Mapping complex optical light field distributions with single fluorescent molecules

Abstract

Single fluorescent molecules, behaving as ideal electric dipole emitters, are powerful nanoscopic probes of complex optical fields. Here, this property is exploited to precisely map the polarization and vectorial structure of tightly focused laser beams, utilizing both linear and circular polarization states. The resulting three-dimensional fluorescence excitation maps strikingly reveal the intrinsic chiral and non-chiral structure of the light field, in perfect quantitative agreement with a full vectorial wave-optical model. This precise correspondence not only enables the reliable determination of absolute molecular orientations but also allows for the accurate characterization of the field's properties. These results fundamentally advance our understanding of light-matter interaction at the single-molecule level and open new avenues for characterizing complex light fields, with broad applications in super-resolution microscopy and nanophotonics.

Figures (3)

• Figure 1: Schematic of the custom-built confocal laser scanning microscope used for single-molecule experiments. The microscope is optimized for imaging individual terrylene diimide (TDI) molecules embedded in a thin (30 nm) polystyrene (PS) film. In contrast to a standard confocal setup, the pinhole is removed to maximize photon collection efficiency. Polarization control is achieved by rotating the $\lambda/2$ and $\lambda/4$-plates, enabling left- and right-handed circular polarization as well as linear polarization with an adjustable orientation $\Psi$. On top is a sketch of the sample defining the corresponding parameters. A detailed description of the setup and sample preparation is provided in the Supplemental Material Sections A and B.
• Figure 2: Experimental and corresponding theoretical patterns for three TDI molecules with fixed orientations embedded in a thin PS film. For each molecule, images were recorded at different focal positions $z$ of the objective. Negative $z$-values correspond to moving the objective closer to the sample. The first molecule was measured using left-handed circular polarization, the second with right-handed circular polarization, and the third with linear polarization oriented at $\Psi=82^\circ$. All images are normalized to their respective maximum intensity. The exact parameters used for calculating the theoretical patterns are provided in the Supplemental Material Section D. The scale bar is $1~$µ m.
• Figure 3: Left: Theoretical patterns at the focal position $z=-0.35~$µ m for different out-of-plane angles $\beta$ and polarizations (from top to bottom: left-handed circular, right-handed circular, linear $\Psi =0^\circ$ and linear $\Psi=90^\circ$). Details of the parameters used for the calculations are provided in the Supplemental Information. Right: Experimental patterns of three different TDI molecules inside a PS film. They are measured at $z=-0.35~$µ m with left- and right-handed circular, and two linear polarizations (from top to bottom). The directions of the linear polarizations $\Psi$ were chosen such that $\Psi=\alpha$ for the second images from the bottom and $\Psi=\alpha+90^\circ$ for the bottom images. For comparison, the experimental patterns are rotated by their estimated angle $\alpha$ (see Supplemental Material, Table S2). The scale bar is $1~$µ m.