Optical vortex

An optical vortex is characterized by a helical phase front described by exp(iℓφ), where φ is the azimuthal angle and ℓ is the topological charge. The phase singularity at the beam axis forces the intensity to zero, creating a dark core. The total angular momentum of light separates into spin (polarization) and orbital parts; in 1992, Allen et al. showed that a Laguerre-Gaussian beam carries OAM of ℓℏ per photon.¹

Common generation methods include spiral phase plates, fork gratings (computer-generated holograms), and q-plates. Spiral phase plates impose a helical delay, while fork gratings diffract light into vortex orders. Q-plates, made of liquid crystals, convert spin to orbital angular momentum. Each method offers trade-offs in efficiency, wavelength agility, and tunability.²

Optical vortices have revolutionized optical tweezers: the OAM can rotate microscopic particles, acting as an "optical spanner." In microscopy, stimulated emission depletion (STED) uses a vortex beam to quench fluorescence, achieving resolution beyond the diffraction limit. In communications, OAM multiplexing encodes data on multiple orthogonal modes, dramatically increasing bandwidth; experiments have reached terabit-per-second rates.³

Astronomy exploits vortex coronagraphs to block starlight and image exoplanets. Quantum information uses OAM states for high-dimensional encoding, enhancing security and information density. Additionally, vortex beams have been used in material processing for drilling high-aspect-ratio holes and in chiral discrimination of molecules.⁴

Beyond simple integer charges, fractional vortices (non-integer ℓ) exhibit radial phase discontinuities and are used in particle manipulation. Vector vortices have spatially varying polarization, leading to complex field structures. In 2016, researchers generated acoustic vortices that transfer OAM to matter, opening acoustofluidics.⁵

Historically, the concept of phase singularities dates to 1974, when Nye and Berry proposed them in wavefields. In 1996, the first vortex was generated in a Bose-Einstein condensate. Recent work has created vortices in electron beams and even in neutron beams, extending the concept beyond photons.⁶

Practical deployment faces challenges: atmospheric turbulence distorts vortex modes, limiting free-space OAM communication. Mode crosstalk and loss in fibers require careful design. Researchers are developing adaptive optics and special fibers to mitigate these issues.⁷

Future directions include integrated photonic circuits for OAM generation and detection, and exploiting OAM for quantum computing. The ability to manipulate OAM at the nanoscale could lead to ultra-compact devices. As the field matures, optical vortices may become as ubiquitous as polarization in photonics.⁸

Topological charge

Integer ℓ denoting the number of 2π phase twists per revolution.

Orbital angular momentum (OAM)

Angular momentum carried by the beam's spatial structure.

Phase singularity

Point where phase is undefined and intensity is zero.