Metafiber transforming arbitrarily structured light

3D-printed fiber-based perfect vortex beam generator

An all-fiber focused perfect vortex beam (PVB) generator is reported and fabricated by using 3D printing technology. The generator is constructed by integrating a designed spiral axicon zone plate (SAZP) on the fiber-end facet. The ring diameters of the generated beams are independent of the topological charges and positively correlated with the wave vectors in the transverse direction. By controlling the axicon angle, the ring diameter can be freely adjusted. The experimental results are consistent with the simulation results, confirming the rationality of the design. These generated PVBs are expected to be useful in applications such as high-capacity fiber-optic communication and multifunctional optical tweezers.

Nanofabrication for Nanophotonics

Nanofabrication, a pivotal technology at the intersection of nanoscale engineering and high-resolution patterning, has substantially advanced over recent decades. This technology enables the creation of nanopatterns on substrates crucial for developing nanophotonic devices and other applications in diverse fields including electronics and biosciences. Here, this mega-review comprehensively explores various facets of nanofabrication focusing on its application in nanophotonics. It delves into high-resolution techniques like focused ion beam and electron beam lithography, methods for 3D complex structure fabrication, scalable manufacturing approaches, and material compatibility considerations. Special attention is given to emerging trends such as the utilization of two-photon lithography for 3D structures and advanced materials like phase change substances and 2D materials with excitonic properties. By highlighting these advancements, the review aims to provide insights into the ongoing evolution of nanofabrication, encouraging further research and application in creating functional nanostructures. This work encapsulates critical developments and future perspectives, offering a detailed narrative on the state-of-the-art in nanofabrication tailored for both new researchers and seasoned experts in the field.

High-density multicore fiber with single-mode cores and low-mode crosstalk for visible light applications between 565 nm and 650 nm

This study presents what we believe to be a novel 37-core multicore fiber designed for the visible range (565 nm … 650 nm), featuring a hexagonal arrangement of identical cores, single-mode guidance, low intercore crosstalk, low-mode attenuation (< 0.1 dB/m), and high-core density (1.2 × 10-3). Experimental characterizations, including intercore coupling, mode attenuation, core positioning accuracy (<120 nm), bending loss, group velocity dispersion, and mode field diameter, combined with supermode dispersion simulations, uncover the fiber performance. These results highlight the potential of visible-wavelength multicore fibers for advancing high-resolution imaging, optical trapping, and minimally invasive diagnostics.

Customizing Multicolored Orbital Angular Momentum Combs

Current orbital angular momentum (OAM) combs generating technology is hindered by bulky optical systems, limited control, and lack of multicolored information, impeding system integration and practical applications. We present a metasurface approach to realizing multicolored OAM comb engineering along the light propagation direction. The OAM combs are measured based on the intensity of bright spots in the generated intensity patterns that correspond to the weights of the OAM modes. Three OAM combs with different colors are generated at different observation planes. The positioning of transition points along the azimuthal direction is the key to shaping the OAM distribution of the generated beams. OAM combs with customized mode spacings and broad OAM spectra are obtained. Our approach provides a compact platform to realize OAM combs with multidimensional information in the domains of the OAM spectra, frequency, and space, which can significantly enhance the information capacity for potential applications in optical communications.

Polarization-multiplexing metafiber for dual-mode bright-field and dark-field microscopy

Bright-field and dark-field microscopy are typically used together as complementary techniques to provide comprehensive information about biological specimens with different optical absorption properties. However, switching between these two modes usually involves replacing several bulk optical components, which inevitably increases system complexity, introduces alignment challenges, and results in longer switching times. Herein, we propose a new, to the best of our knowledge, polarization-multiplexing metafiber device for dual-mode bright-field and dark-field microscopy. Utilizing a polarization-multiplexing metalens, two tailored beams (i.e., Gaussian and OAM beam) can be generated, simply by changing the handedness of the incident circularly polarized light. By integrating such metalens onto the tip of a large-mode-area photonic crystal fiber, we experimentally demonstrated that this compact and flexible metafiber can realize the dual-mode bright-field and dark-field microscopy using raspberry trichomes and pine stem, without the need to replace any optical components. The ultra-compact and flexibility features of the proposed metafiber-based dual-mode microscopy pave the way for promising applications in portable and in vivo biological imaging.

