Wavefront Shaping for Fast Focusing Light through Scattering Media Based on Parallel Wavefront Optimization and Superpixel Method

When light travels in biological tissues,it undergoes multiple scattering and forms speckles,which seriously restricts the penetration depth of optical imaging in biological tissues.With wavefront shaping method,by modulating the wavefront of incident light to compensate for the wavefront aberration,light focusing and scanning imaging through scattering media can be achieved.However,wavefront shaping must be accomplished within the speckle decorrelation time.Considering the short speckle decorrelation time of living tissues,the speed of wavefront shaping is rather essential.We propose a new iterative optimization wavefront shaping method to improve the speed of wavefront shaping in which the existing parallel optimization wavefront shaping method is improved and is combined with the superpixel method.Compared with the traditional multi-frequency parallel optimization method,the modulation rate of our method is doubled.Moreover,we combine the high frame rate amplitude modulator,i.e.,the digital micromirror device(DMD),with the superpixel method to replace the traditional phase modulator(i.e.,spatial light modulator),which further increases the optimization speed.In our experiment,when the number of the optical modes is 400,light focusing is achieved with only 1000 DMD superpixel masks and the enhancement factor reaches 223.Our approach provides a new path for fast light focusing through wavefront shaping.

Identification of surface nanobubbles and resolving their size-dependent stiffness

We report a comparative investigation of the topographic features and nanomechanical responses of surface nanobubbles,polymeric nanodrops,and solid microparticles submerged in water and probed by atomic force microscopy in different operating modes.We show that these microscopic objects exhibit similar topographies,either hemispherical or hemiellipsoidal,in the standard tapping mode,and thus are difficult to distinguish.However,distinct differences,caused not only by their different mechanical properties but also by different cantilever tip-sample mechanical interactions that are affected by tip wettability,were observed in successive topographic imaging with controlled scanning forces and the nanoindentation tests,allowing for the identification of surface nanobubbles.Based on the indentation force-distance curves,we further extrapolated the stiffness of surface nanobubbles spanning a wide range of sizes and then developed a simple theoretical model to explain this size dependence.We also demonstrate how size-dependent stiffness can be used to determine the surface tension of nanobubbles,which was found to be much lower than the bulk value of water.