Optical vortices 30 years on: OAM manipulation from topological charge to multiple singularities

Thirty years ago, Coullet et al. proposed that a special optical field exists in laser cavities bearing some analogy with the superfluid vortex. Since then, optical vortices have been widely studied, inspired by the hydrodynamics sharing similar mathematics. Akin to a fluid vortex with a central flow singularity, an optical vortex beam has a phase singularity with a certain topological charge, giving rise to a hollow intensity distribution. Such a beam with helical phase fronts and orbital angular momentum reveals a subtle connection between macroscopic physical optics and microscopic quantum optics. These amazing properties provide a new understanding of a wide range of optical and physical phenomena, including twisting photons, spin–orbital interactions, Bose–Einstein condensates, etc., while the associated technologies for manipulating optical vortices have become increasingly tunable and flexible. Hitherto, owing to these salient properties and optical manipulation technologies, tunable vortex beams have engendered tremendous advanced applications such as optical tweezers, high-order quantum entanglement, and nonlinear optics. This article reviews the recent progress in tunable vortex technologies along with their advanced applications.


Introduction
Vortices are common phenomena that widely exist in nature, from quantum vortices in liquid nitrogen to ocean circulation and typhoon vortices and even to spiral galaxies in the Milky Way, manifesting themselves not only in macroscopic matter but also in structured electromagnetic and optical fields. This year is the 30th anniversary of the birth of optical vortices (OVs). In 1989, Coullet et al. 1 found the vortex solutions of the Maxwell-Bloch equations and created the concept of OVs, inspired by hydrodynamic vortices. Before the proposal of OVs, the analogy between laser physics and fluids/superfluids was already recognized 2 as early as 1970 by reducing the laser equations to complex Ginzburg-Landau equations (CGLEs), which constitute a class of universal models describing pattern formation in a vast variety of phenomena such as superconductivity, superfluidity, and Bose-Einstein condensation 3 . Later, many hydrodynamic features, such as chaos, multistability, and turbulence, were analogically studied in optical fields [4][5][6] and observed in laser systems [7][8][9] . Among the various hydrodynamic effects, the vortex soliton is quite attractive due to its distinctive structure carrying a singularity [5][6][7] . Analogous to the flow singularity in a fluid vortex, an optical vortex soliton has a phase singularity that appears as an isolated dark spot possessing the topological charges (TCs) of a helical phase 5,10 . Novel optical vortex solitons were intensively explored based on CGLEs. For instance, stable vortex solitons 11 and dissipative vortex solitons with trapping potentials 12 can be solved by two-dimensional CGLEs. Topologically multicharged rotating vortex solitons 13 and vortex excitation with feedback 14 in lasers were also studied by CGLEs. Moreover, complicated threedimensional toroidal dissipative vortex solitons 15 can also be characterized by CGLEs with high-order nonlinearity. In 1992, Allen et al. 16 proposed the orbital angular momentum (OAM) in vortex beams (VBs) where the OVs propagate in paraxial beams, which unveiled a new understanding of the connection between macroscopic optics and quantum effects. As a typical representative of OVs, a VB has become a classical tool to study the properties of OVs because its generation can be easily realized in the laboratory 17 . VBs characterized by Hilbert factor exp(iℓθ), e.g., the Laguerre-Gaussian (LG) modes, can carry OAM equivalent to ℓℏ per photon (ℓ is an integer number), and this angular momentum (AM) can be much greater than the spin angular momentum (SAM) related to the photon spin 10 . The general results of these investigations created a new chapter of modern optics, i.e., singular optics 18 , which is a great leap forward in the development of traditional optics.
In the first 10 years, 1989-1999, the studies on OVs mainly focused on establishing fundamental theories and exploring basic physical phenomena, paving the way for further studies of the light-matter interaction, topological structures, and quantum nature of light. For instance, the dynamics of transverse pattern formation 5,6 , the interaction and OAM transfer between OVs and particles [19][20][21] , vortex solitons in a nonlinear medium [22][23][24] , nonlinear OAM-frequency transformation 25,26 , the topological phase in OVs 27 , the rotational Doppler effect 28 , and multisingularity arrays or vortex crystals 5,29 were thoroughly studied. These novel theories lay the foundation for extending further widespread applications by using the unique properties of OVs.
In the second 10 years, 1999-2009, with the development of OAM manipulation, tremendous new applications rapidly emerged. In 2001, Zeilinger's group 30 realized the OAM-entangled photon pair, bringing OVs or twisted photons into quantum applications 31 . In 2002, Dholakia's group trapped particles with controlled rotation 32 and a three-dimensional structure 33 by VBs, expanding the applications of optical tweezers 34 . In 2003, Harwit 35 demonstrated astrophysical OAM light generation and its applications in astrometry. In 2004, Zhuang 36 used VBs as tweezers to assemble DNA biomolecules, opening up biomedical applications of OVs. In 2005, Ritsch-Marte's group 37 used OAM in microscopy and imaging, and Tamburini et al. 38 reported a superdiffraction-limit imaging approach using OAM. In 2008, Barreiro et al. 39 presented a coding technology using OAM, giving VBs great advantage for use in optical communications. During the decade, OVs were extended to almost every field of advanced optics.
In the last 10 years, 2009-2019, vortex and OAM applications have made many breakthroughs in rapid succession. In 2010, the optical lattice in far-field diffraction of OVs was unveiled as a very prompt and handy way to detect the TC 40 . In 2011, Capasso's group 41 proposed the generalized laws of reflection and refraction, guiding OV generation in nanoscale metasurfaces. In 2012, OAM beams were directly generated in a nanoscale emitter 42 . In 2013, Willner's group demonstrated terabitscale high-capacity optical communication via OAM multiplexing in both free space 43 and fibres 44 . In 2016, Zeilinger's group 45 generated extreme OAM states of over 10,000ℏ and realized quantum entanglement of these states. In the recent three years, increasing numbers of tunable properties of OVs have been flexibly controlled at the nanoscale, including SAM-OAM conversion for classical 46 and quantum light 47 , tunable wavelength from visible 48,49 to X-ray light 50 , ultra-broadband tunable OAM 51 , and tunable chirality 52 . Moreover, the timevarying OAM was recently revealed in extreme-ultraviolet VB with time-delay-tunable high harmonic generation 53 . To date, OVs have brought about numerous innovations in various fields and are still enabling great novelties with improved tunability.
Throughout the roadmap summarized above and depicted in Fig. 1, we can divide the 30-year development into three stages: the first 10 years, the fundamental theories stage; the second, the application development stage; and the third, the technology breakthrough stage. These three stages share one common theme of pursuing improved tunability of OVs because the realization of a broader tunable range of OVs can always benefit the birth of new applications. Thus, we propose the tunability of OVs as a better way to describe state-of-the-art achievements of OVs. Traditionally, tunable light always means that the wavelength can be tuned and sometimes means that the pulse width can be tuned for a pulsed laser; however, the tunability of OVs should be expanded to more dimensions due to their exotic properties. The tunability of OVs includes not only the spectral and temporal tunability but also the OAM-, chirality-, TC-, and singularity-distribution tunability. We present reviews in succession on the historical progress in the new tunable methods of OVs driven by the fundamental theories and then the numerous novel applications engendered by the improved tunability of OVs. In the "Properties of OVs" section, we review the fundamental theories and properties of OVs, providing a better understanding of the corresponding applications enabled by the unique properties. In "Progress in vortex generation, tuning, and manipulation" section, we review the generation and manipulation methods of OVs, developed from tuning the TC of a single singularity to controlling a multi-singularity array, including wavelength-, temporal-, and OAM-tuning technologies.
In "Advanced applications of tunable VBs" section, we comprehensively review various advanced applications derived from vortex manipulation. Concluding remarks and prospects are given in the "Conclusions and perspectives" section.

