On‐Demand Subwavelength‐Scale Light Sculpting Using Nanometric Holograms

Structured light – electromagnetic waves with a strong spatial inhomogeneity of amplitude, phase, and polarization – has occupied far-reaching positions in both optical research and applications. Terahertz (THz) waves, due to recent innovations in photonics and nanotechnology, became so robust that it was not only implemented in a wide variety of applications such as communications, spectroscopic analysis, and non-destructive imaging, but also served as a low-cost and easily implementable experimental platform for novel concept illustration. In this work, we show that structured nonparaxial THz light in the form of Airy, Bessel, and Gaussian beams can be generated in a compact way using exclusively silicon diffractive optics prepared by femtosecond laser ablation technology. The accelerating nature of the generated structured light is demonstrated via THz imaging of objects partially obscured by an opaque beam block. Unlike conventional paraxial approaches, when a combination of a lens and a cubic phase (or amplitude) mask creates a nondiffracting Airy beam, we demonstrate simultaneous lensless nonparaxial THz Airy beam generation and its application in imaging system. Images of single objects, imaging with a controllable placed obstacle, and imaging of stacked graphene layers are presented, revealing hence potential of the approach to inspect quality of 2D materials. Structured nonparaxial THz illumination is investigated both theoretically and experimentally with appropriate extensive benchmarks. The structured THz illumination consistently outperforms the conventional one in resolution and contrast, thus opening new frontiers of structured light applications in imaging and inverse scattering problems, as it enables sophisticated estimates of optical properties of the investigated structures.

Structured light refers to light fields that are specially customized by manipulating their properties across multiple physical dimensions, demonstrating immense potential in applications such as optical communications, precision measurement, advanced imaging, and quantum information. Structured light lasers, capable of directly generating light fields with complex spatial distributions, polarization states, and topological structures, provide efficient and versatile tools for light field manipulation and its applications. Unlike structured light generation techniques that rely on external modulation, structured light lasers achieve compact and high-quality structured light output through direct intracavity control, significantly enhancing system integration and efficiency. This review summarizes the latest research progress in structured light lasers, exploring their fundamental principles, design methodologies, and applications. It introduces the basic concepts of structured light, including typical structured beams such as vortex beams, vector beams, and propagation-invariant beams, as well as more complex topological and quantum structures like optical skyrmions, non-separable states, and ray-wave duality. The review elaborates on the categories of high-dimensional structured light lasers, covering design schemes such as on-cavity structures and intracavity geometries, and provides insight into different architectures ranging from free-space and fiber-based systems to integrated devices. We also discuss the broad applications and future prospects of structured light lasers, highlighting the challenges and opportunities in this rapidly evolving field. This review aims to serve as a resource for researchers who seek to design and develop structured light lasers, as well as for those looking to harness their potential to drive innovation in photonic techniques.

Arrays of Gaussian vortex, Bessel and Airy beams by computer-generated hologram

Flat lens also known as metasurface lens is drawing considerable attention nowadays. Thus far, the metasurface lens has been achieved by engineering local phase profile in deep subwavelength scale according to resonant or non‐resonant (or both) responses of nanoscatters to light. Such local phase mappings require precise control over each nanoscatter. In addition, there is currently no metasurface lens that exhibits multiple functionalities. Here, a novel working mechanism, named as a global phase‐modulation mechanism, for the flat‐lens design is demonstrated. To illustrate this concept, a binary amplitude‐only hologram is introduced, resulted from an interference between an angular cosine wave and a spherical wave. The resultant flat lens can be made extremely thin (60 nm) and is comprised of spatially shaped apertures. Such a technique allows to globally manipulate the diffractive wave vectors in the reciprocal space. Multiple focusing performances with a single flat lens are demonstrated under different illumination beams including the plane‐wave beam, vortex beam, and vector beam. In addition to imaging, the flat lens is able to Fourier transforming a complex self‐accelerating Airy beam, which was not considered before. The presented global mechanism facilitates new capabilities and holds for multifunctional flat lens, with potential applications in areas such as imaging and optical information processing.

