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16 pages, 3676 KiB

Open AccessArticle

Design and Structure of a Non-Coaxial Multi-Focal Composite Fresnel Acoustic Lens for Synergistic Ultrasound Stimulation of Multiple Brain Regions

by Ruiqi Wu, Fangfang Shi, Juan Tao, Jiajia Zhao, Jinying Zhang, Xianmei Wu and Jingjing Xu

Sensors 2025, 25(11), 3299; https://doi.org/10.3390/s25113299 - 24 May 2025

Viewed by 517

Abstract

Transcranial focused ultrasound (TcFUS) neuromodulation is hindered by skull-induced acoustic limitations. To enable synergistic multi-region brain stimulation, we designed non-coaxial multi-focal composite Fresnel acoustic lenses, including an overlapping Fresnel lens (OFL) and an alternating-segmented Fresnel lens (ASFL). These lenses convert planar ultrasound into [...] Read more.

Transcranial focused ultrasound (TcFUS) neuromodulation is hindered by skull-induced acoustic limitations. To enable synergistic multi-region brain stimulation, we designed non-coaxial multi-focal composite Fresnel acoustic lenses, including an overlapping Fresnel lens (OFL) and an alternating-segmented Fresnel lens (ASFL). These lenses convert planar ultrasound into multiple foci. Based on Fresnel theory, acoustic fields were analyzed via simulations and experiments, validating the generation of four non-coaxial foci (10/30 mm focal lengths) from a 1 MHz planar wave using both OFL and ASFL. The influence of lens parameters on focal pressure distribution was investigated, and morphology was quantified using a linear least-squares method. Significant differences in focal morphology and intensity between OFL and ASFL provide crucial guidance for optimizing multi-target TcFUS strategies. Full article

(This article belongs to the Section Biomedical Sensors)

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12 pages, 2383 KiB

Open AccessEditor’s ChoiceArticle

Novel Focusing Performances of High-Numerical-Aperture Micro-Fresnel Zone Plates with Selective Occlusion

by Qiang Liu, Yunpeng Wu, Yuanhao Deng, Junli Wang, Wenshuai Liu and Xiaomin Yao

Photonics 2025, 12(4), 372; https://doi.org/10.3390/photonics12040372 - 13 Apr 2025

Viewed by 526

Abstract

In this study, novel focusing performances of high-numerical-aperture (NA) micro-Fresnel zone plates (FZPs) with selective occlusion are identified and investigated through numerical calculations based on vectorial angular spectrum (VAS) theory, and further rigorously validated using the finite-difference time-domain (FDTD) method. The central occlusion [...] Read more.

In this study, novel focusing performances of high-numerical-aperture (NA) micro-Fresnel zone plates (FZPs) with selective occlusion are identified and investigated through numerical calculations based on vectorial angular spectrum (VAS) theory, and further rigorously validated using the finite-difference time-domain (FDTD) method. The central occlusion of a standard micro-FZP can significantly extend the depth of focus while keeping the lateral size of the focusing spot essentially unchanged. When a standard micro-FZP only retains two separated transparent rings and all other rings are obstructed, it will result in multi-focus phenomena; at the same time, the number of focal points is equal to the difference in number between the two separated transparent rings. Furthermore, a focusing light needle can be generated by combining the central occlusion and wavelength shift of a standard micro-FZP. This study not only provides new ideas for the design and optimization of micro-FZPs but also provides reference for the expansion of practical applications of FZPs. Full article

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13 pages, 27964 KiB

Open AccessArticle

Enhanced Terahertz Sensing via On-Chip Integration of Diffractive Optics with InGaAs Bow-Tie Detectors

by Karolis Redeckas, Vytautas Jakštas, Matas Bernatonis, Vincas Tamošiūnas, Gintaras Valušis and Linas Minkevičius

Sensors 2025, 25(1), 229; https://doi.org/10.3390/s25010229 - 3 Jan 2025

Viewed by 953

Abstract

The practical implementation of terahertz (THz) imaging and spectroscopic systems in real operational conditions requires them to be of a compact size, to have enhanced functionality, and to be user-friendly. This work demonstrates the single-sided integration of Fresnel-zone-plate-based optical elements with InGaAs bow-tie [...] Read more.