Singularity and topological charge
The salient properties of OVs are mostly related to the topological phase structure. Early in the 1970s before OVs were first observed, the topological structure in the wave phase was already under study. Nye and Berry 54 demonstrated that wave trains with dislocations could induce a vortex structure where a singularity could be solved in the wave equation, which laid the foundation for the study of vortices in air, water, and even light waves, pushing the discovery of OVs. To understand the profound topology in a plain way, we can refer to a familiar artwork exhibiting a similar structure. Escher's painting Ascending and Descending shows an impossible scenario where the stairs are ascending clockwise yet have a seamless connection to their origin after a roundtrip, which is an artistic implementation of the Penrose Stairs 55 , as illustrated in Fig. 2. This structure is impossible in real space but possible in phase space. If the phase angle continually increases clockwise along a closed loop from 0 to 2πℓ and returns to the origin, where the integer ℓ is called the TC, the angle zero is exactly equal to 2πℓ, forming a continuous phase distribution along the closed loop, similar to the topology of the well-known Möbius strip 56 . The centre spot of the closed loop where the phase cannot be defined is a phase singularity. The definition of the TC of a singularity for the phase distribution φ is given by: where C is a tiny closed loop surrounding the singularity. For the light field with phase distribution exp(iℓθ) carrying OAM of ℓℏ per photon, the TC of the centre phase singularity is ℓ. The effect of TC is actually commonly seen in our daily life, e.g., the time distribution on earth has a singularity at the North Pole with a TC of 24 h, the duration that the earth takes to rotate one cycle. The continuous phase along the closed loop results in an integer TC. However, as a peculiar case, a non-integer TC was also experimentally and theoretically investigated in OVs 57,58 . A phase singularity with a certain TC is a representation of a very simple vortex soliton yet acts as an important unit element in that more complex hydrodynamic vortices with chaos, attractors, and  59 , and neutron 60 vortex fields.

Orbital angular momentum and vortex beams
A VB is a paraxial light beam possessing Hilbert factor exp(iℓθ) and carrying OVs along the propagation axis. OVs are not restricted to VBs, yet as typical OVs, VBs carrying OAM, also called OAM beams, are almost the most attractive form of OVs due to their unique quantum-classical-connection properties. There are already many review articles on OAM, especially on vortex generation 61,62 , OAM on metasurfaces 63 , and basic OAM theories and applications 64,65 . However, few studies have focused on vortex tunability, which is the main theme of this article. For the introduction of basic theories of OAM, previous reviews usually used the well-known Poynting picture to describe the AM of the photon 66,67 , which leads to some difficulties, such as complex expressions of OAM and SAM, incompatibility with quantum optics, and the Abraham-Minkowski dilemma 68 . Here, we review the recently proposed new theory of the canonical picture 69,70 , which can overcome these difficulties, to introduce basic properties of OAM. The canonical momentum of light is represented as where H is the magnetizing field. Gaussian units with g ¼ ð8πωÞ À1 ,ε ¼ ε þ ωdε=dω, andμ ¼ μ þ ωdμ=dω are used. The canonical SAM and OAM densities are expressed as The total AM of light is J = S + L. For a light beam, a rotating polarization leads to SAM, while a rotating wavefront leads to OAM. Consider a VB propagating along the z-axis: The average SAM and OAM can be derived as 69,70 where the power density W ¼ gω 2ε E j j 2 þμ H j j 2 À Á and σ ¼ 2ImðmÞ 1þ m j j 2 . σ = +1 (−1) and 0 correspond to left (right) circularly polarized light and linearly polarized light, respectively. Thus, Eq. (4) reveals that left (right) circularly polarized light carries an SAM of + ℏ (−ℏ) per photon; the light with Hilbert factor exp(iℓθ) carries an OAM of ℓℏ (ℓ = 0, ±1, ±2, …) per photon, where " ± " reveals the chirality of the vortex, as demonstrated in Fig. 3. This is consistent with the AM quantization in quantum optics, i.e., the eigenvalues of SAM and OAM for the photon eigenstate yieldL z ψ j i ¼ ' h ψ j i andŜ z ψ j i ¼ σ h ψ j i. Therefore, the phase factor exp(iℓφ) provides a basic frame of VBs.