Space-time wave packets in multimode optical fibers with controlled dynamic motions and tunable group velocities

Space-time wave packets (STWPs) with correlated spatial and frequency degrees of freedom exhibit time-dependent spatial interference, thereby giving rise to interesting dynamic evolution behaviors. While versatile spatiotemporal phenomena have been demonstrated in freely propagating fields, coupling spatiotemporal light into multimode fibers remains a fundamental experimental challenge. Whereas synthesizing freely propagating STWPs typically relies on a continuum of plane-wave modes, their multimode-fiber counterparts must be constructed from the discrete set of fiber modes whose propagation constants depend on fiber structures. Here, we demonstrate STWPs with axially controllable motion of the transverse profile and reconfigurable group velocity in graded-index multimode fibers. This is accomplished by introducing a linear association between frequency comb lines and corresponding fiber modes. The synthesized STWPs present dynamic rotation and translation with a 4.8-ps period. Simultaneously, the group velocity can be tuned from positive subluminal and superluminal to negative values (e.g., 0.870, 1.35, 10, and -3.3 × 108 m/s, respectively).

Metasurface higher-order poincaré sphere polarization detection clock

Accurately and swiftly characterizing the state of polarization (SoP) of complex structured light is crucial in the realms of classical and quantum optics. Conventional strategies for detecting SoP, which typically involves a sequence of cascaded optical elements, are bulky, complex, and run counter to miniaturization and integration. While metasurface-enabled polarimetry has emerged to overcome these limitations, its functionality predominantly remains confined to identifying SoP within the standard Poincaré sphere framework. The comprehensive detection of SoP on the higher-order Poincaré sphere (HOPS), however, continues to be a huge challenge. Here, we propose a general polarization metrology method capable of fully detecting SoP on any HOPS through a single measurement. The underlying mechanism relies on transforming the optical singularities and Stokes parameters into visualized intensity patterns, facilitating the extraction of all parameters that fully determine a SoP. We actualize this concept through a novel meta-device known as the metasurface photonics polarization clock, which offers an intuitive display of SoP using four distinct pointers. As a proof of concept, we theoretically and experimentally demonstrate fully resolving SoPs on the 0th, 1st, and 2nd HOPSs. Our implementation opens up a new pathway towards real-time polarimetry of arbitrary beams featuring miniaturized size, a simple detection process, and a direct readout mechanism, promising significant advancements in fields reliant on polarization.

Advanced remote focus control in multicore meta-fibers through 3D nanoprinted phase-only holograms

In this study, we present an unexplored approach for remote focus manipulation using 3D nanoprinted holograms integrated on the end face of multi-core single-mode fibers. This innovative method enables precise focus control within a monolithic metafiber device by allowing light coupled into any of the 37 cores to be precisely focused at predefined locations. Our approach demonstrates significant advances over conventional lenses and offers unique functionalities through computationally designed holograms. This research marks the first successful use of multi-core fibers for remote focus control via 3D nanoprinting, achieving crosstalk-free operation at visible wavelengths. Key findings include strong agreement between design, simulation, and experimental results, highlighting the potential of this technology to improve applications in fields such as biological optics, laser micromachining, telecommunications, and laser surgery. This work opens new avenues for the development of advanced optical systems with superior focus control capabilities.

Extensive Q-factor tuning for leaky modes with minimal frequency variation in asymmetric slab grating structures

We investigated an asymmetric slab grating structure to achieve significant tuning of the quality (Q) factor for a leaky mode while minimizing frequency variation. This structure comprises two identical gratings placed on the top and bottom of a slab waveguide, with one grating laterally shifted to introduce asymmetry. Simulations demonstrate that lateral shifting of one grating induces extensive changes in the Q-factor with minimal frequency variation, particularly near the band-flip filling fraction because the band-flip filling fraction remains unaffected by the shifting. The independence of the band-flip filling fraction from lateral shifting is attributed to the superposition property of Bragg scattering processes in the asymmetric grating structure. Experimental verification in the terahertz range confirms significant control over the Q-factor of the leaky mode of the structure. The proposed asymmetric slab grating structure offers possibilities for mechanically controllable optical devices, which are applicable to tunable filters and sensors. This study advances our understanding and application of leaky modes in asymmetric grating structures, revealing a previously unexplored aspect of asymmetric optical lattice.

Optical skyrmions from metafibers with subwavelength features

Optical skyrmions are an emerging class of structured light with sophisticated particle-like topologies with great potential for revolutionizing modern informatics. However, the current generation of optical skyrmions involves complex or bulky systems, hindering the development of practical applications. Here, exploiting the emergent "lab-on-fiber" technology, we demonstrate the design of a metafiber-integrated photonic skyrmion generator. We not only successfully generate high-quality optical skyrmions from metafibers, but also verify their remarkable properties, such as topology switchability and topology stability with subwavelength polarization features beyond the diffraction limits. Our flexible fiber-integrated optical skyrmions platform paves the avenue for future applications of topologically-enhanced remote super-resolution microscopy and robust information transfer.

