Introduction
At the invitation of Photonics Insights' founding editor, Prof. Jian Wang from Huazhong University of Science and Technology authored a review article titled "Integrated structured light manipulation". The article was published in the 3rd issue of Photonics Insights in 2024 and was selected as the "On the Cover". (Jian Wang, Kang Li, and Zhiqiang Quan. Integrated structured light manipulation [J]. Photonics Insights, 2024, 3(3): R05)
This review begins with a brief introduction to the fundamental concepts and development history of structured light, followed by a systematic summary of four key aspects of integrated structured light manipulation: the generation of various types of integrated structured light, the processing of diverse integrated structured light, the detection of multiple forms of integrated structured waves, and the applications of integrated structured light. Finally, the review discusses future trends, opportunities, challenges, and potential solutions, offering insights into the outlook for integrated structured light manipulation.
1. Basic concept and development history
Structured light beams refer to a unique set of light fields, including vortex beams, Bessel beams, Airy beams, vector beams, and spatiotemporal beams. These beams exhibit specific spatial amplitude, spatial phase, spatial polarization, and even more general spatiotemporal structures. Unlike traditional plane waves and Gaussian beams with uniform field distributions, structured light with non-uniform field distributions offers richer degrees of freedom and additional physical dimensions for exploration, thus providing more opportunities in various advanced applications. The concept of structured light can be traced back to the early 19th century, over 200 years ago, during Thomas Young's double-slit interference experiment. In this experiment, the interference fringes produced by plane waves passing through two slits can be viewed as a type of structured light with a one-dimensional intensity distribution. In principle, structured light can extend to all spatial and temporal degrees of freedom of light waves, including spatial amplitude, phase, polarization distribution, and spatiotemporal structure, leading to the generation of various types of structured light beams. In recent years, the development of structured light beams has accelerated, with numerous types reported, including vortex beams carrying orbital angular momentum (OAM), higher-order linearly polarized beams, Laguerre-Gaussian beams, Hermite-Gaussian beams, Bessel beams, Mathieu beams, Ince-Gaussian beams, Airy beams, bottle beams, needle beams, pin-shaped beams, array beams, spatiotemporal beams, light bullet beams, planar waveguide modes, vector beams, Skyrmion beams, Hopfion beams, and more general arbitrary structured beams, as illustrated in Figure 1. It is evident that various structured beams display non-uniform spatial amplitude, phase, and polarization distributions, as well as complex spatiotemporal structures.
Notably, the diverse applications and immense development potential of structured light have propelled research into structured light manipulation technologies, encompassing generation, processing, and detection. Traditionally, manipulation of structured light has relied on large, discrete, and cumbersome equipment. While these relatively large devices excel in structured light control, they hinder the development and application of integrated structured light systems. Looking ahead, the reported compact photonic devices capable of flexibly controlling degrees of freedom for traditional light waves across various platforms suggest that miniaturization and integration are inevitable trends, equally applicable to spatial degrees of freedom in structured light manipulation. Recent advancements, illustrated in Figure 2, have emerged from various fundamental theories and principles, including spiral phase plate theory, mode field superposition theory, phase control theory, holography, coupling mode theory, whispering gallery mode theory, non-Hermitian theory, phased array theory, multi-plane optical conversion theory, geometric coordinate transformation theory, nonlinear interaction theory, chiral mode conversion theory, surface plasmon polariton effects, photoelectric effects, thermoelectric responses of the rotating Hall effect, diffraction deep neural networks, and inverse design methods. These advancements have led to the rapid development of miniaturized and integrated structured light manipulation technologies, attracting increasing attention in generation, processing, and detection. For example, in-plane and out-of-plane integrated structured light generation can be achieved using micro-nano spiral phase plates, holographic gratings, 3D waveguides, trench waveguides, nanoantenna waveguides, microrings with angular gratings, metamaterials, subwavelength structures, phased arrays, and asymmetric directional couplers. Flexible integrated structured light processing can be realized through multimode waveguides, multi-phase planes, nonlinear photonic devices, two-dimensional materials, 3D photonic chips, silicon photonic integrated circuits, and inverse-designed micro-nano structures. Diverse integrated structured light detection can be achieved with plasmonic and dielectric micro-nano structures, photoconductive detectors with U-shaped electrodes, thermoelectric detectors with rotating Hall couplers, hybrid optoelectronic neural networks, networks on silicon substrates, and inverse-designed subwavelength structures. These typical studies demonstrate the remarkable performance of shaping light in an integrated and compact manner, stimulating rapid and vigorous development in the cutting-edge field of integrated structured light manipulation.