The practical implementation of terahertz (THz) imaging and spectroscopic systems in real operational conditions requires them to be of a compact size, to have enhanced functionality, and to be user-friendly. This work demonstrates the single-sided integration of Fresnel-zone-plate-based optical elements with InGaAs bow-tie diodes directly on a semiconductor chip. Numerical simulations were conducted to optimize the Fresnel zone plate’s focal length and the InP substrate’s thickness to achieve constructive interference at 600 GHz, room-temperature operation and achieve a sensitivity more than an order of magnitude higher—up to 24.5 V/W—than that of a standalone bow-tie detector. Investigations revealed the strong angular dependence of the incident radiation on the Fresnel zone plate-integrated bow-tie diode’s response. These findings pave a promising avenue for the further development of single-sided integration of flat optics with THz detectors, enabling improved sensitivity, simplified manufacturing processes, and reduced costs for THz detection systems in a more compact design scheme. Full article

(This article belongs to the Special Issue Topical Advisory Panel Members' Collection Series: "Terahertz Sensors")

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14 pages, 5809 KiB

Open AccessArticle

Creating Tunable Micro-Optical Components via Photopolymerization 3D Printing Combined with Polymer-Dispersed Liquid Crystals

by Sheng-Yuan Zhang, Hsi-Fu Shih, Chuen-Lin Tien and Han-Yen Tu

Micromachines 2025, 16(1), 26; https://doi.org/10.3390/mi16010026 - 28 Dec 2024

Viewed by 1305

Abstract

Based on additive manufacturing via photopolymerization, this study combines polymer-dispersed liquid crystal (PDLC) technology with 3D printing technology to produce tunable micro-optical components with switchable diffraction or focusing characteristics. The diffraction grating and Fresnel zone plate are the research targets. Their structures are [...] Read more.

Based on additive manufacturing via photopolymerization, this study combines polymer-dispersed liquid crystal (PDLC) technology with 3D printing technology to produce tunable micro-optical components with switchable diffraction or focusing characteristics. The diffraction grating and Fresnel zone plate are the research targets. Their structures are designed and simulated to achieve expected optical functions. A liquid crystal display (LCD) 3D printer is used to produce structures on transparent conductive substrates. The printed structures are filled with PDLCs and covered with transparent conductive substrates to achieve tunable functions. The proposed configurations are implemented and verified. The experimental results show that the diffraction efficiency of the 0th order increases from 15% to 50% for the diffraction grating and the focusing spot intensity decreases from 74% to 12% after the application of an electric field. These results demonstrate the feasibility of the proposed tunable optical component configurations. Full article

(This article belongs to the Section D3: 3D Printing and Additive Manufacturing)

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23 pages, 128368 KiB

Open AccessArticle

Optimization of Soft X-Ray Fresnel Zone Plate Fabrication Through Joint Electron Beam Lithography and Cryo-Etching Techniques

by Maha Labani, Vito Clericò, Enrique Diez, Giancarlo Gatti, Mario Amado and Ana Pérez-Rodríguez

Nanomaterials 2024, 14(23), 1898; https://doi.org/10.3390/nano14231898 - 26 Nov 2024

Cited by 1 | Viewed by 1410

Abstract

The ability to manufacture complex 3D structures with nanometer-scale resolution, such as Fresnel Zone Plates (FZPs), is crucial to achieve state-of-the-art control in X-ray sources for use in a diverse range of cutting-edge applications. This study demonstrates a novel approach combining Electron Beam [...] Read more.

The ability to manufacture complex 3D structures with nanometer-scale resolution, such as Fresnel Zone Plates (FZPs), is crucial to achieve state-of-the-art control in X-ray sources for use in a diverse range of cutting-edge applications. This study demonstrates a novel approach combining Electron Beam Lithography (EBL) and cryoetching to produce silicon-based FZP prototypes as a test bench to assess the strong points and limitations of this fabrication method. Through this method, we obtained FZPs with 100 zones, a diameter of 20 µm, and an outermost zone width of 50 nm, resulting in a high aspect ratio that is suitable for use across a range of photon energies. The process incorporates a chromium mask in the EBL stage, enhancing microstructure precision and mitigating pattern collapse challenges. This minimized issues of under- and over-etching, producing well-defined patterns with a nanometer-scale resolution and low roughness. The refined process thus holds promise for achieving improved optical resolution and efficiency in FZPs, making it viable for the fabrication of high-performance, nanometer-scale devices. Full article

