Optical tweezers were introduced by Askhin in 1986, when he proposed a way for capturing particles by means of focalized light, through the mechanism of optical trapping. Over the years, optical tweezers have assumed a prominent role in scientific research, covering the most disparate fields, from physics to biology and materials nano-engineering. For this reason, Askhin was awarded the Nobel Prize in 2018. At the same time, over the last decade multimode optical fibres have attracted a growing interest, thanks to the possibility of propagating high power laser beams, thus unveiling previously unforeseen nonlinear effects.
SPOTLIGHT aims at merging these two major branches of research, by proposing optical fibres as a new practical tool for generating microstructured light for 3D nonlinear optical trapping applications.
Our project is based on the exploitation of a recent discovery by the PI and his coworkers: the emission of a beam with a spiral shaped intensity profile in the far-field from the output facet of an optical fibre. Such a spiral emission is only obtained for proper laser-fibre coupling conditions, so that the beam propagating inside the fibre carries unconventional longitudinal orbital angular momentum (OAM). Although spiral emission is a linear phenomenon, i.e., it can be achieved with low power light sources, it is subject to nonlinear effects when high intensity lasers are used. Specifically, the interplay of radial self-phase modulation, Imbert-Fedorov shift and supercontinuum generation leads to spatial self-organization in the optical frequency domain, or rainbow-like spiral emission.
The main goal of SPOTLIGHT is developing a 3D optical trapping device, which exploits rainbow spiral emission. This will lead to breakthrough advantages in optical trapping, leading to a significant improvement of the optical tweezers technology, such as for example a new class of micromixers, which will find application in medicine and nano-bioengineering.
SPOTLIGHT is based on the theoretical and experimental demonstration of SE for OT. Our project aims at providing a leap forward in several fields, ranging from optics and photonics to engineering and biology. We shall exploit, for the first time, helical beam propagation in annular structures, e.g., the cladding of OFs, for developing a new class of SE-based optical devices. Indeed, SE is a promising candidate for the development of a new generation of optical tweezers, which mainly find an application in the treatment of biological samples.
At first, we shall develop theoretical models for the description of rainbow SE generation. In this sense, we would like to underline that a theory of nonlinear propagation in the cladding of multimode fibres is still lacking in the literature. Our studies will also provide the mathematical basis for answering several open questions related to beam propagation in annular structures, such as the generation of Bessel beams in fibre Bragg gratings. Unfolding these studies will lead to additional fascinating questions, which will further move forward the knowledge of the scientific community. For example, studies about spiral shaped nanoantennas have been recently carried out. However, a full mathematical description of the interaction of light with such shaped structures has not yet been developed.
A novelty introduced by the project is the use of unconventional OAM beams. These have only recently been introduced in the literature [1], and for this reason, they represent a frontier research topic. In this sense, the project success will be the first achievement of OT with beams carrying unconventional OAM. This will lead to the development of micromixers, that will allow for a full three-dimensional control of the rotational properties of trapped particles.
The potential and feasibility of using SE in the field of OT are undoubtful. However, there are still many fundamental questions that remain open. As a matter of fact, novel kinds of structured light, such as light beams carrying uOAM, have only recently been discovered by the PI and coworkers [1]. Therefore, the exploitation of the potential of such a discovery has only scratched the surface. In order to get a full understanding of the physical mechanisms behind the rainbow SE, in-depth theoretical and numerical studies of the phenomenon are necessary.
Indeed, we dedicated a specific task of the project to the development of a theory for spiral beam-based OT. It is worth noting that, although several works on OT with OAM and Gaussian beams in the Mie regime have been proposed, nowadays a full theory is still missing.
As mentioned above, the scope of SPOTLIGHT is the demonstration of optical tweezers for biological and biomedical applications. For this reason, we will not limit our studies to the cases of dielectric particles, as it has often been done in the literature. Conversely, we will explore the optical properties of biological specimens in the broad rainbow supercontinuum spectrum, that ranges from the UV to the NIR.
The main innovation brought by SPOTLIGHT will be the improvement of optical tweezer devices. To do so, we will combine the advantages introduced by light confinement to OT with rainbow SE, that exploits the nonlinear properties of OFs and LC waveguides. Using the spiral shape and the color separation will permit us to demonstrate a new class of micromixer devices, which will provide a new approach for the development of three-dimensional optical tweezers.
Within this project, we will provide the first theoretical, numerical and experimental demonstration of OT with rainbow polychromatic spiral beams. These beams possess several advantages with respect to what is in use with state-of-the-art technology. Specifically, SE in the linear regime will allow for implementing OT, without encountering the cumbersomeness and cost issues of current setups, that use costly and bulky devices, such as SLM or phase-plates.
On the other hand, rainbow SE in the nonlinear regime is a brand new field of research, and we envisage that it will lead to previously unforeseen advances in both fundamental and applied sciences.