Coherent state preparation and observation of Rabi oscillations in a single molecule

We report on the excitation of single molecules via narrow zero-phonon transitions using short laser pulses. By monitoring the Stokes-shifted fluorescence, we studied the excited state population as a function of the delay time, laser intensity, and frequency detuning. A $\pi$-pulse excitation was demonstrated with merely 500 photons, and 5 Rabi cycles were achieved at higher excitation powers. Our findings are in good agreement with theoretical calculations and provide a first step toward coherent manipulation of the electronic states of single molecules with few photons.

Figure 1

(Color online) (a) The arrangement of two cascaded acousto-optical modulators to obtain short laser pulses with a high signal to background ratio. (b) The optical setup inside the cryostat with a solid immersion lens (SIL). (c) The level scheme of a single molecule. (d) Typical examples of CW fluorescence and transmission spectra given by black and red curves, respectively. (e) Pulse intensity after the AOMs (black curve) and phase (red curve) of the complex electric field function used for the simulation calculations in Figs. 3c, 3d.

Figure 2

(Color) Raw Stokes-shifted fluorescence of a single molecule (red curve) and a theoretical fit (black curve). (a) As a function of delay with respect to the optical pulse. The blue dashed line displays the measured excitation pulse intensity. (b) As a function of the square root of the excitation pulse power using an optical pulse of $4\mathrm{ns}$ length. The data correspond to the vertical cut at $\Delta =70\mathrm{MHz}$ in Fig. 3a.

Figure 3

(Color) (a) Time-averaged raw fluorescence signal of a single molecule under a $4\mathrm{ns}$ pulsed excitation as a function of the excitation laser frequency (horizontal axis) and electric field (vertical axis). The vertical dashed line indicates the cross section shown in Fig. 2b. (c) Theoretical simulation of the data in (a). (b) and (d) are spectral profiles taken at field strengths marked by the horizontal dashed lines in panels (a) and (c), respectively.