(This article belongs to the Special Issue Mechanical Properties and Applications for Nanostructured Alloys)

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46 pages, 19002 KiB

Open AccessArticle

³Cat-8 Mission: A 6-Unit CubeSat for Ionospheric Multisensing and Technology Demonstration Test-Bed

by Luis Contreras-Benito, Ksenia Osipova, Jeimmy Nataly Buitrago-Leiva, Guillem Gracia-Sola, Francesco Coppa, Pau Climent-Salazar, Paula Sopena-Coello, Diego Garcín, Juan Ramos-Castro and Adriano Camps

Remote Sens. 2024, 16(22), 4199; https://doi.org/10.3390/rs16224199 - 11 Nov 2024

Viewed by 3242

Abstract

This paper presents the mission analysis of ³Cat-8, a 6-Unit CubeSat mission being developed by the UPC NanoSat Lab for ionospheric research. The primary objective of the mission is to monitor the ionospheric scintillation of the aurora, and to perform several technological [...] Read more.

This paper presents the mission analysis of ³Cat-8, a 6-Unit CubeSat mission being developed by the UPC NanoSat Lab for ionospheric research. The primary objective of the mission is to monitor the ionospheric scintillation of the aurora, and to perform several technological demonstrations. The satellite incorporates several novel systems, including a deployable Fresnel Zone Plate Antenna (FZPA), an integrated PocketQube deployer, a dual-receiver GNSS board for radio occultation and reflectometry experiments, and a polarimetric multi-spectral imager for auroral emission observations. The mission design, the suite of payloads, and the concept of operations are described in detail. This paper discusses the current development status of ³Cat-8, with several subsystems already developed and others in the final design phase. It is expected that the data gathered by ³Cat-8 will contribute to a better understanding of ionospheric effects on radio wave propagation and demonstrate the feasibility of compact remote sensors in a CubeSat platform. Full article

(This article belongs to the Special Issue Advances in CubeSats for Earth Observation)

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12 pages, 2404 KiB

Open AccessArticle

Investigation of an Active Focusing Planar Piezoelectric Ultrasonic Transducer

by Qiao Wu, Bin You, Xu Zhang and Jun Tu

Sensors 2024, 24(13), 4082; https://doi.org/10.3390/s24134082 - 23 Jun 2024

Cited by 1 | Viewed by 1948

Abstract

Ultrasonic focusing transducers have broad prospects in advanced ultrasonic non-destructive testing fields. However, conventional focusing methods that use acoustic concave lenses can disrupt the acoustic impedance matching condition, thereby adversely affecting the sensitivity of the transducers. In this paper, an active focusing planar [...] Read more.

Ultrasonic focusing transducers have broad prospects in advanced ultrasonic non-destructive testing fields. However, conventional focusing methods that use acoustic concave lenses can disrupt the acoustic impedance matching condition, thereby adversely affecting the sensitivity of the transducers. In this paper, an active focusing planar ultrasonic transducer is designed and presented to achieve a focusing effect with a higher sensitivity. An electrode pattern consisting of multiple concentric rings is designed, which is inspired by the structure of Fresnel Zone Plates (FZP). The structural parameters are optimized using finite element simulation methods. A prototype of the transducer is manufactured with electrode patterns made of conductive silver paste using silk screen-printing technology. Conventional focusing transducers using an acoustic lens and an FZP baffle are also manufactured, and their focusing performances are comparatively tested. The experimental results show that our novel transducer has a focal length of 16 mm and a center frequency of 1.16 MHz, and that the sensitivity is improved by 23.3% compared with the conventional focusing transducers. This research provides a new approach for the design of focusing transducers. Full article

(This article belongs to the Section Industrial Sensors)

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11 pages, 2614 KiB

Open AccessArticle

The Generation of Equal-Intensity and Multi-Focus Optical Vortices by a Composite Spiral Zone Plate

by Huaping Zang, Jingzhe Li, Chenglong Zheng, Yongzhi Tian, Lai Wei, Quanping Fan, Shaoyi Wang, Chuanke Wang, Juan Xie and Leifeng Cao

Photonics 2024, 11(5), 466; https://doi.org/10.3390/photonics11050466 - 15 May 2024

Cited by 1 | Viewed by 1713

Abstract

We propose a new vortex lens for producing multiple focused coaxial vortices with approximately equal intensities along the optical axis, termed equal-intensity multi-focus composite spiral zone plates (EMCSZPs). In this typical methodology, two concentric conventional spiral zone plates (SZPs) of different focal lengths [...] Read more.

