Optical Sciences

Biomolecules and nanostructures

The Optical Sciences group studies the interaction of light and matter at the nanoscale. We do this by exploring ways to shape light and its environment. It's what we call active and passive control. Our current focus is on the interaction of light with biomolecules and nanostructures. We are part of Twente University's Department of Science and Technology and member of the MESA+ institute.
We participate in the EU-COST actions MP1102: Coherent Raman microscopy (MicroCor) and CM1202: Supramolecular photocatalytic water splitting (PERSPECT-H2O)


Holographic design

  • Herman Offerhaus - Scientific Staff
  • Martijn de Kat - Former member

  • Everything that oscillates has amplitude and phase. Controlling the phase means controlling the oscillation. Even structures that might not seem to oscillate, such as patterns on crystals and masks, often contain periodicities and those periodicities are defined by a phase. This research proposal focuses on the manipulation of phase to control a variety of optical situations, from multiple traps that contain interacting particles to laser pulses with special structures in time. Phase plates with well-defined phase structures on them can be used to shape fields, such as laser beams or laser pulses. Examples of phase plates include holograms for image creation, spectral shapers for pulse creation and periodic poling for phase matching.
    A fundamental difference between a phase plate and a photographic (amplitude) plate for image creation is that the phase plate redirects the energy to the desired areas whereas the photographic plate only rejects energy that is in the wrong place. The efficiency of the phase plate is therefore much better, as is the ability to create homogeneous images.

    holographic beamsplitter
    The advantage of holographic beamsplitters

    Holography and other light-shaping techniques can generally be modeled using Fourier-type calculations, where the resulting field is related to the Fourier transform of the plate. However, phase plates are not trivial. The resulting field is directly related to the Fourier transform of the plate but since only the phase of the light is modulated, the outcome is the Fourier transform of a field in which the modulation is in the (imaginary part of the) exponent. Minimizing the mean square root error of the intensities leads to a problem that may well have no exact solution, which leads to many possibilities for approximate solutions which up to now have not been identified by systematic direct calculation.
    Perhaps the easiest phase structure to understand is the binary phase plate or binary hologram. Such a hologram consists of a plate with a surface relief pattern on it such that an image is produced in the far field when the plate is inserted into a laser beam. An example is a hologram that splits the beam into equal portions that will be focused into separate spots by a lens so that way multiple holes can be drilled simultaneously. The image is related to the pattern in such a way that spots in the image correspond to periodicities in the pattern for the phase plate. If the beam has to be split into separate parts in the conventional way (using beam splitters and mirrors) then the generation of more than a few spots will require many components, extensive alignment and induce extensive losses. A single phase-plate can split into any pattern of beams.

    Wafer with holographic 4x4 beamsplitter
    Wafer with holographic 4x4 beamsplitter: upper left: lithography mask, upper right: etched splitter, below: magnified pattern

    Phase is not just the connection between object and image. Phase manipulation can also be used to transform the spectrum of a laser pulse in such a way that the temporal shape of the pulse is converted to a specific pattern. The patterns in periodically poled materials ensure the correct conversion of the frequency of the light. Special spectral phase filters encode and extract signals code-division multiplexing (CDM) in telecommunications, a much more secure encoding than wavelength division or time division multiplexing.
    We propose to develop the algorithms for direct optimization of a phase plates in any of these situations. Besides that we will also design, manufacture and test these plates for three applications: hole drilling, pulse shaping and periodic poling.
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