Polarization and vector vortices
The previous part focuses on the scalar light field, where the polarization is separable from the space. In scalar vortices, there are topological spatial phase structures, but the polarization is unchanged; e.g., Fig. 4a shows that a circularly polarized OV can be expressed as the product of a spatially varying vortex phase state and a circular polarization state 71 . If the polarization state has a spatially varying vector distribution forming vortex-like patterns, then the corresponding optical field is called polarization vortices or vector vortices, and the corresponding singularity is called a polarization singularity or a vector singularity 72,73 . Based on the various topological disclinations of polarization, vector vortices can be categorized into many types, such as C-point, V-point, lemons, star, spider, and web, according to the actual vector morphology 74 . In contrast to the phase vortices carrying OAM, the vector vortices are always related to a complex SAM-OAM coupling; e.g., Fig. 4b shows a spider-like vector OV formed by the superposition of opposite phase   Fig. 4 Formation of vector beam with space-polarization nonseparability. a Circularly polarized OV with an azimuthally varying phase distribution. Such a state is considered separable, as it can be represented as the product of a spatially varying vortex phase and a polarization state vector. b Spider-like vector vortex represented as the superposition of the state of a with another state with the opposite phase variation and the opposite circular polarization. From ref. 71 . Reprinted with permission from AAAS variations and opposite circular polarizations, where the total OAM is zero due to the sum of the two opposite phase variations but there is a complex SAM entangled with the space 71 .

Classical models of OVs
LG and Hermite-Laguerre-Gaussian modes LG modes with circular symmetry are the earliest reported VBs carrying OAM 16 and can be included in the general family of Hermite-Laguerre-Gaussian (HLG) modes with elliptical vortices [75][76][77] , thus accommodating the transform from the HG to LG mode, which has recently played increasingly important roles because the exploration of the more general structure of OVs always leads to novel applications: where HL n,m (·) is a Hermite-Laguerre (HL) polynomial 75 , and z R is the Rayleigh range. For α = 0 or π∕ 2, the HLG n,m mode is reduced to the HG n,m or HG m,n mode. For α = π∕ 4 or 3π∕ 4, the HLG n,m mode is reduced to LG p,±ℓ mode For the other interposed states, the HLG mode has multiple singularities with a total TC of ℓ. As illustrated in Fig. 5, the LG p,ℓ mode can be decomposed into a set of Hermite-Gaussian (HG) modes 16,17 : LG which also interprets the transformation to an LG p,ℓ mode from an HG n,m mode through an astigmatic mode converter (AMC) 17 .

Helical-Ince-Gaussian and singularities hybrid evolution nature mode
The Ince-Gaussian (IG) mode 78 is the eigenmode of the paraxial wave equation (PWE) separable in elliptical coordinates (ξ, η) 79 : where C e,o are normalization constants (the superscripts e and o refer to even and odd modes), I e;o u;v Á; ϵ ð Þ are the even and odd Ince polynomials, with 0 < v < u for even functions, 0 < u < v for odd functions, and À1 ð Þ uÀv ¼ 1 for both, and ϵ 2 0; 1Þ ð is the eccentricity. The special superposition of these modes can form a multi-singularity LG 0,2 HG 2,0 HG 1,1 HG 0,2 which carries multiple singularities with unit TC, having a total TC of v. Sharing the singularities hybrid evolution nature (SHEN) of the HIG and HLG modes, the SHEN mode is a very general family of structured Gaussian modes including the HG, LG, HLG, and HIG modes, the expression of which is 83 The SHEN mode is reduced to the HIG mode when β = ±π/2, to the HLG mode when γ = 0, to the HG mode when (β, γ) = (0,0) or (π, 0), and to the LG mode when (β, γ) = (±π/2, 0). In addition, there is a graphical representation, the so-called SHEN sphere, to visualize the topological evolution of multi-singularity beams. Thus, the SHEN mode has great potential to characterize more general structure beams.

Bessel and Mathieu modes
Using the non-diffraction assumption in solving the PWE, we can also solve a set of eigenmodes. Under separable conditions in circular coordinates, the Bessel mode can be obtained as 84 Bessel beams with ℓ ≠ 0 are VBs carrying ℓℏ OAM. Another non-diffraction solution separable in elliptical coordinates is the Mathieu modes 85 , where C m and S m are normalization constants, Je m and Jo m are radial Mathieu functions, and ce m and se m are angular Mathieu functions. Analogous to deriving the HIG mode, a helical Mathieu (HM) beam 86 High-order Bessel and HM beams are often called nondiffractive VBs, whose unique properties have been extended to a great number of applications, such as particle assembly and optical communication 88,89 .

SU(2) geometric modes
When a resonator cavity fulfils the reentrant condition of a coupled quantum harmonic oscillator in SU(2) Lie algebra 90 , the laser mode undergoes frequency degeneracy with a photon performing as an SU(2) quantum coherent state coupled with a classical periodic trajectory 91 , which is called an SU(2) geometric mode (GM) 92 . The frequency degeneracy means that Δf T =Δf L ¼ P=Q ¼ Ω should be a simple rational number, where P and Q are two coprime integers, and Δf T (Δf L ) is the longitudinal (transverse) mode spacing. The wave-packet function of a planar GM is given by 92 where phase ϕ 0 is related to the classical periodic trajectory. ψ HG ð Þ n;m;s represents the HG n,m mode considering the frequency-dependent wavenumber k n;m;s ¼ 2πf n;m;s =c, If the HG bases are transformed into LG bases, then the circular GM is obtained 92 : where ψ LG ð Þ p;';s represents the LG p,ℓ mode considering the frequency-dependent wavenumber. The vortex circular GM has many unique properties, such as an exotic 3D structure, multiple singularities, and fractional OAM 92,93 . Note that there are other types of SU(2) modes related to OAM with special properties, such as Lissajous modes 94 , trochoidal modes 95 , polygonal VBs 96 , and SU(2) diffraction lattices 97 as shown in Fig. 6d, e.
The above forms are classical VBs in free space, which are just optical modes carrying OAM. In addition, there are OVs that are formed by non-OAM beams, as reviewed in the following.