We propose a new vortex lens for producing multiple focused coaxial vortices with approximately equal intensities along the optical axis, termed equal-intensity multi-focus composite spiral zone plates (EMCSZPs). In this typical methodology, two concentric conventional spiral zone plates (SZPs) of different focal lengths were composited together and the alternate transparent and opaque zones were arranged with specific m-bonacci sequence. Based on the Fresnel–Kirchhoff diffraction theory, the focusing properties of the EMCSZPs were calculated in detail and the corresponding demonstration experiment was been carried out to verify our proposal. The investigations indicate that the EMCSZPs indeed exhibit superior performance, which accords well with our physical design. In addition, the topological charges (TCs) of the multi-focus vortices can be flexibly selected and controlled by optimizing the parameters of the zone plates. These findings which were demonstrated by the performed experiment may open new avenues towards improving the performance of biomedical imaging, quantum computation and optical manipulation. Full article

(This article belongs to the Special Issue Space Division Multiplexing Techniques)

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9 pages, 5379 KiB

Open AccessCommunication

Generation of a Focused THz Vortex Beam from a Spintronic THz Emitter with a Helical Fresnel Zone Plate

by Xiaoqiang Zhang, Yong Xu, Bin Hong, Fan Zhang, Anting Wang and Weisheng Zhao

Nanomaterials 2023, 13(14), 2037; https://doi.org/10.3390/nano13142037 - 10 Jul 2023

Cited by 6 | Viewed by 1865

Abstract

Similar to optical vortex beams, terahertz (THz) vortex beams (TVBs) also carry orbital angular momentum (OAM). However, little research has been reported on the generation of TVBs. In this paper, based on the detour phase technique, we design a series of spintronic terahertz [...] Read more.

Similar to optical vortex beams, terahertz (THz) vortex beams (TVBs) also carry orbital angular momentum (OAM). However, little research has been reported on the generation of TVBs. In this paper, based on the detour phase technique, we design a series of spintronic terahertz emitters with a helical Fresnel zone plate (STE-HFZP) to directly generate focused TVBs with topological charges (TCs) of l = ±1, ±2 and ±3, respectively. The STE-HFZP is a hybrid THz device composed of a terahertz emitter and a THz lens, and it has a high numerical aperture (NA), achieving subwavelength focal spots. Its focus properties are surveyed systemically through accurate simulations. This STE-HFZP can also generate focused TVBs with higher order TCs. More importantly, the components of the focused electric field with OAM make up the majority of the intensity and have potential applications in the field of THz communications, THz imaging and atom trapping. Full article

(This article belongs to the Special Issue Laser-Matter Interaction for Nanostructuration and Characterization: From Fundamentals to Sensing and Energy Applications)

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14 pages, 4473 KiB

Open AccessArticle

Silicon Fresnel Zone Plate Metalens with Subwavelength Gratings

by William Fraser and Winnie N. Ye

Sensors 2023, 23(8), 4137; https://doi.org/10.3390/s23084137 - 20 Apr 2023

Cited by 4 | Viewed by 3582

Abstract

Metalenses are planar optical components that have demonstrated immense potential for integrated optics. In particular, they are capable of high-efficiency subwavelength focusing without the bulkiness of traditional lenses. Dielectric metalenses operating in the C-band typically employ relatively tall, amorphous silicon structures arranged in [...] Read more.

