Journal of Optics

The Zernike polynomials are a complete set of continuous functions orthogonal over a unit circle. Since first developed by Zernike in 1934, they have been in widespread use in many fields ranging from optics, vision sciences, to image processing. However, due to the lack of a unified definition, many confusing indices have been used in the past decades and mathematical properties are scattered in the literature. This review provides a comprehensive account of Zernike circle polynomials and their noncircular derivatives, including history, definitions, mathematical properties, roles in wavefront fitting, relationships with optical aberrations, and connections with other polynomials. We also survey state-of-the-art applications of Zernike polynomials in a range of fields, including the diffraction theory of aberrations, optical design, optical testing, ophthalmic optics, adaptive optics, and image analysis. Owing to their elegant and rigorous mathematical properties, the range of scientific and industrial applications of Zernike polynomials is likely to expand. This review is expected to clear up the confusion of different indices, provide a self-contained reference guide for beginners as well as specialists, and facilitate further developments and applications of the Zernike polynomials.

We first review basic equations of plasmonics in anisotropic media. We recall the origin of Maxwell's gradient index fisheye lens. We then apply tools of transformation optics to the design of a cyclic concentrator and a variety of plasmonic carpet-cloaks. We further give a brief account of the discovery of spoof plasmon polaritons (SfPPs) by Pendry et al (2004 Science 305 847–8) 150 years after Maxwell invented the fisheye lens. Finally, we experimentally demonstrate a concept of a fisheye lens for SfPPs at microwave frequencies. We stress that perfect metal surfaces perforated with dielectrics offer a playground for moulding surface waves in many areas of physics.

The Covid-19 pandemic showed forcefully the fundamental importance broadband data communication and the internet has in our society. Optical communications forms the undisputable backbone of this critical infrastructure, and it is supported by an interdisciplinary research community striving to improve and develop it further. Since the first 'Roadmap of optical communications' was published in 2016, the field has seen significant progress in all areas, and time is ripe for an update of the research status. The optical communications area has become increasingly diverse, covering research in fundamental physics and materials science, high-speed electronics and photonics, signal processing and coding, and communication systems and networks. This roadmap describes state-of-the-art and future outlooks in the optical communications field. The article is divided into 20 sections on selected areas, each written by a leading expert in that area. The sections are thematically grouped into four parts with 4–6 sections each, covering, respectively, hardware, algorithms, networks and systems. Each section describes the current status, the future challenges, and development needed to meet said challenges in their area. As a whole, this roadmap provides a comprehensive and unprecedented overview of the contemporary optical communications research, and should be essential reading for researchers at any level active in this field.

Subluminal and superluminal propagation of light, and normal and anomalous rotary photon drag are reported in this manuscript using control fields of the Milnor polynomial. A crater-type absorption spectrum is reported, having a central peak at the origin with a variation of positions. The normal and anomalous dispersion are associated with the low and high absorption regions. The group index varies in the range of $-1 \times 10^{7} \unicode{x2A7D} n_{\mathrm{g}} \unicode{x2A7D} 1 \times 10^{7}$. This corresponds to a group velocity of $v_{\mathrm{g}} = c/n_{\mathrm{g}}$ varying in the range of $-30\,\mathrm{m}\,\mathrm{s}^{-1} \unicode{x2A7D} v_{\mathrm{g}} \unicode{x2A7D} 30\,\mathrm{m}\,\mathrm{s}^{-1}$. The maximum positive and negative delay times are measured to $\pm0.002\,\mathrm{s}$. It is reported that slow light propagation generates normal photon drag and fast light propagation generates anomalous rotary photon drag in a spinning medium. The rotary photon drag varies in the range of $-0.005\,\mathrm{rad} \unicode{x2A7D} \theta_{\mathrm{d}} \unicode{x2A7D} 0.005\,\mathrm{rad}$, where both normal and anomalous drag is also observed. The modified results will be useful in optical and biomedical sensors, and imaging and telecommunication technologies.