Optical Möbius strips
A direct idea is to arrange the optical parameter into the form of Möbius strips, one of the classical topological models. This type of OV is called an optical Möbius strip (OMS). A simple vortex phase with integer TC can be seen as a phase OMS. In addition to phase vortices, more OMSs can be obtained by arranging the polarization: the major and minor axes of the polarization ellipses that surround singular lines of circular polarization in threedimensional optical ellipse fields can be organized into an OMS, as theoretically proposed 98,99 and experimentally observed 49 . Currently, multitwist OMSs can be controlled in both paraxial and nonparaxial vector beams 56,100 . By combining other spatial and optical parameters into OMSs, more complex structures, such as 3D solitons and topological knots, can be proposed for OVs 101 .

Vortex knots
The vortex core of an OV can not only be distributed along the propagation axis of a beam but also form   There are many other forms of OVs that cannot be fully covered in this paper. For instance, there are many freespace VB modes that carry OVs and OAM, such as elegant HLG beams 106 , Airy beams 107 , Pearcey beams 108 , and parabolic beams 109 . There are many morphologies of the non-beam spatial distribution of OVs with singularities fractality 110 . It is highly expected that many new formations of OVs will be reported and investigated in future explorations.

Properties of VBs Reflection and refraction
The reflection of a VB generally does not satisfy the classical reflection law, i.e., the angle of incidence θ i does not equal the angle of reflection θ r . Instead, the reflected light has a spatial deflection effect related to the OAM of the VB 111 . The difference between θ i and θ r is related to the OAM of the beam, obeying the generalized law of reflection 41 where λ and ϕ are the wavelength and phase of the light beam, respectively, and n is the refractive index of the medium. In addition, the refraction of VBs does not satisfy Snell's law, i.e., n t sinθ t ≠ n i sinθ i . The refraction is related not only to the angles of incidence and refraction (θ i and θ t ) and the refractive indices but also to the OAM, obeying the generalized law of refraction 41 Interference For conventional laser beams, the equal-inclination interference pattern is equispaced fringes, and the equalthickness interference pattern is Newton's rings. However, for a VB, the pattern of equal-inclination interference with a plane wave is not equispaced fringes but fringes with bifurcation at the singularity of the vortex, and the morphology of the bifurcation is related to the OAM of the beam 66 . The equal-thickness interference pattern of a VB with a plane wave is not Newton's rings but spiral stripes extending outward from the vortex singularity, the number of which is related to the OAM 112 . The selfinterference pattern can also show some bifurcation fringes 112 . These special interference fringes can be used in detection and measurement methods of vortices.

Diffraction
VBs have unique diffraction properties, the aperture diffraction patterns of which are coupled with the actual OAM. Since Hickmann et al. 40 unveiled in 2010 the exotic lattice pattern in triangular-aperture far-field diffraction of VBs, it has been used as an effective method for OAM detection and measurement of femtosecond vortices 113 , non-integer charge vortices 114 , and elliptical VBs 115 . Many other unique far-field diffraction patterns were investigated through a slit 116 , a square aperture 117 , a diamond-shaped aperture 118 , a circular aperture 119 , an off-axis circular aperture 120 , an isosceles right triangular aperture 121 , a sectorial screen 122 , and so on. The Fresnel diffraction of VBs was also studied 123 . Some special VBs, such as vector VBs 124 and SU(2) VBs 97 , can even bring about special lattice structures in diffraction patterns. These special diffraction patterns can be used in vortex detection and measurement methods.

Polarization
The polarization states of conventional beams can be represented on the Poincaré sphere. VBs can have complex transverse structures involving polarization vortices. Upon combining structured polarization with VBs, the vector VBs can demonstrate more amazing properties and more extensive applications 74 . To characterize a classical family of vector VBs, Holleczek et al. proposed a classicalquantum-connection model to represent cylindrically polarized beams on the Poincaré sphere 125 ; this model was then extended to the high-order Poincaré sphere (HPS) 126 , which can reveal SAM-OAM conversion and more exotic vector beams, including radial and azimuthal polarization beams. In an experiment, controlled generation of HPS beams was realized 127 as illustrated in Fig.  8f. As an improved formation of the HPS, the hybridorder Poincaré sphere was theoretically proposed 128 , and the corresponding experimental controlled generation methods were also presented 129,130 .

Quantum properties
Twisted photons 31 are associated with the quantum behaviour of macroscopic VBs. Akin to the conventional Heisenberg uncertainty, there is also the formation of uncertainty for twisted photons; i.e., the product of the uncertainties in the angle and the OAM is bounded by Planck's constant, ΔϕΔL ≥ ℏ/2 131,132 . The general Fourier relationship between the angle and the OAM of twisted photons was also investigated 133 . In contrast to the polarization-entangled state with two dimensions, the OAM-entangled state can be high dimensional as Combining the polarization and OAM of the photon, more complex SAM-OAM entangled photon pairs were realized 47,135 . There are many other new quantum properties related to OAM beams, such as the spin-orbit interaction [136][137][138] , the Hanbury-Brown-Twiss effect 139 , quantum interference 140,141 , and the spin Hall effect 142,143 .

Measurements of OVs
As mentioned above, OVs can be measured by adopting the interference and diffraction properties of VBs. Counting the stripes and lattices in the special interferogram and diffraction patterns serves as a toolkit to measure the TC, OAM, and singularity distributions of corresponding OVs. In addition, for measuring phase vortices, one can use a spatial light modulator (SLM) to carry out phase transformations, reconstructing the target phase to detect the TC and OAM. Typical realizations include the forked diffraction grating detector 144 , the OAM sorter 145 , and spiral transformation 146 . For polarization vortices, the measurement should also consider the detection of the vector field. By introducing a spacevariant structure into a half-wave plate to modulate the polarization, the TC of the polarization singularity in vector VBs can be measured 147 . For measuring more properties of vector OVs, Forbes' group introduced quantum measurement methods to classical light and realized more precise measurement of properties such as the non-separability, SAM-OAM coupling, and vector factors of vector beams 148,149 , which is widely applicable to more structured OVs.

