As an alternative of information carrier, orbital angular momentum (OAM) modes enable us to promote the development of high-capacity classical optical communication because there are an infinite number of orthogonal eigenstates in OAM beam in theory. In addition, high-dimensional quantum systems have attracted much interest of many scientists in recent years because of their higher channel capacity and more robust noise resistance. The OAM’s excellentdimension scalability makes it an important role in realization of high-dimensional systems. Recent research advances in photon OAM states transmission are reviewed, with main focus on the distribution of quantum superposition and entangled states in various transmission ways such as free space, fiber and underwater. And intractable problems in its practical use as well as several approaches to mitigating these conundrums are also elaborated, which can provide reference for relevant researchers.

Orbital angular momentum (OAM) is an important degree of freedom of light. As having unique phase and intensity distribution, OAM-carrying light has wide applications in high capacity optical communication, metrology, imaging, optical tweezers and quantum information processing. In one hand, frequency conversion with OAM-carrying light in nonlinear process shows new optical physics different from that of Gaussian beam. On the other hand, nonlinear optics offers various tool kits to manipulate different content of light, which can be used to generate specific light field for target applications. In this article, the main progress for frequency conversion of OAM-carrying light in quasi-phase matching nonlinear crystals is reviewed, including the conservation, propagation, evolution and interference of OAM-carrying light in frequency conversion, high efficiency nonlinear frequency conversion for OAM-carrying light, realization of frequencey conversion efficiency independent of the input signal modes, frequency conversion of vector vortex beam, and generation of high dimensional OAM entangled states without post selection based on pump beam engineering. Finally, some discussions and outlooks for future research trends in this field are put forward.

As a non-diffractive beam with self-accelerating characteristics, Airy beam has application potentials in many fields such as optical tweezers, optical filamentation, imaging and surface plasma excitation. The introduction of vortex on Airy beam can enrich the characteristics of light field. Taking advantage of the characteristics of both Airy beam and vortex phase, greater flexibility for the tuning of light field distribution could be achieved, as well as more interesting dynamic evolution features. Starting with the characteristics and applications of Airy beams, the parameter tuning and evolution characteristics of a single Airy vortex are systematically introduced, including its evolution characteristics, generation ways, derived light field and applications, and also the peculiar properties of the superposition beams of Airy vortex in rectangular and polar coordinates respectively, which provides a reference for the systematic understanding of Airy vortex parameters and dynamic characteristics, lays a foundation for the generation of light field with richer properties, and has guiding significance for expanding the practical application field of Airy vortex.

Information security is a crucial part of a nation’s core competitiveness. Based on the fundamental principle of quantum mechanics, quantum key distribution (QKD) realizes information encryption being independent of computational complexity, thus becomes an important element of the next-generation technology for information security. With the development during the last thirty years, QKD technology, combined with the decoy-state method, has become mature. Especially, the BB84-protocol QKD system has been deployed for practical application. Seeking and pursuing better underlying schemes and fundamental technologies is the best way to promote the development of QKD. By utilizing high-dimensional quantum states for the encoding and decoding procedures, high-dimensional QKD (HD-QKD) can transfer a larger amount of information and possess stronger noise resistance ability. Therefore, it has been a fast-growing branch of QKD and strongly supports further development of QKD. The degree of freedom of orbital angular momentum (OAM) of photons possesses a Hilbert space with infinite dimensionality in principle, and hence is a potential resource for HD-QKD. In this paper, the development history and important progress of HD-QKD technology based on OAM photons are reviewed, and the challenges to be overcome and the trends of HD-QKD technology before practical applications are also discussed and prospected.

Light can carry both spin and orbital angular momenta, which are determined by light’s polarization and spatial degrees of freedom respectively. In paraxial fields, optical spin and orbital angular momenta are mutually independent and conserved under free-space propagation. By contrast, in nonparaxial fields, such as tightly focused or scattered fields, the coupling and transformation between the spin and orbital angular momenta occur. Particularly, because it is widely involved in applications such as optical trapping, microscopy and sensing, spin-orbit interactions (SOIs) occurring in tight focusing systems have gained much attention recently. This review presents the theoretical calculation of spin and orbital angular momenta in the tightly focused fields, reveals the relationship between SOIs and the incident structured light fields, and briefly introduces the latest progress of the related applications.

Orbital angular momentum (OAM) of photons is characterized with both optics vortices and high dimension, and shows great application potential in both classical and quantum fields. The experimental realizations of high-dimensional quantum gates accomplished in author’s group are summarized, such as three-qubit Toffoli gate, Fredkin gate, and the modulations to phase and amplitude of different OAM modes by using hybrid-entanglement between polarizationand OAM. Moreover, a deep-learning method is introduced to accurately recognize OAM modes under the misalignment coordinate. Furthermore, the quantum state transfer experiments of transferring a polarization state to another photon in the OAM state, and the preparation of four-dimensional Bell states are also reported in this review.

Vectorial vortex beams (VVBs) are new type of structured optical fields with helical phase and spatial inhomogeneous polarization. It has already shown broad applications in domains like optical tweezers, rotator detection, optical communication, high-resolution imaging, quantum information and other fields. With the continuous deepening of research, the requirements for more complex optical fields of VVBs are getting higher and higher, which brings huge challenges to the generation technology of VVBs. In addition, how to represent such complex optical fields of VVBs more practically and completely is also an important basis for its application. The author’s team has been engaged in the research on the manipulation and application of structured beams like VVBs, and has proposed a variety of extra- and intra-cavity generation of VVBs with selective output modes. In this paper, recent advances of VVB representation and intra-cavity generation are introduced and reviewed, including the four-parameter representation, the intra-cavity regulation of transverse and longitudinal modes of VVBs, and the eye-safe solid VVB laser as well.

