Quadratic electroabsorption spectroscopy as a probe of χ(3) mechanisms in dye-doped polymers

Research aimed at addressing materials issues relevant to making doped polymer optical devices is well known.[1,2] Poled doped polymers that exhibit second harmonic generation were first demonstrated by Singer and coworkers.[3] In these materials, the polar order of the dopant chromophores is stabilized when the polymer composite is cooled below its glass transition temperature in the presence of a static electric field. Later studies found that the polar order of such a poled polymer - as probed with second harmonic generation - relaxes over time and that the characteristic time of decay depends on the free volume of the polymer host.[4J Early work aimed at decreasing relaxation rates focused on attaching the dopant molecules to the polymer.[5] In these second-order susceptibility studies, stabilization of slow molecular reorientation was of interest. The faster reversible reorientational effects of molecular dopants on millisecond time scales were first studied by Kuzyk and coworkers using second harmonic generation. Both isotropic and poled polymers were investigated below their glass transition temperature. [6] Later, Boyd and coworkers did similar mobility experiments but modeled the response as a free volume effect.[7] The temperature dependence of Boyd's free volume model and Kuzyk's elastic model showed the same qualitative behavior. In contrast to the slow relaxation process which has impact on the stability of an electrooptic device, the faster reorientational response is of interest owing to its contribution to the third-order nonlinear-optical susceptibility. While this reorientational response may be large in many material systems, materials in which faster mechanisms dominate - such as the fast electronic response would find applications in ultra-high speed devices. The reorientational response in organic liquids is well characterized.{8] Elastic reorientational mechanisms in polymers have only recently been addressed with quadratic electrooptic modulation (a third-order effect).[9,10J When the contribution of these reorientational effects are properly accounted for and removed from quadratic electrooptic measurements, the third order nonlinear-optical susceptibility so determined is found to be consistent with third harmonic measurements.[9,10][1 1] Because the third harmonic measures only the electronic response, it follows that the electrooptic modulation experiment can be used to estimate the third-order response if the reorientational effects are properly accounted for. In retrospect, the agreement between the low frequency electrooptic technique (1 3kHz) and the fast third harmonic results (w 1015Hz) is astounding considering that the comparison spans twelve decades of frequency. Q uadratic electroabsorption studies were first applied to doped polymers by Havinga and coworker to study the orientational order of the dopants.[llJ More recently, Poga and coworkers have reformulated this response as the imaginary part of the third-order susceptibility and applied the technique . . . . (3) (3)to study molecular mobility.[12,13,14] In these studies, the value of the tensor ratio a =X3333/Xii is found to be an indicator of the mechanism of response. Such experiments are well suited for materials physics investigations of the microscopic properties of a polymer as well as how these microscopic properties affect the third-order response. In this contribution, we extend the quadratic electroabsorption technique by measuring the magnitude and phase of the response over a broad range of wavelengths (modulating frequency dependence will be reported later); and, show that quadratic electroabsorption spectroscopy is a useful tool at shedding light on response mechanisms. As an example of how this technique can be used to study the nature of excited states that leads to a nonlinear-optical response, we show that a temperature-independent peak in the quadratic electroabsorption spectrum (which is not observed in the linear spectrum) is evidence of a two-photon state in the squarylium dye ISQ doped in poly(methyl methacrylate) (PMMA). A significant observation is that the different mechanisms dominate at different wavelengths. Indeed, it is this dispersion in the mechanisms that allows them to be more easily separated. Given the high signal-to-noise ratio of our data, we show that a Kramers-Kronig transformation can be used to obtain the real part of the thirdorder susceptibility. Furthermore, the relationships between the phase of the response and the electrostrictive contribution is briefly discussed.