Metalenses are planar optical components that have demonstrated immense potential for integrated optics. In particular, they are capable of high-efficiency subwavelength focusing without the bulkiness of traditional lenses. Dielectric metalenses operating in the C-band typically employ relatively tall, amorphous silicon structures arranged in a periodic array. Phase control spanning from 0 to 2π is accessed by varying the geometry of these scattering structures. The full 2π phase range is necessary to impose a hyperbolic focusing phase profile, but this is difficult to achieve without custom fabrication practices. In this work, we propose a binary phase Fresnel zone plate metalens designed for the standard 500 nm silicon-on-insulator platform. Our design uses subwavelength gratings with trapezoidal segmentation to form concentric rings. The effective index of the grating is set with the duty cycle using a single full-etch step to form the binary phase profile of the zone plate. The metalens design can be easily tuned to achieve longer focal lengths at different wavelengths. It offers a simple platform for high-throughput wavelength-scale focusing elements in free-space optics, including for microscopy and medical imaging. Full article

(This article belongs to the Section Optical Sensors)

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9 pages, 2511 KiB

Open AccessArticle

Comparative Study of Numerical Methods for Solving the Fresnel Integral in Aperiodic Diffractive Lenses

by Adrián Garmendía-Martínez, Francisco M. Muñoz-Pérez, Walter D. Furlan, Fernando Giménez, Juan C. Castro-Palacio, Juan A. Monsoriu and Vicente Ferrando

Mathematics 2023, 11(4), 946; https://doi.org/10.3390/math11040946 - 13 Feb 2023

Cited by 7 | Viewed by 3139

Abstract

In this work, we present a comparative analysis of different numerical methods to obtain the focusing properties of the zone plates based on Fibonacci and Cantor sequences. The Fresnel approximation was solved numerically in order to obtain the axial irradiance provided by these [...] Read more.

In this work, we present a comparative analysis of different numerical methods to obtain the focusing properties of the zone plates based on Fibonacci and Cantor sequences. The Fresnel approximation was solved numerically in order to obtain the axial irradiance provided by these diffractive lenses. Two different methods were applied. The first one is based on numerical integration, specifically the Simpson integration method and the two-dimensional Gaussian quadrature. The second consisted in the implementation of the Fast Fourier Transform in both one and two dimensions. The axial irradiance of the lenses, the relative error with respect to the analytical solution, and the calculation time required by each method are analyzed and compared. From this analysis it was concluded that the Gauss method presents the best balance between accuracy and computation time. This analysis could be useful to decide the most convenient numerical method to be used for the study of more complex diffractive structures. Full article

(This article belongs to the Special Issue Numerical Analysis and Modeling)

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12 pages, 6146 KiB

Open AccessArticle

An Annular Fresnel Zone Plate without Central Spots Fabricated by Femtosecond Laser Direct Writing

by Xiaoyan Sun, Fang Zhou and Lian Duan

Micromachines 2022, 13(8), 1285; https://doi.org/10.3390/mi13081285 - 10 Aug 2022

Cited by 2 | Viewed by 3122

Abstract

In recent years, micro-annular beams have been widely used, which has expanded the possibilities for laser processing. However, the current method of generating an annular beam still has shortcomings, such as spot energy at the center of the produced beam. In this study, [...] Read more.

In recent years, micro-annular beams have been widely used, which has expanded the possibilities for laser processing. However, the current method of generating an annular beam still has shortcomings, such as spot energy at the center of the produced beam. In this study, a Fresnel zone plate with an annular structure was machined using a femtosecond laser. After focusing, an annular laser beam without a spot in the center was obtained, and the radius and focal length of the annular beam could be easily adjusted. In addition, two annular Fresnel zone plates were concentrically connected to obtain a concentric double-ring beam in the same focal plane. The simulation and experimental results were consistent, providing effective potential for applications related to nontraditionally shaped laser beams. Full article

(This article belongs to the Special Issue Micro/Nano Structures and Systems: Analysis, Design, Manufacturing, and Reliability)

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12 pages, 7586 KiB

Open AccessArticle

Investigation of Quadrate Fresnel Zone Plates Fabricated by Femtosecond Laser Direct Writing

by Xiaoyan Sun, Fang Zhou, Lian Duan and Ji-an Duan

Appl. Sci. 2022, 12(15), 7788; https://doi.org/10.3390/app12157788 - 2 Aug 2022

Cited by 5 | Viewed by 2020

Abstract

The circular Fresnel zone plate (FZP) has been extensively used in micro-optics due to its outstanding focusing performance. Despite that, the curved edge of the circular zone has drawbacks limiting its use in terahertz imaging, array generator, and micro/nano-manufacturing. Therefore, a circular structure [...] Read more.

