We investigate how certain organic molecules can very efficiently mediate the interaction between different light waves. We use different techniques such as Degenerate Four-Wave Mixing or Z-scan at various wavelengths from the visible to the near and far infrared to determine how an organic molecule can interact with three photons at the same time to generate a fourth one when the energy of the photons is changed from a value much smaller than the first optically accessible excited state of the molecule, to a value that approaches half the distance to the excited state (this gives information on two-photon absorption cross sections) to even higher energies.

As an example, we determined the third order polarizability of the molecule shown on the right (Green=Carbon, Red=Nitrogen, Blue=Hydrogen). This molecule is synthesized in the Group of Prof. Diederich at ETH Zurich, Switzerland, and it has some exceptional capabilities to mediate light-light interaction.

The third order polarizability (sometimes called the “second hyperpolarizability”) measures the ability of a molecule to support the interaction of three photons to create a fourth one. It can be determined using degenerate four-wave mixing, so that all photons have the same frequency, and we did so for a photon wavelength of 1.5 micrometers, in the infrared. This measures the so-called zero-frequency limit of the third order polarizability where all photon energies are much smaller than the energy distance to the first excited state. Measurements in this limit are necessary in order to avoid the influence of real electronic excitations and to be able to compare different materials.

An amazing fact about these molecules is that their nonlinear response approaches the theoretical upper limit and is very strong compared to the size of the molecules. Such a theoretical upper limit has been calculated from first principles by Prof. Kuzyk at Washington State University.

The above figures give the structure of some more molecules of the same family (Donor-Substituted Cyanoethynylethenes) and the corresponding third order polarizability (blue circles). The red line in the plot corresponds to the upper limit for centrosymmetric molecules divided by 50. Note the strength of molecule 6. The other data points correspond to the largest molecular nonlinearities measured to date, but the molecules were they were observed were heavier and larger. See May et al, Optics Letters 30 (2005) for more details.

The molecules investigated above are extraordinary for many reasons:

• The polarizability of the best molecules, including the centrosymmetric dimer 6, as measured at a wavelength of 1.5 micrometers, is only a factor of 50 below the absolute theoretical limit, which corresponds to the case of an ideal two-state system.
• The conjugated backbone that is responsible for the nonlinearity via its delocalized electron system takes up almost the whole molecule.
• The specific third order polarizability of the best molecules of this family are much larger than for any other molecule investigated to date.

The specific third order polarizability is the third order polarizability divided by the molecular mass and it is roughly proportional to the third order susceptibility that we would expect in a dense solid-state assembly of molecules.

These results are of great interest towards expanding our design capabilities for more efficient molecules for third order nonlinear optics and data processing and towards the creation of efficient third-order nonlinear optical materials for integrated nonlinear optics.

Press releases can be found here or here.

References:

• J. C. May, J. H. Lim, I. Biaggio, N. N. P. Moonen, T. Michinobu, F. Diederich,Highly Efficient Third-Order Optical Nonlinearities in Donor-Substituted Cyanoethynylethene Molecules, Optics Letters 30, 3057 (2005).

See also:

• Degenerate Four-Wave Mixing.
• Researchers approach quantum limit in third-order nonlinear light-light interaction
• Whoosh! New Molecules Fastest Ever For Optical Technologies