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.

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.

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.

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.

As a tutorial, we examine the absolute brightness and number statistics of photon pairs generated in spontaneous parametric down-conversion (SPDC) from first principles. In doing so, we demonstrate how the diverse implementations of SPDC can be understood through a single common framework, and use this to derive straightforward formulas for the biphoton generation rate (pairs per second) in a variety of different circumstances. In particular, we consider the common cases of both collimated and focused Gaussian pump beams in a bulk nonlinear crystal, as well as in nonlinear waveguides and micro-ring resonators. Furthermore, we examine the number statistics of down-converted light using a non-perturbative approximation (the multi-mode squeezed vacuum), to provide quantitative formulas for the relative likelihood of multi-pair production events, and explore how the quantum state of the pump affects the subsequent statistics of the down-converted light. Following this, we consider the limits of the undepleted pump approximation, and conclude by performing experiments to test the effectiveness of our theoretical predictions for the biphoton generation rate in a variety of different sources.

High-dimensional quantum entanglement is an important resource for emerging quantum technologies such as quantum communication and quantum computation. The scalability of metres-long experimental setups limits high-dimensional entanglement in bulk optics. Advancements in quantum technology hinge on reproducible, and reconfigurable quantum devices—including photon sources, which are challenging to achieve in a scalable manner using bulk optics. Advances in nanotechnology and CMOS-compatible integration techniques have enabled the generation of entangled photons on millimeter-scale chips, significantly enhancing scalability, stability, replicability, and miniaturization for real-world quantum applications. In recent years we have seen several chip-scale demonstrations with different discrete degrees of freedom including path, frequency-bin, time-bin, and transverse modes, on many material platforms. A complete quantum photonic integrated circuit requires the generation, manipulation, and detection of quantum states, involving various active and passive quantum photonic components which further increase the degree of complexity. Here, we focus on the high-dimensional versions of qubits—qudits—and review the nonlinear optical processes that facilitate on-chip high-dimensional entangled photon sources, and the currently used material platforms. We discuss a range of current implementations of on-chip high-dimensional entangled photon sources and demonstrated applications. We comment on the current challenges due to the limitations of individual material platforms and present future opportunities in hybrid and heterogeneous integration strategies for the next generation of integrated quantum photonic chips.

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.

Optical parametric amplifiers (OPAs) exploit second-order nonlinearity to transfer energy from a fixed frequency pump pulse to a variable frequency signal pulse, and represent an easy way of tuning over a broad range the frequency of an otherwise fixed femtosecond laser system. OPAs can also act as broadband amplifiers, transferring energy from a narrowband pump to a broadband signal and thus considerably shortening the duration of the pump pulse. Due to these unique properties, OPAs are nowadays ubiquitous in ultrafast laser laboratories, and are employed by many users, such as solid state physicists, atomic/molecular physicists, chemists and biologists, who are not experts in ultrafast optics. This tutorial paper aims at providing the non-specialist reader with a self-consistent guide to the physical foundations of OPAs, deriving the main equations describing their performance and discussing how they can be used to understand their most important working parameters (frequency tunability, bandwidth, pulse energy/repetition rate scalability, control over the carrier-envelope phase of the generated pulses). Based on this analysis, we derive practical design criteria for OPAs, showing how their performance depends on the type of the nonlinear interaction (crystal type, phase-matching configuration, crystal length), on the characteristics of the pump pulse (frequency, duration, energy, repetition rate) and on the OPA architecture.

This paper proposes a method to extend the depth range of integral imaging systems by representing three-dimensional (3D) images in both virtual-real and real–real forms. The approach combines two integral imaging systems using a transmission-type retroreflector and a beam splitter (BS). The transmission-type retroreflector relays the integrated images from one display to form a real full-color 3D aerial images while preserving the resolution at extended depths. The BS combines the second integral imaging system across various depth ranges, providing additional real or virtual 3D images. The proposed method enhances the applicability of integral imaging systems in mixed-reality environments without requiring complex equipment.

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.

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.

