Angular momentum holography via a minimalist metasurface for optical nested encryption

Stretchable Dual-Axis Terahertz Bifocal Metalens with Flexibly Polarization-Dependent Focal Position and Direction

Varifocal lenses are essential components in any optical system, while traditional lenses suffer from bulky volume, fixed focal position, and limited working spectra. As well-arranged subwavelength structures, metalenses overcome the abovementioned obstacles and exhibit merits of ultrathin thickness, flexible focal length, and multifocus. The electromagnetic responses of metasurfaces, including metalens, rely on the phase distributions of phase-shifting elements. The steerable focal direction is investigated to obtain the combinations of focusing and anomalous refraction phase distribution. To fully explore the flexibility of focal length and direction, seven designs of double layers of terahertz (THz) bifocal metalenses are proposed and investigated in this study. They exhibit dependent and independent relationships of tunable focal length and direction with flexible tuning mechanisms. Along with polarization multiplexing, two different focuses can be obtained when the incident waves are x-linear and y-linear polarization states, respectively. The simulation results agreed well with the theoretical predictions. These designs provide a new method to modulate the focal position precisely with promising applications in wireless communication, imaging, and on-chip optical integration systems.

Cylindrical Vector Beam Holography without Preservation of OAM Modes

Orbital angular momentum (OAM) multiplexed holograms have attracted a great deal of attention recently due to their physically unbounded set of orthogonal helical modes. However, preserving the OAM property in each pixel hinders fine sampling of the target image in principle and requires a fundamental filtering aperture array in the detector plane. Here, we demonstrate the concept of metasurface-based vectorial holography with cylindrical vector beams (CVBs), whose unlimited polarization orders and unique polarization distributions can be used to boost information storage capacity. Although CVBs are composed of OAM modes, the holographic images do not preserve the OAM modes in our design, enabling fine sampling of the target image in a quasi-continuous way like traditional computer-generated holograms. Moreover, the images can be directly observed by passing them through a polarizer without the need for a fundamental mode filter array. We anticipate that our method may pave the way for high-capacity holographic devices.

Ultra-robust informational metasurfaces based on spatial coherence structures engineering

Optical information transmission is vital in modern optics and photonics due to its concurrent and multi-dimensional nature, leading to tremendous applications such as optical microscopy, holography, and optical sensing. Conventional optical information transmission technologies suffer from bulky optical setup and information loss/crosstalk when meeting scatterers or obstacles in the light path. Here, we theoretically propose and experimentally realize the simultaneous manipulation of the coherence lengths and coherence structures of the light beams with the disordered metasurfaces. The ultra-robust optical information transmission and self-reconstruction can be realized by the generated partially coherent beam with modulated coherence structure even 93% of light is recklessly obstructed during light transmission, which brings new light to robust optical information transmission with a single metasurface. Our method provides a generic principle for the generalized coherence manipulation on the photonic platform and displays a variety of functionalities advancing capabilities in optical information transmission such as meta-holography and imaging in disordered and perturbative media.

Multifunctional Meta-Devices for Full-Polarization Rotation and Focusing in the Near-Infrared

The creation of multi-channel focused beams with arbitrary polarization states and their corresponding optical torques finds effective applications in the field of optical manipulation at the micro-nanoscale. The existing metasurface-based technologies for polarization rotation have made some progress, but they have been limited to single functions and have not yet achieved the generation of full polarization. In this work, we propose a multi-channel and spatial-multiplexing interference strategy for the generation of multi-channel focusing beams with arbitrary polarization rotation based on all-dielectric birefringent metasurfaces via simultaneously regulating the propagation phase and the geometric phase and independently controlling the wavefronts at different circular polarizations. For the proof of concept, we demonstrate highly efficient multi-channel polarization rotation meta-devices. The meta-devices demonstrate ultra-high polarization extinction ratios and high focusing efficiencies at each polarization channel. Our work provides a compact and versatile wavefront-shaping methodology for full-polarization control, paving a new path for planar multifunctional meta-optical devices in optical manipulation at micro-nano dimensions.

Dynamic Spatial-Selective Metasurface with Multiple-Beam Interference

The emergence of the metasurface has provided a versatile platform for the manipulation of light at the nanoscale. Recent research in metasurfaces has explored a plethora of dynamic control and switching of multifunctionalities, paving the way for innovative applications in fields such as imaging, sensing, and communication. However, current dynamic multifunctional metasurfaces face challenges in terms of functional scalability and selective activation. In this work, we introduce and experimentally demonstrate a strategy that utilizes multiple plane waves to create arbitrary periodic patterns on the metasurface, thus enabling the dynamic and arbitrary spatial-selective activation of its embedded multiplexed functionalities. Furthermore, our strategy facilitates dynamic light control through mechanical translation, as demonstrated by a high-speed, dynamically switchable beam deflection scenario. Our method effectively overcomes the limitations associated with traditional spatially multiplexing techniques, offering greater flexibility and selectivity for dynamic control in multifunctional metasurfaces.

