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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - VoidTrap (Advanced studies of trapping and rotation of nanoparticles in vacuum)

Teaser

Summary

The proposal originated as a project to explore optical fibres as a future experimental platform for studying quantum phenomena with mesoscopic particles. While quantum states of atoms and small molecules have been extensively studied, an important and yet unachieved goal of quantum physics is to observe quantum behaviour for large object consisting of millions of atoms. And yet, such observations could bridge the gap between classical and quantum worlds, which is a holy grail of modern quantum physics. As an example, building quantum computing systems requires maintaining many particles in well-defined quantum states simultaneously.

One potential path towards this goal is to use optical levitation of such mesoscopic object, which allows to cool it optically and bring it to quantum state, and to remove the interaction with the surroundings by putting it in vacuum. We initially proposed that specialized micro-structured optical fibres which can provide a very tight optical trap for efficient optical cooling, and also allow for simpler and cleaner experimental conditions and cleaner vacuum, which would allow a step towards reaching the quantum state.

In line with the initial proposal, the action resulted in a substantial advancement of the field of optical trapping with fibres, and major milestones have been achieved. Additionally, the technical expertise acquired by the fellow on wavefront control of light to deliver controlled fibre outputs, as a part of the project, has brought additional important results for the broader optical community. The MCSA fellow has developed strong independence and both deepened and broadened his expertise in light shaping, complex media photonics, optical trapping and computation microscopy.

This work has so far resulted in 2 peer-reviewed articles, 1 conference proceedings and has been disseminated for broad audience on multiple international conferences and seminars, including invited and plenary talks (UK, France, Japan, Australia, US). We systematically acknowledged the EU funding on every occasion, and we ensured open access to the publications.

Work performed

We started our work by exploring the capabilities of fibres to generate optical traps in liquid and in air and vacuum. Between two potential approaches for air â€“ optical trapping and gravity-assisted optical levitation â€“ we have initially chosen the optical levitation as a technique which is not as demanding as three dimensional trapping in terms of fibre numerical aperture parameter (NA).

To implement levitation, we developed and mastered the techniques of wavefront shaping through low-NA fibres. This work has provided us with necessary foundation (experimental setup, practical experience, software) for all subsequent fibre trapping work. Notably, as an intermediate goal, we have achieved 2D trapping in liquid which is a natural prerequisite for air/vacuum trapping.

The expertise in wavefront shaping we acquired had much broader application prospective for optical sciences, notably for imaging and endoscopy. We have seen this as an opportunity to create additional value and to bring bonus publications to the project. Notably, we have used wavefront correction to implement Raman spectroscopy imaging with a single fibre. Raman scattering â€“ a label free spectroscopic approach with intrinsic chemical specificity â€“ is particularly promising for biomedical studies as it does not require labelling. Having the motivation for miniaturization in mind (i.e. use for minimally invasive surgery), we have developed a fibre imaging Raman endoscope, and tested it for imaging of bacteria and pharmaceutics.

Separately, using the same technology we showed how the fibres can help imaging in highly corrosive environments. An important example of such corrosive media are those used for optical clearing â€“ a technique extensively used in neurosciences to image large 3D volumes (e.g. a whole mouse brain). In this respect, we used fibres to image in cleating solutions and also in harsh conditions (sulphuric acid immersion).

After mastering the wavefront control in fibres, we connected with new collaborators that provided us with custom fibres of high focusing capability (NA). With those fibres capable not only of levitation but also of more demanding 3D trapping, we continued along the initial goal and have recently achieved a major milestone â€“ true 3D particle trapping with a single fibre in liquid.

During this work however, we have faced a major practical impediment to monitor trapped particles, as they virtually become invisible when placed over the high-NA fibre. To solve this problem, we have developed a method to observe the trapped particle through the same fibre as the one with which it is trapped. This has remedied to the particle tracking problem, and also contributed to design of a compact trapping chamber as no extra objective for tracking would be needed.

Surprisingly, this particular method happened to be of significant impact for microendoscopy, as it allowed for fast fluorescent imaging without any complex wavefront shaping. This is particularly relevant for functional imaging of brain activity in neuroscience. We have performed pilot experiments and now collecting more data and preparing a publication.

Final results

First of all, we have achieved a major milestone towards fibre quantum experiments, in demonstrating a stable optical trap with just a single optical fibre. This paves the way towards trapping and cooling of the mesoscopic particles in vacuum. Additionally, we have demonstrated Raman spectroscopy through the trapping fibre, which creates a promising tool for studies of trapped matter in variety of settings, including vacuum.

Our publication on Raman imaging with multimode fibres pushed the state of art in several aspects. First, it was the first system to use Raman scattering with a single fibre for imaging. Not only this is new by itself, it also made our system the smallest Raman microscope in the world, with footprint of only 125 microns and ~50x50 pixels resolution. Additionally, we performed advanced data treatment algorithms, to obtain high fidelity Raman images and spectra in noisy conditions. Finally, our system featured variable magnifications and field of view larger than the footprint, which is not available in modern microendoscopes.

For the work on the use of multimode fibres for optical clearing, we demonstrated that cheap multimode fibres are a viable alternative to specialised and expensive microscope objectives for optically cleared tissues. As a wider impact to the society, our work provided a different view on the use of fibres and complex media in microscopy, broadening the current paradigm imaging.

We believe that our new results which are not yet published will push the state of the art significantly. Notably, we are developing a single-shot wide-field system for imaging luminescent particles through a fibre. On one hand, this advances the state of art for optical trapping, because it allows to monitor the trapped objects without the need of additional imaging system. On another hand, this enables fast fluorescence endoscopy without any wavefront shaping, which is also beyond of what is possible now with the state of the art.

With all the prerequisites now completed and prototype air/vacuum chambers in place, we expect to soon achieve 3D trapping in air and vacuum, which would be a major breakthrough and a milestone for quantum studies.