Progress in vortex generation, tuning, and manipulation
Brief review of vortex generation The vortex generation methods can be divided into passive vortex generators (converting the fundamental Gaussian beams into VBs by using dynamic or geometric phase plates, metasurfaces, SLMs, etc.) and active vortex laser generators (such as free space or fibre vortex lasers and nanointegrated OAM generators) 61,112,150 . There have already been some recent reviews on vortex and OAM beam generation [61][62][63]112,150 . However, a review focused on vortex generation with tunable and multi-singularity properties is rare. Hereinafter, we specifically review active vortex generation with tunable properties, including wavelength-, temporal-, and OAM-tunable beams. In particular, the OAM-tunable beams include TC-tunable and multi-singularity-tunable beams.

Wavelength-and OAM-tunable VBs
OAM-tuning of VBs can be realized by gain-loss control 151 , off-axis pumping 92,152 , or the use of a spiral phase plate (SPP) 153 , a Q-plate 154,155 , or an SLM 144 . A wavelength-tunable VB can be achieved by designing special liquid crystal devices 156 , microcavities 157 , or onchip gratings 158 or using nonlinear frequency conversion 159,160 . However, more methods to simultaneously realize wavelength and OAM tuning for novel applications, such as high-capacity optical communication using wavelength-and mode-division multiplexing, are still required.
In 2016, Zhang's group 161 presented a wavelength-and OAM-tunable system by employing a tunable fibre laser with an acousto-optic fibre grating with a wavelengthtunable range of 1540-1560 nm and an OAM of ± 1ℏ, as shown in Fig. 9a  realizing vortex generation with a wavelength-tunable width of 37.5 nm and an OAM of 0~3ℏ. In the same year, Liu et al. 163 reported a ring-pumped Er:YAG solid-state laser generating an 8.4-nm wavelength-tunable width and 0~±2ℏ OAM-tunable VB. In 2018, Yao et al. 164 invented a new optical fibre combiner for combining two polarization-controllable fundamental modes into a VB with chiral control, obtaining a 30-nm wavelength-tunable width and 0~±1ℏ OAM. Our group 165 proposed solid-state vortex generation utilizing a dual-off-axis pumped ultra-wide-band Yb:CALGO laser, reaching a wavelength-tunable width of over 10 nm and an OAM range of 0~±15ℏ, as depicted in Fig. 10a. This system was adapted to generate tunable dual-wavelength VBs 166 .
Recently, Wang et al. 167 improved the output efficiency and reduced the threshold of a similar system by using a Z-cavity and a birefringent plate in the cavity design, and a 14.5-nm wavelength-tunable width and a 0~±14ℏ OAM range were achieved. Wang's group 168  In addition to the abovementioned wavelength-and OAM-tunable OVs from lasers, there are vortex generators that are known to be intrinsically broadband, which can also be used to obtain wavelength-and OAMtunable OVs. For instance, vortex generation from anisotropic solid crystals, both uniaxial and biaxial, can be related to complex SAM-OAM coupling and gain competition effects, leading to the tunability of vector OVs 72,73,169 . Similar tunable OVs can be generated in chiral liquid crystals in the regime of circular Bragg reflection 170,171 . Taking advantage of space-variant anisotropic liquid crystals that can be electrically controlled, wavelength-and OAM-tunable OVs can be generated within a wide spectral tunable range 156,172 . Overall, a wider tunable range controlled by a more convenient OV generation method is still required in the current explorations.

Pulsed VBs
High-peak-power pulsed VBs with different levels of duration have great potential for use in advanced applications, such as optical machining 173,174 , nonlinear optics 25,26,48,49 , strong-field physics 175 , and optical tweezers 176 . Hereinafter, we review the recent compact and effective pulsed VB sources.

Nanosecond-level VBs
The generation of nanosecond-level VBs has always been combined with Q-switched lasers. An earlier method involved selecting the LG modes using an intracavity aperture in a Q-switched solid-state laser 177 , with which obtaining high mode purity is difficult. In 2013, Kim et al. 178 used an etalon and a Brewster plate in an acoustic-optic Q-switched laser and generated highquality chirality-controlled LG beams with an~250 μJ pulse energy and an~33 ns duration. In 2016, Zhao et al. 179 controlled the pump position in an Er:YAG acoustic-optic Q-switched laser, generating a nearly 1-mJ and 50-ns pulsed VB. In 2017, Chen's group 180 designed a nanosecond vortex laser system employing a compact Nd: YVO 4 /Cr 4+ :YAG passively Q-switched laser with an external AMC. In 2018, He et al. 181 presented a Cr,Nd: YAG self-Q-switched microchip laser to directly generate low-threshold nanosecond high-repetition-rate vortex pulses without an AMC, where the chirality was   165,166 . Reproduced by permission of IOP Publishing. b Reprinted with permission from ref. 167 , OSA Publishing controlled by a tilted output mirror. Our group 182 recently reported a pulsed vortex output directly from a room-temperature diode-pumped Er,Yb:glass microchip laser with an extremely compact structure.

Picosecond-level VBs
Picosecond-level VBs have always been realized in a mode-locking laser using a narrow-band gain medium. In 2011, Koyama et al. 183 realized a VB in a stressed Ybdoped fibre amplifier seeded by a picosecond mode-locked Nd:YVO 4 laser with a pulse width of 8.2 ps and a peak power of 34.2 kW. However, the master oscillator poweramplifier structure limited the compactness. The discovery of a self-mode-locking effect in neodymium-doped crystals provided an alternative way to generate picosecond pulses with a quite compact structure 184 . In 2009, Liang et al. 185 reported an OV with a pulse width of 20-25 ps and a repetition rate of 3.5 GHz using off-axis-pumped selfmode-locked Nd:GdVO 4 lasers with an AMC. In 2013, the same group 186 improved this system via cavity control and realized the self-mode-locked SU(2) vortex GM with pulse widths of 22.2 ps and 21.1 ps for Ω = 1/4 and Ω = 1/3, respectively 92 . In 2017, Tung et al. 187 reported the direct generation of picosecond large-OAM SU(2) vortex pulses in self-mode-locking Nd:YVO 4 lasers without the help of an AMC, which largely enhanced the compactness. In 2018, Huang et al. 188 reported an 8.5 ps pulsed VB generated from a mode-locked fibre laser, where the polarization could be controlled at arbitrary states on the HPS.