The circular Fresnel zone plate (FZP) has been extensively used in micro-optics due to its outstanding focusing performance. Despite that, the curved edge of the circular zone has drawbacks limiting its use in terahertz imaging, array generator, and micro/nano-manufacturing. Therefore, a circular structure is not very practical to use. In this paper, Quadrate FZPs were proposed, and diffraction properties of the linear FZP (L-FZP), orthogonal FZP (O-FZP), and square FZP (S-FZP) were studied. Theoretically, the relationship between half side length of S-FZP and the radius of circular FZP is established, and the focal length formula of the S-FZP is derived. The linear and crossline focusing properties of quadrate FZPs were studied experimentally along with simulations. With the addition of blockers and phase shifting, the square and rectangular diffraction patterns were successfully obtained. Full article

(This article belongs to the Special Issue Laser-Based Micro/Nano Manufacturing Technology)

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12 pages, 7132 KiB

Open AccessCommunication

Amplitude Zone Plate in Adaptive Optics: Proposal of the Principle

by Vasily Matkivsky, Alexsandr Moiseev, Pavel Shilyagin and Grigory Gelikonov

Photonics 2022, 9(3), 163; https://doi.org/10.3390/photonics9030163 - 7 Mar 2022

Viewed by 2588

Abstract

One of the main elements in hardware-based adaptive optics systems is a deformable mirror. There is quite a large number of such mirrors based on different principles and exhibiting varying performance. They constitute a significant portion of the cost of the final optical [...] Read more.

One of the main elements in hardware-based adaptive optics systems is a deformable mirror. There is quite a large number of such mirrors based on different principles and exhibiting varying performance. They constitute a significant portion of the cost of the final optical devices. In this study, we consider the possibility of replacing an adaptive mirror with the adaptive amplitude Fresnel zone plate, implemented using a digital light-processing matrix. Since such matrices are widely used in mass industry products (light projectors), their costs in large batches are 1–2 orders of magnitude lower than the cost of inexpensive deformable mirrors. Numerical modeling for scanning an optical coherence tomography system with adaptive optics is presented. It is shown that wavefront distortions with high spatial frequencies and large amplitudes can be corrected using an amplitude Fresnel zone plate. The results are compared with piezoelectric and microelectromechanical system mirrors. Full article

(This article belongs to the Special Issue Advances in Modern Photonics)

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13 pages, 5354 KiB

Open AccessArticle

Flexible Plenoptic X-ray Microscopy

by Elena Longo, Domenico Alj, Joost Batenburg, Ombeline de La Rochefoucauld, Charlotte Herzog, Imke Greving, Ying Li, Mikhail Lyubomirskiy, Ken Vidar Falch, Patricia Estrela, Silja Flenner, Nicola Viganò, Marta Fajardo and Philippe Zeitoun

Photonics 2022, 9(2), 98; https://doi.org/10.3390/photonics9020098 - 8 Feb 2022

Cited by 2 | Viewed by 3821

Abstract

X-ray computed tomography (CT) is an invaluable technique for generating three-dimensional (3D) images of inert or living specimens. X-ray CT is used in many scientific, industrial, and societal fields. Compared to conventional 2D X-ray imaging, CT requires longer acquisition times because up to [...] Read more.

X-ray computed tomography (CT) is an invaluable technique for generating three-dimensional (3D) images of inert or living specimens. X-ray CT is used in many scientific, industrial, and societal fields. Compared to conventional 2D X-ray imaging, CT requires longer acquisition times because up to several thousand projections are required for reconstructing a single high-resolution 3D volume. Plenoptic imaging—an emerging technology in visible light field photography—highlights the potential of capturing quasi-3D information with a single exposure. Here, we show the first demonstration of a flexible plenoptic microscope operating with hard X-rays; it is used to computationally reconstruct images at different depths along the optical axis. The experimental results are consistent with the expected axial refocusing, precision, and spatial resolution. Thus, this proof-of-concept experiment opens the horizons to quasi-3D X-ray imaging, without sample rotation, with spatial resolution of a few hundred nanometres. Full article

(This article belongs to the Special Issue Advances in X-ray Optics)

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