This work innovatively designs an all-dielectric metasurface fabricated using 3D printing technology with alumina as the substrate material. This metasurface successfully generates vectorial non-diffracting Bessel beams at a specific frequency of 0.1THz. Initially, through comprehensive simulation analysis of the interaction between the metasurface and the incident beam, it was discovered that the metasurface can precisely manipulate the properties of the transmitted beam based on the polarization state of the incident beam. Specifically, when illuminated by a left-handed circularly polarized (LCP) Gaussian beam, the transmitted beam manifests as a radially polarized Bessel beam. Conversely, under right-handed circularly polarized (RCP) Gaussian beam illumination, the transmitted light transforms into an azimuthally polarized Bessel beam. Building upon the theoretical simulations and considering experimental constraints, further experimental validation was conducted using linearly polarized light, confirming the effectiveness and feasibility of the proposed design. Experimental results demonstrate that the presented all-dielectric alumina metasurface, fabricated via 3D printing, successfully achieves the generation of vector nondiffracting Bessel beams. On one hand, the combination of alumina material and 3D printing technology significantly reduces manufacturing costs, laying a foundation for large scale application. On the other hand, leveraging the high precision of 3D printing ensures the accurate fabrication of the metasurface structure, thereby guaranteeing the high quality and stability of the generated vector beams. This work provides a novel approach for the efficient generation of Bessel beams and is expected to have profound implications in the field of terahertz technology.

In this paper, we study the characteristics of the supercontinuum (SC) induced by the filamentation of two typical vortex beams (i.e., Laguerre-Gaussian (LG) and Bessel-Gaussian (BG) beams) in water. By moving the cuvette filled with water along the laser propagation path, we measure the SC induced by the filamentation of the two vortex beams at different positions in water. The results show that the degree of spectral broadening induced by the filamentation of LG beams hardly changes with the change of position, while for BG beams, the spectral broadening induced by filamentation is weak on both sides and strong in the middle. The value of topological charge (TC) affects the length of the filament formed by BG beams; however, its effect on the spectral broadening induced by the filamentation of LG and BG beams is negligible.

Traditional optics usually studies the uniform polarization state of light. Compared with uniform vector beams, non-uniform vector beams have more polarization information. Most of the research on generating cylindrical vector beams using metasurfaces focuses on generating transmitted beams using the geometric phase. However, the geometric phase requires the incident light to be circularly polarized, which limits the design freedom. Here, an all-dielectric reflective metasurface is designed to generate different output light according to the different polarization states of the incident light. By combining the two encoding arrangements of the dynamic phase and the geometric phase, the output light is a radial vector beam when the linearly polarized light is incident along the x-direction. Under the incidence of linearly polarized light along the y-direction, the generated output light is an azimuthal vector beam. Under the incidence of left-handed circularly polarized light, the generated output light is a vortex beam with a topological charge of -1. Under the incidence of right-handed circularly polarized light, the generated output light is a vortex beam with a topological charge of +1. The proposed reflective metasurface has potential applications in generating vector beams with high integration.

Two-photon flow cytometry with laser scanning two-dimensional airy beams

The propagation characteristics of Airy beams is investigated and fully\ndescribed under the traveling waves approach analogous to that used for\nnon-diffracting Bessel beams. This is possible when noticing that Airy\nfunctions are in fact Bessel functions of fractional order 1 . We show how\nphysical 3 principles impose restrictions such that the non-diffracting Airy\nbeams cannot be of infinite extent as has been argued, and introduce for the\nfirst time quantitative expressions for the maximum transverse and longitudinal\nextent of Airy beams. We show that under the appropriate physical conditions it\nis possible to obtain higher-order Airy beams.\n

Explicit analytical expressions for the electromagnetic field components of typical structured light beams

Vector beams are light beams with spatially variant polarization [1]. During the last decade vector beams have become an indispensable tool in many areas of science and technology such as optical trapping, quantum memories, and quantum optics. In particular, radially and azimuthally polarized light beams are the paradigm of vector beams. Radial vector beams are especially interesting due to the non-vanishing longitudinal electric field component present in tightly focusing systems, which allows to sharply focus light below the diffraction limit [2]. On the other hand, azimuthal vector beams can induce longitudinal magnetic fields with potential applications in spectroscopy and microscopy. However, the spectral limitations of the generation techniques of vector beams based on linear optics prevent their efficient generation in the extreme-ultraviolet (EUV) and x-ray regimes, which would further extend their applications down to the nanometric scale. High-order harmonic generation (HHG) is known as a unique non-perturbative frequency up-conversion process for the generation of coherent EUV and soft x-ray radiation. A remarkable aspect of HHG is its fully coherent nature, mapping the characteristics of the driving field to the high frequency spectral region and thus allowing to harness the angular momentum properties of the harmonic radiation through modifications of the driving field [3-5].