Optical vortex carries optical orbital angular momentum because its Poynting vector is featured by a spiral phase rotating around its propagation axis, and thus its generation and transformation are always accompanied by the change of orbital angular momentum. Orbital angular momentum has attracted intensive attention in the fields of classical optics and quantum information. At present, a series of mature methods of orbital angular momentum generationand modulation have been developed, of which photoaligned liquid crystal fork grating is one important type. Photoalignment is suitable for high-resolution and free fabrication of liquid crystal microstructures, greatly enhancing the capability of generating and modulating of optical vortices and their arrays. This paper summarizes researches on generations and modulations of optical vortices with photoaligned liquid crystal fork gratings, mainly focuses on the recent progress of binary and polarization fork gratings, Damman encoded fork gratings and spiral structured Damman fork gratings in the generations of optical vortices and their arrays, as well as broadband applications.

In order to study the orbital angular momentum of higher-order diffraction beams, based on the method of computer generated holography, the perfect vortex beam with different diffraction orders is generated on the Fourier plane of space light modulator, and its topological charge is measured by spherical wave interferometry. Theoretical and experimental results show that the topological charge l of integer and fractional perfect vortex beams at different diffraction order p satisfies the relation of l=mp , where m is the topological charge of the phase mask. Furthermore, the optical vortex array with different diffraction order is also studied experimentally, the results show that the topological charge of optical vortices satisfies the relation of l=p , and the higher-order diffraction beam has greater orbital angular momentum than the diffraction beam at the +1 order. This study provides theoretical and experimental reference for further research and application of optical vortices and optical vortex arrays.

Because they can provide a unique light-matter interface, various vortex beams carrying net orbital angular momentum have become a hot issue in structured light science. With the developing of the field, “perfect flattop vortex beams" that have both homogeneous intensity and phase distribution are particularly desired in the field of optical tweezers and nonlinear optics. Regarding to the challenge, based on super-Gaussian function, a complex-amplitude-computed holography for generating the “perfect flattop vortex beams" is theoretically proposed, the principle is experimentally proofed in a digital Fourier transform system based on a phase-only spatial light modulator, and the method to obtain the beam with a smooth vortex singularity in a finite-aperture system is mainly focused on.

Hollow beam is an important structured light and also one of the hot research topics in the field of light field manipulation. As a typical hollow beam, Laguerre-Gaussian beam is also named vortex beam because of its helical phase. Moreover, because of carrying orbital angular momentum, vortex beam is tremendously valuable in optical communication, quantum entanglement and super-resolution imaging. In this work, based on the all-solid-state two-mirror concave-plane Nd:YVO4 laser experimental platform,the continuous-wave 16th-order vortex laser is obtained by fabricating some spot defects onto the output coupler to suppress the low-order Gaussian mode and force the operation of high-order Laguerre-Gaussian vortex laser. A maximum output power of 280 mW is achieved with a slope efficiency of 18.6% at an absorbed power of 3.3 W. Furthermore, a passive Q-switched high-order Laguerre-Gaussian laser with positive and negative handedness is demonstrated for the first time by inserting a Cr:YAG crystal into the laser cavity as a saturable absorber, and the shortest pulse width of the pulsed laser is 232 ns at a repetition rate of 229.1 kHz. The research indicates that spot defect mirror is a stable and reliable measures for direct generation of high-order vortex lasers. Moreover, it can be combined with other solid-state laser technology such as passive Q switching to produce spatially structured light operating at different regimes.

To promote the coding capacity of quantum light sources in quantum information processing is an urgent problem to be solved in the development of quantum technology. As a unique degree of freedom for photons, the spatial pattern distribution of orbital angular momentum will form an infinite-dimensional orthogonal solution in Hilbert space. Therefore, encoding with the orbital angular momentum of photons can greatly increase the capacity of informationprocessing and is an important resource for high dimensional quantum information processing. A quantum light source with a two-level system can be equivalent to a point light source, and modulating the quantum light source to generate orbital angular momentum at the micro-nano scale is one of the effective ways to expand the coding dimension of on-chip integrated quantum light source. This article mainly discusses the properties of the superposition state of orbital angular momentum generated by point light source in micro-ring resonator. Firstly, the modulation mechanism on the orbital angular momentum generated by the point light source in the microring resonator with angular gratings is introduced. Then, the influence of the cavity quantum electrodynamics effect of the microring resonator on the radiation rate and collection efficiency of the point light source emitter is investigated, and thesingle photon purity is also discussed. Finally, the electric field distribution, phase distribution and vector polarization characteristics of the superposition state of orbital angular momentum produced by the point light source are investigated. Not only the effect of the point light source at different positions in the microring cavity on the cavity mode and the properties of orbital angular momentum, but also the mode purity of the single-photon orbital angular momentum states is analyzed in detail.This research provides an effective method for the development of integrated optical quantum devices carrying orbital angular momentum, which will promote the further understanding of the mechanism and properties of the orbital angular momentum generatedat the micro-nano scale.