In this review, we comprehensively examine the latest advancements in integrated structured light manipulation, organized into four key areas: generation, processing, detection, and applications, as shown in Figure 3.
Fig. 1. Schematic illustration of various types of structured light and their beam profiles.
Fig. 2. Various basic theories and principles of integrated structured light manipulation.
Fig. 3. Four aspects of integrated structured light manipulation: generation, processing, detection and application.
2. Integrated structured light generation
For the generation of integrated structured light, we first introduce the fundamental theories and principles of structured light generation. These include spiral phase plate theory, mode field superposition theory, phase control theory (resonant phase, geometric phase, propagation phase, and composite phase), holography, coupling mode theory, whispering gallery mode theory, and non-Hermitian theory.
Next, we present various schemes for integrated generation of orbital angular momentum (OAM) beams based on both passive and active integrated platforms. We categorize the significant research based on whether the beams are located within the waveguide plane, dividing them into four categories: in-plane to in-plane OAM beam generation, out-of-plane to in-plane OAM beam generation, in-plane to out-of-plane OAM beam generation, and out-of-plane to out-of-plane OAM beam generation.
Fig. 4. Typical examples of in-plane to in-plane OAM generation.
For in-plane to in-plane OAM beam generation, three-dimensional waveguide structures are commonly used. Examples include hybrid devices formed by coupling silicon planar lightwave circuits (PLC) with three-dimensional waveguide structures, asymmetric directional couplers embedded in photonic chips for emitting vortex beams, and trench waveguides on silicon/silica chips, as shown in Figure 4. For out-of-plane to in-plane OAM beam generation, diffraction structures are typically employed, such as dielectric antenna waveguides and grating couplers, illustrated in Figure 5. Similarly, diffraction structures are frequently utilized for in-plane to out-of-plane OAM generation. Figure 6 displays four different structures commonly used for in-plane to out-of-plane OAM generation, including grating arrays, microring resonators, fork gratings, and subwavelength structures. For out-of-plane to out-of-plane OAM beam generation, the primary design schemes rely on micro-nano structures, including metasurfaces, spiral phase plates, and holographic gratings, as shown in Figure 7.
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Fig. 5. Typical examples of out-of-plane to in-plane OAM generation.
Fig. 6. Typical examples of in-plane to out-of-plane OAM generation.
Fig. 7. Typical examples of out-of-plane to out-of-plane OAM generation.
For integrated OAM emitters on passive platforms, an external light source is always required. In recent years, active OAM emitters based on integrated active photonic platforms have garnered increasing attention, as these devices do not rely on external light sources and are suitable for various emerging OAM photonic technologies. Among them, integrated OAM lasers have emerged as a typical active device for OAM generation, representing an important and valuable area of research for diverse OAM production. Common types of OAM lasers include vertical cavity surface-emitting lasers (VCSELs) using spiral phase plates, non-Hermitian lasers, micro-nano-etched lasers, topological lasers, and supersymmetric micro-laser arrays.
Fig. 8. The integrated spiral phase plate OAM laser.
For integrated OAM lasers, a direct approach involves combining a passive OAM generator with a compact laser on a single chip. VCSELs offer numerous advantages, making them ideal for integration with passive OAM generators. Figure 8 illustrates an OAM laser that combines a VCSEL with an 8×8 spiral phase plate array, capable of emitting 64 different OAM modes. Microring lasers are also a significant focus of research due to their ability to support high-order OAM modes within their cavity. However, due to their rotational symmetry, microring structures excite both clockwise and counterclockwise whispering-gallery modes simultaneously, resulting in two OAM modes with opposite topological charges that effectively cancel each other out. To achieve independent OAM, mechanisms that break rotational symmetry must be introduced. Non-Hermitian symmetries with gain or loss represent an emerging and promising solution, as they disrupt rotational symmetry and lift the degeneracy of the two spin-orbit modes, facilitating the generation of OAM beams with the desired handedness. Figure 9 presents several non-Hermitian single-mode OAM lasers based on microring structures.