Femtosecond-level VBs
In contrast to picosecond pulses, femtosecond pulse generation always requires more extreme operating conditions, such as a tightly focused pumping spot, a wide emission band, and a high nonlinear coefficient of the gain medium. Utilizing the external modulation method, flexible temporal shaping of femtosecond VBs was recently realized 189 . Considering the improvements in compactness and cost, the self-mode-locking laser oscillator scheme is still desirable. In 2013, Chen's group 190 reported a self-mode-locked monolithic Yb:KGW laser with a duration of 850 fs and a repetition rate of 22.4 GHz. In 2016, they 191 improved the system to directly generate a sub-picosecond VB carrying OAM by selective pumping. In 2018, Zhang et al. 192 proposed an all-fibre mode-locked femtosecond LG 0,±1 vortex laser with a pulse width of 398 fs. In the same year, Wang et al. 193 realized direct emission of an ultrafast LG 0,±1 VB via a z-type cavity design in an SESAM mode-locking Yb:QX laser with a pulse width of 360 fs, as shown in Fig. 11c. These structures have recently been improved by using a Yb:KYW oscillator with a defect-spot mirror, obtaining a 298-fs VB 194 . Direct generation of sub-100-fs VBs may be a future target.

Complex OAM manipulation
In addition to TC-tunable VBs, beams with multiple singularities can induce exotic tunable OAM. The multisingularity optical field with a vortex array is also known as a vortex lattice or a vortex crystal 5,29,73 . Strong requirements of multi-singularity beams have been put forward because of the boom of special applications such as multiple particle manipulation 82,195 , 3D displays 196 , and optical modulation and communication 197 .
A singularity splitting phenomenon was found in an AMC when the phase matching condition in the AMC was not satisfied 198,199 . A large number of matrix optics theories were put forward 200,201 , deriving the HLG mode to describe the controllable generation of a vortex array in the AMC system 76,77 . Similar to the HLG mode, the HIG mode is also a multi-singularity VB, which can be generated in special cavities with selective pumping 80 and an SLM 81 . Recently, our group proposed a method of tuning the periodic orbits of an SU(2) GM in a degenerate cavity and further tuning the multi-singularity OAM of SU(2) VBs [202][203][204] , as shown in Fig. 12b. In addition to HIG, HLG and SU(2) VBs, many other multi-singularity VBs with special mathematical formulations were generated with different control methods, such as trochoidal VBs 95 , transverse-mode-locking vortex lattices 202,203 , and polygonal VBs 96 .
In addition to the above multi-singularity modes, people are pursuing more freely tailored methods for arbitrary singularity distributions. SLM modulation combined with a laser source for on-demand modes has been favoured 205 . Recently, increasing numbers of tailored singularity distributions have been designed and realized via SLMs, such as rectangular and circular multi-singularity arrays 206,207 and arbitrary curvilinear arrays 208 , and quadrantseparable singularity control 209 , as presented in Fig. 13.
There are still many novel methods of tuning the multi-singularity OAM in more types of exotic OVs. For instance, various OV arrays can be generated by coherent combining technology with digital control 210 . Infinite scalar and vector OV arrays can be realized in fractional OAM VBs 211,212 . On-demand multi-singularity VBs can be generated based on the appropriate combination of optical scattering and discrete rotational symmetries of optical isotropic masks 213 and can be electrically and optically controlled via anisotropic masks 214,215 .
Despite the numerous multi-singularity manipulation methods, the realization of universal and versatile tunability will be the everlasting target in the future.

Optical tweezers
Optical tweezers that trap particles using an optical force were proposed by Ashkin 216 , who won the Nobel Prize in 2018. Benefitting from the study of OAM interactions with matter, OVs were first used in 1995 in optical tweezers and extended to the optical spanner 19 , where particles can be trapped and driven to move around the singularity. Then, the transformation from optical OAM to mechanical AM was widely studied [32][33][34] .
With the improvement of vortex tunability, newgeneration tweezers with OVs have shown distinct advantages 34,217 . As demonstrated in Fig. 14, the novel vortex tweezers can conveniently manipulate not only the spatial positions of particles but also the multiple degrees of freedom of particles, largely extending the automated guiding, assembly, and sorting technology 217,218 . With the control of multi-singularity VBs, many new techniques were designed and applied to trap multiple particles 82,217,218 , including the fractional optical VB for optical tweezers 219 . With femtosecond VBs, the tweezers carrying special nonlinear properties can be used to manipulate optical Rayleigh particles 220 . Furthermore, with femtosecond vector VBs, nonlinearity-induced multiplexed optical trapping and manipulation was designed 221 , where the number of traps and their orientations could be flexibly controlled. In addition to dielectric particles, metal particles can also be manipulated by novel plasmonic vortex tweezers 222 , where the vortex field of surface plasmon polaritons can be generated by focusing vector VBs onto a metal film. Plasmonic vortex tweezers as depicted in Fig. 15d, e were shown to be superior in manipulating metal particles with large flexibility 223 .

Optical communication
In addition to the polarization, amplitude, pulse shape, and wavelength of light, the OAM can be used as an alternative degree of freedom for multiplexing modulation, enlarging the capacity of optical communication 39 , which is also referred to as mode/spatial-division multiplexing (MDM/SDM) 224 . Optical communication by OAM multiplexing has enabled breaking the Tbit level 43,44 , much beyond the conventional scheme, thus greatly broadening the application scope 225,226 . With the study of VB propagation in the atmosphere, free-space communication using vortices was gradually improved [227][228][229] . Furthermore, a sidelobe-modulated OV method was proposed for free-space communication with a significant increase in the data transmission capacity 230

