Contents Diffraction, Gratings, and Holography, 26 Article(s)
Bi-layered coded metasurface for multi-functional hologram with broadband transmission and efficient reflection
Lei Zhu, Xusheng Li, Liang Dong, Wenjing Xie, Guanyu Shang, Shah Nawaz Burokur, and Xumin Ding

A multi-functional full-space metasurface based on frequency and polarization multiplexing is proposed. The metasurface unit consists of metallic patterns printed on the two faces of a single-layered dielectric substrate. The unit cell can control electromagnetic wavefronts to achieve a broadband transmission with amplitudes greater than 0.4 from 4.4 to 10.4 GHz. Meanwhile, at 11.7 GHz and 15.4 GHz, four high-efficiency reflection channels with a reflection amplitude greater than 0.8 are also realized. When illuminated by linearly polarized waves, five different functions can be realized at five different frequencies, which are demonstrated by theoretical calculations, full-wave simulations, and experimental measurements.

Chinese Optics Letters
May. 17, 2024, Vol. 22 Issue 5 050502 (2024)
Linear mathematical model for the unique solution of 3D ptychographic iterative engine
Liqing Wu, Naijie Qi, Chengcheng Chang, Hua Tao, Xiaoliang He, Cheng Liu, and Jianqiang Zhu

Diffraction intensities of the 3D ptychographic iterative engine (3PIE) were written as a set of linear equations of the self-correlations of Fourier components of all sample slices, and an effective computing method was developed to solve these linear equations for the transmission functions of all sample slices analytically. With both theoretical analysis and numerical simulations, this study revealed the underlying physics and mathematics of 3PIE and demonstrated for the first time, to our knowledge, that 3PIE can generate mathematically unique reconstruction even with noisy data.

Chinese Optics Letters
May. 17, 2024, Vol. 22 Issue 5 050501 (2024)
Experimental implementation of phase triplicator gratings in a spatial light modulator
Shang Gao, María del Mar Sánchez-López, and Ignacio Moreno

In this work, we compare different methods for implementing a triplicator, a phase grating that generates three equi-intense diffraction orders. The design with optimal efficiency features a continuous phase profile, which cannot be easily reproduced, and is typically affected by quantization. We compare its performance with binary and sinusoidal phase profiles. We also analyze the effect of quantizing the phase levels. Finally, a random approach is adopted to eliminate the additional harmonic orders. In all cases, a liquid-crystal-on-silicon spatial light modulator is employed to experimentally verify and compare the different approaches.

Chinese Optics Letters
Feb. 21, 2024, Vol. 22 Issue 2 020501 (2024)
Femtosecond laser direct written fiber Bragg gratings with narrow bandwidth and high sideband suppression
Wenping Qiu, Shuang Liu, Guanghua Cheng, Huan Zhan, Guodong Zhang, Guanpin Ren, Zhongrui Sun, and Min Zhang

Aiming for suppressing side-mode and spectrum broadening, a slit beam-shaping method and super-Gaussian apodization processing for femtosecond laser point-by-point (PbP) inscription technology of fiber Bragg gratings (FBGs) are reported here. High-quality FBGs, featuring narrow bandwidth of less than 0.3 nm, high reflectivity above 85%, low insertion loss (0.21 dB), and low cladding loss (0.82 dB), were obtained successfully. By a semi-automatic PbP inscription process, an array consisting of six FBGs, exhibiting almost no side-mode peaks with high suppression ability and narrow bandwidth, was fabricated along three independently developed single-mode fibers with an interval of 20 mm.

Chinese Optics Letters
Jan. 18, 2024, Vol. 22 Issue 1 010501 (2024)
Babinet’s meta-apertures for holographic bi-imaging
Syed Yasir Azeem, Dong Zhao, Ruixing Xia, and Kun Huang

Complementary metasurfaces based on Babinet’s principle have shown remarkable performance in optical applications like polarization conversion and split ring resonators by dynamically reversing the properties of light in both transmission and reflection modes. However, complementary diffractive metasurfaces for different holographic images have not yet proven to be effective because Babinet’s principle predicts identical diffraction patterns from complementary surfaces. Here, we report carefully designed complementary metasurfaces consisting of an engineered metallic aluminum layer sitting on a transparent quartz substrate. Upon illumination, both complementary devices output entirely different diffractive intensity profiles from each other, yielding two holographic images at visible wavelengths from 430 nm to 650 nm. It provides experimental evidence for encoding two images into complementary metasurfaces, indicating an exception of Babinet’s principle in the Fresnel region of complementary apertures.

