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.


Resolving disorder by coherent single molecule excitation

  • Herman Offerhaus - Chair
  • Niek van Hulst - Former member

  • (FOM projectruimte 2004, N.F. van Hulst)
    Single molecules and nanoparticles are rapidly becoming key players in the assembly of nanodevices for molecular photonic and electronic applications. Unfortunately, so far only isolated prototypes are demonstrated, mostly operating at specific and restricted conditions.
    One of the main obstacles for successful operation is disorder, which limits the spatial extent of energy and charge transfer, especially at room temperature. Even in individual molecular assemblies, selected from the heterogeneous ensemble, dynamic and static disorder are present, in direct competition with energy transfer on the femtosecond and nanometer scale. Only by ultrafast observation the disorder can be overcome.
    This project will address individual molecules on the femtosecond scale. We will create fs-ps laser pulses with a dedicated spectral and time structure, to control the molecular excitation/de-excitation path through its electronic and vibrational states, in direct competition with dephasing and incoherent decay paths. Specifically we will explore coherent excitation routes to disentangle dynamic and static disorder. Energy transfer in isolated molecular complexes will be studied on fs-ps timescale, where the transfer efficiency can be suitably optimised using phase control. Addressing single systems will prove essential to disentangle different dynamic conformations.
    The results of this study will have implications in the study and control of molecular photonic systems, molecular switches, complex (bio)molecules, and the generation of single photon sources.
    The picture shows an introductory experiment on vibrational relaxation by Erik van Dijk.
    (A) The ground state |1, v=0> is coupled to an electronically and vibrationally excited state (|2,v>) by saturation with a broad band laser pulse. In the absence of other states the balance between stimulated absorption and emission gives equal state probability for both states. However, the excited state |2,v> couples to other vibrational states |2,v'> with sub-picosecond redistribution time and the |2,v'> states will ultimately decay by spontaneous emission with nanosecond decay time. The states |2,v'> have reduced coupling constants for stimulated emission by the laser pulse to the ground state, and therefore a new equilibrium between level |1, v=0> and |2,v > will be reached. Employing two equal pulses with a controllable delay the stimulated photo cycle can be manipulated resulting in enhanced spontaneous decay. (B) A train of short pulses (280 fs, circularly polarized) is split by a 50/50 beam splitter (BS), and recombined after reflection on two corner mirrors (CM). One mirror is placed on a translation stage to provide a variable delay (Δt) between the two beams. The combined "double pulse" beam is coupled into a confocal microscope to excite single molecules (SM). The single molecular fluorescence is collected by a high NA objective (Obj) and passed via a dichroic beam splitter (DBS) onto a photon counting detector (Det). (C) The fluorescence probability for a single saturating pulse is 0.5 (solid blue line). In the delay interval from -2 to 2 ps the calculated response for two femtosecond pulses is plotted where the relaxation time from level |2,v> to |2,v'> is set to 90 fs (solid red line). The fluorescence probability increases from 0.5 at zero delay to 0.75 for longer delay, resulting in a dip in the fluorescence intensity for zero delay. For a realistic longer pulse duration (280 fs, green dashed line,) the dip becomes convoluted with the pulse width, less pronounced and the femtosecond decay time is retrieved by deconvolution.
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