Metasurfaces are thin two-dimensional metamaterial layers that allow or inhibit the propagation of electromagnetic waves in desired directions. For example, metasurfaces have been demonstrated to produce unusual scattering properties of incident plane waves or to guide and modulate surface waves to obtain desired radiation properties. These properties have been employed, for example, to create innovative wireless receivers and transmitters. In addition, metasurfaces have recently been proposed to confine electromagnetic waves, thereby avoiding undesired leakage of energy and increasing the overall efficiency of electromagnetic instruments and devices. The main advantages of metasurfaces with respect to the existing conventional technology include their low cost, low level of absorption in comparison with bulky metamaterials, and easy integration due to their thin profile. Due to these advantages, they are promising candidates for real-world solutions to overcome the challenges posed by the next generation of transmitters and receivers of future high-rate communication systems that require highly precise and efficient antennas, sensors, active components, filters, and integrated technologies. This Roadmap is aimed at binding together the experiences of prominent researchers in the field of metasurfaces, from which explanations for the physics behind the extraordinary properties of these structures shall be provided from viewpoints of diverse theoretical backgrounds. Other goals of this endeavour are to underline the advantages and limitations of metasurfaces, as well as to lay out guidelines for their use in present and future electromagnetic devices.

This Roadmap is divided into five sections:

1. Metasurface based antennas. In the last few years, metasurfaces have shown possibilities for advanced manipulations of electromagnetic waves, opening new frontiers in the design of antennas. In this section, the authors explain how metasurfaces can be employed to tailor the radiation properties of antennas, their remarkable advantages in comparison with conventional antennas, and the future challenges to be solved.

2. Optical metasurfaces. Although many of the present demonstrators operate in the microwave regime, due either to the reduced cost of manufacturing and testing or to satisfy the interest of the communications or aerospace industries, part of the potential use of metasurfaces is found in the optical regime. In this section, the authors summarize the classical applications and explain new possibilities for optical metasurfaces, such as the generation of superoscillatory fields and energy harvesters.

3. Reconfigurable and active metasurfaces. Dynamic metasurfaces are promising new platforms for 5G communications, remote sensing and radar applications. By the insertion of active elements, metasurfaces can break the fundamental limitations of passive and static systems. In this section, we have contributions that describe the challenges and potential uses of active components in metasurfaces, including new studies on non-Foster, parity-time symmetric, and non-reciprocal metasurfaces.

4. Metasurfaces with higher symmetries. Recent studies have demonstrated that the properties of metasurfaces are influenced by the symmetries of their constituent elements. Therefore, by controlling the properties of these constitutive elements and their arrangement, one can control the way in which the waves interact with the metasurface. In this section, the authors analyze the possibilities of combining more than one layer of metasurface, creating a higher symmetry, increasing the operational bandwidth of flat lenses, or producing cost-effective electromagnetic bandgaps.

5. Numerical and analytical modelling of metasurfaces. In most occasions, metasurfaces are electrically large objects, which cannot be simulated with conventional software. Modelling tools that allow the engineering of the metasurface properties to get the desired response are essential in the design of practical electromagnetic devices. This section includes the recent advances and future challenges in three groups of techniques that are broadly used to analyze and synthesize metasurfaces: circuit models, analytical solutions and computational methods.

Lightwave communications is a necessity for the information age. Optical links provide enormous bandwidth, and the optical fiber is the only medium that can meet the modern society's needs for transporting massive amounts of data over long distances. Applications range from global high-capacity networks, which constitute the backbone of the internet, to the massively parallel interconnects that provide data connectivity inside datacenters and supercomputers. Optical communications is a diverse and rapidly changing field, where experts in photonics, communications, electronics, and signal processing work side by side to meet the ever-increasing demands for higher capacity, lower cost, and lower energy consumption, while adapting the system design to novel services and technologies. Due to the interdisciplinary nature of this rich research field, Journal of Optics has invited 16 researchers, each a world-leading expert in their respective subfields, to contribute a section to this invited review article, summarizing their views on state-of-the-art and future developments in optical communications.

