Fluorescence Phase Microscopy (FPM) and Nanoscopy - Biomedical Optical Phase Microscopy and Nanoscopy - page 375

demonstrate their application to optical tomography with mesoscopic resolution along an
extended penetration depth (hundreds of micrometers), as well as their use in 3D
localization of single fluorescent quantum dots (QDs) with nanometer-scale precision.
Finally, we provide a summary and outlook.
18.2 The Fluorescence Self-Interference Process
Single fluorescent emitters such as fluorescent proteins and QDs are intrinsic quantum
sources. As a result, a single fluorescent photon can provide both a phase-modulated signal
and a local reference beam that are combined interferometrically to convert phase
information into intensity information. Specifically, an emitted fluorescent photon can
simultaneously travel two different optical paths, thereby generating two beams that are
subsequently combined to produce an interference pattern. The relative phase (or path-
length difference) between the two beams encodes the axial (depth) position of the emitter
and can be directly retrieved from the detected self-interference pattern (fringe).
Figure 18.1 shows a single-photon fluorescence self-referencing interferometer. Consider a
fluorescent point source placed between two opposing lenses such that the emitted
fluorescent light is collected from both sides of the source and then directed to a beam
splitter using mirrors where the clockwise and counterclockwise fluorescence fields are
Figure 18.1
The fluorescence self-interference process. Fluorescent light waves emitted from an excited
fluorophore located between two matched, opposing lenses are directed using mirrors to a beam
splitter where they recombine. An interference pattern appears when the fluorophore is close to
point II—near the zero differential path-length point (z 0 ) of the self-referencing interferometer. For
positions far from z 0 (points I and III) only the constant fluorescence intensity is recorded. Source:
This figure is reproduced from figure 2 of Ref. [40] with permission of John Wiley & Sons Ltd.