Understanding the exact shape of light has always been a complex challenge, ever since Newton imagined that sunlight was composed of particles. Today, we know that light is an electromagnetic wave, and it is easier to give it a shape: we commonly come into contact with light objects of a defined shape, such as holograms or laser beams, which can be considered light rays.

But it is also possible to change the shape of a laser beam by acting on the electromagnetic radiation's wavefront (i.e. at points where the wave's phase is constant) to obtain so-called structured light, which can take on the most varied shapes (or structures).

Of the infinite number of possible structures that can be given to light, spiral structures are particularly well studied. Due to their unique characteristics, spiral laser beams are used in frontier fields such as biophysics and quantum technologies.

A new study published in the journal Scientific Reports and coordinated by Stefan Wabnitz of the Department of Information Engineering, Electronics and Telecommunications at Sapienza University of Rome, proposes a method for creating spiral-shaped beams from small segments of optical fibre: by exploiting the cylindrical geometry of the optical fibre to guide the light along a helical path, the wavefront needed to emit a spiral beam ends up being produced spontaneously. This is an inexpensive and simple method that does not require any nanofabrication: all that is needed are a few easily available items such as a laser, a converging lens and a few centimetres of standard optical fibre.

The work, developed in collaboration with the University of Brescia, the XLIM University Institute in Limoges and the Southern Methodist University in the USA, is part of the Horizon 2020 STEMS project funded by the European Research Council.

The realisation of structured light commonly requires dedicated optical systems: it is necessary to provide a laser beam with the correct wavefront each time to generate the desired structuring. This is done using ad hoc masks, which are still not very flexible. There are also more flexible methods using instruments based on liquid crystals. However, these technologies are costly and bulky.

"One of the key elements of our research is the linearity of the phenomenon," says Stefan Wabnitz, "which means that the generation of spiral beams using this method does not depend on the power of the laser used. Suffice to say, we were able to produce a spiral beam in the laboratory using an ordinary laser pointer, which can be bought in any electronics shop as a source," Wabnitz adds.

If, on the other hand, high-powered lasers are used to generate non-linear effects, it is possible to observe a very unusual phenomenon from a chromatic point of view: the spiral, originally of a single colour, acquires all shades, from red to violet.

"These colours spontaneously organise themselves to form a spiral-shaped rainbow," highlights Mario Ferraro, a Sapienza University of Rome researcher. "This peculiar multicoloured shape cannot be achieved using conventional methods and can be applied in a wide range of fields, from quantum optics to telecommunications."

References:

Rainbow Archimedean spiral emission from optical fibres – F. Mangini, M. Ferraro, M. Zitelli, V. Kalashnikov, A. Niang, T. Mansuryan, F. Frezza, A. Tonello, V. Couderc, A. B. Aceves & S. Wabnitz – Scientific Reports 2021. DOI: https://doi.org/10.1038/s41598-021-92313-w

Further Information

Stefan Wabnitz
Department of Information Engineering, Electronics and Telecommunications
stefan.wabnitz@uniroma1.it