Neural network-assisted meta-router for fiber mode and polarization demultiplexing

Advancements in computer science have propelled society into an era of data explosion, marked by a critical need for enhanced data transmission capacity, particularly in the realm of space-division multiplexing and demultiplexing devices for fiber communications. However, recently developed mode demultiplexers primarily focus on mode divisions within one dimension rather than multiple dimensions (i.e., intensity distributions and polarization states), which significantly limits their applicability in space-division multiplexing communications. In this context, we introduce a neural network-assisted meta-router to recognize intensity distributions and polarization states of optical fiber modes, achieved through a single layer of metasurface optimized via neural network techniques. Specifically, a four-mode meta-router is theoretically designed and experimentally characterized, which enables four modes, comprising two spatial modes with two polarization states, independently divided into distinct spatial regions, and successfully recognized by positions of corresponding spatial regions. Our framework provides a paradigm for fiber mode demultiplexing apparatus characterized by application compatibility, transmission capacity, and function scalability with ultra-simple design and ultra-compact device. Merging metasurfaces, neural network and mode routing, this proposed framework paves a practical pathway towards intelligent metasurface-aided optical interconnection, including applications such as fiber communication, object recognition and classification, as well as information display, processing, and encryption.

Capillary-assisted flat-field formation: a platform for advancing nanoparticle tracking analysis in an integrated on-chip optofluidic environment

Here, we present the concept of flat-field capillary-assisted nanoparticle tracking analysis for the characterization of fast diffusing nano-objects. By combining diffusion confinement and spatially invariant illumination, i.e., flat-fields, within a fiber-interfaced on-chip environment, ultra-long trajectories of fast diffusing objects within large microchannels have been measured via diffraction-limited imaging. Our study discusses the design procedure, explains potential limitations, and experimentally confirms flat-field formation by tracking gold nanospheres. The presented concept enables generating flat-fields in a novel on-chip optofluidic platform for the characterization of individual nano-objects for fundamental light/matter investigations or applications in bioanalytics and nanoscale material science.

Beyond the lab: a nanoimprint metalens array-based augmented reality

A see-through augmented reality prototype has been developed based on an ultrathin nanoimprint metalens array, opening up a full-colour, video-rate, and low-cost 3D near-eye display.

Arbitrary engineering of spatial caustics with 3D-printed metasurfaces

Caustics occur in diverse physical systems, spanning the nano-scale in electron microscopy to astronomical-scale in gravitational lensing. As envelopes of rays, optical caustics result in sharp edges or extended networks. Caustics in structured light, characterized by complex-amplitude distributions, have innovated numerous applications including particle manipulation, high-resolution imaging techniques, and optical communication. However, these applications have encountered limitations due to a major challenge in engineering caustic fields with customizable propagation trajectories and in-plane intensity profiles. Here, we introduce the "compensation phase" via 3D-printed metasurfaces to shape caustic fields with curved trajectories in free space. The in-plane caustic patterns can be preserved or morphed from one structure to another during propagation. Large-scale fabrication of these metasurfaces is enabled by the fast-prototyping and cost-effective two-photon polymerization lithography. Our optical elements with the ultra-thin profile and sub-millimeter extension offer a compact solution to generating caustic structured light for beam shaping, high-resolution microscopy, and light-matter-interaction studies.

Nanoprinted microstructure-assisted light incoupling into high-numerical aperture multimode fibers

The coupling of light into optical fibers is limited by the numerical aperture (NA). Here, we show that large-area polymer axial-symmetric microstructures printed on silica multimode fibers improve their incoupling performance by two to three orders of magnitude beyond the numerical aperture limit. A ray-optical mathematical model describing the impact of the grating-assisted light coupling complements the experimental investigation. This study clearly demonstrates the improvement of incoupling performance by nanoprinting microstructures on fibers, opening new horizons, to the best of our knowledge, for multimode fiber applications in life sciences, quantum technologies, and "lab-on-fiber" devices.

Information multiplexing from optical holography to multi-channel metaholography

Holography offers a vital platform for optical information storage and processing, which has a profound impact on many photonic applications, including 3D displays, LiDAR, optical encryption, and artificial intelligence. In this review, we provide a comprehensive overview of optical holography, moving from volume holography based on optically thick holograms to digital holography using ultrathin metasurface holograms in nanophotonics. We review the use of volume holograms for holographic multiplexing through the linear momentum selectivity and other approaches and highlight the emerging use of digital holograms that can be implemented by ultrathin metasurfaces. We will summarize the fabrication of different holographic recording media and digital holograms based on recent advances in flat meta-optics and nanotechnology. We highlight the rapidly developing field of metasurface holography, presenting the use of multi-functional metasurfaces for multiplexing holography in the use of polarization, wavelength, and incident angle of light. In the scope of holographic applications, we will focus on high bandwidth metasurface holograms that offer the strong sensitivity to the orbital angular momentum of light. At the end, we will provide a short summary of this review article and our perspectives on the future development of the vivid holography field.