Emerging all-optical methods provide unique possibilities for noninvasive studies of physiological processes at the cellular and subcellular scale. On the one hand, superresolution microscopy enables observation of living samples with nanometer resolution. On the other hand, light can be used to stimulate cells due to the advent of optogenetics and photolyzable neurotransmitters. To exploit the full potential of optical stimulation, light must be delivered to specific cells or even parts of cells such as dendritic spines. This can be achieved with computer generated holography (CGH), which shapes light to arbitrary patterns by phase-only modulation. We demonstrate here in detail how CGH can be incorporated into a stimulated emission depletion (STED) microscope for photostimulation of neurons and monitoring of nanoscale morphological changes. We implement an original optical system to allow simultaneous holographic photostimulation and superresolution STED imaging. We present how synapses can be clearly visualized in live cells using membrane stains either with lipophilic organic dyes or with fluorescent proteins. We demonstrate the capabilities of this microscope to precisely monitor morphological changes of dendritic spines after stimulation. These all-optical methods for cell stimulation and monitoring are expected to spread to various fields of biological research in neuroscience and beyond.
Aerosol tweezing with a super-continuum laser source has been successfully demonstrated. Salt-water droplets
in the range between 3 and 7 microns in diameters are trapped with a 300nm-wide super-continuum spectrum.
As the spectrum covers a few Mie resonances, the optical force is averaged and the trapping efficiency varies
smoothly with the square of the radius as in the case of the ray optics approximation. On-axis elastically back-scattered
spectrum allows a direct and precise determination of the trapped droplet. Evaporation of a single
droplet is precisely followed using this method. Alternative spectroscopic droplet sizing techniques are proposed
Light induced binding energy between particles in the Rayleigh range in laser fields varies as 1/r. This long range
dependence suggests a diverging increase of the binding force when the number of interacting particles becomes
large. Theoretical studies performed for 1D periodic chains of dipoles reveals large binding field enhancement for
an optimum number of interacting particles. When the number of dipoles overshoots this number, the binding
intensity collapses. This result is consistent with experimental observations of periodicity defects in large 2D
optically built crystals. Some solutions are explored to keep large binding field enhancement for large crystals.
Shrinking the coherence length and phase modulating the incident trapping light are among the proposed schemes.
Laser trapped mirrors may offer an interesting alternative for very large space telescopes. The use of sub-wavelength structures raises many problems concerning electromagnetism, solid state physics and out of equilibrium micro-scale thermodynamics. Optical trapping in low index media and damping in vaccum are among the main technical difficulties that must be addressed. New experimental techniques are explored. Theoretical issues are discussed, relating to Van der Waals cohesion energy and optical forces in the Mie and Rayleigh ranges.
Laboratory testing is undertaken for exploring the feasibility of "Laser Trapped Mirrors". These involve a soft membrane or two-dimensional array of nano-spheres having sub-micrometer thickness. It is constrained to an accurate optical figure by radiation pressure trapping in polychromatic standing waves formed by a pair of diverging and counter-propagating laser beams. The optical design and resulting properties are discussed as well as the mission aspects for several architectures of telescopes and hypertelescopes at scales from meters to hundreds of kilometers.
Corrugated feedhorns are commonly used with reflector antennas, either for emission or reception purposes, because of their very low side lobe beam patterns, their very good E-H plane symmetry and their important bandwidth. Unfortunately, the electroforming technique that is generally used to fabricate them requires the machining of single-use mandrel. Direct milling of the horn is also possible, either in a single block or in a split block, but this also requires to machine each desired horn. At submillimeter frequencies, machining of small corrugations in a mandrel or in a block is costly. We present in this paper a cost-effective technique to fabricate helicoidal corrugated feed horns, which consists in machining one reusable mandrel and to mold as many horns as needed.