Spatiotemporal sculpturing of light pulse with ultimately sophisticated structures represents a major goal of the everlasting pursue of ultra-fast information transmission and processing as well as ultra-intense energy concentration and extraction. It also holds the key to unlock new extraordinary fundamental physical effects. Traditionally, spatiotemporal light pulses are always treated as spatiotemporally separable wave packet as solution of the Maxwell's equations. In the past decade, however, more generalized forms of spatiotemporally nonseparable solution started to emerge with growing importance for their striking physical effects. This roadmap intends to highlight the recent advances in the creation and control of increasingly complex spatiotemporally sculptured pulses, from spatiotemporally separable to complex nonseparable states, with diverse geometric and topological structures, presenting a bird's eye viewpoint on the zoology of spatiotemporal light fields and the outlook of future trends and open challenges.

We review holography techniques related to imaging and sensing. Holography has been actively researched as three-dimensional (3D) imaging and 3D display techniques. Because of the successive evolutions of electronic and optical devices, digital holographic and quantitative 3D measurements with high accuracy and realistic 3D motion-picture image display without glasses have been realized. Moreover, holography has led to breakthroughs in various applications in the fields of measurement and processing through the development of holographic light-wave modulation techniques. We briefly introduce various applications of holography and then review imaging and sensing techniques with holography, focusing on quantitative phase imaging with daily-use light, spatially incoherent digital holography, holographic display, and microscopy with holographic light modulation.

Optical skyrmion fields inherit the exceptional dynamic shaping capabilities of structured light. This distinguishes them from their counterparts in other physical systems, yet ongoing research remains largely centred on well-known textures such as Néel-type skyrmions, Bloch-type skyrmions and bimerons. We chart topologically equivalent optical skyrmion fields and discuss how this property could potentially be used for communication applications. We previously showed that Stokes phase plots are effective tools to characterize skyrmion fields experimentally, we develop this theory and examine skyrmion beams through the lens of constellations of polarization singularities.

It is widely recognized that optically captured images can be degraded under non-ideal imaging conditions, such as blurring, defocusing, and the presence of scattering media. In this study, we introduce a novel architecture that integrates an adaptive encoder to enhance the general restoration of optical images. We refer to this architecture as AdaptiveNet, which is capable of adaptively identifying different types of non-ideal imaging conditions and learning to restore images accordingly. Our extensive experimental results indicate that AdaptiveNet outperforms existing methods across a range of optical image restoration tasks.

Multi-Gaussian Schell-model (MGSM) beam sources, due to their flattened correlations, has found various uses in many fields of optics such as in free space optical communications. Here, we introduce the double MGSM (DMGSM) source to redistribute the beam intensity profile of Schell-model beams. Theory of coherence and genuine spatial correlation functions are used to formulate the model. Specifically, we consider the multi-Gaussian functional form both in the spectral density and in the correlations part of the partially coherent beams. The model is called the DMGSM source in the sense that the source's complete flatness helps the beam to have a localized and enhanced intensity in the far field relative to the source spectrum. The source model and the field expressions in the far-field are derived and analyzed numerically. The primary outcome of this study is the derivation of the far-field spectrum equation, which characterizes the far-field spectral properties of scalar DMGSM sources. This equation provides a comprehensive analytical framework for understanding spectral variations influenced by source parameters, with numerical evaluations serving to illustrate and elucidate the theoretical formula. Comparison of the DMGSM source to the MGSM source and to the Gaussian Schell-model source is presented.