Chinese Optics Letters
Aug. 14, 2023, Vol. 21 Issue 9 090501 (2023)
Suppressing defocus noise with U-net in optical scanning holography
Haiyan Ou, Yong Wu, Kun Zhu, Edmund Y. Lam, and Bing-Zhong Wang

Optical scanning holography (OSH) records both the amplitude and phase information of a 3D object by a 2D scan. To reconstruct a 3D volumetric image from an OSH hologram is difficult, as it suffers from the defocus noise from the other sections. The use of a random phase pupil can convert defocus noise into speckle-like noise, which may require further processing in sectional image reconstruction. In this paper, we propose a U-shaped neural network to reduce this speckle haze. Simulation results show that the proposed method works effectively and efficiently both in simple and complex graphics.

Chinese Optics Letters
Aug. 07, 2023, Vol. 21 Issue 8 080501 (2023)
Self-healing of holographically generated moiré lattice wave fields
Siwei Tang, Chunlei Shang, Zhaofeng Liu, Chengzhen Lu, Yangjian Cai, Yuanmei Gao, and Zengrun Wen

Self-healing in optics generally refers to the ability to reconstruct itself and restore the original state after encountering obstacles in the propagation of the light field. In this research, we observe the processes of the wave fields from perfect to defect in front of the focal plane of the 4f system, finally returning to an intact situation after the plane. According to simulations and experimental results, there is a minimum self-healing distance for the moiré lattice field that positively associates with the radius of the defect (obstacle) in the nondiffracting transmission range. Furthermore, it is observed that the defect self-healing is a process of “repairing the center and then repairing the edges.” These findings can be applied in areas such as optical imaging, capture, and information processing.

Chinese Optics Letters
Editors' PickOct. 12, 2022, Vol. 21 Issue 3 030502 (2023)
Binary diffractive lens with subwavelength focusing for terahertz imaging
Ran Ning, Dayong Wang, Lu Rong, Jie Zhao, Yunxin Wang, and Shufeng Lin

The converging lens is one of the key components in high-resolution terahertz imaging. In this Letter, a binary diffractive lens is proposed for the scanning imaging system working at 278.6 GHz, in which a convergent beam with a waist diameter of 0.65 mm is generated, and 1 mm lateral imaging resolution is realized. This low-cost terahertz lens, constituted by concentric rings with different radii, is optimized by stimulated annealing algorithm and fabricated by three-dimensional printing. Compared with the conventional transmissive convex lens, higher resolution and enhanced imaging quality are achieved via smaller focal spot of the illumination beam. This type of lens would promote terahertz imaging closer to practical applications such as nondestructive testing and other scenarios.

Chinese Optics Letters
Nov. 03, 2022, Vol. 21 Issue 3 030501 (2023)
Wavelength and polarization dual-multiplexed imaging based on holographic metasurfaces
Jilian Xu, Zhiyuan Yue, Peiyao Lu, Rui Wu, Kun Jiang, Xiquan Jiang, and Shuyun Teng

In light of the powerful light manipulation ability of holographic metasurfaces, optical imaging with wavelength multiplexing and polarization multiplexing is performed in this paper. The metasurface is composed of identical rectangular nanoholes etched in silver film. Three imaging effects, including the in-plane color imaging, three-dimensional wavelength-encrypted imaging, and polarization-multiplexing wavelength-encrypted imaging, are realized. The designed metasurface has compact structure, and the obtained image has lower noise. The simulation and experiment results give the verification. Multiple images, including spatial multiplexing, wavelength multiplexing, and polarization multiplexing, exhibit immense potentialities of metasurfaces, and this work is helpful for expanding the applications of metasurfaces.

Chinese Optics Letters
Oct. 03, 2023, Vol. 21 Issue 10 100501 (2023)
Electrically switchable structural patterns and diffractions in a dual frequency nematic liquid crystal
Zhenpeng Song, Ziyang Li, Xiaohu Shang, Chaoyi Li, Lingling Ma, Yanqing Lu, and Bingxiang Li

Electrically driven structural patterns in liquid crystals (LCs) have attracted considerable attention due to their electro-optical applications. Here, we disclose various appealing reconfigurable LC microstructures in a dual frequency nematic LC (DFNLC) owing to the electroconvection-induced distortion of the LC director, including one-dimensional rolls, chevron pattern, two-dimensional grids, and unstable chaos. These patterns can be switched among each other, and the lattice constants are modulated by tuning the amplitude and frequency of the applied electric field. The electrically switchable self-assembled microstructures and their beam steering capabilities thus provide a feasible way to tune the functions of DFNLC-based optical devices.

