Generally Applicable Holographic Torque Measurement for Optically Trapped Particles.

We present a method to measure the optical torque applied to particles of arbitrary shape such as micrometer-sized micro-organisms or cells held in an optical trap, inferred from the change of angular momentum of light induced by the particle. All torque components can be determined from a single interference pattern recorded by a camera in the back focal plane of a high-NA condenser lens provided that most of the scattered light is collected. We derive explicit expressions mapping the measured complex field in this plane to the torque components. The required phase is retrieved by an iterative algorithm, using the known position of the optical traps as constraints. The torque pertaining to individual particles is accessible, as well as separate spin or orbital parts of the total torque.

Spectral image scanning microscopy.

For decades, the confocal microscope has represented one of the dominant imaging systems in biomedical imaging at sub-cellular lengthscales. Recently, however, it has increasingly been replaced by a related, but more powerful successor technique termed image scanning microscopy (ISM). In this article, we present ISM capable of measuring spectroscopic information such as that contained in fluorescence or Raman images. Compared to established confocal spectroscopic imaging systems, our implementation offers similar spectral resolution, but higher spatial resolution and detection efficiency. Color sensitivity is achieved by a grating placed in the detection path in conjunction with a camera collecting both spatial and spectral information. The multidimensional data is processed using multi-view maximum likelihood image reconstruction. Our findings are supported by numerical simulations and experiments on micro beads and double-stained HeLa cells.