The rapid evolution of 5G and emerging 6G technologies demands high-frequency millimeter-wave (MMW) signal generation to meet growing requirements for high-capacity, low-latency communications. However, current frequency multiplication techniques often require complex filtering and high modulation indices, introducing distortion and limiting system performance. A scheme is proposed based on a dual-parallel polarization modulator (DP-PolM) to generate a frequency 16-tupling MMW without a filter. The scheme can be divided into two parts. In the first part, the RF drive signal of 8 GHz is used to drive DP-PolM1. The MMW of 32 GHz can be generated by controlling the modulation index (MI) and the phase difference between the upper and lower arms of DP-PolM1 and the angle of the polarizer (Pol). In the second part, the MMW generated in the first part is used as the remodulation signal to drive the DP-PolM2, and the output power of the optical carrier can be adjusted to produce a 128 GHz MMW with optical sideband suppression ratio and radio frequency spur suppression ratio of 55.02 dB and 46.25 dB, respectively. In addition, the stability of the remodulated signal and the generated frequency 16-tupling MMW signal under different optical carrier power and the MI are analyzed. The results show that the system can generate a frequency of 16-tupling MMW when the optical carrier power, MI offset, phase difference of the RF local oscillator signal driving DP-PolM, and polarization controller angular deviation are less than $\pm 44\%$, $\pm 38\%$, 40°, and 26°, respectively. Compared with existing schemes, the proposed approach offers a simpler and more efficient solution for high-frequency MMW generation by removing the need for complex filtering and achieving better performance with a smaller MI. It shows strong potential for addressing challenges in future 5G and 6G communication systems.

Accurately predicting turbulence phase information from distorted beam intensity is crucial for investigating anti-turbulence interference in wireless optical communications. In this study, we investigated ocean turbulence and developed a predictor of the phase of ocean turbulence (POT) based on a convolutional neural network (CNN) (called the CNN-POT predictor). Furthermore, the influence of different temperature dissipation rates and radial indices on the POT prediction effect was analyzed. Meanwhile, the CNN is optimized by adding efficient attention mechanism and mixed loss function. When orders p = 3, the average test loss rate of the mixing test dataset with varying temperature dissipation rates was 0.0576. This demonstrated the reliability of the CNN-POT predictor predictions in various ocean turbulence environments. We innovatively demonstrated that high-order radial LG beams can improve the accuracy of the CNN-POT predictor in predicting POT. These results not only provide a new method for accurately predicting POT, but also provide a theoretical basis for the study of anti-turbulence interference in applications such as quantum information and wireless optical communications.

To effectively apply passive speckle reduction methods, it is essential to use an illumination system that maximally exploits the non-ideal temporal coherence and angular diversity (spatial coherence reduction) of laser light. This study examines the necessary conditions for these factors to act independently to achieve maximum speckle reduction. A novel design for decoherent focusing of laser illumination into a rectangular, uniformly lit spot is proposed. The design is based on an array of rectangular prisms of varying heights and a Fresnel lens comprising crossed 1D cylindrical Fresnel lenses with flat facets. Mathematical modeling demonstrates that such a lens can effectively focus a Gaussian beam into a rectangular uniform illuminated spot, even for lenses with a high numerical aperture (NA = 0.2). A planar implementation of this lens is also proposed. The comb-like spectrum of a laser diode is shown to limit the capabilities of passive methods, as it is challenging to generate a significant number of decorrelated laser sub-beams beyond the available spectral modes. Experimental results confirm that applying a slight modulation (∼15%) to the laser diode current during intensity integration transforms the comb spectrum into a continuous one. This enables the generation of an unlimited number of decorrelated laser beams, independent of the laser's spectral modes, to achieve the desired number for a specific optical device.

A novel polarization conversion metasurface (PCM) is designed and used for radar cross section (RCS) reduction. The PCM unit can implement polarization conversion ratio over 97% from 10.1 to 21.1 GHz. By loading the PCM array into the centre of the dual-band CP antenna which works in the X- and Ku-band simultaneously, a new low-RCS antenna is proposed. Two bands are at 9.8 GHz (relative bandwidth 18.0%) and 15.2 GHz (relative bandwidth 11.0%) respectively. The corresponding 3 dB axial ratio bandwidth is 5.1% and 4.6%. Meanwhile, other radiation performances are also well maintained. In particular, the scattering ability is effectively enhanced. The average monostatic RCS reduction reaches 10.3 dB at normal incidence from 7.3 to 21.7 GHz (99.3%). Furthermore, bistatic RCS also shows apparent reduction under different incidence angles which greatly improves its practical value.

