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Digital optical holograms can achieve nanometre-scale resolution as a result of recent advances in metasurface technologies. This has raised hopes for applications in data encryption, data storage, information processing and displays. However, the hologram bandwidth has remained too low for any practical use. To overcome this limitation, information can be stored in the orbital angular momentum of light, as this degree of freedom has an unbounded set of orthogonal helical modes that could function as information channels. Thus far, orbital angular momentum holography has been achieved using phase-only metasurfaces, which, however, are marred by channel crosstalk. As a result, multiplex information from only four channels has been demonstrated. Here, we demonstrate an orbital angular momentum holography technology that is capable of multiplexing up to 200 independent orbital angular momentum channels. This has been achieved by designing a complex-amplitude metasurface in momentum space capable of complete and independent amplitude and phase manipulation. Information was then extracted by Fourier transform using different orbital angular momentum modes of light, allowing lensless reconstruction and holographic videos to be displayed. Our metasurface can be three-dimensionally printed in a polymer matrix on SiO2 for large-area fabrication.

Holography has been identified as a vital platform for three-dimensional displays, optical encryption, microscopy and artificial intelligence through different physical dimensions. However, unlike the wavelength and polarization divisions, orbital angular momentum (OAM) of light, despite its helical wavefront being an independent physical dimension, has not been implemented as an information carrier for holography due to the lack of helical mode index selectivity in the Bragg diffraction formula. Here, we demonstrate OAM holography by discovering strong OAM selectivity in the spatial-frequency domain without a theoretical helical mode index limit. As such, OAM holography allows the multiplexing of a wide range of OAM-dependent holographic images with a helical mode index spanning from −50 to 50, leading to a 10 bit OAM-encoded hologram for high-security optical encryption. Our results showing up to 210 OAM-dependent distinctive holographic images mark a new path to achieving ultrahigh-capacity holographic information systems harnessing the previously inaccessible OAM division. The orbital angular momentum degree of freedom is used to demonstrate 10 bit holographic images with a helical mode index spanning from −50 to 50.

The ability to engineer nonlinear optical processes in all-dielectric nanostructures is both of fundamental interest and highly desirable for high-performance, robust, and miniaturized nonlinear optical devices. Herein, we propose a novel paradigm for the efficient tuning of second-harmonic generation (SHG) process in dielectric nanoantennas by integrating with chalcogenide phase change material. In a design with Ge$_{2}$Sb$_{2}$Te$_{5}$ (GST) film sandwiched between the AlGaAs nanoantennas and AlO$_{x}$ substrate, the nonlinear SHG signal from the AlGaAs nanoantennas can be boosted via the resonantly localized field induced by the optically-induced Mie-type resonances, and further modulated by exploiting the GST amorphous-to-crystalline phase change in a non-volatile, multi-level manner. The tuning strategy originates from the modulation of resonant conditions by changes in the refractive index of GST. With a thorough examination of tuning performances for different nanoantenna radii, a maximum modulation depth as high as 540$\%$ is numerically demonstrated. This work not only reveals out the potential of GST in optical nonlinearity control, but also provides promising strategy in smart designing tunable and reconfigurable nonlinear optical devices, e.g., light emitters, modulators, and sensors.

Allowing subwavelength-scale-digitization of optical wavefronts to achieve complete control of light at interfaces, metasurfaces are particularly suited for the realization of planar phase-holograms that promise new applications in high-capacity information technologies. Similarly, the use of orbital angular momentum of light as a new degree of freedom for information processing can further improve the bandwidth of optical communications. However, due to the lack of orbital angular momentum selectivity in the design of conventional holograms, their utilization as an information carrier for holography has never been implemented. Here we demonstrate metasurface orbital angular momentum holography by utilizing strong orbital angular momentum selectivity offered by meta-holograms consisting of GaN nanopillars with discrete spatial frequency distributions. The reported orbital angular momentum-multiplexing allows lensless reconstruction of a range of distinctive orbital angular momentum-dependent holographic images. The results pave the way to the realization of ultrahigh-capacity holographic devices harnessing the previously inaccessible orbital angular momentum multiplexing., Conventional hologram designs lack orbital angular momentum selectivity. Here, the authors design metasurface holograms consisting of GaN nanopillars with discrete spatial frequency distributions allowing the reconstruction of distinctive orbital angular momentumdependent holographic images.