This paper demonstrates a dual-functional terahertz metamaterial that utilizes the phase transition properties of vanadium dioxide (VO₂). By altering the conductivity of VO₂, the role of the proposed metamaterial changes from absorption to reflective polarization conversion. The unit cell is the stacked structure with different VO₂ patterns, polyimide-slabs (PI), gold pattern and gold plate. When VO₂ is metallic state, the proposed metamaterial demonstrates ultra-broadband absorption within the 5.8–17.5 THz range, achieving absorption rate superior than 90%. In the non-metallic state of VO₂, The metamaterial displays outstanding performance in converting linear-to-linear polarization. This performance is observed in the range of 6.1–20.9 THz, where the polarization conversion rate exceeds 90%. The operating principle of the metamaterial is clarified by analyzing the electric field in the absorption state and the surface currents during the polarization conversion state. Furthermore, the ultra-broadband absorption of the metamaterial is explained using impedance matching theory and multiple interference theory. Mechanisms behind polarization conversion are clarified through the use of Jones vectors and the Jones matrix. Theoretical calculations align closely with simulations, validating their accuracy. The proposed metamaterial excels in ultra-broadband absorption and polarization conversion. As a result, it holds promising prospects for applications in terahertz stealth technology, communication systems, radar, and other advanced domains.

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.

In this paper, we propose a novel architecture for an ultrathin-film solar cell, integrating silicon multi-snowflake fractals into the electron-transport layer. In the proposed structure, each dielectric snowflake fractal is designed to trap sunlight within specific wavelength bands using multiple orders of Mie resonances and branch coupling, collectively covering the solar cell's operation bandwidth. This structure is numerically investigated using full-wave simulation with the finite difference frequency domain method and by solving the drift and diffusion equations. The calculations demonstrate enhanced absorption across the whole wavelength range of 300 nm to 1100 nm, leading to an increased photo-generated current for both transverse electric (TE) and transverse magnetic (TM) polarizations of the incident light and at various angles of incidence. Simulation results illustrate a short-circuit current of 15.37 mA cm⁻² for the proposed structure, which is enhanced by a factor of 5.12 compared to a simple solar cell without fractal nanostructures inside.

The polarization of electromagnetic waves is a fundamental property that influences their propagation direction and state, represents a critical physical parameter in modern applications such as signal transmission, target recognition, and precision measurement. Conventional polarization control devices based on metamaterials are often sensitive to structural perturbations, presenting challenges for practical applications. In contrast, topologically protected photonic devices have demonstrated that photon topological edge states can effectively overcome scattering losses caused by disorder and structural perturbations. The prospect of utilizing the robustness of topological edge states to design reliable electromagnetic wave polarization devices has thus become an increasingly significant research focus. We design and construct a curved photonic dimer chain composed of subwavelength resonators and experimentally verify that its topological edge states are immune to internal disorder perturbations, enabling efficient long-range electromagnetic wave polarization conversion. Based on this study, future investigations could explore more complex topological architectures, such as quasiperiodic or trimer chains, to achieve enhanced multifunctional polarization control. Furthermore, we could consider integrating exceptional points in non-Hermitian systems to design chiral polarization devices.

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.

A flexible optical communication network is needed to realize a backbone transport network for 6G communication and further higher generation communication technologies. However, the practical implementation of the higher generation network experiences some serious challenges due to the existing multicarrier generation technology i.e. an array of multiple discrete laser sources (less spectrally efficient, complex, bulkier and costlier). Recently, a multicarrier generation technique using the optical frequency comb has been extensively researched. It can reduce the complexity, cost, and size compared to the existing multicarrier generator. Moreover, it increases the utilization of available spectral efficiency due to its capability to tune the operating frequency and carrier spacing. So, considering these advantages, we reviewed the multiple optical frequency comb generation techniques, categorized as mode-locked laser, microresonator and electro-optic modulator based frequency combs. We identify the salient features of different frequency comb generation techniques by keeping the requirements of a flexible optical network in mind. We also reviewed the drawbacks and possible solutions proposed to improve the characteristics of the optical frequency comb. Further, we reviewed the optical frequency comb expansion techniques to broaden the spectrum of the optical frequency comb, which is the requirement in optical frequency comb suitable for communication applications. At last, we summarize the progress in the practical implementation of the optical frequency comb as a multichannel source in a flexible optical network.