We review holography techniques related to imaging and sensing. Holography has been actively researched as three-dimensional (3D) imaging and 3D display techniques. Because of the successive evolutions of electronic and optical devices, digital holographic and quantitative 3D measurements with high accuracy and realistic 3D motion-picture image display without glasses have been realized. Moreover, holography has led to breakthroughs in various applications in the fields of measurement and processing through the development of holographic light-wave modulation techniques. We briefly introduce various applications of holography and then review imaging and sensing techniques with holography, focusing on quantitative phase imaging with daily-use light, spatially incoherent digital holography, holographic display, and microscopy with holographic light modulation.

Optically transparent microwave absorbers based on metamaterials demonstrate exceptional microwave absorption performance while maintaining high optical transmittance, showcasing significant potential for applications in modern communication, defense, and architectural fields. Transparency in the visible light spectrum is primarily achieved through material selection and structural optimization. The artificially designed metamaterials based on transparent resistive films can be used to achieve devices with excellent wave absorption characteristics in the microwave frequency band. In this paper, we systematically review the research progress in the domain of optically transparent microwave metamaterial absorbers. We first introduce the implementation principles of optically transparent microwave metamaterial absorbers from the perspectives of transparency and wave absorption, laying the foundation for the in-depth discussions in subsequent sections. Subsequently, we focus on the research progress of optically transparent microwave metamaterial absorbers. In this paper, microwave metamaterial absorbers are classified into three types: passive absorbers, tunable absorbers and adaptive absorbers. Passive and tunable absorbers are further discussed based on their structural classifications. This paper summarizes the current research status and technical bottlenecks of optically transparent microwave absorbers while envisioning their extensive applications in stealth technology, wireless communication, and multifunctional devices. While challenges persist in balancing thickness, bandwidth and transmittance, future advancements in novel material, innovative structural designs, and manufacturing processes are expected to enable the realization of efficient, intelligent, multifunctional absorbers.

This review focuses on novel phenomena that emerge when optical pulses propagate through a spatiotemporal dispersive medium whose refractive index is modulated, both in space and time, in a traveling-wave fashion. Using optical fibers as an example of a dispersive medium, we first derive an equation governing the evolution of short pulses in such a medium. This equation is used to discuss the phenomena such as temporal reflection and refraction, total internal reflection, and waveguiding from a moving boundary with different refractive indices on its two sides. The use of solitons, forming through the Kerr effect, shows how such effects can be observed with silica fibers by employing a pump-probe configuration. A pair of solitons provide the temporal analog of a waveguide or a Fabry–Perot resonator. A new kind of grating, called a spatiotemporal Bragg grating, is formed when a train of pump pulses creates periodic high-index regions inside an optical fiber moving at the speed of pump pulses. The interaction of probe pulses with such a Bragg grating is studied both within and outside of momentum gaps. It is also shown that a photonic analog of Anderson localization is possible when disorder is introduced into a spatiotemporal Bragg grating.

Skyrmions are topologically protected quasi-particles that have aroused substantial interest in nuclear physics and condensed matter physics. For instance, magnetic skyrmions are regarded as having potential applications in high-density information storage due to their ultracompact size, topologically protected stability, and low driven current. Recently, optical analogs have been discovered in light field, known as optical skyrmions. With similar intriguing properties, research on optical skyrmions has grown dramatically. Several types of optical skyrmions defined by various optical parameters have been uncovered. Along with the fundamental physics studies, methods for generating, modifying, and detecting optical skyrmions have also been developed, which in turn enriches the toolkit for light field modulation and detection. It has shown promising applications in high-precision positioning, information storage, and optical communication. In this paper, we begin with the fundamental theory and then introduce generalized classes of optical skyrmions, with a particular emphasis on optical spin skyrmions. We discuss their generation, modulation, and detection methods. Additionally, we highlight the emerging applications of optical skyrmions, showcasing the potential of these unique properties for future advancements.