Nonlinear holography has been identified as a vital platform for optical multiplexing holography because of the appearance of new optical frequencies. However, due to nonlinear wave coupling in nonlinear optical processes, the nonlinear harmonic field is coupled with the input field, laying a fundamental barrier to independent control of the interacting fields for holography. We propose and experimentally demonstrate high-dimensional orbital angular momentum (OAM) multiplexing nonlinear holography to overcome this problem. By dividing the wavefront of the fundamental wave into different orthogonal OAM channels, multiple OAM and polarization-dependent holographic images in both the fundamental wave and second-harmonic wave have been reconstructed independently in the spatial frequency domain through a type-II second harmonic generation process. Moreover, this method can be easily extended to cascaded χ2 nonlinear optical processes for multiplexing in more wavelength channels, leading to potential applications in multicasting in optical communications, multiwavelength display, multidimensional optical storage, anticounterfeiting, and optical encryption.

To manipulate the wave front of the harmonic wave, nonlinear photonic crystals (NPCs) have been employed in constructing nonlinear holography. Through designing a multi-channel ${\ensuremath{\chi}}^{2}$ structure, nonlinear multiplexing holography is reported here in NPCs. We propose a multi-channel nonlinear holography in an orbital-angular-momentum- (OAM-)multiplexing NPC. In experiment, three images encoded in a single NPC are reconstructed in second-harmonic waves distinctively, when the necessary Fourier-domain OAM-matching conditions are met, respectively. Our method not only conceptually extends nonlinear holography, but also provides a multifunctional platform for high-capacity security storage.

A nonlinear photonic crystal (NPC)1 possesses space-dependent second-order nonlinear coefficients, which can effectively control nonlinear optical interactions through quasi-phase matching2. Lithium niobate (LiNbO3) crystal is one of the most popular materials from which to fabricate NPC structures because of its excellent nonlinear optical properties3–5. One- and two-dimensional LiNbO3 NPCs have been widely utilized in laser frequency conversion6,7, spatial light modulation8–12 and nonlinear optical imaging13,14. However, limited by traditional poling methods, the experimental realization of three-dimensional (3D) NPCs remains one of the greatest challenges in the field of nonlinear optics1,15. Here, we present an experimental demonstration of a 3D LiNbO3 NPC by using a femtosecond laser to selectively erase the nonlinear coefficients in a LiNbO3 crystal16,17. The effective conversion efficiency is comparable to that of typical quasi-phase-matching processes. Such a 3D LiNbO3 NPC provides a promising platform for future nonlinear optical studies based on its unique ability to control nonlinear interacting waves in 3D configuration. By selectively erasing the nonlinear coefficients in a lithium niobate crystal using a femtosecond laser, a 3D nonlinear photonic crystal, with an effective conversion efficiency comparable to that of the typical quasi-phase-matching processes, is demonstrated.

Optical orbital angular momentum (OAM) provides an additional dimension for photons to carry information in high-capacity optical communication. Although the practical needs have informed the generations of miniaturized devices to manipulate the OAM modes in various integrated platforms, it is still a challenge for on-chip OAM detection to match the newly developed compact OAM emitter and OAM transmission fiber. Here, we demonstrate an ultracompact device, i.e., a single plasmonic nanohole, to efficiently measure an optical beam’s OAM state in a simple and nondestructive way. The device size is reduced to a few hundreds of nanometers, which can be easily fabricated and installed in the current OAM devices. It is a flexible and robust way for in situ OAM monitoring and detection in optical fiber networks and long-distance optical communication systems. With proper optimization of the nanohole parameters, this approach could be further extended to discriminate the OAM information multiplexed in multiple wav...

We report generation of tunable vortex beams in the blue spectral range, with a 3.3 nm spectral tuning range, by frequency tripling of the near-infrared (IR) wave at around 1.34 um in a LiTaO3 optical superlattice. The nonlinear crystal used in this work has a chirped dual-periodical structure which can provide two expanded reciprocal vectors for tunable performance of the cascaded third harmonic generation (THG). The maximum THG efficiency reaches about 1.4%.

We experimentally investigate the second-harmonic generation of a high-order Laguerre–Gaussian (LG) mode under the quasi-phase-matching (QPM) configuration. First, we introduce a simple method to observe the azimuthal (l) and radial (p) indices of the high-order LG modes. Based on the astigmatic transformation technique, l and p are revealed in the number of dark stripes of the converted pattern in the focal plane. Then, using this efficient method of measurement, we demonstrate in experiments a second-harmonic LG mode with its radial and azimuthal indices being twice those of the inputted fundamental wave through QPM in a periodically poled KTP crystal. Our results provide a feasible way to obtain simultaneously the LG modes with larger radial and azimuthal indices.

