be repatterned for each kind of relative intensity [Opt. Express23, 29855 (2015)]. Besides, the focusing efficiency of the proposed metalens is as high as 72%, and the separate distance between those two foci can be designed at will, which may find itself significant applications in optical tomography technique, optical data storage, and so on.

Monitoring algorithm of tilt angle based on sub-block plane fitting for high-resolution imaging.

The limitation of mechanical structure and misoperations can result in a small tilt angle formed by the sample and the focal plane, which will decrease the resolution of the imaging system. Moreover, the small tilt angle is difficult to be observed. In order to solve this problem, a monitoring algorithm of tilt angle based on sub-block plane fitting for high-resolution imaging systems has been proposed, which is used to measure the initial angle of most 2D samples before imaging and assist users to determine the tilt degree of the sample. Experiments demonstrate that the proposed method can measure the tilt angle with a high measurement precision of 0.007° and a low residual tilt angle of 0.004°, indicating that the algorithm has high measurement precision and stability. Further results show that the quality of the image will be improved by 20%-27% when the tilt angle is 0.3056°, which means that the small degree of tilt of the sample can seriously damage the image quality. Therefore, the study of tilt angle measurement has great significance for high-resolution imaging systems.

Correction for color artifacts using the RGB intersection and the weighted bilinear interpolation.

Due to the defects of optical systems, image sensors, and imperfect algorithms for image acquisition, compression, and restoration, color artifacts often appear in images obtained by imaging devices such as digital cameras and scanners. Moreover, color artifacts are difficult to eliminate because of technical limitations, even in some mature commercial cameras. On the basis of red, green, and blue (RGB) intersection (RGBI), a correction method for color artifacts is proposed in this paper, where the RGB intersection-based method can effectively detect various types of color artifacts. Also, by combining the object information with weighted bilinear interpolation, the continuity of the image is kept while restoring the real color. Experiments demonstrate that the RGBI method, which is applicable to all color images, can eliminate various types of color artifacts with accurate detection and less artifact residue, even if the image has severe color distortion or the areas of the color artifacts are small and discrete.

Dispersion engineering in unidirectional excitation of the surface wave of photonic crystal.

The discovery of transverse spin angular momentum (SAM) of evanescent and guided modes presents a novel spin-orbit interaction (SOI), i.e., transverse SOI, to affect and control the intensity distribution and propagation path of light. In this Letter, we first theoretically verify the transverse SAM property of the surface wave of a photonic crystal (PhC) slab. Then we realize the polarization-controllable unidirectional excitation of such (forward) surface wave by means of transverse SOI. Furthermore, taking advantage of dispersion engineering of PhC, we design another PhC slab capable of sustaining a backward surface wave and find that, compared to a forward surface wave, the backward surface wave is related to inverse unidirectional excitation with incident of a circularly polarized beam. In other words, dispersion engineering of PhC provides another route to control the excitation direction of surface modes. The combination of dispersion engineering and transverse SOI will facilitate the design of functional devices based on PhC in the field of nanophotonics and nanoplasmonics.

Right- and left-handed rules on the transverse spin angular momentum of a surface wave of photonic crystal.

By investigating the surface wave of photonic crystal, we put forward two sets of rules: the right-handed screw rule, judging the transverse spin angular momentum (SAM) directions according to the propagation direction of the surface wave; and the left-handed rule, judging the excitation direction of the surface wave in accordance to the SAM direction of incident circularly polarized light and the relative position of the dipole-like scatterer with respect to the interface where the surface wave propagates. Both right- and left-handed rules apply to the interface consisting of opposite-sign-permittivity materials. With the help of these two sets of rules, it is convenient to judge the direction of the transverse SAM and the excited surface wave, which facilitate the application involving transverse SAM of the surface wave.

[Locally Dynamically Moving Average Algorithm for the Fully Automated Baseline Correction of Raman Spectrum].

The baseline correction is an, extremely important spectral preprocessing step and can significantly improve the accuracy of the subsequent spectral analysis algorithm. At present most of the baseline correction algorithms are manual and semi-automated. The manual baseline correction depends on the user experience and its accuracy is greatly affected by the subjective factor. The semi-automated baseline correction needs to set different optimizing parameters for different Raman spectra, which will be inconvenient to users. In this paper, a locally.dynamically moving average algorithm (LDMA) for the fully automated baseline correction is presented and its basic ideas.and steps are demonstrated in detail. In the LDMA algorithm the modified moving averaging algorithm (MMA) is used to strip the Raman peaks. By automatically finding the baseline subintervals of the raw Raman spectrum to divide the total spectrum range into multi Raman peak subintervals, the LDMA algorithm succeed in dynamically changing the window half width of the MA algorithm and controlling the numbers of the smoothing iterations in each Raman peak subinterval. Hence, the phenomena of overcorrection and under-correction are avoided to the most degree. The LDMA algorithm has achieved great effect not only to the synthetic Raman spectra with the convex, exponential, or sigmoidal baseline but also to the real Raman spectra.

[Spectral Smoothing with Adaptive Multiscale Window Average].

In order to smooth the spectra automatically and reliably, a spectral smoothing algorithm with adaptive multiscale window average (AWMA) is demonstrated. In this method, different positions of the spectra are smoothed by windows of different width, and the width of the windows will directly affect smoothing. The window with inappropriate width may cause excessive denoising (peak distortion or loss) or inadequate denoising (the flat region of the spectra still contains a lot of noise). So, how to get the right width of the window is the key of spectral smoothing. The algorithm optimized the width of windows by an iterative method, and verified whether the width is the best according to statistical Z-test. In order to increase the reliability of the algorithm, a comprehensive comparison of the thresholds of hypothesis according to simulation data of different SNR was performed. When the threshold is set to 1. 1, the denoising effect can be the best. In this work, the AMWA algorithm was tested by simulated spectra and real syectra, and it can automatically adapt to different spectral shape and different noise intensity. A comprehensive comparison of AMWA smoothing, Savitzky-Golay smoothing and moving average smoothing was performed in this paper, and the AMWA algorithm is better than the other two algorithms. Results show that the AMWA algorithm not only has better denoising effect, but also has higher accuracy and fidelity. This method has achieved great effect not only to simulated spectra but also to real spectra.

[Fast Discrimination of Drugs by Improved Projection Algorithm Based on Raman Spectroscopy].

The projection algorithm used in mixture analysis to determine whether there is unknown disturbance existing in grey system can not accurately identify different samples and similar samples at the same time when it is used in the identification of drugs, because of the insufficient criteria. In the present study, one of its criteria for whether the size of measurement error of testing sample is at a limited level is improved for whether the size and distribution of measurement error is equal and similar between testing sample and standard sample. By testing 6 kinds of normal drugs (including BAYER Aspirin Enteric-coated Tablets, TYLENOL Acetaminophen Sustained Release Tablets, BAYER Compound Paracetamol Tablets(II), HUAZHONG Compound Vitamin C, HUAZHONG Vitamin B and MADINGLIN Demperidone Tablets) and 3 kinds of similar drugs of aspirin (including BAYER Aspirin Enteric-coated Tablets, Shanghai SINE Aspirin Enteric-coated Tablets and Bamyl Aspirin Effervescent Tablets), it was found that the un-improved projection algorithm directly used in discrimination of drugs shows poor performance with many problems existing, however, the improved projection algorithm can discriminate different drugs and similar drugs with accuracy up to 100%. The improved projection algorithm can be a universal, accurate and reliable automated pharmaceutical identification algorithm and can provide a reference for the study on identification of substance.