Machine learning techniques, notably various deep neural network methods, are instrumental in processing extensive and intricate data sets in engineering and scientific fields. This paper shows how deep neural networks can inversely design cascaded-mode converting systems, particularly waveguide gratings that implement selective mode conversion upon reflection. Neural networks can map the grating's physical features to the scattering parameters of the modes reflected from the grating. The trained networks can then be utilized to inversely design waveguide grating mode converters based on the desired values of the scattering parameters. The process of the inverse design involves using the technique of gradient descent of a defined loss function. Minimizing this loss function leads to calculating more accurate features fulfilling the desired scattering parameters.

Tailoring the reactive helicity and momentum of electromagnetic fields has emerged as a unique way to control light-matter interaction. In this paper, we explore these reactive quantities in hybrid polarized vector (HPV) beams that are tightly focused through a high NA objective. By precisely controlling polarization parameter and topological charge, we are able to modulate the hybrid state of polarization for the focused HPV beams, which allows for controllable generation of the reactive helicity and momentum. Notably, we create a purely longitudinal reactive momentum which arises on the beam axis. We also generate a three-dimensional focused spot of the reactive helicity, the size of which can shrink beyond the diffraction limit imposed on light intensity spots. These findings provide new insights into the dynamic properties of structured light, and would have implications for optical manipulation techniques including particle trapping, pulling and rotation.

Computer-generated holography is of significant interest in various scientific and engineering problems: holographic displays, optical tweezers, high-quality 2D- and 3D-information visualization, beam shaping, laser micromanipulation and etc. In this work a method for iterative hologram generation improvement is proposed. It mutually takes into account various quantitative image quality metrics to improve object reconstruction. Mean squared error (MSE), structural similarity index measure (SSIM), normalized standard deviation (NSTD), correlation coefficient (CC), and diffraction efficiency (DE) values were taken into account with different weights. To demonstrate the method effectiveness, the iterative amplitude binary inline Fresnel hologram generation technique was improved. It was obtained that large weight of NSTD value provides the best reconstruction quality. According to the modeling, NSTD is reduced in 1.8 times and equal to 0.09 for binary images and 0.05 for halftone images. It was obtained that the choice of the internal metric has a great influence on the reliability of the reconstructed image quality. The generated holograms were optically reconstructed by its displaying in divergent beams onto digital micromirror device. The results confirm modeling conclusions. Our findings demonstrated the effectiveness of the proposed approach and providing a framework for future developments in computer-generated holography.

In this paper, we propose a topological plasmonic 1D nanocavity, which comprises a dielectric nanoparticle chain placed on a metallic substrate. The structure is designed based on the topological characteristics of the plasmonic photonic crystals. A topological edge state exists at the interface between two distinct structures. In comparison to a non-topological defect mode, the topological edge mode exhibits better properties including good confinement and robustness against perturbations in scales and length-width ratio of the nanoparticles.

Direct writing systems equipped with a confocal micro-light-emitting diode array can employ active illumination by the array for alignment purposes. Based on the principles of single-pixel imaging, structured illumination can be used to obtain position information. The positions of markers with certain spectral signatures can be extracted from the recorded hyperspectral data through spectral filtering, spectral fitting, or principal component analysis. Application to direct writing is demonstrated by creating individual photolithography patterns aligned with different types of fluorophores on the same substrate.

Despite the complex optical lens system, classical optical microscopy can't achieve high spatial resolution and a large field of view (FOV). The lensless optical ptychographic microscopy technique solves the contradiction between resolution and FOV with a simple optical path. However, position errors between LED array and sample, caused by misalignment of optical imaging axis and manufacture errors of LED array, decrease reconstruction quality. Due to the difficulty of hardware correction, we propose an adaptive genetic strategy to correct LED array position error for lensless ptychographic imaging. Firstly, the coarse positions of the LEDs are predicted by Kalman filter, thus we can determine the position range. Secondly, the precise positions of LEDs are calculated by our adaptive genetic algorithm, which combines the fitness function according to the previous predicted positions. Finally, we incorporate the LED array position correction into the Gerchberg-Saxton (GS) phase recovery method to achieve improved reconstructed image quality. Compared with similar methods, both simulation and real experiments indicate that our method GAadapt-GS (adaptive Genetic algorithm-based Gerchberg-Saxton) method has higher robustness in position errors correction, and creates results with better quality. This method is applied to other state-of-art reconstruction algorithms as a pre-processing step, highly improve the image quality.

