Anisotropic stimulated emission cross-section measurement in Nd:YVO_4

As a crucial parameter in the design and analysis of laser performances, stimulated emission(SE) cross-section is currently considered to be dependent on several factors, such as temperatures and eigen-polarizations for anisotropic crystals. In contrast with these factors, impact of propagating directions upon SE cross-section has garnered less attention.In this paper, to investigate the SE cross-section in arbitrary propagating directions, fluorescence spectra for the transition ~4F_(3/2)→~4I_(11/2) in Nd:YVO_4 are measured in different propagating directions. Based on Fuchtbauer–Ladenburg equation model, the propagating direction-dependent SE cross-section spectra in Nd:YVO_4 are obtained for the first time, to our best knowledge. A novel concept of anisotropic SE cross-section is proposed to interpret the propagating direction-dependent effect. The experiment results reveal that for an arbitrary propagating direction the SE cross-section of e light around 1064 nm can be expressed as a superposition from two principle axial propagating directions with a weight of plane projection.

Multidimensional optical tweezers synthetized by rigid-body emulated structured light

Structured light with more extended degrees of freedom(DoFs)and in higher dimensions is increasingly gaining traction and leading to breakthroughs such as super-resolution imaging,larger-capacity communication,and ultraprecise optical trapping or tweezers.More DoFs for manipulating an object can access more maneuvers and radically increase maneuvering precision,which is of significance in biology and related microscopic detection.However,manipulating particles beyond three-dimensional(3D)spatial manipulation by using current all-optical tweezers technology remains difficult.To overcome this limitation,we theoretically and experimentally present six-dimensional(6D)structured optical tweezers based on tailoring structured light emulating rigid-body mechanics.Our method facilitates the evaluation of the methodology of rigid-body mechanics to synthesize six independent DoFs in a structured optical trapping system,akin to six-axis rigid-body manipulation,including surge,sway,heave,roll,pitch,and yaw.In contrast to previous 3D optical tweezers,our 6D structured optical tweezers significantly improved the flexibility of the path design of complex trajectories,thereby laying the foundation for next-generation functional optical manipulation,assembly,and micromechanics.

Topological transformation and free-space transport of photonic hopfions

Structured light fields embody strong spatial variations of polarization,phase,and amplitude.Understanding,characterization,and exploitation of such fields can be achieved through their topological properties.Three-dimensional(3D)topological solitons,such as hopfions,are 3D localized continuous field configurations with nontrivial particle-like structures that exhibit a host of important topologically protected properties.Here,we propose and demonstrate photonic counterparts of hopfions with exact characteristics of Hopf fibration,Hopf index,and Hopf mapping from real-space vector beams to homotopic hyperspheres representing polarization states.We experimentally generate photonic hopfions with on-demand high-order Hopf indices and independently controlled topological textures,including Néel-,Bloch-,and antiskyrmionic types.We also demonstrate a robust free-space transport of photonic hopfions,thus showing the potential of hopfions for developing optical topological informatics and communications.

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.

Towards higher-dimensional structured light

Structured light refers to the arbitrarily tailoring of optical fields in all their degrees of freedom(DoFs),from spatial to temporal.Although orbital angular momentum(OAM)is perhaps the most topical example,and celebrating 30 years since its connection to the spatial structure of light,control over other DoFs is slowly gaining traction,promising access to higher-dimensional forms of structured light.Nevertheless,harnessing these new DoFs in quantum and classical states remains challenging,with the toolkit still in its infancy.In this perspective,we discuss methods,challenges,and opportunities for the creation,detection,and control of multiple DoFs for higher-dimensional structured light.We present a roadmap for future development trends,from fundamental research to applications,concentrating on the potential for larger-capacity,higher-security information processing and communication,and beyond.

