Hohlstrahl

Hohlstrahl, literally “hollow beam” in German, designates a beam whose intensity profile is annular rather than Gaussian, featuring a central void. The term is used across optics, particle and plasma physics, and jet or spray technologies to describe beams or jets with a ring-shaped cross section.

Generation methods vary by domain. In optics, hollow beams can be produced by transforming a Gaussian beam with phase masks or spiral phase plates that impart orbital angular momentum, by using axicons, or by exciting higher-order spatial modes such as Laguerre–Gauss modes with nonzero azimuthal index. Spatial light modulators and combinations of lensing elements can also yield donut-shaped focuses or ring-shaped intensity along the propagation axis. In charged-particle or plasma contexts, annular beams can arise from aperture shaping, magnetic lensing, or downstream phase-space manipulation that concentrates energy in a ring away from the axis.

Applications span several fields. In imaging and microscopy, annular illumination can reduce photodamage to central features and improve contrast for thin samples. In optical trapping, a ring trap can confine particles along a circular path. In laser micromachining and materials processing, hollow beams enable the creation of hollow or ring-like structures. In accelerator and plasma physics, hollow beams are explored to tailor beam–plasma interactions, reduce core density effects, or shape energy deposition.

Challenges include lower overall efficiency, sensitivity to optical aberrations, and precise control of ring radius and thickness. Maintaining a stable hollow profile over propagation and suppressing unwanted side lobes require careful mode management and system alignment. Related concepts include donut beams, optical vortices, Bessel beams, and annular apertures.