Additionally, lasers with micro-nano-etched structures have been proposed for OAM mode generation, including lasers etched with chromium micro-teeth on silicon pillars, hexagonal lasers made from In0.05Ga0.95As quantum wells, subwavelength-etched metasurface lasers located on top of microrings, and microring lasers based on distributed feedback semiconductor designs (as shown in Figure 10). Furthermore, advanced physical concepts have been introduced in the design of OAM lasers to enhance device performance, such as topological photonics and supersymmetry theory. Topological photonics, originating from topological phases and phase transitions in condensed matter, has garnered significant attention in integrated optics, nonlinear optics, and quantum optics, and can be applied to OAM microlasers to generate high-quality spin-momentum-locked edge modes and vortex/antivortex modes, as illustrated in Figure 11. Supersymmetry theory can also be utilized for the coupling of multiple lasers into an array, enabling precise mode control and nonlinear power scaling, as depicted in Figure 12.
Fig. 9. The integrated non-Hermitian-controlled OAM laser.
Fig. 10. The integrated micro-etching OAM laser.
Fig. 11. The integrated topological OAM laser.
Fig. 12. Higher-dimensional supersymmetric microlaser arrays.
In addition, we also introduced the integrated generation of other structured light forms beyond OAM, such as chiral light, higher-order LP modes, LG beams, HG beams, non-diffracting beams (including Bessel beams, Mathieu beams, Airy beams, and needle beams), vector beams, array beams, optical vortex lattices, spatiotemporal beams, optical binding beams, in-plane waveguide modes, and reconfigurable structured beams.
3. Integrated structured light processing
In our discussion of integrated structured light processing, we introduced the theories and principles underlying the manipulation of structured light, including multi-plane light conversion theory, geometric coordinate transformation theory, nonlinear interaction theory, and chiral mode conversion theory. Subsequently, we presented several integrated structured light processing functionalities, which can primarily be categorized into multiplexing and conversion.
Fig. 13. Out-of-plane/ in-plane OAM multiplexing.
Multiplexing is one of the most important and powerful techniques for efficiently expanding capacity in optical communication, where multiple collinear propagating beams (each carrying different data) are combined to enhance transmission capacity. Given the diversity of structured light fields, structured light multiplexing is significant for achieving high-capacity optical communication across various applications. Integrated structured light multiplexing has gained widespread attention due to its compactness, encompassing both out-of-plane and in-plane multiplexing. For OAM multiplexing, out-of-plane (de)multiplexing can be achieved using multiple phase planes, while in-plane (de)multiplexing can be realized through 3D waveguides. In the case of in-plane mode multiplexing, multiplexers typically utilize cascaded microring resonators and cascaded asymmetric directional couplers.
Fig. 14. The in-plane waveguide mode multiplexing
Fig. 15. Fraction OAM multiplication and division
Fig. 16. The in-plane waveguide mode processing (chiral mode switching)
Fig. 17. Transformation of different types of structured light.
The goal of structured light conversion is to transform one type of structured light into another or to produce the same type of structured light at different orders, facilitating optical data processing with structured light. For the conversion of similar types of structured light, the multiplication and division operations in OAM mode transformation can achieve not only integer multiples but also fractional multiples by applying the principle of coordinate transformation. Figure 15 illustrates the fractional multiplication and division conversions of OAM. Notably, when converting in-plane waveguide modes, the introduction of non-Hermitian concepts can enable asymmetric transmission of waveguide modes. In this case, the output mode is determined solely by the direction of the input, independent of the input mode itself, as shown in Figure 16. In addition to conversions between similar types of structured light, conversions between different types of structured light can also be achieved. As illustrated in Figure 17, multilayer phase plates fabricated using femtosecond laser processing can facilitate the asymmetric transmission of OAM modes and LP modes. Furthermore, heterogeneous multi-level inverted cone structures can enable direct conversion between in-plane modes and LP modes.
4. Integrated structured light detection
In the field of integrated structured light detection, we first elucidate the theories and principles behind structured light detection, including the effects of surface plasmon polaritons, photoelectric current effects, thermoelectric responses to the rotating Hall effect, diffraction deep neural networks, and inverse design methods.
The structured light detection scheme based on surface plasmon polaritons relies on the unique polarization and phase distribution of structured light, which influences the surface plasmon polariton fields generated at metal-dielectric interfaces, enabling the detection of spin angular momentum and orbital angular momentum. By designing micro-nano structures etched onto metal surfaces, near-field microscopy can be employed to gather information about the incident structured light, as shown in Figure 18. While metallic micro-nano structures demonstrate good performance in structured light detection, the significant Ohmic losses in metals lead to weaker transmission power. This implies that detection devices must possess robust noise suppression capabilities to identify the topological charge and polarization information of the incident structured light. Due to the significantly lower inherent losses in dielectric metasurfaces compared to metals, structured light detection schemes based on dielectric metasurfaces are more advantageous.