Optical vectorial-mode parity Hall effect: a case study with cylindrical vector beams

The vectorial optical field (VOF) assumes a pivotal role in light-matter interactions. Beyond its inherent polarization topology, the VOF also encompasses an intrinsic degree of freedom associated with parity (even or odd), corresponding to a pair of degenerate orthogonal modes. However, previous research has not delved into the simultaneous manipulation of both even and odd parities. In this study, we introduce and validate the previously unexplored parity Hall effect for vectorial modes using a metasurface design. Our focus lies on a cylindrical vector beam (CVB) as a representative case. Through the tailored metasurface, we effectively separate two degenerate CVBs with distinct parities in divergent directions, akin to the observed spin states split in the spin Hall effect. Additionally, we provide experimental evidence showcasing the capabilities of this effect in multi-order CVB demultiplexing and parity-demultiplexed CVB-encoded holography. This effect unveils promising opportunities for various applications, including optical communication and imaging.

Metasurface for Engineering Superimposed Ince-Gaussian Beams

Ince-Gaussian beams (IGBs) are the third complete family of exact and orthogonal solutions of the paraxial wave equation and have been applied in many fields ranging from particle trapping to quantum optics. IGBs play a very important role in optics as they represent the exact and continuous transition modes connecting Laguerre-Gaussian and Hermite-Gaussian beams. The method currently in use suffers from the high cost, complexity, and large volume of the optical system. The superposition of IGBs can generate complicated structured beams with multiple phase and polarization singularities. A metasurface approach is proposed to realizing various superpositions of IGBs without relying on a complicated optical setup. By superimposing IGBs with even and odd modes, multiple phase, and polarization singularities are observed in the resultant beams. The phase and polarization singularities are modulated by setting the initial phase in the design and controlling the incident linear polarization. The compactness of the developed metasurface devices and the unique properties of the generated beams have the potential to impact many practical applications such as particle manipulation, orbital angular momentum spectrum manipulation, and optical communications.

Nanostructure-based orbital angular momentum encryption and multiplexing

The orthogonality among the OAM modes provides a new degree of freedom for optical multiplexing communications. So far, traditional Dammann gratings and spatial light modulators (SLMs) have been widely used to generate OAM beams by modulating electromagnetic waves at each pixel. However, such architectures suffer from limitations in terms of having a resolution of only a few microns and the bulkiness of the entire optical system. With the rapid development of the electromagnetic theory and advanced nanofabrication methods, artificial nanostructures, especially optical metasurfaces, have been introduced which greatly shrink the size of OAM multiplexing devices while increasing the level of integration. This review focuses on the study of encryption, multiplexing and demultiplexing of OAM beams based on nanostructure platforms. After introducing the focusing characteristics of OAM beams, the interaction mechanism between OAM beams and nanostructures is discussed. The physical phenomena of helical dichroism response and spatial separation of OAM beams achieved through nanostructures, setting the stage for OAM encryption and multiplexing, are reviewed. Afterward, the further advancements and potential applications of nanophotonics-based OAM multiplexing are deliberated. Finally, the challenges of conventional design methods and dynamic tunable techniques for nanostructure-based OAM multiplexing technology are addressed.

Switchable unidirectional emissions from hydrogel gratings with integrated carbon quantum dots

Directional emission of photoluminescence despite its incoherence is an attractive technique for light-emitting fields and nanophotonics. Optical metasurfaces provide a promising route for wavefront engineering at the subwavelength scale, enabling the feasibility of unidirectional emission. However, current directional emission strategies are mostly based on static metasurfaces, and it remains a challenge to achieve unidirectional emissions tuning with high performance. Here, we demonstrate quantum dots-hydrogel integrated gratings for actively switchable unidirectional emission with simultaneously a narrow divergence angle less than 1.5° and a large diffraction angle greater than 45°. We further demonstrate that the grating efficiency alteration leads to a more than 7-fold tuning of emission intensity at diffraction order due to the variation of hydrogel morphology subject to change in ambient humidity. Our proposed switchable emission strategy can promote technologies of active light-emitting devices for radiation control and optical imaging.