It has been almost 10 years since Fourier single-pixel imaging (FSI) was proposed in 2015. FSI has been extended to 3D single-pixel imaging, full-color imaging, multi-modality imaging, microscopy, and light-field imaging. It has also been applied to through-fire imaging, image-free object edge detection, image-free fast-moving object tracking, high-speed rotating object imaging, image-free autofocusing, photovoltaic device functionality imaging, and more. This paper addresses students and researchers in different disciplines interested in learning about FSI. We aim to provide the readers with a tutorial that teaches fundamental principles and technical details that help the researchers better understand, analyze, and implement FSI.

It is widely recognized that optically captured images can be distorted 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.

The paper proposes a novel mathematical model of emission and spatial distribution of the optical radiation that carries orbital angular momentum, i.e. optical vortex beams, by microring resonators. The considered microring resonator features a diffraction grating formed by holes perforated on its surface, facilitating the emission of vortex beams. The emission process is analytically examined using the theory of scattering electromagnetic waves by permittivity inhomogeneities, applying the Rayleigh approximation. We use the vector representation of the electromagnetic fields utilizing the Kirchhoff-Helmholtz integral theorem to describe the field spatial distribution. Unlike already known ones, the proposed model considers the impact of the microring resonator and the diffraction grating parameters, including its inhomogeneities configuration, on the emitted field. Numerical simulations conducted using the Finite Difference Time Domain method in the Ansys Lumerical 2020 R2.4 environment support the validity of the proposed analytical model. The results we obtained enable us to identify the area with the highest energy flux density from the resonator at a specified distance analytically and with high accuracy. This capability can greatly streamline and enhance the development and optimization of vortex field emitters based on micro-ring resonators.

The modulation of electron beams is a critical area for investigating the interaction between electrons and matter. By harnessing the inelastic scattering process of free electrons and surface plasmon polaritons (SPPs), we can effectively manipulate the electron beam. Our study shows that electron beams can be compressed and deflected through electron-plasmon field interaction, which adds to the existing research on spatial control of electron beams. Specifically, considering the interaction between an electron beam and counterpropagating SPPs with two wave-vectors, asymmetric diffraction and transverse compression of coherent Gaussian electron beams are predicted employing semi-classical theory. This effect can be understood by analyzing the symmetry breaking and exchange properties between momentum and position in the Fourier plane. Our findings demonstrate a linear superposition of transverse momentum associated with electron and structural plasmon fields, offering the potential for active modulation to program transverse electron wave functions and presenting alternative solutions for accelerator science.

This paper proposes an optical encryption system that integrates optical orbital angular momentum holography with nonlinear authentication techniques. The scheme employs orbital angular momentum beams with different topological charges to illuminate the segmented patterns, subsequently forming optical orbital angular momentum multiplexed holograms. These multiplexed holograms are then decomposed into a series of matrices and combined with corresponding random matrix ciphertexts. Finally, the ciphertext serves as an illumination pattern to perform ghost imaging on the authentication images, yielding the authentication key. During decryption, the system requires pre-authentication, and only upon successful authentication can the decryption key be authorized for retrieval. This method effectively enhances the security of information transmission and demonstrates the system's effectiveness through computer simulation experiments.

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.

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.

It is widely recognized that optically captured images can be distorted 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.

Electro-optic modulators are widely used for the generation of optical sidebands for various applications. Here, we report on a technique enabling control of the relative amplitudes of optical sidebands generated by electro-optic modulators. The technique makes use of a phase modulator and Mach–Zehnder amplitude modulator, connected in series to break the symmetry of the sideband amplitudes. The generated optical sideband spectrum can be controlled by the two radio frequency (RF) modulation amplitudes, the attenuation level of the amplitude modulator, and the relative RF phase of the two modulations. We demonstrate near-complete suppression of one first-order sideband and with simultaneously achieving equal amplitudes of the carrier and the other first-order sideband with $\gt\,$94% purity. The technique can be utilised to produce a spectrum with effectively two frequency components. This enables application in atomic physics experiments such as the minimisation of off-resonant light shifts that can limit the performance of atomic clocks and interferometers. We demonstrate operation in the near-infrared, at 795 nm, using commercial-off-the-shelf components and circumventing the need for frequency doubling of lasers in the optical communications region.

