Context:

Mechanical effects of light, known since Kepler’s explanation of comets tails, have brought to a real scientific revolution with the advent of the laser [1-4]. Optical tweezers [1,2] (OT),in their simplest configuration (Fig. 1a), are instruments based on laser beams focused by high numerical aperture (NA) lenses that are capable to trap, manipulate, and characterize a wide range of microscopic and nanoscopic particles, in liquids, air, and vacuum [3,4]. Key applications of this contactless manipulation technique have been developed in a wide range of fields: from biology, soft matter, and ultra-sensitive spectroscopy to atomic physics,nanoscience, photonics, spectroscopy, and aerosols science [2]. A key advancement has been the realization of Raman tweezers, i.e., the coupling of OT with a Raman spectrometer [5,6]. This allows the chemical and physical analysis of a trapped particle through its vibrational fingerprints.

Despite this tremendous experimental progress, the accurate light scattering modeling of OT, that takes into account particle size, shape, and composition, has been developed only recently [4,7,8]. Optical forces in OT can be easily understood in the limiting cases of particles much smaller or much larger than the laser wavelength where two components are identified [4]: a gradient force, proportional to the intensity gradient of the laser spot, responsible for trapping, and a scattering force, proportional to the light intensity that tends to push particles away from the trap destabilizing single-beam trapping of particles with large extinction. Such detrimental effects can be suppressed through the use of two counter-propagating beams to null the opposite scattering forces [9]. These dual-beam traps (Fig. 1b) are based on the use of low-NA lenses and allow the trapping of particles with reduced incident power in a focal region that is wider than for standard OT [10]. Thus they are well suited for operation in air or vacuum where detrimental effects by radiation pressure are enhanced by the reduced viscous damping [11,12].

Not withstanding the remarkable improvement in optical trapping techniques, their application to planetary exploration is still to be developed, even though already conceived by, e.g.,NASA [13]. The development of the optical trapping technique to collect and analyze in situ or return to Earth a variety of extraterrestrial particles will open doors to information on space materials that is currently unreachable without bias (e.g., the volatile component missing as for Rosetta/ESA dust instruments and/or contamination issues, as for Stardust/NASA). In space missions several techniques have been used to collect samples to be analyzed in situ, e.g., GIADA, MIDAS, COSIMA, the 3 dust instruments on board Rosetta/ESA space mission, or trapped them in aerogel to be returned to Earth, Stardust/NASA space probe [14,15]. Though these techniques have been very successful, they are limited because of samples bouncing/modification or particle contamination due to collecting media.

In the proposed research we aim at developing methods for using OT to trap and spectroscopically characterize (Raman Tweezers) extraterrestrial dust particles and/or their analogs. This study will provide solid ground for the application of OT techniques in the near future to solar system study, e.g., cometary particles analyses including the volatile component, dust particles in the Martian atmosphere and/or on the Martian, Lunar surfaces. Such application will also be strategic for the "clean" handling/preliminary characterization of restricted and unrestricted samples of planetary bodies returned from space missions in curatorial facilities.

Objectives and workpackages:

The main objective of our project is to exploit current OT techniques, that are being developed in our laboratories, to trap and spectroscopically characterize individual particulate matter of extraterrestrial nature and their analogs. To this aim, we will bring together complementary expertises and know-how from very different communities and research fields such as optics and photonics, astrophysics, and geology. The project work will be developed along the following workpackages (WP) (Fig 2).