Chinese Optics Letters
Sep. 07, 2022, Vol. 21 Issue 1 010501 (2023)
Double-groove rectangular gratings for high-efficiency wideband vertical coupling in planar-integrated optical systems
Guoqing Ma, Changhe Zhou, Yongfang Xie, Ge Jin, Rongwei Zhu, Jin Zhang, Junjie Yu, and Guohai Situ

A planar-integrated optical system (PIOS) represents powerful optical imaging and information processing techniques and is a potential candidate for the realization of a three-dimensional (3D) integrated optoelectronic intelligent system. Coupling the optical wave carrying information into a planar transparent substrate (typically fused silica) is an essential prerequisite for the realization of such a PIOS. Unlike conventional grating couplers for nano-waveguides on the silicon-on-insulator platform, the grating couplers for PIOS enable to obtain a higher design freedom and to achieve much higher coupling efficiency. By combining the rigorous coupled wave algorithm and simulated annealing optimization algorithm, a high-efficiency asymmetric double-groove grating coupler is designed for PIOS. It is indicated that, under the condition of the normal incidence of TE polarization, the diffraction efficiency of the -1st order is over 95%, and its average value is 97.3% and 92.8% in the C and C+L bands. The simulation results indicate that this type of grating coupler has good tolerance and is expected to be applied in optical interconnections, waveguide-based augmented reality glasses, and planar-integrated 3D interconnection optical computing systems.

Chinese Optics Letters
Jun. 14, 2022, Vol. 20 Issue 9 090501 (2022)
Machine-learning-based high-speed lensless large-field holographic projection using double-sampling Fresnel diffraction method
Chentianfei Shen, Tong Shen, Qi Chen, Qinghan Zhang, and Jihong Zheng

Machine learning can effectively accelerate the runtime of a computer-generated hologram. However, the angular spectrum method and single fast Fresnel transform-based machine learning acceleration algorithms are still limited in the field-of-view angle of projection. In this paper, we propose an efficient method for the fast generation of large field-of-view holograms combining stochastic gradient descent (SGD), neural networks, and double-sampling Fresnel diffraction (DSFD). Compared with the traditional Gerchberg–Saxton (GS) algorithm, the DSFD-SGD algorithm has better reconstruction quality. Our neural network can be automatically trained in an unsupervised manner with a training set of target images without labels, and its combination with the DSFD can improve the optimization speed significantly. The proposed DSFD-Net method can generate 2000-resolution holograms in 0.05 s. The feasibility of the proposed method is demonstrated with simulations and experiments.

Chinese Optics Letters
Mar. 25, 2022, Vol. 20 Issue 5 050502 (2022)
Large area patterning of ultra-high thermal-stable structural colors in transparent solids
Jie Zhang, Dezhi Tan, Kaiqiang Cao, Tianqing Jia, and Jianrong Qiu

Printing stable color with a lithography-free and environment-friendly technique is in high demand for applications. We report a facile strategy of ultrafast laser direct writing (ULDW) to produce large-scale embedded structural colors inside transparent solids. The diffraction effect of gratings enables effective generation of structural colors across the entire visible spectrum. The structural colors inside the fused silica glass have been demonstrated to exhibit excellent thermal stability under high temperature up to 1200°C, which promises that the written information can be stable for long time even with unlimited lifetime at room temperature. The structural colors in the applications of coloring, anti-counterfeiting, and information storage are also demonstrated. Our studies indicate that the presented ULDW allows for fabricating large-scale and high thermal-stability structural colors with prospects of three-dimensional patterning, which will find various applications, especially under harsh conditions such as high temperature.

Chinese Optics Letters
Jan. 12, 2022, Vol. 20 Issue 3 030501 (2022)
Generating large topological charge Laguerre–Gaussian beam based on 4K phase-only spatial light modulator
Ruijian Li, Yuan Ren, Tong Liu, Chen Wang, Zhengliang Liu, Jie Zhao, Rusheng Sun, and Ziyang Wang

The resolution of the spatial light modulator (SLM) screen and the encoding algorithm of the computer-generated hologram are the primary limiting factors in the generation of large topological charge vortex beams. This paper attempts to solve these problems by improving both the hardware and the algorithm. Theoretically, to overcome the limitations of beam waist radius, the amplitude profile function of large topological charge Laguerre–Gaussian (LG) beam is properly improved. Then, an experimental system employing a 4K phase-only SLM is set up, and the LG beams with topological charge up to 1200 are successfully generated. Furthermore, we discuss the effect of different beam waist radii on the generation of LG beams. Additionally, the function of the LG beam is further improved to generate an LG beam with a topological charge as high as 1400. Our results set a new benchmark for generating large topological charge vortex beams, which can be widely used in precise measurement, sensing, and communication.

