Optical Fabrication Technology Based on Vector Optical Field(Invited)
Significance As the basic properties of light,the degrees of freedom provided by polarization,amplitude,and phase play an important role in light modulation.Vector optical fields(VOFs)with spatially structured polarization,amplitude,and phase have been widely applied in various fields because of their unique properties,which differ from those of traditional optical fields.In recent years,new vector optical fields with more abundant spatiotemporal characteristics have attracted intense attention.The emergence of such optical fields enriches the types of vector optical beams and provides a new degree of freedom for light modulation,thereby bringing about a new opportunity for optical processing.Traditional laser processing mainly focuses on the energy characteristics of the laser.Nevertheless,momentum exchange occurs in addition to energy absorption during the interaction of light and matter.Compared with the scalar optical field,the vector optical field can converge to the focal spot beyond the diffraction limit.Moreover,the spot size is smaller.Hence,the processing accuracy can be higher.Furthermore,the light field with the photonic angular momentum can exchange momentum with matter.For instance,a vector vortex light that carries photonic orbital angular momentum can drive a particle to rotate along a fixed axis.Therefore,the momentum characteristics of vector optical fields are promising and attractive for applications in the field of laser processing,such as the induction of complex patterns or chiral structures.Progress This review covers the rapid development of optical processing technology based on vector optical fields in recent years,including vector beam-based laser micro/nanofabrication and geometric phase liquid crystal planar device processing technology.The spatial topology of the vector optical field in the inherent features of the beam,such as polarization,amplitude,and phase,allows for the fine manipulation of the interaction between the laser and matter,resulting in a diverse and intricate processing structure.The use of vector optical fields can also improve the accuracy,efficiency,and even break limits of laser micro/nano manufacturing.Polarization has a direct impact on how laser light interacts with materials.Radial polarization significantly improves laser absorption via a resonant absorption process,and the laser cutting efficiency is 1.5 to 2 times that of planar P-polarized and circularly polarized beams.Radially polarized lasers have emerged as an appealing technique for laser drilling.The focus spot sizes and depths of traditional Gaussian beams are theoretically bound to each other,which severely restricts the machining efficiency and depth/width ratios.Bessel beams,which are the most frequent type of non-diffracting beam,are formed by a cone-shaped superposition of plane waves whose on-axis energy can remain relatively constant over long distances.As a result,Bessel beams are known for their large focusing depth and have been used in glass,metal,sapphire,silicon,and other materials to rapidly manufacture high depth/width ratio micro/nano channels.Radial/angular polarized beams produce sharper structural edges than do linearly polarized beams,indicating better machining quality.When compared with those of the conventional Gaussian beam machining of stainless steel and titanium,the cylindrical vector beam machining efficiency increases by 80％,and the average surface roughness decreases by more than 94％.In addition,vector light fields have demonstrated considerable versatility in the construction of 2D and 3D complex structures,including 2D and 3D chiral structures and spiral nanostructures.In a recent study,using the ultra-diffracted focusing property of the vector light field with variable longitudinal polarization,ultra-diffracted nanopores with a diameter of 10-30 nm and a depth/width ratio of over 16:1 was created on sapphire substrates.Another important application area for vector light fields is the fabrication of geometric phase liquid crystal planar devices,such as liquid crystal gratings,liquid crystal lenses,and vortex phase liquid crystal devices.The liquid crystal element can attain high diffraction efficiency near the theoretical limit because of the continuous variation of the principal axis of the liquid crystal molecular axis,which gives the liquid crystal element a continuous variation of the phase modulation distribution.This continuous variation is similar to that of the catenary metasurface.The geometric phase liquid crystal element can effectively modulate circularly polarized light to create the necessary phase delay when its thickness meets the half-wave requirement.The orientations of liquid crystal molecules serve as the foundation for liquid crystal planar optical components.However,the common orientation methods are constrained by low efficiency and high costs.When nematic liquid crystals are exposed to a vector light field,the arrangement of liquid crystal molecules will follow and record the polarization distribution of the vector light field.Hence,well-designed vector light fields are critical to the fabrication of large-area,low-cost liquid crystal planar lenses.However,achieving high stability and purity over a large area in liquid crystal remains a considerable challenge.Conclusions and Prospects Vector light fields feature non-uniform distributions of spatial polarization,and they offer a fresh perspective on the relationship between light and matter as well as new avenues for the development of micro and nano optical processing technologies.The relevant research effort has started only recently because of the limitations of vector light field generation technology,among other reasons.However,it has demonstrated a wide range of applications in laser micro-nano manufacturing,vector field exposure,etc.In recent years,with the emergence and rapid development of new optical field manipulation technologies,such as metasurfaces,spatiotemporal multidimensional vector light field control has become possible,bringing about new opportunities for optical processing.Moreover,these technologies are expected to further improve the performance of optical processing.
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