hotorefractive Polymers - New Materials & Effects

Klaus Meerholz

Institut f¸r Physikalische Chemie, LMU Munich,
Sophienstr. 11 D - 80333 Munich
fax +49-89-5902 290

Photorefractive materials have many potential applications, including
dynamic holographic storage and image processing. Recently, the new class
of amorphous organic photorefractive materials has emerged, offering wide
structural flexibility, easy processability, and low cost at very high
performance levels. Progress in this field has led to absorption-limited
complete diffraction for the readout of a hologram stored in materials of
only 100-150 mm thickness and to extremely large net gain coefficients of
more than
200 cm-1 [1] compared to 40-50 cm-1 in the best inorganic photorefractive
crystals known to date. These excellent properties occur in materials with
low glass transition temperatures and result from refractive index
modulations as large as Dn ª 10-2, mostly originating from a Kerr-type
orientational birefringence rather than the electro-optic effect as in
traditional photorefractive crystals. The materials can be adjusted for
photorefractivity over the entire visible spectrum and in the near
infrared. The sensitivity is excellent enabling the use of low-power laser
sources, such as HeNe laser or laser diodes.
We will present a series of PR guest/host materials based on
poly(N-vinylcarbazole) using merocyanine dyes as the electrooptic
chromophores [2]. After optimization we obtained a composite with excellent
stability and an one order of magnitude larger index modulation amplitude
at 790 nm compared with the reference composite DMNPAA:PVK:ECZ:TNF measured
under identical conditions [3].
Furthermore, we have discovered that under certain conditions the
performance of the devices can be asymmetric with respect to the applied
electric field direction [4]. For one field direction, light is coupled
into the polymer layer and can be detected on the side of the devices. For
the other, the originally Gaussian-shaped writing beams show a shoulder or
even split into two. The strength of the asymmetry depends on the diameter
of the beams writing the hologram. We demonstrate that this effect is due
to beam fanning. Finally, we demonstrate the existence of six independent
PR gratings, which compete inside the organic PR devices. This results from
the internal reflection of the holographic beams on the internal
polymer/ITO interface [5].

1. K. Meerholz, B. Volodin, Sandalphon, B. Kippelen, N. Peyghambarian,
Nature 371, 497 (1994).
2. F. W¸rthner, R. Wortmann, R. Matschiner, K. Lukaszuk, K. Meerholz, R.
Bittner, Angew. Chem. Int. Ed. Engl. 36, 2765 (1997).
3. K. Meerholz, Y. De Nardin, R. Bittner, F. W¸rthner, R. Wortmann, Appl.
Phys. Lett., 73, 4 (1998).
4. K. Meerholz, R. Bittner, Y. De Nardin, Opt. Commum., 150, 205 (1998).
5. Meerholz, R. Bittner, E. Mecher, Y. De Nardin, J. Opt. Soc. Am. B, 15,
2114 (1998).