Chinese Optics Letters
Aug. 10, 2022, Vol. 20 Issue 12 120501 (2022)
Measuring the topological charge of optical vortices with a single plate
Jingyin Zhao, Yunxia Jin, Fanyu Kong, Dongbing He, Hongchao Cao, Wang Hao, Yubo Wu, and Jianda Shao

Measuring the topological charge (TC) of optical vortex beams by the edge-diffraction pattern of a single plate is proposed and demonstrated. The diffraction fringes can keep well discernible in a wide three-dimensional range in this method. The redundant fringes of the diffracted fork-shaped pattern in the near-field can determine the TC value, and the orientation of the fork tells the handedness of the vortex. The plate can be opaque or translucent, and the requirement of the translucent plate for TC measurement is analyzed. Measurement of TCs up to ±40 is experimentally demonstrated by subtracting the upper and lower fringe numbers with respect to the center of the light. The plate is easy to get, and this feasible measurement can bring great convenience and efficiency for researchers.

Chinese Optics Letters
Jun. 26, 2022, Vol. 20 Issue 11 110501 (2022)
Dynamically taming focal fields of femtosecond lasers for fabricating microstructures
Meng-Qiang Cai, Qiang Wang, Cheng-Hou Tu, Yong-Nan Li, and Hui-Tian Wang

Micromachining based on femtosecond lasers usually requires accurate control of the sample movement, which may be very complex and costly. Therefore, the exploration of micromachining without sample movement is valuable. Herein, we have illustrated the manipulation of optical fields by controlling the polarization or phase to vary periodically and then realized certain focal traces by real-time loading of the computer-generated holograms (CGHs) on the spatial light modulator. The focal trace is composed of many discrete focal spots, which are generated experimentally by using the real-time dynamically controlled CGHs. With the designed focal traces, various microstructures such as an ellipse, a Chinese character “Nan”, and an irregular quadrilateral grid structure are fabricated in the z-cut LiNbO3 wafers, showing good qualities in terms of continuity and homogeneity. Our method proposes a movement free solution for micromachining samples and completely abandons the high precision stage and complex movement control, making microstructure fabrication more flexible, stable, and cheaper.

Chinese Optics Letters
Oct. 09, 2021, Vol. 20 Issue 1 010502 (2022)
Holographic visualization of volume data based on adjustable ray to optical-wave conversion
Xin Zhao, Xinzhu Sang, Hui Li, Duo Chen, Yuanhang Li, Cheng Peng, and Binbin Yan

A holographic visualization of volume data based on adjustable ray to optical-wave conversion is presented. Computer-generated holograms are generated by emitting multiple rays to sample the volumetric field. Equal interval sampling, object light wave adjustment, and information composition are sequentially performed during the march of rays. The program is accelerated in parallel to reduce the total time, and the reconstructions are dynamically adjusted to express different parts of an object. Optical experiments verify that the proposed method can holographically reconstruct the surface and interior information of objects.

Chinese Optics Letters
Sep. 22, 2021, Vol. 20 Issue 1 010501 (2022)
Aperture synthesis based solely on phase images in digital holography
Jun Long, Ping Cai, Chiyue Liu, Weijuan Qu, and Hao Yan

Aperture synthesis is an important approach to improve the lateral resolution of digital holography (DH) techniques. The limitation of the accuracy of registration positions between sub-holograms affects the quality of the synthesized image and even causes the failure of aperture synthesis. It is a major issue in aperture synthesis of DH. Currently intensity images are utilized to find the registration positions of sub-holograms in aperture synthesis. To improve the accuracy of registration positions, we proposed a method based on similarity calculations of the phase images between sub-holograms instead of intensity images. Furthermore, a quantitative indicator, degree of image distortion, was applied to evaluate the synthetic results. Experiments are performed and the results verify that the proposed phase-image-based method is better than the state-of-the-art intensity-image-based techniques in the estimation of registration positions and provides a better synthesized final three-dimensional shape image.

