Optical Sciences

Biomolecules and nanostructures

The Optical Sciences group studies the interaction of light and matter. Our current focus is on detection and sensing/imaging with an emphasis on the development of integrated photonics. We are part of Twente University's Department of Science and Technology and member of the MESA+ institute.


Single Molecule Pump Probe (SM2P)

  • Erik van Dijk - Former member

  • Energy transfer and intra-molecular energy redistribution in molecular systems occur on a femtosecond time scale. Until now, this time scale has been the exclusive domain of ensemble pump-probe techniques. Which always yields the ensemble average over a large number of molecules. In the last decade single molecule detection has shown that chemically identical molecules can behave remarkable differently on all studied time scales. Unfortunately so far it has not been possible to study femtosecond processes with single molecule sensitivity. This is caused by the fact that detection of individual molecules traditionally relies on the detection of the background free fluorescence, which only occurs on a nanosecond time scale. We have developed a new method that, still based on fluorescence detection, is able to circumvent the normal time limitation.
    We employ two intense saturating femtosecond pulses with variable delay to study the initial redistribution of the excitation energy over the different vibrational modes within the molecule. A change in the balance between absorption and stimulated emission results in an increase of the fluorescence when the second pulse arrives after the energy is redistributed over different modes (see fig. 1).

    Fig. 1. Fluorescence signal of a single molecule as a function of the delay between two exciting femtosecond pulses. A clear dip is visible at zero delay. The line shows a fit yielding a molecular energy redistribution time of 290 fs (taking into account the finite pulse width).
    We find that chemically identical molecules in the same sample can have redistribution times that vary up to a factor of three from the average value. These variations are attributed to conformational difference due to the nano-environment leading to variations in the coupling between the electronic excitation and the vibrational mode [1].
    When two identical chromophores are linked to form a new coupled quantum system, we find that the intra-molecular energy redistribution is reduced. When the coupling was disrupted the ultra fast redistribution times recovers. These results show for the first time directly the predicted reduction of the vibrational coupling in excitonic systems on the level of a single molecule [2].
    The new single-molecule pump-probe (SM2P) approach will proof powerful for the local investigation of ultrafast energy flow in biomolecules and molecular photonics.
    [1] E.M.H.P. van Dijk, J. Hernando, M.F. García-Parajó and N.F. van Hulst "Single molecule pump-probe (SM2P) experiments reveal variations in ultrafast energy redistribution" Submitted to J. Chem. Phys.
    [2] E.M.H.P. van Dijk, J. Hernando, J.J. García-López, M. Crego-Calama, D.N. Reinhoudt, L. Kuipers, M.F. García-Parajó and N.F. van Hulst "Single-molecule pump-probe detection resolves ultrafast pathways in individual and coupled quantum systems" Phys. Rev. Lett. in press


    The following articles have been published regarding this project:

    Single-molecule pump-probe experiments reveal variations in ultrafast energy redistribution

    (abstract) (full pdf)
    E.M.H.P. van Dijk, J. Hernando, M.F. García-Parajó, and N.F. van Hulst
    the journal of Chemical Physics
    vol. 123 issue 6 p064703 may 2 2005

    Single-molecule pump-probe detection resolves ultrafast pathways in individual and coupled quantum systems

    (abstract) (full pdf)
    Erik M.H.P. van Dijk, Jordi Hernando, Juan-José García-López, Mercedes Crego-Calama, David N. Reinhoudt, Laurens Kuipers, María F. García-Parajó Niek F. van Hulst
    Physical Review Letters
    vol 94 p078302-1 - p078302-4 25 feb 2005
    Printable version