Origins of the Kerr Effect in Organic Materials: Correct Experimental Assessment by Z-Scan Method
As the demand for higher bandwidth telecommunication systems grows, new technological approaches for information and communication technologies are necessary. As the existing electro-optical telecommunication system is reaching its limits great attention is given to the possibility to implement an all-optical system. For such a system to be realized, one of the most essential aspects is nonlinear optical (NLO) materials that can mediate the interaction of photons. At this moment the main bottleneck for all-optical telecommunication system implementation is lack of materials with necessary NLO properties – a high Kerr and Two-photon absorption efficiency.
This work presents an outline for a correct approach to Kerr effect studies. A nonlinear optical property characterization of a wide spectrum of novel organic materials that have not been previously studied is presented in this work as well. By employing the Z-scan method with polarization-resolved and laser pulse repetition rate dependent measurements the thermo-optical effect was separated from the Kerr effect. At the same time polarization-resolved measurements allow to parcel out electronic and molecular reorientation contribution to the Kerr effect. A comparison of experimentally obtained Kerr coefficient values to values calculated by Gaussian 09 with inbuilt density functional theory model shows up a significant underestimation of effect by Quantum Chemical modelling. The magnitude of this discrepancy grows in correlation with the two-photon absorption cross-section.
Thesis:
- Separation of Kerr and thermo-optical contributions to refractive index changes by the Z-scan method can be done by polarisation and pulse repetition rate dependent measurements.
- To correctly separate the electronic and the nuclear contributions to the Kerr coefficient for organic chromophores dissolved in solvents polarisation-resolved measurements must be used.
- When using Quantum Chemical calculations for predicting Kerr effect values for organic chromophores, molecular reorientation contribution can be calculated accurately from values of linear polarizability while a significant error for electronic contribution arises due to disregard of two-photon contribution.