Visualizing and controlling vibrational wave packets of single molecules
(17 June 2010)
24 November 2009
20 April 2010
The active steering of the pathways taken by chemical reactions and the optimization of energy conversion processes provide striking examples of the coherent control of quantum interference through the use of shaped laser pulses. Experimentally, coherence is usually established by synchronizing a subset of molecules in an ensemble 1 , 2 , 3 with ultra-short laser pulses 4 , 5 , 6 , 7 . But in complex systems where even chemically identical molecules exist with different conformations and in diverse environments, the synchronized subset will have an intrinsic inhomogeneity that limits the degree of coherent control that can be achieved. A natural—and, indeed, the ultimate—solution to overcoming intrinsic inhomogeneities is the investigation of the behaviour of one molecule at a time. The single-molecule approach 8 has provided useful insights into phenomena as diverse as biomolecular interactions 9 , 10 , cellular processes 11 , 12 , 13 and the dynamics of supercooled liquids 14 and conjugated polymers 15 . Coherent state preparation of single molecules has so far been restricted to cryogenic conditions 16 , whereas at room temperature only incoherent vibrational relaxation pathways have been probed 17 . Here we report the observation and manipulation of vibrational wave-packet interference in individual molecules at ambient conditions. We show that adapting the time and phase distribution of the optical excitation field to the dynamics of each molecule results in a high degree of control, and expect that the approach can be extended to achieve single-molecule coherent control in other complex inhomogeneous systems. 18
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Figures at a glance
Figure 1: Ultrafast coherent excitation of single molecules.
a, Spectra of the fluorophore (DNQDI ; dissolved in toluene) and broad-band excitation laser used (see Methods). 29 b, Single fluorescent molecules were imaged and investigated in an epi-confocal microscope. Each individual molecule was excited with tailored sequences of 15-fs (full-width at half-maximum) pulses defined by the inter-pulse time delay Δ t and phase shift Both φ. Δ t and were controlled with a double-pass 4f pulse shaper based on a spatial light modulator. φ c, The fluorescence intensity of the single molecules was recorded for different combinations of Δ t and (for example, ( φ Δ t, i ), ( φ i Δ t, k ), and so on), which were applied sequentially, separated by periods of no illumination and repeated until the molecule photobleached. φ k
Figure 2: Single-molecule wave-packet interference.
a, Fluorescence images of single molecules excited with two mutually delayed ( Δ t), phase-locked laser pulses. b, Integrated intensity as a function of Δ t for the fluorescence emission of the three molecules marked in a. A typical background trace is shown for reference. The traces are normalized to their respective average in order to visualize fluctuations in the intensity. Error bars, ±1 s.d. c, Averaged response of 52 molecules compared to the theoretical prediction based on the bulk absorption spectrum. d, Result of the Fourier analysis of 52 single-molecule traces. Distributions and scatter plot of the main frequency component ( f) and its corresponding phase ( ). Marker size and bin width include the experimental errors. θ
Figure 3: Phase control of single-molecule wave packets.
Single-molecule fluorescence intensity as a function of the time delay between two in-phase (
= 0) and in-antiphase ( φ = φ π) excitation pulses. The excitation intensity was constant for all ( Δ t, ) combinations except for the (excluded) points near zero delay with φ = φ π. a, The fits to the ( = 0) and ( φ = φ π) traces are based on the bulk absorption spectrum. Fourier analysis of the fits shows that f = 31 THz in both traces and the difference in is θ π. b, Some molecules present fluctuations even for time delays Δ t as long as 120 fs and with more than one frequency component. Error bars, ±1 s.d.
Figure 4: Single-molecule time-phase coherent excitation maps.
a– d, Time-phase fluorescence excitation maps of four different molecules excited with a four-pulse sequence. Δ t and are the time delay and phase shift between each consecutive pulse in the sequence. The fluorescence intensity (colour scale) is normalized to the average. The maximum/minimum ratio, that is, the contrast achievable through control of coherent excitation, is shown in the upper right corner of each plot. φ