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.

Photonic crystal fiber (PCF) can be used as a microfluidic platform for surface-enhanced Raman spectroscopy (SERS)-based measurement of different samples. PCF can provide a long area of interaction for light, metal nanoparticles (NPs), and analytes incorporated into its holes. This paper provides a COMSOL simulation model for suspended-core PCF used to evaluate the SERS intensity depending on the fiber's geometry and NPs' coverage density. The simulation is applied to compare three PCFs with varying sizes and lengths. SERS measurements of adenine and DNA validate the simulation outcomes and demonstrate the potential of PCFs for SERS-based detection of diverse samples.

Kerr self-focusing is one of the most common and fundamental nonlinear phenomena during high-power light pulses propagating through transparent media. It occurs usually when the pulse peak power exceeds a certain critical power and the Kerr nonlinearity overcomes the diffraction effect. In this paper, the nonlinear propagation of terahertz (THz) pulses in dispersive media is studied via numerical simulations. It is found that, for few-cycle THz pulses, the Kerr self-focusing is suppressed dramatically, and a substantially higher THz intensity than that defined by the well-known formula of self-focusing critical power is required to enable an observable spatial self-focusing behaviour. By theoretical modelling and numerically analysing the time-domain evolution of broadband THz pulses in media, the underlying physical cause is attributed to the dominance of the significant dispersion effect over the diffraction effect, resulting in that the Kerr nonlinearity competes with dispersion instead of diffraction. A modified analytical expression for the dispersion-mediated self-focusing critical intensity is derived and shows good agreement with simulation results. The influences of THz pulse and medium dispersion parameters on the THz propagation dynamics are systematically studied such as the THz cycle number, intensity, initial chirp and absolute phase as well as the medium group velocity dispersion, high-order dispersion and their sign. These results have important implications for the study of strong-field THz wave—matter interactions and THz nonlinear optics.

This paper proposes a method to extend the depth range of integral imaging systems by representing three-dimensional (3D) images in both virtual-real and real–real forms. The approach combines two integral imaging systems using a transmission-type retroreflector and a beam splitter (BS). The transmission-type retroreflector relays the integrated images from one display to form a real full-color 3D aerial images while preserving the resolution at extended depths. The BS combines the second integral imaging system across various depth ranges, providing additional real or virtual 3D images. The proposed method enhances the applicability of integral imaging systems in mixed-reality environments without requiring complex equipment.

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.

Polaritonic interactions are pivotal in advancing sensing technologies, optical devices, and waveguides. This study presents a theoretical investigation into polaritonic interactions at the interface of chiral-loaded temperature-sensitive materials (TSMs). Indium antimonide (InSb), known for its temperature-dependent phase-transition optical properties, is utilized as the TSM. The electromagnetic (EM) behavior of InSb is described using the extended Drude model, while the isotropic chiral medium is characterized through coupled constitutive relations. By applying tangential boundary conditions for EM field continuity at the chiral–InSb interface, the dispersion relation governing hybrid polaritons is derived. Numerical computations performed in Wolfram Mathematica, utilizing the contour plot technique, reveal the dispersion characteristics, effective mode index, and field distributions under varying temperatures. The findings demonstrate the existence of two distinct polaritonic regimes: (i) hybrid polariton–phonon coupling at temperatures below 200 K, and (ii) hybrid polariton–plasmon coupling at temperatures exceeding 260 K. Additionally, the effects of chirality and temperature on the dispersion curves, effective mode indices, and field profiles are systematically analyzed. Results reveal that polaritonic surface modes can be dynamically tuned by manipulating external temperature and material chirality. These insights hold significant promise for the development of temperature-responsive terahertz-infrared sensors, enantiomeric detectors, thermo-optical surface waveguides, and near-field imaging systems.