Researchers Structure Propagation-Invariant Light Fields Using Caustics

Researchers at the University of Münster, the University of Birmingham, and the University of Marseille demonstrated a way to shape propagation-invariant light beams arbitrarily by using a smart-beam design based on caustics. Their approach generalizes caustic light from a small subset to a complete set of tailored, propagation-invariant caustics with intensities that can be concentrated around any desired curve. Applications in material processing, optical trapping, and cell microscopy could benefit from this new approach to engineering light shapes of arbitrary intensity.

The researchers developed methods to tailor the light’s wavefront, forming invariant, customized caustics as the envelope for families of rays. The team developed two complementary methods and demonstrated various propagation-invariant beams, ranging from simple geometric shapes to complex image configurations such as words and letters. These self-healing beams propagated robustly in the presence of perturbations.

This photo shows the fabrication procedure of a nondiffracting of a light field using a desired transversed caustic. Courtesy of WWU/Alessandro Zannotti.

The researchers’ approach is bio-inspired, borrowing from light structures that can be seen in rainbows or when light is transmitted through glass. These ray structures, called caustics, are bright focus lines that overlap, creating networks that can be used for nondiffractive propagation. “We implement an approach inspired by nature, in which any desired intensity structure can be specified by its boundaries,” professor Cornelia Denz said.

Using caustics as a basis for generating arbitrary structures, the researchers have created a way to deliberately manipulate ray propagation. Their approach to shaping light beams could enable new types of laser beams to be formed at the nanoscale, allowing new perspectives in optical materials processing, multidimensional signal transmission, and high-resolution imaging.

Applications such as high-resolution microsopy and nanoscale material processing require customized laser beams that do not change during propagation. This has represented an immense challenge since light typically diffracts during propagation. Photonic science has been able to create concentric ring structures, like the Bessel beam, in a propagation-invariant way for a while, and it has theoretically predicted a whole class of beams whose transverse shape could be generated on elliptical or parabolic trajectories. Although there has long been a need for such customized light beams with these properties, the team said, few of these beams have been produced experimentally because the invariance of the transverse intensity structure must be maintained during propagation.

The research was published in Nature Communications (www.doi.org/10.1038/s41467-020-17439-3).