Strong Extinction of a Laser Beam by a Single Molecule

We present an experiment where a single molecule strongly affects the amplitude and phase of a laser field emerging from a subwavelength aperture. We achieve a visibility of $-6\%$ in direct and $+10\%$ in cross-polarized detection schemes. Our analysis shows that a close to full extinction should be possible using near-field excitation.

Figure 1

(a) Schematics of the experimental setup. The polarizer was inserted for the data in Fig. 3; for details see text. (b) The molecular energy level scheme. ${\nu }_{12}$ denotes the zero-phonon line frequency of the 0-0 vibrational transition. $k_{21}$ describes the decay rate of $|2⟩$ to $|1⟩$, which includes both the zero-phonon and the phonon-wing transitions. Manifold $|3⟩$ signifies the Stokes shifted vibrational levels of the electronic ground state. (c) An electron microscope image of a typical SNOM probe.

Figure 2

Symbols show simultaneously recorded near-field fluorescence excitation (a) and transmission (b) spectra. The solid red curves display fits. In the legend of the right vertical axis, cps stands for counts per second.

Figure 3

Tip emission and Stokes shifted molecular fluorescence for different orientations of polarizer in the detection.

Figure 4

(a) Frequency spectra recorded on ${\mathrm{PD}}_{21}$ for 30 polarizer angles spanning 180°. The visibility is color coded. (b), (c) Cross sections from (a) for $\theta =75°$ and $\theta =90°$ with fit functions according to Eq. (6).