Tailoring the reactive helicity and momentum of electromagnetic fields has emerged as a unique way to control light-matter interaction. In this paper, we explore these reactive quantities in hybrid polarized vector (HPV) beams that are tightly focused through a high NA objective. By precisely controlling polarization parameter and topological charge, we are able to modulate the hybrid state of polarization for the focused HPV beams, which allows for controllable generation of the reactive helicity and momentum. Notably, we create a purely longitudinal reactive momentum which arises on the beam axis. We also generate a three-dimensional focused spot of the reactive helicity, the size of which can shrink beyond the diffraction limit imposed on light intensity spots. These findings provide new insights into the dynamic properties of structured light, and would have implications for optical manipulation techniques including particle trapping, pulling and rotation.

To effectively apply passive speckle reduction methods, it is essential to use an illumination system that maximally exploits the non-ideal temporal coherence and angular diversity (spatial coherence reduction) of laser light. This study examines the necessary conditions for these factors to act independently to achieve maximum speckle reduction. A novel design for decoherent focusing of laser illumination into a rectangular, uniformly lit spot is proposed. The design is based on an array of rectangular prisms of varying heights and a Fresnel lens comprising crossed 1D cylindrical Fresnel lenses with flat facets. Mathematical modeling demonstrates that such a lens can effectively focus a Gaussian beam into a rectangular uniform illuminated spot, even for lenses with a high numerical aperture (NA = 0.2). A planar implementation of this lens is also proposed. The comb-like spectrum of a laser diode is shown to limit the capabilities of passive methods, as it is challenging to generate a significant number of decorrelated laser sub-beams beyond the available spectral modes. Experimental results confirm that applying a slight modulation (∼15%) to the laser diode current during intensity integration transforms the comb spectrum into a continuous one. This enables the generation of an unlimited number of decorrelated laser beams, independent of the laser's spectral modes, to achieve the desired number for a specific optical device.

Direct writing systems equipped with a confocal micro-light-emitting diode array can employ active illumination by the array for alignment purposes. Based on the principles of single-pixel imaging, structured illumination can be used to obtain position information. The positions of markers with certain spectral signatures can be extracted from the recorded hyperspectral data through spectral filtering, spectral fitting, or principal component analysis. Application to direct writing is demonstrated by creating individual photolithography patterns aligned with different types of fluorophores on the same substrate.

Subluminal and superluminal propagation of light, and normal and anomalous rotary photon drag are reported in this manuscript using control fields of the Milnor polynomial. A crater-type absorption spectrum is reported, having a central peak at the origin with a variation of positions. The normal and anomalous dispersion are associated with the low and high absorption regions. The group index varies in the range of $-1 \times 10^{7} \unicode{x2A7D} n_{\mathrm{g}} \unicode{x2A7D} 1 \times 10^{7}$. This corresponds to a group velocity of $v_{\mathrm{g}} = c/n_{\mathrm{g}}$ varying in the range of $-30\,\mathrm{m}\,\mathrm{s}^{-1} \unicode{x2A7D} v_{\mathrm{g}} \unicode{x2A7D} 30\,\mathrm{m}\,\mathrm{s}^{-1}$. The maximum positive and negative delay times are measured to $\pm0.002\,\mathrm{s}$. It is reported that slow light propagation generates normal photon drag and fast light propagation generates anomalous rotary photon drag in a spinning medium. The rotary photon drag varies in the range of $-0.005\,\mathrm{rad} \unicode{x2A7D} \theta_{\mathrm{d}} \unicode{x2A7D} 0.005\,\mathrm{rad}$, where both normal and anomalous drag is also observed. The modified results will be useful in optical and biomedical sensors, and imaging and telecommunication technologies.

