Contrast enhancement by tailored phase matching

The CARS process requires energy as well as the momentum conservation to be fulfilled. The latter condition means that the propagation angles of all contributing beams (i.e. the pump, probe, Stokes and anti-Stokes beams) must take very specific values. A possible geometry - the extremely folded BOX CARS geometry - is shown in Fig. 1a; its experimental realization is depicted in Fig. 1b.

Fig. 1: Beam geometry in our wide field CARS microscope.

Because of the large volumes that are simultaneously excited, phase matching plays a major role in wide field CARS microscopy. Misaligning the pump beam propagation angle by only as much as a degree can lead to a signal drop of already 50%. This high sensitivity can be exploited to alter the signal contrast between particle species of different sizes or to highlight sub-micron sized particles against the undesired but usually dominant signal of the solvent that embeds them. Demonstrations of this effect are shown in figures 2 and 3:

Fig. 2: Left: comparison of the NA-dependence of the total CARS signal of PS beads with 2.8μm and 0.5μm diameter; right: the CARS images show beads of 1.4 and 2.8μm, taken at NA=1.27 and NA=1.24, the plots below show vertical cross-sections through the beads.

The graph on the left of Fig. 2 shows the CARS signal strength generated in polystyrene beads of different sizes in water, measured at different pump beam incidence angles (expressed in terms of NA). Setting the NA to specific values alters the signal ratio between the small and large beads.

Fig. 3: Imaging a polystyrene bead (500nm diameter) in agarose gel with fulfilled phase matching (left) and slightly detuned pump beam angles (right).

Fig. 3 contains two CARS images of a 500nm sized polystyrene bead embedded in a layer of agarose gel. When imaged at phase matching, the non-resonant signal of the agarose gel completely overwhelms that of the bead. Its presence is only revealed by the agarose signal that illuminates it from behind, thus providing a common wide field image of the bead. Slight pump angle detuning strongly suppresses the bulk signal, leaving the CARS signal of the bead behind.


A. Jesacher, C. Roider, S. Khan, G. Thalhammer, S. Bernet, and M. Ritsch-Marte, Contrast enhancement in widefield CARS microscopy by tailored phase matching using a spatial light modulator, Opt. Lett. 36, 2245 (2011).