Figure 19 illustrates various dielectric micro-nano structures for detecting structured light, including dielectric metasurfaces, Dammann gratings, and waveguide gratings. Among these, dielectric metasurfaces enable efficient and broadband detection of OAM modes, while Dammann gratings can detect the angular momentum of vortex beams through vector beam holography calculations, overcoming the limitations of uneven power distribution in traditional optical vortex grating detection devices. This method not only increases the number of multiplexing and demultiplexing channels in OAM optical communication but also expands the range of parallel OAM detection. Additionally, waveguide gratings can couple different OAM modes into distinct output waveguides based on the principle of phase matching, thus allowing the detection of various OAM modes.
Fig. 18. Integrated structured light detection based on metal micro-nano structures.
Fig. 19. Integrated structured light detection based on dielectric micro-nano structures.
Fig. 20. Integrated structured light detection based on orbital photogalvanic effect.
Numerous theoretical and experimental studies indicate that the interaction between light's orbital angular momentum (OAM) and atomic media results in new selection rules and optical responses. These studies reveal that optical phase gradients can alter the excitation process, yet these findings have not directly translated into photoelectric current generation in OAM-sensitive photodetectors. This is because the photoelectric current response itself does not carry phase information, and the slow variation of the vector potential associated with the light's OAM limits its influence on microscopic processes relative to the size of the Brillouin zone. Research has shown that light's OAM can induce strong non-local interactions between electromagnetic waves and matter. Consequently, researchers are exploring how OAM generates photoelectric current during the photoelectric conversion process, designing vortex light photodetectors capable of directly detecting phase, as illustrated in Figure 20. This design features a photodetector based on tungsten diteluride (WTe2), with carefully engineered electrode geometries to directly characterize the topological charge of the light's OAM.
In addition to metal and dielectric micro-nano structures, plasmonic metamaterials, and photoelectric current detectors with U-shaped electrodes, we also introduce several integrated structured light detection schemes, such as digital micromirror devices (DMD) and diffusers, hybrid optoelectronic neural networks, silicon-based Mach-Zehnder interferometer (MZI) networks, inverse-designed sub-wavelength structures, and silicon nanowire optomechanical systems.
5. Integrated structured light applications
In the realm of integrated structured light applications, we begin by introducing the key theories and operational principles of structured light in various contexts, including structured light modulation communication, multiplexed communication, multicast communication, holography, and optical manipulation and capture.
One prominent application of integrated structured light is high-speed optical communication across multiple scenarios, encompassing analog signal transmission, digital signal transmission for data carrying, high-speed spatial light modulation communication, optical interconnections between chips, chip-fiber-chip links, direct optical fiber vector mode multiplexing, mode division multiplexing (MDM) within chips, multidimensional data transmission and processing, as well as free-space and multimode fiber communication. Notably, silicon-based photonic processors can generate arbitrary optical modes and identify modes after they have traversed a transmission medium, showcasing a method for transmitting any orthogonal modes, as illustrated in Figure 21.
The spatial degrees of freedom of structured light can also be utilized for holographic encryption. For example, the orbital angular momentum (OAM) of light with a helical phase front can serve as a holographic information carrier, as shown in Figure 22. Thus, holograms employing multiple OAM modes can achieve highly secure optical data encryption, providing significant information security for data sharing.
Structured light projection represents a promising technique for rapid, non-contact three-dimensional imaging. Figure 23 displays the design and fabrication of multi-wavelength dot lattices aimed at enhancing the speed and resolution of 3D imaging. This dielectric metasurface can generate dense lattices simultaneously at three wavelengths while maintaining high transmission efficiency. Furthermore, the multi-wavelength approach significantly improves lateral resolution.
Fig. 21. Structured light application in arbitrary multimode communication with two silicon photonic processors.
Fig. 22. Structured light application in OAM-multiplexing holography for high-security encryption
Fig. 23. Structured light application in 3D imaging using multi-wavelength dots array.
