Photorefractive Polymers

Figure 1: Writing holograms in a photorefractive polymer crystal.

This project focuses on the chemistry and physics of novel materials for optical storage and optical processing applications. Interests here span from photochromics, to spectral hole-burning materials, to a particularly exciting new class of materials called photorefractive polymers, in which light-induced generation of mobile charges, transport, and trapping are combined with second-order optical nonlinearity in a polymeric composite to form dynamic holograms. The charge separation produces internal electric fields which locally alter the refractive index through the second-order optical nonlinearity.

More specifically, how does this photorefractive effect work? The key idea of the photorefractive process is this: two beams intersect in the material, and holes are generated where the light is bright. These holes move under the influence of an applied electric field over distances on the order of the grating spacing of a micron or two. The resulting charge distribution produces a sinusoidally varying electric field. Finally, due to the presence of the nonlinear optical molecules, this electric field changes the index of refraction. A modulated index of refraction is a hologram. In our work, this hologram is not intended for optical storage because the charges are not stored for a long time; rather it is useful for optical processing applications such as image amplification.

Now, how does the amplification occur? The hologram described above is actually a very special hologram, in that the locations in the sample where the light is bright do not line up with the locations where the index change is largest. When this nonlocal effect occurs, two light beams passing through the sample can exchange energy - one is amplified, and one is attenuated. This is the mechanism responsible for the image amplification: the power present in a "power supply" beam is transferred to an image-bearing beam.

The interest in photorefractive polymers stems from the fact that up until only a few years ago, the only materials showing the photorefractive effect were difficult-to-grow, specialized inorganic crystals such as lithium niobate, barium titanate, bismuth silicate, and strontum barium niobate. The discovery of the first photorefractive polymer composites in Dr. Moerner's lab at IBM Research in 1990 opened up a completely new materials class. Polymer photorefractives are expected to be cheaper, easier to modify, and easier to fabricate into novel geometries than the inorganic crystals.

Since photorefractive polymers are only a few years old, many details of the mechanism must be understood in order to design improved materials. In collaboration with synthetic chemistry partners at UCSD and elsewhere, we have learned that both the polarizability anisotropy as well as hyperpolarizability of the nonlinear chromophores are essential to the creation of strong holograms. To store charge (and therefore holograms) for long periods of time, the mysterious trapping states which occur in some of the new photorefractive polymers must be understood and optimized.

In very recent work, the extremely high performance of our latest materials has been demonstrated by the observation of both beam fanning and self-pumped phase modulation, two effects that have previously only been observed in inorganic photorefractive crystals.

Summary: Photoactive and Photorefractive Organic Materials [Slide]

Recent Advances in Photorefractive Polymers: Moerner Lab Milestones in Photorefractive Polymers

  • High Gain, Low Loss, Reasonable Speed 
    • G = 200 cm-1, a = 12 cm - 1, 50 ms growth time, material stable for > 9 months
    • Appl. Phys. Lett. 70, 1515 (1997) [Abstract]
  • Beam Fanning 
    • Fans in plane due to large GL
    • JOSA B 15, 901 (1998) [Abstract]
  • Spatial Phase Shift Determination
    • New analysis required due to strong energy and phase coupling (high gain regime)
    • Opt. Lett. 22, 874 (1997) [Abstract]
  • Large Single-Pass Gain of a Factor of 500 Observed - That's Right, a Factor of 50,000 Percent
    • Utilizes three-layer sample and grating translation
    • Opt. Commun. 145, 145 (1998) [Abstract]
  • High Trap Density (1x1017 cm-3)
    • Orientational Enhancement confirmed
    • JOSA B 15, 905 (1998) [Abstract]
  • Spontaneous Oscillation and Self-Pumped Phase Conjugation 
    • Utilizes two-layer sample, R = 13%
    • Science 277, 549 (July 1997) [Abstract]
  • C60 anion Spectroscopy 
    • [C60-] as well as the active and inactive trap identities determined by near IR Spectroscopy
    • Chem. Phys. Lett. 291, 553-561 (1998) [Abstract]
  • Net gain achieved in polysiloxane bi-functional polymers 
    • Synthetic technique can be optimized combinatorially
    • J. Amer. Chem. Soc. 120, 9680-9681 (1998) [Abstract]
  • Grating growth times as low as 5ms with large G and low a observed in DCST derivatives
    • Speed not limited by orientation
    • Appl. Phys. Lett. 73, 1490-1492 (1998). [Abstract]
  • Laser Based Ultrasound detection demonstrated with photorefractive polymers 
    • Sensitivity within a factor of 3 of the ideal interferometer
    • Opt. Comm. 162, 79-84 (1999) [Abstract]
  • Correlation between speed and chromophore trap depth observed 
    • Chromophore seems to act as a hole trap
    • Chem. Mater. 11, 1784-1791 (1999) [Abstract]
  • Image Amplification and Novelty Filtering in a Photorefractive Polymer  
    • Utilizes video rate capability of material
    • Appl. Phys. Lett. 76, 3358-3360 (2000) [Abstract]

Selected Bibliography

  • Observation of the Photorefractive Effect in a Polymer, by S. Ducharme, J. C. Scott, R. J. Twieg, and W. E. Moerner, Phys. Rev. Lett. 66, 1846 (1991).
  • Orientationally Enhanced Photorefractive Effect in Polymers, by W. E. Moerner, S. M. Silence, F. Hache, and G. C. Bjorklund, J. Opt. Soc. Am. B. 11, 320 (1993).
  • Photorefractive Polymers Based on Dual-Function Dopants, by S. M. Silence, et al., J. Phys. Chem. 99, 4096 (1995).
  • Polymeric Photorefractive Materials, by W. E. Moerner and S.M. Silence, Chem. Revs. 94, 127 (1994).
  • Photorefractive Polymers, by S. M. Silence, D. M. Burland, and W. E. Moerner, Chap. 5 of Photorefractive Effects and Materials, David D. Nolte, Editor (Kluwer, Boston, 1995).
  • Photorefractive Polymers (a review), by W. E. Moerner, A. Grunnet-Jepsen, and C. L. Thompson, Annual Review of Materials Science 27, 585-623 (1997).
  • Spontaneous Oscillation and Self-Pumped Phase Conjugation in a Photorefractive Polymer Optical Amplifier, by A. Grunnet-Jepsen, C. L. Thompson, and W. E. Moerner, Science 277, 549 (1997).
  • A. Grunnet-Jepsen, D. Wright, B. Smith, M. S. Bratcher, M. S. DeClue, J. S. Siegel, and W. E. Moerner, "Spectroscopic Determination of Trap Density in C60-Sensitized Photorefractive Polymers," Chem. Phys. Lett. 291, 553-561 (1998).
  • D. Wright, M. A. Diaz-Garcia, J. D. Casperson, M. DeClue, and W. E. Moerner, "High Speed Photorefractive Polymer Composites," Appl. Phys. Lett. 73, 1490-1492 (1998).
  • A. Goonesekera, D. Wright, and W. E. Moerner, "Image Amplification and Novelty Filtering in a Photorefractive Polymer," Appl. Phys. Lett. 76, 3358-3360 (2000).
  • O. Ostroverkhova, M. He, R. J. Twieg, and W. E. Moerner, "Role of Temperature in Controlling Performance of Organic Photorefractive Glasses," ChemPhysChem 4, 732-744 (2003).
  • O. Ostroverkhova and W. E. Moerner, "Organic Photorefractives: Mechanisms, Materials, and Applications," appearing in Chem. Revs. (2004)