Structured light with inhomogeneous phase, amplitude, and polarization spatial distributions that represent an infinite-dimensional space of eigenstates for light as the ideal carrier can provide a structured combination of photonic spin and orbital angular momentum (OAM). Photonic spin angular momentum (SAM) interactions with matter have long been studied, whereas the photonic OAM has only recently been discovered, receiving attention in the past three decades. Although controlling polarization (i.e., SAM) alone can provide useful information about the media with which the light interacts, light fields carrying both OAM and SAM may provide additional information, permitting new sensing mechanisms and light–matter interactions. We summarize recent developments in controlling photonic angular momentum (AM) using complex structured optical fields. Arbitrarily oriented photonic SAM and OAM states may be generated through careful engineering of the spatial and temporal structures of optical fields. Moreover, we discuss potential applications of specifically engineered photonic AM states in optical tweezers, directional coupling, and optical information transmission and processing.
Light is a viable information carrier employing the various forms of light–matter interactions for numerous applications in data transmission, optical communications, photonics, and optoelectronics. In particular, optical measurement techniques, including interferometry, spectroscopy, and ellipsometry, determine and characterize the optical properties of materials or structures and are widely used in several fields of physics, materials science, microelectronics, biology, etc. Traditionally, light fields with simple spatial distribution such as plane waves and fundamental Gaussian beams have been employed in these applications. In recent years, structured light1 has become an emerging approach in modulating and tailoring the three-dimensional (3D) distributions of a light beam in its multiple degrees of freedom (i.e., amplitude, phase, polarization ratio, and ellipticity) with high spatial diversity. The spatial diversity present within the cross section of structured light offers much higher degrees of freedom in optical system design, which in turn enable high-capacity information transmission or the specific shaping of intensity and phase distributions to achieve the desired optimized light–matter interactions, such as super-resolution imaging, optical tweezing, and optical nanofabrication. In this review, we focus on how structured light can be utilized to generate, tailor, structure, and modify the properties of light angular momentum (AM), sometimes in rather unexpected ways.