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Optics, Volume 6, Issue 2 (June 2025) – 6 articles

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10 pages, 1800 KiB

Open AccessArticle

Automatic Focusing of Off-Axis Digital Holographic Microscopy by Combining the Discrete Cosine Transform Sparse Dictionary with the Edge Preservation Index

by Zhaoliang Liu, Peizhen Qiu and Yupei Zhang

Optics 2025, 6(2), 17; https://doi.org/10.3390/opt6020017 - 6 May 2025

Abstract

Automatic focusing is a crucial research issue for achieving high-quality reconstructed images in digital holographic microscopy. This paper proposes an automatic focusing method that combines the discrete cosine transform (DCT) sparse dictionary with edge preservation index (EPI) criteria for off-axis digital holographic microscopy. Specifically, within a predefined search range, Fresnel transform is utilized to reconstruct the off-axis digital hologram, yielding reconstruction images at various reconstruction distances. Synchronously, the DCT sparse dictionary is employed to reduce speckle noise, and the EPI is calculated between the denoised image and original image. The value of EPI is used as an indicator for assessing the focal position. A single-peak focusing curve is obtained within the search range 10 mm, with a step size of 0.1 mm. Once the optimal focus position is determined, a focused and noise-reduced reconstructed image can be simultaneously achieved. Full article

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16 pages, 16987 KiB

Open AccessArticle

Comparative Molecular Dynamics Study of Graphitization Mechanisms in Nanosecond Laser Irradiation of Single-Crystal and Nanocrystalline Diamond

by Huixin Yuan, Liang Zhao, Weimian Guan, Yuqi Yang and Junjie Zhang

Optics 2025, 6(2), 16; https://doi.org/10.3390/opt6020016 - 25 Apr 2025

Viewed by 121

Abstract

The mechanisms of material removal and structural transformation under laser radiation differ significantly between single-crystal diamond (SCD) and nanocrystalline diamond (NCD). This study employs atomic simulations to investigate the material removal mechanisms and structural transformation behaviors of SCD and NCD when subjected to laser irradiation. We analyze the effects of temperature and stress changes induced by laser radiation on structural transformations, revealing the driving mechanism behind graphitization transitions. Specifically, the thermal–mechanical coupling effect induced by lasers leads to graphitization in SCD, while in NCD, due to the stress concentration effects at the grain boundaries, graphitization preferentially occurs at these boundaries. The material removal processes for both SCD and NCD are attributed to thermal stress concentrations in the regions where the laser interacts with the diamond surface. This investigation provides a theoretical foundation for a more profound understanding of the behavior of diamond materials during laser irradiation. Full article

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15 pages, 7817 KiB

Open AccessArticle

Sparsity-Guided Phase Retrieval to Handle Concave- and Convex-Shaped Specimens in Inline Holography, Taking the Complexity Parameter into Account

by Yao Koffi, Jocelyne M. Bosson, Marius Ipo Gnetto and Jeremie T. Zoueu

Optics 2025, 6(2), 15; https://doi.org/10.3390/opt6020015 - 17 Apr 2025

Viewed by 200

Abstract

In this work, we explore an optimization idea for the complexity guidance of a phase retrieval solution for a single acquired hologram. This method associates free-space backpropagation with the fast iterative shrinkage-thresholding algorithm (FISTA), which incorporates an improvement in the total variation (TV) to guide the complexity of the phase retrieval solution from the complex diffracted field measurement. The developed procedure can provide excellent phase reconstruction using only a single acquired hologram. Full article

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13 pages, 2979 KiB

Open AccessArticle

Growth and Properties of (Yb-Er) Co-Doped ZnO Thin Films Deposited via Spray Pyrolysis Technique

by Abderrahim El Hat, Imane Chaki, Rida Essajai, Abdelmajid Fakhim Lamrani, Boubker Fares, Mohammed Regragui, Aziz Dinia and Mohammed Abd-Lefdil

Optics 2025, 6(2), 14; https://doi.org/10.3390/opt6020014 - 3 Apr 2025

Viewed by 305

Abstract

Yb_xEr_yZnO thin films with a low concentration (x = 5%, y = 0, 1, 3%) were made on glass substrates using the spray pyrolysis method. The films were characterized through the use of specific techniques to investigate their structural, optical, and electrical properties. The XRD structural analysis of the films revealed that they are polycrystalline with a hexagonal wurtzite structure and a preferential orientation in the (002) direction. The optical characterization of the co-doped layers in the range of 200 to 800 nm revealed that co-doping had a significant impact on the values of transmission. A well-defined peak in the infrared domain centered around 980 nm was observed in photoluminescence measurements. This peak signifies the transition between the electronic levels ²F_5/2 (ground state) and ²F_7/2 (excited state), proving that photons are efficiently transferred between the ZnO matrix and the Yb³⁺ ion. All layers exhibited n-type conduction and an electrical resistivity decrease to 6.0 × 10⁻² Ω cm according to Hall effect measurements at room temperature. Full article

(This article belongs to the Special Issue Optoelectronic Thin Films)

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15 pages, 5132 KiB

Open AccessArticle

Theoretical Model of Curved Liquid Surface in the Microholes for Molding Microlenses

by Yuyang Shi, Yan Long, Xuhui Zhang, Li Wei, Bo Dai and Dawei Zhang

Optics 2025, 6(2), 13; https://doi.org/10.3390/opt6020013 - 3 Apr 2025

Viewed by 237

Abstract

Microlenses are essential optics widely used in many fields. The microfluidics-assisted fabrication method provides a rapid, convenient way to manufacture microlenses. However, there is no mathematical model to describe the profile of the liquid surface. This paper provides a theoretical explanation and mathematical model for the formation of the liquid surface of different curvatures in the microholes of different shapes. A numerical model based on the finite–difference time–domain method verifies the mathematical model. Furthermore, the optical properties of the microlenses in different shapes are analyzed through the demolded microlenses. The proposed theoretical model provides an analytical way to study the properties of microlenses. Full article

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8 pages, 1345 KiB

Open AccessArticle

High-Myopia Diagnosis by Reciprocal of Circle Radius in Choroidal Image

by Shuo-Tsung Chen, Ren-Jie Ye, Ching-Fu Chen, Keng-Yuan Chang, Yu-Hung Huang, Sheng-Jie Tseng and Jun-Qi Liu

Optics 2025, 6(2), 12; https://doi.org/10.3390/opt6020012 - 1 Apr 2025

Cited by 1 | Viewed by 273

Abstract

Light-emitting electronic devices are widely used at present, and this increases the risk of myopia. Thus, the early identification of myopia and the prevention of further exacerbation has become an essential issue in ophthalmology. Recently, choroidal imaging has been used to assist in the early identification and prevention of high myopia due to the fact that swept-source optical coherence tomography (SS-OCT) is an essential diagnostic tool in ophthalmology. This study presents a novel method to judge high myopia using the SS-OCT image dataset obtained from a university hospital. In order to relate the proposed method to the region of the SS-OCT image, the curvature analysis of an arbitrary segmented curve similar to the region of the SS-OCT is first illustrated by quadratic functions and matrix operations. Next, the curvature formula is derived and then applied to the choroidal curve obtained manually in each patient’s choroidal image. In particular, we applied the curvature analysis and its results to find the maximal curvature and average curvature of each patient’s choroidal curve. Finally, we used the maximal curvature and average curvature to evaluate high myopia. The accuracy of the proposed maximal curvature method and average curvature method in the experimental results to verify the proposed method. Full article

(This article belongs to the Special Issue Advanced Optical Imaging for Biomedicine)

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