Structured light plays a significant role in medical imaging and pathological detection. Optical resolution photoacoustic microscopy allows for the observation of wavelength-dependent optical absorption at the cellular level. However, due to the tight focusing of the optical excitation beam, this method has limited depth of field, making it challenging to obtain high-resolution images of uneven surfaces or high-quality volumetric images without Z-scanning. To address this limitation, a needle-beam photoacoustic microscopy (NB-PAM) approach is proposed, which enhances depth of field to approximately 28 times the Rayleigh length using specialized diffractive optical elements (DOE). The needle beams generated by these DOEs offer good beam diameter control, uniform axial intensity distribution, and reduced sidelobes, outperforming Gaussian beam photoacoustic microscopy (GB-PAM) in imaging 2D images of uneven surfaces or volumetric imaging of thick specimens.
Fig. 24. Structured light application in medical imaging using needle beam and multifocal beam.
Fig. 25. Structured light application in photo-induced force microscopy using tightly focused azimuthally polarized beam.
Recently, research into the interaction of materials with light via magnetic fields rather than electric fields has garnered significant attention. Manipulating magnetic transitions in nanostructures at optical frequencies can substantially enhance storage capacity and read/write speeds. However, the asymmetry between electric and magnetic effects typically results in a magnetic response that is much weaker than the electric response at optical frequencies. Consequently, developing effective optomagnetic devices has become a critical task, starting with establishing mechanisms for direct detection of nanometer-scale optomagnetic fields. We propose a method that utilizes structured light fields to directly acquire the near magnetic field distribution in the detection area. Focusing circularly polarized light excites unique micro-nano probes' magnetic resonance while suppressing their electric response, enabling pure optomagnetic detection.
Optical interference techniques have been widely applied in precision optical metrology, particularly in 3D sensing and phase information characterization. A novel optical vortex interferometer has been proposed, utilizing vortex beams as reference beams for detecting 3D objects. We present a self-referencing optical vortex interferometer based on a broadband geometric phase element array. To measure the surface profile of transparent samples, a schematic of the self-referencing helical interferometer is constructed. This system employs a multitasking geometric phase element array to create a vortex phase filter. The geometric phase arises from polarization conversion and in-plane rotation of anisotropic units. By adjusting the optical axis distribution of these units, the phase of transmitted light can be efficiently controlled over a broad bandwidth. The vortex contrast filter simultaneously achieves two tasks: it acts as a vortex filter with a central hole and vortex phase, and it can deflect modulated light to achieve high modal purity over a wide bandwidth. Thus, low- and high-frequency spatial components overlap in the image, creating an interference pattern.
Structured light has also found extensive applications in 3D optical manipulation. Traditionally, optical manipulation has relied on bulky optical equipment. Recently, increasing attention has been directed towards developing integrated methods for generating structured light to enable compact optical manipulation. We propose a method for generating two-dimensional Airy beams using metasurfaces, facilitating the 3D manipulation of particles. This approach employs a cubic phase medium metasurface composed of gallium nitride cylindrical nanopillars to produce polarization-independent, vertically accelerated 2D Airy beams in the visible range. Results indicate that microspheres are trapped within the cross-sectional light field. Additionally, experimental evidence demonstrates that these vertically accelerated 2D Airy beams exhibit unique propagation characteristics, including non-diffractive, self-accelerating, and self-repairing properties.
Fig. 26. Structured light application in three-dimensional topography using vortex beam.
The special modes of on-chip waveguides enable the construction of strong optical potential traps, allowing particles to be captured and manipulated at the chip level. The scheme illustrated in Figure 28 utilizes optical forces induced by Bloch modes propagating along silicon superlattice waveguides to capture particles. As a periodic structure, superlattice waveguides support a periodic optical field distribution along the waveguide, making it possible to stably trap multiple nanoparticles. By modifying the grating period of the superlattice waveguide, the spacing between the captured nanoparticles can be easily designed. Similarly, a one-dimensional optical lattice generated along multimode silicon nanophotonic waveguides is used to demonstrate the optical trapping of dielectric particles and bacteria on a chip.
Furthermore, Figure 29 presents an optical chiral separation scheme capable of capturing and separating chiral nanoparticles within oppositely propagating silicon-based slit waveguides. This silicon-based chiral separation platform offers several advantages over previously proposed optical chiral separation methods. On one hand, it employs the gradient forces generated by the slit waveguide modes for chiral separation. This force acts as a conservative force, providing a more significant optical force compared to the non-conservative forces of optical scattering or radiation pressure. On the other hand, the light field tightly confined by subwavelength slit waveguides effectively captures and separates nanoscale chiral particles, thereby circumventing the diffraction limit imposed by free-space optical focusing. The resulting gradient force can alter the equilibrium position of the optical trap, facilitating chiral-dependent separation.