Divergence-degenerate spatial multiplexing towards future ultrahigh capacity,low error-rate optical communications

Spatial mode(de)multiplexiing of orbital angular momentum(OAM)beams is a promising solution to address future bandwidth issues,but the rapidly increasing divergence with the mode order severely limits the practically addressable number of OAM modes.Here we present a set of multi-vortex geometric beams(MVGBs)as high-dimensional information carriers for free-space optical communication,by virtue of three independent degrees of freedom(DoFs)including central OAM,sub-beam OAM,and coherent-state phase.The novel modal basis set has high divergence degeneracy,and highly consistent propagation behaviors among all spatial modes,capable of increasing the addressable spatial channels by two orders of magnitude than OAM basis as predicted.We experimentally realize the tri-DoF MVGB mode(de)multiplexing and data transmission by the conjugated modulation method,demonstrating lower error rates caused by center offset and coherent background noise,compared with OAM basis.Our work provides a potentially useful basis for the next generation of large-scale dense data communication.

Optimization of microbial cell factories for astaxanthin production: Biosynthesis and regulations, engineering strategies and fermentation optimization strategies

The global market demand for natural astaxanthin is rapidly increasing owing to its safety,the potential health benefits,and the diverse applications in food and pharmaceutical industries.The major native producers of natural astaxanthin on industrial scale are the alga Haematococcus pluvialis and the yeast Xanthopyllomyces dendrorhous.However,the natural production via these native producers is facing challenges of limited yield and high cost of cultivation and extraction.Alternatively,astaxanthin production via metabolically engineered non-native microbial cell factories such as Escherichia coli,Saccharomyces cerevisiae and Yarrowia lipolytica is another promising strategy to overcome these limitations.In this review we summarize the recent scientific and biotechnological progresses on astaxanthin biosynthetic pathways,transcriptional regulations,the interrelation with lipid metabolism,engineering strategies as well as fermentation process control in major native and non-native astaxanthin producers.These progresses illuminate the prospects of producing astaxanthin by microbial cell factories on industrial scale.

Observation of spectral modulation coupled with broadband transverse-mode-locking in an Yb:CALGO frequency-degenerate cavity

A frequency-degenerate cavity(FDC) is the resonator that the ratio of transverse and longitudinal mode frequency spacings is a simple rational number. When an optical resonator is close to the FDC, transverse-mode-locking(TML) takes place with drastic changes of laser mode. We report for the first time, to the best of our knowledge,the multi-frequency emission and spectral modulation effects coupled with TML in FDC. The Yb:CaGdAlO_4(Yb:CALGO) crystal with large gain bandwidth was used as a gain medium in an off-axis-pumped hemispherical FDC for realizing broadband emission. Interestingly, the spectrum can transform from a single smooth packet shape to a multi-peak structure; meanwhile, the transverse pattern accordingly transforms into some exotic wave-packet profiles through controlling off-axis displacement in a special degenerate state.

Scalar optical hopfions

Hopfions are three-dimensional(3D)topological states discovered in field theory,magnetics,and hydrodynamics that resemble particle-like objects in physical space.Hopfions inherit the topological features of the Hopf fibration,a homotopic mapping from unit sphere in 4D space to unit sphere in 3D space.Here we design and demonstrate dynamic scalar optical hopfions in the shape of a toroidal vortex and expressed as an approximate solution to Maxwell’s equations.Equiphase lines correspond to disjoint and interlinked loops forming complete ring tori in 3D space.The Hopf invariant,product of two winding numbers,is determined by the topological charge of the poloidal spatiotemporal vortices and toroidal spatial vortices in toroidal coordinates.Optical hopfions provide a photonic testbed for studying topological states and may be utilized as high-dimensional information carriers.

516 mW,nanosecond Nd:LuAG laser Q-switched by gold nanorods

Q-switched operation of an Nd：LuAG laser using gold nanorods（GNRs） as the saturable absorber（SA） is reported, which also produces the highest average power among the nanosecond Nd-doped Q-switched lasers by GNRs-based SA. The applied GNRs are prepared using a seed-mediated growth method and then dropped onto the quartz substrate to fabricate the SA. The average power of the Q-switched laser is 516 mW with the shortest pulse duration of 606.7 ns and the repetition rate of 265.1 kHz.

Shrinking multiplexed orbital angular momentum to the nanoscale

Orbital angular momentum interactions at the nanoscale have remained elusive because the phase structure becomes unresolved.Now researchers have shown how to overcome this with tightly focused beams,demonstrating a recordhigh six-dimensional encoding in an ultra-dense nanoscale volume.