Chinese Optics Letters
Jul. 10, 2021, Vol. 19 Issue 7 070501 (2021)
Pattern-adaptive error diffusion algorithm for improved phase-only hologram generation
Kexuan Liu, Zehao He, and Liangcai Cao

The bidirectional error diffusion (BERD) algorithm is free from random phase modulation that introduces speckle noise on the reconstructed images, compared with other computer-generated phase-only hologram (POH) approaches. During the POH generation process, the amplitudes of all pixels are traditionally set to one for diffusing the errors to their neighborhood of unprocessed pixels. In this paper, we reveal that the reconstruction quality depends on the uniform amplitude value for different object pattern. The pattern-adaptive BERD (PA-BERD) algorithm is proposed for high-quality holographic reconstruction. The optimized amplitude value can be acquired for each object pattern and each propagation distance. The PA-BERD-based POHs have shown higher reconstruction quality than traditional BERD-based POHs in simulations as well as optical experiments.

Chinese Optics Letters
May. 10, 2021, Vol. 19 Issue 5 050501 (2021)
Review of Fresnel incoherent correlation holography with linear and non-linear correlations [Invited]
Vijayakumar Anand, Tomas Katkus, Soon Hock Ng, and Saulius Juodkazis

Fresnel incoherent correlation holography (FINCH) is a well-established incoherent imaging technique. In FINCH, three self-interference holograms are recorded with calculated phase differences between the two interfering, differently modulated object waves and projected into a complex hologram. The object is reconstructed without the twin image and bias terms by a numerical Fresnel back propagation of the complex hologram. A modified approach to implement FINCH by a single camera shot by pre-calibrating the system involving recording of the point spread function library and reconstruction by a non-linear cross correlation has been introduced recently. The expression of the imaging characteristics from the modulation functions in original FINCH and the modified approach by pre-calibration in spatial and polarization multiplexing schemes are reviewed. The study reveals that a reconstructing function completely independent of the function of the phase mask is required for the faithful expression of the characteristics of the modulating function in image reconstruction. In the polarization multiplexing method by non-linear cross correlation, a partial expression was observed, while in the spatial multiplexing method by non-linear cross correlation, the imaging characteristics converged towards a uniform behavior.

Chinese Optics Letters
On the CoverFeb. 10, 2021, Vol. 19 Issue 2 020501 (2021)
Real-time gradation-expressible amplitude-modulation-type electroholography based on binary-weighted computer-generated hologram
Ren Noguchi, Kohei Suzuki, Yoshiki Moriguchi, Minoru Oikawa, Yuichiro Mori, Takashi Kakue, Tomoyoshi Shimobaba, Tomoyoshi Ito, and Naoki Takada

In amplitude-modulation-type electroholography, the binary-weighted computer-generated hologram (BW-CGH) facilitates the gradation-expressible reconstruction of three-dimensional (3D) objects. To realize real-time gradation-expressible electroholography, we propose an efficient and high-speed method for calculating bit planes consisting of BW-CGHs. The proposed method is implemented on a multiple graphics processing unit (GPU) cluster system comprising 13 GPUs. The proposed BW-CGH method realizes eight-gradation-expressible electroholography at approximately the same calculation speed as that of conventional electroholography based on binary computer-generated holograms. Consequently, we were able to successfully reconstruct a real-time electroholographic 3D video comprising approximately 180,000 points expressed in eight gradations at 30 frames per second.

Chinese Optics Letters
Nov. 10, 2021, Vol. 19 Issue 11 110501 (2021)
Dual-band independent phase control based on high efficiency metasurface [Invited]
Jinxing Li, Yueyi Yuan, Qun Wu, Shah Nawaz Burokur, and Kuang Zhang

A general method to realize arbitrary dual-band independent phase control is proposed and demonstrated in this paper. A double-layered C-shape reflective meta-atom is designed to realize independent phase control with high efficiency. As a proof of concept, we propose two functional metasurfaces in the microwave region; the first metasurface performs beam steering in different directions, and the second metasurface generates achromatic beam steering at two distinct frequencies. Both simulation and measurement results agree well with the theoretical pre-setting. The maximum measured efficiency is 88.7% and 92.3% at 6.8 GHz and 8.0 GHz, respectively, for one metasurface, and 91.0% and 89.8% at 6.9 GHz and 8.6 GHz, respectively, for the other.

