Dynamic structured illumination for confocal microscopy

Structured illumination enhances the resolution of a standard microscope by encoding the high spatial frequencies of an object's image into lower spatial frequencies through the use of a carefully selected pattern. In essence, it modifies the optical transfer function (OTF), which is the Fourier Transform of the point spread function (PSF), to increase sensitivity to high spatial frequencies. In [G. Noetinger et al, Arxiv 2306.14631 (2023)], the authors introduce a novel technique that further leverages time by incorporating a temporal periodic modulation, specifically through the use of a rotating mask, to encode multiple transfer functions within the temporal domain. This methodology is exemplified using a confocal microscope setup. At each scanning position, a temporal periodic signal is captured, enabling the construction of multiple images of the same object. The image carried by each harmonic is a convolution of the object with a phase vortex of topological charge, similar to the outcome when using a vortex phase plate as an illumination. This enables the collection of chosen high spatial frequencies from the sample, thereby enhancing the spatial resolution of the confocal microscope.

Subwavelength light focusing through a scattering medium

[J. H. Park et al., Nat. Photon., (2013)]

After the first experiment of light focusing through a scattering medium using wavefront shaping (see A pioneer experiment), the same group demonstrated in [I. M. Vellekoop et al., Nat. Photon., 4, 320, (2010)] that a random medium can improve the sharpness of the focus. The scattering in a medium behind a lens randomizes the direction of the light. The speckle pattern shows high spatial frequencies not allowed by the lens alone because of its finite numerical aperture. After optimization of the input wavefront, the focus spot obtained is sharper than the resolution limit of the lens. In these experiments, the intensity profile was always measured in the far field, i.e. at least several wavelengths away from the surface, where only the propagating waves contribute to the optical field. In the present paper, J. H. Park and his colleagues optimize the input wavefront impinging on turbid media to increase the intensity measured in the near field at a given position. Subwavelength focusing is achieved thanks to the contributions of the evanescent waves.