We introduce a new class of nondiffracting fractional vortex beams that connect Bessel beams of successive order in a smooth transition. The new fractional-order beams preserve the same nondiffracting feature of Bessel beams of integer order, and their orbital angular momentum per photon can take any value in a continuous range. The propagation of the more physically realizable fractional-order Bessel–Gauss beams, i.e. fractional Bessel beams apodized by a Gaussian transmittance, through general ABCD optical systems is studied in detail and is complemented by the experimental generation of several instances of fractional beams which in turn confirms our theoretical predictions.

Single-pass cutting of glass with a curved edge using ultrafast curving bessel beams and oblong airy beams

A convenient and versatile scheme to generate continuously adjustable cylindrical vector (CV) and vortex beams by multiplexing and cascading passive vortex half-wave plates (VHPs) is proposed. (\ensuremath{-}24)--24 order CV or vortex beams are generated only based on VHPs with m = 1, 3, and 8. The coaxial and small-angle interference are introduced to detect orders. For coaxial interference of ${l}_{1}$-order and ${l}_{2}$-order vortex beams, the intensity distribution is spirally fan shaped. The partition number is N = |${l}_{2}$\ensuremath{-} ${l}_{1}$|, and the spiral direction can distinguish the sign of topological charge with higher value. Fork-shaped fringes appear in the center for small-angle interference between vortex beams with incident angle ${\ensuremath{\beta}}_{1}$ and ${\ensuremath{\beta}}_{2}$. The forking number between two fringes meets N = |${l}_{2}$\ensuremath{-} ${l}_{1}$|, and the forks face upward when (${l}_{2}$\ensuremath{-} ${l}_{1}$)(${\ensuremath{\beta}}_{2}$\ensuremath{-} ${\ensuremath{\beta}}_{1}$) 0, and face downward when (${l}_{2}$\ensuremath{-} ${l}_{1}$)(${\ensuremath{\beta}}_{2}$\ensuremath{-} ${\ensuremath{\beta}}_{1}$) > 0. Particularly, when one of the beams is a Gaussian beam, such as ${l}_{1}$ = 0, the value and sign of the topological charge ${l}_{2}$ can be directly detected. And for the interference between CV and Gaussian plane beams, the partition number N is positively correlated with the polarization order P, with N = |P| or N = 2|P| for coaxial or small-angle interference, respectively, and corresponding sign can be identified by the polarization azimuth distributions measured by Stokes parameters. It is experimentally demonstrated that the proposed scheme can effectively generate continuously adjustable CV and vortex beams by programming limited VHPs, greatly expanding order numbers with low cost, low energy consumption, and high utilization, and the coaxial and small-angle interference method can conveniently detect the corresponding orders without borrowing redundant devices.

Topological defects in vector fields constitute polarization singularities that have numerous applications in classical and quantum optics. These beams are inhomogeneously polarized and are shown to self-heal under symmetric amplitude perturbations. Polarization singular beams are characterized using a singularity index that can be detected using Stokes polarimetry or other interferometric and diffraction approaches. However, the information about the singularity index is lost when these beams travel through random scattering media; this results in a spatially fluctuating polarization pattern known as polarization speckle. This paper proposes and experimentally demonstrates a new method to detect the topological index of these randomly scattered V-point singularities using higher-order Stokes correlations in a lensless condition. A detailed theoretical basis is developed, and the performance of the technique is demonstrated by retrieving the signature of polarization singularities with Poincaré-Hopf index |η|=1 and |η|=2. We also demonstrate that by studying the intensity-intensity correlations of the polarization speckle, it is possible to differentiate between different vector beams having the same magnitude as the Poincaré-Hopf index.