Integrated 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
Shaped Coherent Anti-Stokes Raman Scattering
Coherent anti-Stokes Raman scattering (CARS) has been successfully used in spectroscopy and microscopy since the development of (tunable) pulsed laser sources. In CARS, molecular vibrations are excited coherently by the pump (ωp) and Stokes (ωs) pulses. Subsequently a probe (ωpr) pulse, which is often derived from the same pulse as the pump, generates the anti-Stokes signal (ωc = ωp - ωs + ωpr). The energy diagram for a narrow band resonant CARS process is given in figure 1.
Figure 1: Narrowband Coherent anti-Stokes Raman Scattering (CARS)
In addition to the resonant CARS signal, there is a nonresonant four-wave mixing contribution, also known as the nonresonant background, which interferes with the detection of the resonant CARS signal.
We apply a broadband CARS method with a broad pump and probe and a narrow Stokes, shown in figure 2, which allows us to excite multiple vibrational coherences at once. With this method we investigate vibrational responses around 3000 cm-1. Using a high resolution phase shaper to scan a p-phase step through the broadband pump/probe spectrum we are able to obtain non-resonant background-free CARS spectra with a precision of 1 cm-1. The non-resonant background is removed by substracting the inverse phase-profile signal from the original scan, exploiting the time-reversal asymmetry of the resonant signal.
Figure 2: Broadband pump/probe CARS and the corresponding nonresonant background
Using spectral phase shaping we can also create molecule-specific pulses where the specificity is based on the interferences between multiple transitions. Using precise positioning of π-phase steps, selective imaging for selected compounds of interest can be easily obtained by altering the phase profile of the broadband pump/probe pulse, as shown in figure 3.
Figure 3: Chemically selective imaging of substances with overlapping resonances. The sample consists of 4μm diameter polystyrene and PMMA beads. Both images are obtained with the same broadband pump/probe pulse exciting all of the Raman resonances shown. (A) Selective excitation of the PMMA beads. (B) Selective imaging of the polystyrene beads.
Imaging with increased selectivity and rejected background will be the aim of further research. Applying more complex phase profiles, that exploit the full vibrational spectrum of the target molecules, will increase specificity and selectivity even more. By lowering the difference frequency between the pump and Stokes pulses the fingerprint (~100 1100 cm-1) region can be accessed.