Hyper-Rayleigh scattering (HRS) is used to measure the first-hyperpolarizability (β) of electro-optic (EO) chromophores. One of the inherent concerns in any HRS measurement is the extent to which resonant enhancement contributes to the observed intensity thereby leading to inaccuracies when evaluating chromophore potential for application in electro-optical devices. One way to address this concern is to employ increasingly longer excitation wavelengths far from resonance. However, in charge-transfer-based non-linear optical chromophores, enhanced β generally correlates with a red-shift of the charge transfer absorption band so that even at the longest excitation wavelengths generally employed in HRS studies, resonant enhancement remains an issue. We have adopted an alternative approach in which the wavelength dispersion of the HRS intensity is determined by performing measurements at a variety of excitation wavelengths. This approach allows one to ascertain the role of resonance enhancement thereby allowing for more accurate correlation of improved β with molecular architecture. We report the results of our HRS studies for nine chromophores employing excitation wavelengths ranging from 780 to 1907 nm. Our HRS results demonstrate good agreement with the predictions of density functional theory. This synthesis of experimental and theoretical techniques has resulted in more effective designs for the next generations of electro-optical chromophores.
In pursuit of greater understanding of structure property relationships in NLO chromophores, a series of molecules consisting of three aromatic rings was synthesized. The relative positions of benzene and thiophene rings in these molecules were varied. Theoretical calculations also suggest that the use of a slightly electron deficient heteroaromatic, such as thiazole, can increase β through the concept of an electronic gradient. The use of this heteroaromatic in the correct orientation can compensate for the energetic barrier that benzene presents during charge-transfer. Hyper-Raleigh Scattering (HRS) measurements on three of these “gradient bridge” type chromophores show that benzene located at the donor end provided the highest hyperpolarizability. The poor solubility of these three-ring systems severely limited their processability and gave considerable synthetic challenges. The difference in theoretical and experimental trends of β are discussed.
Utilizing guidance from quantum and statistical mechanics, the electro-optic coefficients of organic materials have been increased to values greater than 100 pm/V at telecommunication wavelengths (e.g., to 130 pm/V at 1.3 microns). Electro-optic materials now afford significant advantages in terms of bandwidth and electro-optic activity over inorganic materials such as lithium niobate. Moreover, organic materials have also been found to be quite processable permitting the fabrication, by reactive ion etching and photolithographic techniques, of 3-D active waveguide structures and integration with both VLSI semiconductor electronics and silica fiber optics. Stripline, cascaded prism, and microresonator structures have been fabricated, as have low-optical-loss coupling structures. A number of prototype devices demonstrating superior performance have been produced; however, the long-term, in-field performance of such devices still remains to be evaluated. Nevertheless, significant advances have been made in improving the thermal and photochemical stability of organic materials and in defining the mechanisms that define these stabilities (by testing under accelerated conditions). The role of nanoscale architecture in systematically improving stability of organic electro-optic materials, as well as contributing to enhanced electro-optic activity and reduced optical loss, has been clarified.