Extended Data Fig. 9: Knife-edge measurements.
In order to characterize the optical beam morphology, the microscope optical assembly is positioned to focus the 488 nm laser spot on a set of optical windows outside the cryostat that mimic the measurement conditions. The power transmitted through a photolithographically-patterned metal edge in the sample plane is measured as a function of transverse displacements in X axis a and Y axis b. This measurement is repeated for various axial displacements. The power versus transverse position is fitted to an error function. The beam diameter extracted by the error function fit is then examined as a function of axial displacement (inset). A fit to a hyperbolic profile allows for the extraction of the beam waist and Rayleigh range. The beam appears to display coma or spherical aberration, as well as astigmatism. These aberrations are likely the result of errors in alignment through the cryostat window and sample coverslip.
