Unilateral-shift-subtracting confocal microscopy with nanoscale axial focusing precision

A novel unilateral-shift-subtracting confocal microscopy (USSCM) method with nanoscale axial focusing precision is proposed based on the optical arrangement of conventional confocal microscopy (CM). As the two segments of data on both sides of the confocal axial response curve are very sensitive to variations of the axial position, USSCM introduces an axial shift of S for one segment, to intersect it with the other segment. It then separately interpolates the two segments of intersecting data, subtracts the corresponding interpolated data, and selects the data that exhibit a good linearity from all of the subtracted data to fit a straight line. It calculates the zero position of the fitting line and offsets it by S/2, to precisely reveal the focus position of the confocal system, thereby achieving high-precision imaging of the three-dimensional sample's structure. Theoretical analyses and preliminary experiments indicate that, for excitation wavelength of λ=405  nm, numerical aperture of NA=0.95, and normalized axial shift of S=5.21, USSCM achieves an axial resolution of 3 nm and a repetitive focusing precision of 1.5 nm, while it does not change the lateral resolution of CM. Furthermore, compared with conventional CM, under the same noise condition, USSCM is less affected by system aberration, which leads to higher focusing precision. These findings demonstrate that USSCM is a very efficient method for imaging.