Structured light refers to the generation and application of custom light fields. As the tools and technology to create and detect structured light have evolved, steadily the applications have begun to emerge. This roadmap touches on the key fields within structured light from the perspective of experts in those areas, providing insight into the current state and the challenges their respective fields face. Collectively the roadmap outlines the venerable nature of structured light research and the exciting prospects for the future that are yet to be realized.

Light–matter interaction has a long history in both physics and astronomy. About 400 years ago, Kepler observed the deflection of a comet’s tail away from the Sun, which may constitute the first reported conjecture of the radiation force.^{1 – 3} In the 1700s, John Michell attempted to measure radiation pressure,⁴ while Euler hypothesized that light beams induce pressure on illuminated bodies.⁵ In the 1800s, Maxwell predicted that light was an electromagnetic wave,⁶ which was confirmed by the first demonstration of a radiation force originating from thermal light sources by Lebedev^7,8 and Nichols and Hull⁹ in 1901. We now know that light beams can be considered as a large collection of photons, each carrying a quantized amount of momentum, which can be transferred to matter. However, as derived by Poynting in 1906, the radiation pressure is so minute that it only affects small bodies.¹⁰ Shortly afterward, Mie¹¹ and Debye¹² proposed exact physical models to calculate scattering force and radiation pressure of light in 1908 and 1909, respectively. At that stage, no one could imagine any practical value of radiation pressure with it being too weak to overcome frictional forces in most circumstances. For a considerable time, researchers focused their attention on the use of radiation pressure in space, e.g., solar sail propulsion systems,¹³ due to the absence of appreciable friction in space.

Vector vortex beams are structured states of light that are nonseparable in their polarisation and spatial mode, they are eigenmodes of free-space and many fiber systems, and have the capacity to be used as information carriers for both classical and quantum communication. Here, we outline recent progress in our understanding of these modes, from their creation to their characterization and detection. We then use these tools to study their propagation behavior in free-space and optical fiber and show that modal cross-talk results in a decay of vector states into separable scalar modes, with a concomitant loss of information. We present a comparison between probabilistic and deterministic detection schemes showing that the former, while ubiquitous, negates the very benefit of increased dimensionality in quantum communication while reducing signal in classical communication links. This work provides a useful introduction to the field as well as presenting new findings and perspectives to advance it further.

Mode division multiplexing (MDM) is mooted as a technology to address future bandwidth issues and has been successfully demonstrated in free space using spatial modes with orbital angular momentum (OAM). To further increase the data transmission rate, more degrees of freedom are required to form a densely packed mode space. Here we move beyond OAM and demonstrate multiplexing and demultiplexing using both the radial and azimuthal degrees of freedom. We achieve this with a holographic approach that allows over 100 modes to be encoded on a single hologram, across a wide wavelength range, in a wavelength independent manner. Our results offer a new tool that will prove useful in realizing higher bit rates for next generation optical networks.

Complex vector light fields, classically entangled in polarization and phase, have become ubiquitous in a wide variety of research fields. This has triggered the demonstration of a wide variety of generation techniques. Of particular relevance are those based on computer-controlled devices due to their great flexibility. In particular, spatial light modulators have demonstrated their high capabilities to generate any vector beam, with various spatial profiles and polarization distributions. Here, we put forward a novel technique that exploits the superposition principle in optics to enable the simultaneous generation of many vector beams using a single digital hologram. As proof-of-principle, we demonstrated the simultaneous generation of sixteen vector vortex beams with various polarization distributions and spatial shapes on a single SLM, each with their own spatial shape and polarization distribution.