Abstract
Adaptive optics can focus light through opaque media by compensating the random phase delay acquired while crossing a scattering curtain. The technique is commonly exploited in many fields, including astrophysics, microscopy, biomedicine and biology. A turbid lens has the capability of producing foci with a resolution higher than conventional optics, however it has a fundamental limit: to obtain a sharp focus one has to introduce a strongly scattering medium in the optical path. Indeed a tight focusing needs strong scattering and, as a consequence, high resolution focusing is obtained only for weakly transmitting samples. Here we describe a novel method allowing to obtain highly concentrated optical spots even by introducing a minimum amount of scattering in the beam path with semi-transparent materials. By filtering the pseudo-ballistic components of the transmitted beam we are able to experimentally overcome the limits of the adaptive focus resolution, gathering light on a spot with a diameter which is one third of the original speckle correlation function.
Highlights
Structures and wavefront correction by OEi methods have been exploited to produce subwavelength foci[36,37]
The speckle pattern is collected by an objective lens (OBJ) and is projected behind it
At a distance of 150 mm from the rear face of the OBJ a lens L5 is placed to reproduce the speckle pattern at its focal length. On this plane the HP filter is aligned in order to block the central components of the speckle pattern: these components are related to modes which underwent a few scattering events
Summary
Structures and wavefront correction by OEi methods have been exploited to produce subwavelength foci[36,37]. To date and to the best of our knowledge, adaptive foci with a resolution under the limit of the speckle pattern correlation function have never been demonstrated. In semi-transparent media the larger speckle grains correspond to higher intensities, and the focusing process which exploits intensity as a feedback automatically selects configurations with larger grains to increase intensity and producing larger foci. We can produce a focus smaller than the speckle pattern correlation function, effectively overcoming the theoretical limit previously proposed by Vellekop and coworkers[14] for strongly scattering media. A spatial filter is employed for the selection of the appropriate light paths (modes)[40] and by exploiting a standard phase scan method[30] (see Methods) a stronger focusing with a smaller size speckle pattern is obtained. By exploiting our protocol we are able to obtain a focus size approximately 68% smaller than the average speckle grain
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