Progress in neuroscience relies on new techniques for investigating the complex dynamics of neuronal networks. An ongoing challenge is to achieve minimally invasive and high-resolution observations of neuronal activity in vivo inside deep brain areas. Recently introduced methods for holographic control of light propagation in complex media enable the use of a hair-thin multimode optical fibre as an ultranarrow imaging tool. Compared to endoscopes based on graded-index lenses or fibre bundles, this new approach offers a footprint reduction exceeding an order of magnitude, combined with a significant enhancement in resolution. We designed a compact and high-speed system for fluorescent imaging at the tip of a fibre, achieving a resolution of 1.18 ± 0.04 µm across a 50-µm field of view, yielding 7-kilopixel images at a rate of 3.5 frames/s. Furthermore, we demonstrate in vivo observations of cell bodies and processes of inhibitory neurons within deep layers of the visual cortex and hippocampus of anaesthetised mice. This study paves the way for modern microscopy to be applied deep inside tissues of living animal models while exerting a minimal impact on their structural and functional properties.

Holographic optical tweezers (HOT) holds great promise for many applications in modern biophotonics, allowing the creation and measurement of minuscule forces on biomolecules, molecular motors and cells. Optical geometries used in HOT currently make use of bulk optics, and their usage in-vivo is compromised by the optically turbid nature of living tissues-a limiting factor in any advanced high-resolution imaging method. We present an alternative HOT approach in which multiple three-dimensional optical traps are introduced through a high-numerical-aperture multimode optical fibre, thus enabling an equally versatile means of optical manipulation through channels having cross-section comparable to the size of a single cell. Our work demonstrates real-time manipulation of 3-D arrangements of micro-objects, as well as the possibility of manipulating inside otherwise inaccessible cavities. We show that the position of the optical traps can be controlled with nanometric resolution over fibre lengths exceeding 100 mm. The results provide the basis for exploitation of holographic manipulation and other high-numerical-aperture techniques, including advanced forms of microscopy, through single-core-fibre endoscopes deep inside living tissues and other complex environments.

Abstract:The dynamic spatial control of light fields is essential to a range of applications, from microscopy to optical micro-manipulation and communications. Here we describe the use of a single digital micro-mirror device (DMD) to generate and rapidly switch vector beams with spatially controllable intensity, phase and polarisation. We demonstrate local spatial control over linear, elliptical and circular polarisation, allowing the generation of radially and azimuthally polarised beams and Poincaré beams. All of these can be switched at rates of up to 4kHz (limited only by our DMD model), a rate ∼2 orders of magnitude faster than the switching speeds of typical phase-only spatial light modulators. The polarisation state of the generated beams is characterised with spatially resolved Stokes measurements. We also describe detail of technical considerations when using a DMD, and quantify the mode capacity and efficiency of the beam generation. The high-speed switching capabilities of this method will be particularly useful for the control of light propagation through complex media such as multimode fibers, where rapid spatial modulation of intensity, phase and polarisation is required. Alfano et al., "4 × 20 gbit/s mode division multiplexing over free space using vector modes and a q-plate mode (de) multiplexer," Opt. Lett. 40, 1980Lett. 40, -1983Lett. 40, (2015. 8. S. Popoff, G. Lerosey, R. Carminati, M. Fink, A. Boccara, and S. Gigan, "Measuring the transmission matrix in optics: an approach to the study and control of light propagation in disordered media," Phys. Rev. Lett. 104, 100601 (2010). 9. S. Bianchi and R. Di Leonardo, "A multi-mode fiber probe for holographic micromanipulation and microscopy," Lab Chip 12, 635-639 (2012). 10. T.Čižmár and K. Dholakia, "Exploiting multimode waveguides for pure fibre-based imaging," Nat. Commun. 3, 1027Commun. 3, (2012. 11. J. Carpenter, B. J. Eggleton, and J. Schröder, "110x110 optical mode transfer matrix inversion," Opt. Express 22, 96-101 (2014). 12. C. Maurer, A. Jesacher, S. Bernet, and M. Ritsch-Marte, "What spatial light modulators can do for optical microscopy,"Laser Photon. Rev. 5, 81-101 (2011 728-735 (2015). 33. W.-H. Lee, "Binary computer-generated holograms," Appl. Opt. 18, 3661-3669 (1979) 34. J. Courtial, "Self-imaging beams and the guoy effect," Opt. Commun. 151, 1-4 (1998). 35. R. Dorn, S. Quabis, and G. Leuchs, "Sharper focus for a radially polarized light beam," Phys. Rev. Lett. 91, 233901 (2003). 36. L. Allen, M. W. Beijersbergen, R. Spreeuw, and J. Woerdman, "Orbital angular momentum of light and the transformation of laguerre-gaussian laser modes," Phys. Rev. A 45, 8185 (1992). 37. E. Galvez, P. Crawford, H. Sztul, M. Pysher, P. Haglin, and R. Williams, "Geometric phase associated with mode transformations of optical beams bearing orbital angular momentum," Phys. Rev. Lett. 90, 203901 (2003). 38. J. Dyment, "Hermite-gaussian mode patterns in GaAs junction lasers," Appl. Phys. Lett. 10, 84-86 (1967). 39. T.Čižmár, M. Mazilu, and K. Dh...