Quantum entanglement
With the recent mature quantum descriptions of twisted photons 31 , OAM entanglement has engendered plenty of applications 134 . For instance, high-dimensional quantum key distribution (QKD) protocols can be designed based on mutually unbiased bases related to OAM photons 238 , which motivated high-dimensional quantum cryptography for high-security communication 239 . The quantum memory technology for OAM photonic qubits was recently proposed to provide an essential capability for future networks 240 . Because of the inherent infinite dimension of OAM, the OAM of photons has been successfully used to realize quantum storage in various systems, such as atomic ensembles 241 and rare-earth-iondoped crystals 242 , benefiting high-capacity communication. High-dimensional OAM entanglement was also successfully used in high-efficiency digital spiral imaging 243 . Employing the Hong-Ou-Mandel interference of OAM photons, quantum cloning technology for making copies of unknown quantum states was presented 244 . With the development of vector VB manipulation, SAM and OAM were combined for quantum communication to further scale the capacity and speed 245 . Quantum teleportation using OAM can largely improve the technical control of scalable and complicated quantum systems 246 . To date, the entangled photon system with the highest number of qubits (18 qubits with six entangled photons) with OAM as one degree of freedom has been produced 247 . Very recently, as a remarkable breakthrough, quantum entanglement between the SAM and OAM states was realized in a metamaterial 47 .
In addition to scalar phase OVs, vector polarization OVs also have fruitful quantum properties. The non-separable states between the polarization and space share common properties with the entangled state of photons, which is also called the classical entanglement state 71,248 . The quantum tomography, Bell parameter, concurrence count, and linear entropy can be realized in vector OVs akin to corresponding quantum measurements 148,149,248 . Taking advantage of the high-dimensional properties of the nonseparable states, quantum walks can be implemented by vector OV modes of light, enlarging the scalable range 249 . Entanglement beating generated in vector VBs can be used to control spin-orbit coupling in free space 135  dimensional entanglement has also been utilized in coding quantum channels to improve high-capacity optical communication 250 , as illustrated in Fig. 17d, e.

Nonlinear optics
With the development of high-power and large-energy VBs 92,180,251 , the peak power can exceed the threshold of various nonlinear effects, providing conditions to explore novel nonlinear conversion phenomena related to OAM 48,49,251 . Conventionally, the development of nonlinear optics was based mainly on the scattering that obeys momentum conservation (Rayleigh scattering, Brillouin scattering, Raman scattering, etc.), and the corresponding development of nonlinear frequency transformation effects (frequency doubling, frequency summing, four-wave mixing, etc.) has benefited a myriad of applications. In the new century, new transverse nonlinear transformation effects have been developed based on AM conservation, such as TC variation during the processes of frequency doubling 25,26 , summing and mixing 252,253 , tunable OAM highharmonic transform 48,49 , and OAM strong-field physics 175 . Recently, these OAM harmonic generations have been widely applied in nanomaterials for the control of nonlinear phases 254 , the Pancharatnam-Berry phase 255 and beam shaping 256 . In addition, there are many novel physical phenomena coupled with nonlinear OAM-frequency conversion, such as the rotational Doppler effect 257 and rotational nonlinear absorption 258 .

Nanotechnology
Due to the rapid development of nanofabrication and increasing demands for nanotechnology applications, nanointegrated on-chip vortex generators have emerged for emitting VBs at the nanoscale, such as integrated silicon-chip-based VB emitters 259 , vortex vertical-cavity surface-emitting lasers (VCSELs) 260 , angular gratings 42 , micro-nano-OAM laser emitters 261 , and various metasurface designs 262 . Taking advantage of nanoscale VBs, many novel phenomena related to OAM in nanophotonic materials have been demonstrated, such as non-dispersive vortices 263 and SAM-to-OAM conversion effects 46,47 . Combined with new nanomaterials, many vortex-emitting materials and devices with unique functions have been invented, such as vector vortex on-chip generators 264 and parallel OAM processors 265  purifying the OAM quantum states was recently produced, which possesses great potential to develop on-chip quantum calculation 266 .

Optical machining
Due to the nature of high-order modes, VBs show weaker capability in conventional machining processes, such as laser cutting and laser punching, than the fundamental Gaussian beam. However, in some special applications, vortex light has distinct advantages. When a metal surface is processed by different vector VBs, various intriguing new patterns can be selectively displayed under light illumination 267,268 . Moreover, the surface can exhibit different patterns when the illuminated light has different incidence angles 269 . In addition to the angular sensitivity, a polarization-sensitive surface was fabricated based on a similar technique using vortex processing, i.e., different patterns were exhibited when the surface was illuminated by light with different polarizations 268 . Utilizing nanophotonics technology, nanoscale VBs were used in nanostructure fabrication. For instance, the chiral nanoneedle structure can be easily fabricated by a perpendicular VB through the transfer of the consequential torque from OAM light to the object 173,269,270 . Similar methods can produce some other nanostructures, such as helical surface reliefs 271 and monocrystalline silicon needles 272 . Recently, high-power ultrashort OAM-tunable VBs were combined with femtosecond laser direct writing technology to process more special structures, such as multiwaveguide 266 and micro-pipe structures 174 .

Microscopy and imaging
The unique spiral phase of VBs can be used in phasecontrast microscopy, demonstrating high-resolution micro-imaging 37 . Applying OAM analysis in the imaging method, the novel digital spiral imaging technique was proposed to improve the resolution 273 . Currently, imaging using OAM has already realized super-diffraction-limit resolution 38 . In recent years, a growing number of novel microscopy and imaging technologies using VBs have emerged, reaching increasingly higher resolution. For instance, plasmonic structured illumination microscopy using standing surface plasmon waves induced by OVs was proposed, realizing high-resolution wide-field imaging 274   enhance the excitation efficiency and reduce the background noise 275 . With the development of multisingularity beams, a vortex array was used to harness the point-spread function to realize high-resolution farfield microscopy 276 . Specifically, fractional VBs were also used for precise microscopy to reach sub-100-nm resolution 277 . With the advanced vector VBs having a special polarization structure, the super-resolution imaging reached an even higher resolution 278 , as shown in Fig. 18c, d. With the quantum properties of VBs, quantum ghost imaging was combined with twisted photons, opening new routes for imaging techniques 243 . As a remarkable breakthrough of microscopy using OVs, the stimulated emission depletion (STED) microscopy technique proposed by Willig et al. 279 , in which the vortex phase is modulated in STED beams to realize super-resolution, was awarded the 2014 Nobel Prize in Chemistry.