Asymmetric Full-Color Vectorial Meta-holograms Empowered by Pairs of Exceptional Points

Holography holds tremendous promise in applications such as immersive virtual reality and optical communications. With the emergence of optical metasurfaces, planar optical components that have the remarkable ability to precisely manipulate the amplitude, phase, and polarization of light on the subwavelength scale have expanded the potential applications of holography. However, the realization of metasurface-based full-color vectorial holography remains particularly challenging. Here, we report a general approach utilizing a modified Gerchberg-Saxton algorithm to achieve spatially aligned full-color display and incorporating wavelength information with an image compensation strategy. We combine the Pancharatnam-Berry phase and pairs of exceptional points to address the issue of redundant twin images that generally appear for the two orthogonal circular polarizations and to enable full polarization control of the vectorial field. Our results enable the realization of an asymmetric full-color vectorial meta-hologram, paving the way for the development of full-color display, complex beam generation, and secure data storage applications.

"Interesting and useful, extreme and ultimate": an interview with Prof. Huigao Duan

EDITORIAL

He is an explorer in micro-nano manufacturing, an adventurer in interdisciplinary studies, an impassioned educator; an ever-curious reader, a lover of sports, and a coffee connoisseur. In this episode of Light People Interview, we are honored to have Prof. Huigao Duan from Hunan University share his insights on micro-nano manufacturing, planar optics, teaching, nurturing, reading, growing up, and hobbies. "A man with a great smile", "a good teacher," and "an interesting soul" are just some of the descriptions that came through my mind during my conversation with him. Today, let's get to know this outstanding scholar who pursues "interesting and useful, extreme and ultimate" in micro-nano manufacturing and optics.

Optical image conversion and encryption based on structured light illumination and a diffractive neural network

In this paper, an optical image encryption method is proposed based on structured light illumination and a diffractive neural network (DNN), which can realize conversion between different images. With the use of the structured phase mask (SPM) in the iterative phase retrieval algorithm, a plaintext image is encoded into a DNN composed of multiple phase-only masks (POMs) and ciphertext. It is worth noting that ciphertext is a visible image such that the conversion of one image to another is achieved, leading to high concealment of the proposed optical image encryption method. In addition, the wavelength of the illuminating light, all Fresnel diffraction distances, the optical parameters of the adopted SPM such as focal length and topological charge number, as well as all POMs in the DNN are all considered as security keys in the decryption process, contributing to a large key space and high level of security. Numerical simulations are performed to demonstrate the feasibility of the proposed method, and simulation results show that it exhibits high feasibility and safety as well as strong robustness.

Orbital angular momentum holographic multicasting for switchable and secure wireless optical communication links

The physical dimension of orbital angular momentum (OAM) states of light has been successfully implemented as information carrier in wireless optical communication (WOC) links. However, the current OAM data coding strategies in WOC are mainly limited to the temporal domain, rarely involving the degree of freedom of spatial domain to transmit an image directly. Here, we apply OAM holographic multiplexing technology for spatial information encoding in WOC links. Further, we demonstrate the new concept of OAM holographic multicasting, wherein a beam-steering grating has been utilized for information decoding. To distribute the OAM multiplexing information appropriately in the receiving terminal, the beam-steering grating with controllable topological charges and amplitude weighting coefficients of each diffraction order in the spatial frequency domain has been designed. An iterative algorithm has been introduced to obtain the intensity uniformity >98% at target diffraction orders. As such, this scheme experimentally allows four separate users to receive independent images, which can be switched by modulating the topological charges of the beam-steering gratings at each diffraction order. In addition, this leads to a beam-steering grating-encrypted WOC links, wherein the information can only be decoded by the grating phase with 7 pre-set spatial frequency components. Our results mark a new parallel decoding paradigm of OAM multiplexing holography, which opens up the door for future high-capacity and high-security all-optical holographic communications.

High-power femtosecond vortices generated from a Kerr-lens mode-locked solid-state Hermite-Gaussian oscillator

We report the generation of high-order transverse modes from a Kerr-lens mode-locked femtosecond laser. Two different orders of Hermite-Gaussian modes were realized by non-collinear pumping, which were converted into the corresponding Laguerre-Gaussian vortex modes using a cylindrical lens mode converter. The mode-locked vortex beams, with an average power of 1.4 W and 0.8 W, contained pulses as short as 126 fs and 170 fs at the first and second Hermite-Gaussian mode orders, respectively. This work demonstrates the possibility of developing Kerr-lens mode-locked bulk lasers with various pure high-order modes and paves the way for generating ultrashort vortex beams.