The technologies used in the manipulation of light can be used to do analogue simulations of physical systems with wave-like equations of motion. This analogy is maximized by the use of all the degrees of freedom of light. The Helmholtz equation in physical optics and the Schödinger equation in quantum mechanics share the same mathematical form. We use this connection to prepare non-diffracting optical beams representing the spatial and temporal dynamics of a nonlinear physical system: the quantum pendulum. By using the propagation coordinate to represent time in the quantum problem, we are able to analogue-simulate quantum wavepacket dynamics. These manifest themselves in novel optical beams with rich three-dimensional structures, such as rotation and sloshing of the light's intensity as it propagates. Our experimental results agree very well with the predictions from quantum theory, thus demonstrating that our system can be used as a platform to simulate the quantum pendulum dynamics. This three-dimensional light-sculpting capability has the potential to impact fields such as manipulation with light and imaging.

Optical skyrmion fields inherit the exceptional dynamic shaping capabilities of structured light. This distinguishes them from their counterparts in other physical systems, yet ongoing research remains largely centred on well-known textures such as Néel-type skyrmions, Bloch-type skyrmions and bimerons. We chart topologically equivalent optical skyrmion fields and discuss how this property could potentially be used for communication applications. We previously showed that Stokes phase plots are effective tools to characterize skyrmion fields experimentally, we develop this theory and examine skyrmion beams through the lens of constellations of polarization singularities.

We review holography techniques related to imaging and sensing. Holography has been actively researched as three-dimensional (3D) imaging and 3D display techniques. Because of the successive evolutions of electronic and optical devices, digital holographic and quantitative 3D measurements with high accuracy and realistic 3D motion-picture image display without glasses have been realized. Moreover, holography has led to breakthroughs in various applications in the fields of measurement and processing through the development of holographic light-wave modulation techniques. We briefly introduce various applications of holography and then review imaging and sensing techniques with holography, focusing on quantitative phase imaging with daily-use light, spatially incoherent digital holography, holographic display, and microscopy with holographic light modulation.

It is widely recognized that optically captured images can be degraded under non-ideal imaging conditions, such as blurring, defocusing, and the presence of scattering media. In this study, we introduce a novel architecture that integrates an adaptive encoder to enhance the general restoration of optical images. We refer to this architecture as AdaptiveNet, which is capable of adaptively identifying different types of non-ideal imaging conditions and learning to restore images accordingly. Our extensive experimental results indicate that AdaptiveNet outperforms existing methods across a range of optical image restoration tasks.

We consider the Jacobian of a random transverse polarisation field, from the transverse plane to the Poincaré sphere, as a Skyrme density partially covering the sphere. Connected domains of the plane where the Jacobian has the same sign—patches—map to facets subtending some general solid angle on the Poincaré sphere. As a generic continuous mapping between surfaces, we interpret the polarisation pattern on the sphere in terms of fold lines (corresponding to the crease lines between neighbouring patches) and cusp points (where fold lines meet). We perform a basic statistical analysis of the properties of the patches and facets, including a brief discussion of the polarisation analogue to superoscillation in scalar speckle patterns and the percolation properties of the Jacobian domains. Connections with abstract origami manifolds are briefly considered. This analysis combines previous studies of structured skyrmionic polarisation patterns with random polarisation patterns, suggesting a particle-like interpretation of random patches as polarisation skyrmionic anyons.

Photonic nanorods with multiple concentric layers are found to exhibit giant polarization selectivity when absorbing power from near-field sources. An improved version of chaotic accelerated particle swarm optimization is developed and employed to determine various designs for several combinations of alternating dielectric and plasmonic media, operated under visible light of different colors. The spatial distribution of the electromagnetic intensity unveils the nature of the sustained resonances across the cylindrical layers while the robustness of their response against changes in the physical dimensions is checked. The reported setups can be directly utilized as ultra-efficient components in polarization-controlled photonic integrated systems involving a wide spectrum of applications from sensing and multiplexing to analog signal processing and optical detection.