Chinese Optics Letters
Oct. 10, 2021, Vol. 19 Issue 10 100501 (2021)
Multiple super-resolution imaging in the second band of gradient lattice spacing photonic crystal flat lens
Jie Sheng, Jianlan Xie, and Jianjun Liu

Based on the triangular lattice two-dimensional photonic crystal (PC), the lattice spacing along the transverse direction to propagation is altered, and a gradient PC (GPC) flat lens is designed. The band structures and equal frequency curves of the GPC are calculated; then, the imaging mechanism and feasibility are analyzed. The imaging characteristics of the GPC flat lens are investigated. It is observed that the GPC can achieve multiple types of super-resolution imaging for the point source. This GPC lens is allowed to be applied to imaging and other fields such as filtering and sensing.

Chinese Optics Letters
Dec. 10, 2020, Vol. 18 Issue 12 120501 (2020)
Flat transmitted serrated-phase high-contrast-index subwavelength grating beam splitter
Gongqing Li, Xiaofeng Duan, Yongqing Huang, Kai Liu, and Xiaomin Ren

We proposed a method to form a flat transmitted serrated-phase (SP) high-contrast-index subwavelength grating (HCG) beam splitter (HBS) for all dielectric materials, which is to alternately arrange two kinds of grating bars with a phase difference of π. Compared to the typical linear-phase (LP) HBS, which consists of two symmetrical deflecting gratings, the SP-HBS is extensible in size, and can achieve excellent splitting ability regardless of normal incidence or small-angle oblique incidence with large deflection angles, higher diffraction efficiency, lower energy loss, and higher tolerance of fabrication accuracy. Furthermore, the incident light can be split in half at any part of the SP-HBS, and the output beams of light maintain the original shape. In this Letter, we designed an SP-HBS with a 44.8° deflection angle and a 90.28% transmissivity.

Chinese Optics Letters
Nov. 10, 2020, Vol. 18 Issue 11 110504 (2020)
Experimental demonstration of generating arbitrary total angular momentum states
Heng Zhou, Chunqing Gao, Shiyao Fu, Yanwang Zhai, and Jianqiang Zhang

We experimentally demonstrated an approach to generate arbitrary total angular momentum (TAM) states by using two liquid crystal devices. Photons’ TAM, the sum of spin and orbital angular momenta (SAM and OAM) under paraxial approximation, has found many applications in optics and attracted increasing attention in recent years. Our approach is based on the orthogonality of two eigen SAM components, that arbitrary TAM states will be produced through encoding different holograms in one system. The comparison with theoretical predications yields an excellent agreement, including both the separable state and the non-separable state. The proposed scheme takes a step forward for generating complex structured fields and broadens its application to various fields like laser processing and large capacity data transmission.

Chinese Optics Letters
Nov. 10, 2020, Vol. 18 Issue 11 110503 (2020)
Screen printing of upconversion NaYF4:Yb3+/Eu3+ with Li+ doped for anti-counterfeiting application
Dongdong Li, Jianye Mo, Chong Wang, Wei Liu, Haibo Ge, Dongdong Han, Aihua Hao, Baoyu Chai, and Jiangbo She

Li ions affect the upconversion efficiency by changing the local crystal field of the luminescent center. Herein, in order to improve the upconversion efficiency of NaYF4:Yb3+/Eu3+, a series of NaYF4:Yb3+/Eu3+ micro-particles with different Li+ doping concentrations were synthesized by the hydrothermal synthesis method, respectively. Firstly, the structure and morphology of NaYF4:Yb3+/Eu3+ upconversion micro-particles (UCMPs) with different doping concentrations were analyzed by X-ray diffraction and a scanning electron microscope (SEM). SEM results show that the UCMPs are not only highly crystallized, but also have hexagons with different Li+ concentrations of NaYF4:Yb3+/Eu3+. X-ray diffraction shows that the crystal field around Eu3+ changes with the increase of Li+ concentration. Then, the fluorescence spectrum of NaYF4:Yb3+/Eu3+ was studied under the irradiation of a 980 nm laser. The results show that the fluorescence intensity of NaYF4:Yb3+/Eu3+ with 2% Li+ is the strongest, which is twice the intensity of NaYF4:Yb3+/Eu3+ without Li+. Finally, the fluorescence imaging analysis of NaYF4:Yb3+/Eu3+ with 2% Li+ concentration was carried out. The UCMPs are used to screen printing to evaluate the imaging effect on different sample surfaces. The results show NaYF4:Yb3+/Eu3+ (with 2% Li+) has great application prospects in anti-counterfeiting recognition.

Chinese Optics Letters
Nov. 10, 2020, Vol. 18 Issue 11 110501 (2020)
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