Fig. 27. Structured light application in 3D optical manipulation using the 2D Airy beam.
Fig. 28. Structured light application in optical trapping using waveguide modes and optical phased array.
Fig. 29. Structured light application in chiral trapping using silicon-based slot waveguide.
Fig. 30. Structured light application in Doppler cloak by spinning OAM metasurface.
Theoretically, it has been demonstrated that spacetime-modulated metamaterials can be utilized to construct a Doppler invisibility cloak by correcting the Doppler shift to conceal the motion of moving objects. The orbital angular momentum (OAM) of light emitted from rotating objects can be employed to detect the rotational Doppler effect, which is analogous to the linear Doppler effect in an angular context. Spacetime characteristics can be realized by mechanically modulating the reflection phase of metamaterials to generate a rotational Doppler effect. As a two-dimensional spacetime-modulated metamaterial, the polarization-independent OAM metasurface can exploit spatial and temporal dimensions to produce this effect. This approach offers a straightforward method for modifying the reflection phase, enabling the implementation of a Doppler invisibility cloak. Figure 30 illustrates the experimental setup for constructing a Doppler invisibility cloak using a rotating OAM metasurface. Measurement results confirm the ability of spacetime metamaterials to conceal moving objects by generating a rotational Doppler shift that opposes linear motion. Notably, it is anticipated that extending radio frequencies to the optical range may also enable this Doppler invisibility cloak through a rotating OAM metasurface, referred to as a structured light Doppler cloak.
Fig. 31. Structured light application in quantum processing
Metamaterials, as simplified structures of metasurfaces, have recently been proposed as a promising platform for quantum optics. Figure 31(a) shows the creation of entanglement between the spin and orbital angular momentum of photons using dielectric metasurfaces. Single photons transition into entangled states when interacting with the metasurface. Additionally, this metasurface can generate entangled biphoton states, with an experimental conversion efficiency of approximately 72%. Demonstrating the generation of entangled photon states using metamaterials/metasurfaces paves the way for nano-optical quantum information applications. Orthogonal transverse modes represent a significant degree of freedom in photonic integrated circuits, providing potential technology for enhancing communication capabilities in classical and quantum information processing. To construct large-scale on-chip multimodal quantum systems, a control CNOT gate based on transverse mode encoding is required. Figures 31(d)-(g) showcase a multimodal implementation of a 2-qubit CNOT gate with transverse mode encoding, comprising developed multimodal directional couplers and attenuators. The designed multimodal waveguide supports two lowest-order transverse modes, TE0 and TE1, with a cross-section of 760×220 nm². These modes are orthogonal and do not interact with each other, allowing them to be directly used for encoding quantum information. Two mode-dependent photonic devices (transverse mode-dependent directional couplers and multimodal attenuators) have been created to facilitate the construction of the CNOT gate with transverse mode encoding.
6. Summary and Outlook
Finally, we summarize recent advancements and discuss future trends in integrated structured light manipulation across multiple materials, integration technologies, operating frequency bands, types of structured light, processing functions, detection structures, and diverse application scenarios. This review also explores future opportunities, challenges, and solutions, as illustrated in Figure 32.
As advancements continue, integrated structured light control will evolve from fundamental theory to critical technologies, ultimately finding applications in significant fields. Figure 33 depicts a promising vision for integrated structured light manipulation. To foster the growth of this concept, it is essential to enhance the foundational theories and mechanisms between structured light and structured matter. Diverse photonic integration platforms tailored for different materials will continue to develop, providing support for high-performance optoelectronic devices and chip-level structured light manipulation.
Notably, while this review primarily focuses on the spatial degrees of freedom of photons, other degrees of freedom, such as wavelength, polarization, time, and complex amplitude, can also be flexibly manipulated using photonic integrated devices. In addition to the main discussions on generation, processing, detection, and applications, there are many other areas of integrated structured light control that require further development, including display, transmission, computation, and storage. We have discussed generation, processing, detection, and application separately in this review. In the future, the integration of these aspects is expected to propel the realization of ultra-compact integrated structured light manipulation systems. Moreover, the compatibility of the spatial degrees of freedom with other degrees of freedom suggests a high expectation for the integration of spatial, wavelength, polarization, time, and complex amplitude dimensions, driving innovation in advanced multidimensional integrated structured light manipulation for future optical technologies.
Fig. 32. Opportunities, challenges and possible solutions for integrated structured light manipulation.
Fig. 33. A vision of integrated structured light manipulation.
