We report on simultaneous characterization of space charge gratings in photorefractive PVK-based polymer films by means of photo-EMF and Two-Wave Mixing (TWM) of periodically phase modulated beams. 100 micron thick samples of a polymer DMNPAA:PVK:ECZ:TNF with chromophore (DMNPAA) concentration of 5 wt% were investigated at (lambda) equals 633 nm in reflectance configuration. The amplitudes of the unshifted (i.e. drift induced) and the shifted (i.e. diffusion or saturation induced) components of the photorefractive space- charge field grating were evaluated directly by detection of the fundamental and the second harmonic of the TWM signal and indirectly from the corresponding harmonics of the photo-EMF current. The unshifted grating component exhibited approximately linear dependence on the externally applied dc field E₀ and had an amplitude close to E₀, which can be interpreted as absence of any remarkable saturation of trapping centers associated with photorefractive recording. Also growing with E₀, the amplitude of the shifted component did not depend on the applied field direction, but was nearly as big as the unshifted component for the external fields of about approximately equals 50 V/micrometers . We interpret these facts as well as an experimentally observed double change of sign fo the fundamental harmonic photo-EMF signal with the external field as a result of dramatic growth of the Einstein ratio D/(mu) (relating diffusion coefficient D and mobility (mu) of the photogenerated carriers) - at least up to 1 V for the external dc field mentioned above. This allows us to address the observed shifted component as an external field enhanced diffusion grating, rather than the result of trapping centers saturation. Additionally, the (mu) (tau) product for dominating photocarriers (holes) was evaluated as approximately equals 0.3*10^{- 10} cm²/V from the photo-EMF measurements.

Date Published: 16 January 2002
PDF: 9 pages
Proc. SPIE 4462, Nonlinear Optical Transmission Processes and Organic Photorefractive Materials, (16 January 2002);

Published in SPIE Proceedings Vol. 4462:
Nonlinear Optical Transmission Processes and Organic Photorefractive Materials
Christopher M. Lawson; Klaus Meerholz, Editor(s)