Figure 1: Writing holograms in a photorefractive
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
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
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
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
Summary: Photoactive and Photorefractive Organic Materials
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
- 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
- Utilizes video rate capability of material
- Appl. Phys. Lett. 76, 3358-3360 (2000) [Abstract]
- 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)