Abstract 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. 

/pdf/towards-higher-dimensional-structured-light-1aqzgjjk.pdf

Towards higher-dimensional structured light

Controlling the various degrees‐of‐freedom (DoFs) of structured light at both quantum and classical levels is of paramount importance in optics. It is a conventional paradigm to treat diverse DoFs separately in light shaping. While, the more general case of nonseparable states of light, in which two or more DoFs are coupled in a nonseparable way, has become topical recently. Importantly, classical nonseparable states of light are mathematically analogue to quantum entangled states. Such similarity has hatched attractive studies in structured light, for example, the spin‐orbit coupling in vector beams. However, nonseparable classical states of light are still treated in a fragmented fashion, while its forms are not limited by vector beams and its potential is certainly not fully exploited. For instance, exotic space‐time coupled pulses open nontrivial light shaping toward ultrafast time scales, and ray‐wave geometric beams provide new dimensions in optical manipulations. Here, a bird's eye view on the rapidly growing but incoherent body of work on general nonseparable states involving various DoFs of light is provided and a unified framework for their classification and tailoring, providing a perspective on new opportunities for both fundamental science and applications, is introduced.

Nonseparable States of Light: From Quantum to Classical

A highly sensitive light-induced thermoelectric spectroscopy (LITES) sensor based on a multi-pass cell (MPC) with dense spot pattern and a novel quartz tuning fork (QTF) with low resonance frequency is reported in this manuscript. An erbium-doped fiber amplifier (EDFA) was employed to amplify the output optical power so that the signal level was further enhanced. The optical path length (OPL) and the ratio of optical path length to volume (RLV) of the MPC is 37.7 m and 13.8 cm-2, respectively. A commercial QTF and a self-designed trapezoidal-tip QTF with low frequency of 9461.83 Hz were used as the detectors of the sensor, respectively. The target gas selected to test the performance of the system was acetylene (C2H2). When the optical power was constant at 1000 mW, the minimum detection limit (MDL) of the C2H2-LITES sensor can be achieved 48.3 ppb when using the commercial QTF and 24.6 ppb when using the trapezoidal-tip QTF. An improvement of the detection performance by a factor of 1.96 was achieved after replacing the commercial QTF with the trapezoidal-tip QTF.

A highly sensitive LITES sensor based on a multi-pass cell with dense spot pattern and a novel quartz tuning fork with low frequency

Optical skyrmions and other topological quasiparticles of light

Structured light recently highlighted the higher-dimensional control with many tailored degrees of freedom (DoFs) such as amplitude, wavelength, polarization, and orbital angular momentum (OAM), spurring the extension of fundamental science and advanced applications, especially as the novel information carrier to meet the requirement of everlastingly increasing communication bandwidth and security. In addition to the basic DoFs in light, there are also several complex DoFs hidden in spin–orbital coupled vector beams, ray-wave geometric modes, spatiotemporal vortex rings, etc., extending the structured light manipulation. In this Review, we summarize a systematic technical roadmap on how to extend multiple DoFs step-by-step beyond the conventional OAM beams, creating new forms of ultra-DoF structured light, and present a tutorial on how to exploit multiple DoFs of light as advanced information carriers, especially with the recent assistance of artificial intelligence or deep learning method, enhancing the capacity, security, and dimension for the next-generation information technologies. 

https://pubs.acs.org/doi/pdf/10.1021/acsphotonics.2c01640

Ultra-Degree-of-Freedom Structured Light for Ultracapacity Information Carriers

Abstract Orbital angular momentum (OAM) detection underpins almost all aspects of vortex beams’ advances such as communication and quantum analogy. Conventional schemes are frustrated by low speed, complicated system, limited detection range. Here, we devise an intelligent processor composed of photonic and electronic neurons for OAM spectrum measurement in a fast, accurate and direct manner. Specifically, optical layers extract invisible topological charge information from incoming light and a shallow electronic layer predicts the exact spectrum. The integration of optical-computing promises us a compact single-shot system with high speed and energy efficiency (optical operations / electronic operations ~ $${10}^{3}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mn>3</mml:mn> </mml:msup> </mml:math> ), neither necessitating reference wave nor repetitive steps. Importantly, our processor is endowed with salient generalization ability and robustness against diverse structured light and adverse effects (mean squared error ~ $$10^{(-5)}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>(</mml:mo> <mml:mo>-</mml:mo> <mml:mn>5</mml:mn> <mml:mo>)</mml:mo> </mml:mrow> </mml:msup> </mml:math> ). We further raise a universal model interpretation paradigm to reveal the underlying physical mechanisms in the hybrid processor, as distinct from conventional ‘black-box’ networks. Such interpretation algorithm can improve the detection efficiency up to 25-fold. We also complete the theory of optoelectronic network enabling its efficient training. This work not only contributes to the explorations on OAM physics and applications, and also broadly inspires the advanced links between intelligent computing and physical effects.

/pdf/intelligent-optoelectronic-processor-for-orbital-angular-3gc3aj4q.pdf

Intelligent optoelectronic processor for orbital angular momentum spectrum measurement

Utilization of coal gangue power generation industry by-product CFA in cement: Workability, rheological behavior and microstructure of blended cement paste

Yb:CaGdAlO4, or Yb:CALGO, a new laser crystal, has been attracting increasing attention recently in a myriad of laser technologies. This crystal features salient thermal, spectroscopic and mechanical properties, which enable highly efficient and safe generation of continuous-wave radiations and ultrafast pulses with ever short durations. More specifically, its remarkable thermal-optic property and its high conversion efficiency allow high-power operation. Its high nonlinear coefficient facilitates study of optimized mode locking lasers. Besides, its ultrabroad and flat-top emission band benefits the generation of complex structured light with outstanding tunability. In this paper, we review the recent advances in the study of Yb:CALGO, covering its physical properties as well as its growing applications in various fields and prospect for future development.

Advances of Yb:CALGO Laser Crystals

We propose and generate a class of structured light fulfilling the mathematical form of a SU(2) coherent state based on a set of circular Airy vortex modes. Such wave packets possess strong focus with both radial and angular self-accelerations, which exploit more general 3D inhomogeneous velocity control with global spatial symmetry of multilayer rotation akin to galactic kinematics, termed galaxy waves. Galaxy waves are endowed with higher degrees of freedom to control strong focusing and acceleration, which opens a direction of multi-dimensional accelerating of 3D structured light field, promising numerous applications in optical trapping, manufacturing, and nonlinear optics.

https://aip.scitation.org/doi/pdf/10.1063/5.0104188

Airy coherent vortices: 3D multilayer self-accelerating structured light

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