Biomedicine and chemistry
Using OV tweezers, one can manipulate and assemble some proteins and other biomolecules, greatly advancing the development of structural chemistry and biomedical photonics 34,36 . Note that VBs and some organic molecules all have chirality, and the chirality of the vortex phase can interplay with that of a biomolecule, which has promoted a number of applications in biomedicine and chemistry 280 . For instance, VBs can be used to assemble DNA 36 and resolve enantiomers 281 due to the chirality coupling effect. By applying this method to chiral metamaterials, novel sensing technology was proposed to detect many enantiomers or biomolecules, such as amino acids, sugars, and nucleotides 282 . Additionally, the functionalities of transporting subcellular organelles and exerting less photodamage on the trapped particle was developed for vortex tweezers, which have been used in sophisticated single-cell nanosurgery 283 . The advanced microscopy brought about by VBs was also used for observing biological cell structures with high resolution 279 . Most recently, vortices were directly generated from organic materials 284 , with further development of organic illumination and chemical detection technologies expected in the future.  Fig. 18 High-resolution imaging using OVs. a Setup of the plasmonic structured illumination microscopy technique, and b imaging results with super-resolution 277 . c Setup of superoscillation focusing imaging using a vector VB, and d imaging results with super-resolution 278 . a, b Reprinted with permission from ref. 277 , OSA Publishing. c, d Reprinted with permission from ref. 278 , OSA Publishing

Metrology
Based on the light-matter interaction through which the OAM of light can be coupled with the mechanical momentum, VBs can be used to detect object motion, including spin motion 285 and lateral motion 286 . With recent advances in nanophotonics and nanofabrication, the precision of detection has reached the nanoscale, and VBs can be used for label-free single-molecule detection in metamaterials 287 . Recently, the OAM spectrum, acting as a new powerful tool, was used in optical detection, in which the difference between the OAM spectra of incident and outgoing light revealed the topography of the target 288 , as depicted in Fig. 19h. Similar OAM-spectrum methods have been successfully applied to detect complicated turbulence in the atmosphere 289 and ocean 290 . Recently, with the study of the interaction between OVs and plasmonic nanoslits 291 , VBs have been used to detect the nanostructure on metal films, opening the door for onchip compact OAM detection 292 . There are also several devices and structures for detecting OAM states. For instance, a virtual rotational antenna structure was designed to generate the rotational Doppler effect, and the signal of the Doppler shift could be detected to reveal the OAM of the corresponding OV 293 . The on-chip plasmonic nanoslit structure can produce different scattering effects for OVs with different TCs, serving as a useful tool for the discrimination of OAM 294 .   Fig. 19 Nanotechnology, nonlinear conversion, and detection using OVs. a Various integrated nanoscale vortex emitters 259 . b Sorting and resolving enantiomers or molecules with different chiralities 281 . c Machining a nanoscale needle using VBs with d clockwise rotation and e counterclockwise rotation structures 270 . f Schematic diagram of the rotational Doppler effect in nonlinear optics 257 . g Schematic diagram of the OAM conservation process in the high-harmonic generation of VBs 49 . h Schematic diagram of object detection using the OAM spectrum 288 . a From ref. 259 . Reprinted with permission from AAAS. b Reprinted/adapted from ref. 281  Moreover, some on-demand metasurface 262 and liquid crystal 170,171,265 devices have shown great potential for detecting OAM, enabling the further development of precise metrology technologies.

Astronomy
OVs not only have been artificially created in laser beams but also naturally exist in the cosmic microwave background 35 . In 2003, Harwit described astrophysical processes of OAM light generation, including photon scattering and vortex generation in the environments surrounding energetic sources, e.g., masers, pulsars, and quasars 35 . To make an astronomical survey that took advantage of OVs, an OV coronagraph was designed 295 and experimentally verified 296 by Swartzlander's group, which has made many breakthroughs in astronomical demonstration 297 . In addition to the scalar vortex masks used in these coronagraph devices, vectorial masks were also implemented in coronagraphs at nearly the same time as Swartzlander's work in 2005 298 . With the development of vector OVs, the vortex coronagraph implemented in international ground-based telescope facilities has been based on vectorial vortex masks to obtain higher sensitivity and lower aberrations 299 . With the recent development of multi-singularity tunability, adaptive multiple-vortex coronagraph masks have been developed for multiple-star detections 300,301 . In 2011, Tamburini et al. 302 reported the OAM light effect around rotating black holes, which provided a new method to detect black holes, as shown in Fig. 20d. Interestingly, astronomical applications are always accompanied by sci-fi themes, and vortex light has been declared to be a fast, furious and perfect tool for talking to aliens and detecting alien civilizations due to its unique properties 303 .

Other advances
OVs indeed demonstrate various characteristics, not only as VBs analysed under the paraxial approximation but also as a general spatial singular field with fractality of singularities. In addition, OVs are not restricted to linear space but have been extensively studied in nonlinear media in connection with optical solitons 7,22-24 . Moreover, topological vortex waves can be studied in other spectra in addition to the light field, such as microwave vortices 304 , acoustic vortices 305 and X-ray vortices 50 . Vortex electron beams 59 and neutron beams 60 with unique OAM properties were also produced and investigated. Very recently, gravitational waves with AM were observed and could be used for trapping and guiding cosmic bodies 306 . Overall, there are currently numerous promising and amazing applications related to OVs with unlimited possibilities that require further exploration.

Conclusions and perspectives
This review article is dedicated to commemorating the 30th anniversary of the birth of OVs, covering the development history from fundamental theories to tunable vortex techniques and then to widespread scientific applications. We first reviewed the theoretical foundation of OVs and emphasized the unique properties related to OAM, TC, and singularities. Then, we reviewed the recent advances in tunable VBs, where the tunability includes not only wavelength tunability and temporal tunability but also OAM tunability. Recent vortex generation methods with different kinds of tunability were reviewed, revealing the development of optical field manipulation. Taking advantage of the advanced vortex manipulation techniques, widespread novel applications have boomed in the new century. We reviewed the various applications in different branches of science as comprehensively as possible. The development tendency of OVs is a typical example that theories guide new applications and that application demands inspire new theories. To date, OVs are still hot topics and have high potential for both theories and applications.