Abstract
Isotropic optical focusing - the focusing of light with axial confinement that matches its lateral confinement, is important for a broad range of applications. Conventionally, such focusing is achieved by overlapping the focused beams from a pair of opposite-facing microscope objective lenses. However the exacting requirements for the alignment of the objective lenses and the method's relative intolerance to sample turbidity have significantly limited its utility. In this paper, we present an optical phase conjugation (OPC)-assisted isotropic focusing method that can address both challenges. We exploit the time-reversal nature of OPC playback to naturally guarantee the overlap of the two focused beams even when the objective lenses are significantly misaligned (up to 140 microns transversely and 80 microns axially demonstrated). The scattering correction capability of OPC also enabled us to accomplish isotropic focusing through thick scattering samples (demonstrated with samples of ~7 scattering mean free paths). This method can potentially improve 4Pi microscopy and 3D microstructure patterning.
Highlights
Optical focusing is important for a broad range of applications, such as biological imaging [1,2,3], particle trapping [4], and optical data storage [5]
We report the application of optical phase conjugation (OPC) in an isotropic focusing scheme
Our implementation of OPC-assisted isotropic focusing involved the incorporation of a digital optical phase conjugation (DOPC) into the conventional isotropic focusing scheme
Summary
Optical focusing is important for a broad range of applications, such as biological imaging [1,2,3], particle trapping [4], and optical data storage [5]. 1) The two objective lenses need to be precisely aligned so that their focused spots overlap exactly, 2) optical aberrations (including sample scattering) in the intervening medium between the two objective lenses can disrupt the focus overlap or, worse, deteriorate either one or both of the focused beams so that the light beam(s) no longer come to the focus This second challenge is especially problematic and restricts 4Pi imaging application to thin sample sections (~50 microns). One research group recently showed that the time-reversal theory can be employed in non-adaptive way to achieve isotropic focusing with a single objective lens and a plane mirror [16] This modification does not mollify the optical aberration challenge. This approach may enable high-resolution laser-etching of 3D microstructure
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