With experimental results, we demonstrate the generation of high-order Laguerre–Gaussian modes with non-zero radial indices using a metal meta-surface, which is composed of a series of rectangle nanoholes with different orientation angles. The phase shift after transmission through the metasurface is determined by the orientation angle of the nanohole. This device works over a broad wavelength band ranging from 700 to 1000 nm. Moreover, we achieve a LG mode with a radial mode index of 10. Our results provide an integrated method to obtain high-order LG modes, which can be used to enhance the capacity in optical communication and manipulation.

We experimentally demonstrate multiple generations of high-order orbital angular momentum (OAM) modes through third-harmonic generation in a 2D nonlinear photonic crystal. Such third-harmonic generation process is achieved by cascading second-harmonic generation and sum-frequency generation using the non-collinear quasi-phase-matching technique. This technique allows multiple OAM modes with different colors to be simultaneously generated. Moreover, the OAM conservation law guarantees that the topological charge is tripled in the cascaded third-harmonic generation process. Our method is effective for obtaining multiple high-order OAM modes for optical imaging, manipulation, and communications.

We report the cascaded conversion of orbital-angular-momentum (OAM) states through quasi-phase-matching (QPM) in 1D and 2D periodically poled LiTaO3 crystals. Multiple OAM modes with different colors are simultaneously generated by QPM second- and third-harmonic generations.

We experimentally generate multiple orbital angular momentum (OAM) modes with different color in 2D nonlinear photonic crystal. The process is achieved by the cascaded third-harmonic generation using the non-collinear quasi-phase-matching technique.

We experimentally demonstrate the orbital angular momentum (OAM) conversion by the coupled nonlinear optical processes in a quasi-periodically poled LiTaO3 crystal. In such a crystal, third-harmonic generation (THG) is realized by the coupled second-harmonic generation (SHG) and sum-frequency generation (SFG) processes, i.e., SHG is dependent on SFG and vice versa. The OAMs of the interacting waves are proved to be conserved in such coupled nonlinear optical processes. As we increase the input OAM in the experiment, the conversion efficiency decreases because of the reduced fundamental power density. Our results provide better understanding for the OAM conversions, which can be used to efficiently produce an optical OAM state at a short wavelength.

We experimentally observed conical third harmonic generation (THG) in a two-dimensional periodically poled LiTaO3 nonlinear photonic crystal. The conical THG was attributed to cascaded quasi-phase matching (QPM) processes assisted by scattering fundamental waves in the nonlinear photonic crystal. The characteristics of the generated conical beam, such as the emit angle and beam size, varied with the fundamental wavelength because it changed the scattering-assisted QPM configuration. Our experimental results demonstrated that scattering light can be effectively involved in a cascade nonlinear optical process through QPM, which provides a possible way to produce short-wavelength conical beams to meet the demands of practical applications.

We experimentally demonstrate the plasmon-assisted second-harmonic generation of an optical orbital angular momentum (OAM) beam. Because of the shape resonance, the plasmons in a periodic array of rectangular metal holes greatly enhance the nonlinear optical conversion of an OAM state. The OAM conservation (i.e., 2l1 = l2 with l1 and l2 being the OAM numbers of the fundamental and second-harmonic waves, respectively) holds well under our experimental configuration. Our results provide a potential way to realize nonlinear optical manipulation of an OAM mode in a nano-photonic device.

We experimentally demonstrate multiple copies of optical orbital angular momentum (OAM) states through quasi-phase-matched (QPM) second-harmonic (SH) generation in a 2D periodically poled LiTaO3 (PPLT) crystal. Since the QPM condition is satisfied by involving different reciprocal vectors in the 2D PPLT crystal, collinear and noncollinear SH beams carrying OAMs of l2 are simultaneously generated by the input fundamental beam with an OAM of l1. The OAM conservation law (i.e., l2 = 2l1) holds well in the experiment, which can tolerate certain phase-mismatch between the interacting waves. Our results provide an efficient way to obtain multiple copies of the wavelength-converted OAM states, which can be used to enhance the capacity in optical communications.