Process for photorefractive index grating formation

Details Process for photorefractive index grating formation Process for photorefractive index grating formation

1994-04-21

2001-08-28

Angebranndt, Martin (Department: 1756)

Radiation imagery chemistry: process, composition, or product th

Holographic process, composition, or product

C430S002000, C430S394000, C359S003000, C385S005000, C385S130000, C385S141000

Reexamination Certificate

active

06280884

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an improved process for photorefractive index grating formation.
BACKGROUND OF THE INVENTION
The photorefractive effect involves light-induced charge redistribution in a nonlinear optical material to produce internal electric fields which by virtue of the optical nonlinearity, produce local changes in the index of refraction such that dynamic, erasable holograms are formed which diffract light. The photorefractive effect is achieved by exposing the material to an optical intensity pattern consisting of bright and dark regions, such as formed by interfering two coherent laser writing beams of the same polarization. Mobile charge generated in the material migrates under the influence of diffusion and drift processes to form internal space charge electric fields which create refractive index variations due to the electrooptic effect. These variations in refractive index in the photorefractive material are known as index gratings. The index gratings diffract light and are useful for a variety of applications, including storage of holographic images, diffractive optical elements, and photorefractive two-beam coupling.
Inorganic crystals exhibiting the photorefractive effect are well known in the art as described in Gunter and Huignard, “Photorefractive Materials and Their Applications”, Vols. I and II (“Topics in Applied Physics”, Vols. 61 and 62) (Springer, Berlin, Heidelberg, 1988). Inorganic photorefractive crystals have been fabricated into optical articles for the transmission and control (change in phase, intensity, or direction of propagation) of electromagnetic radiation, as well as for holographic image and data storage.
However, it is technically difficult to fabricate such crystals into desired large area samples or thin-layered devices such as optical wave guides or multiple-layer stacks. Further, it is difficult to dope crystalline material with large concentrations of dopants in order to achieve desired property improvements, such as increase in the speed and/or magnitude of the photorefractive effect, because dopants are often excluded from the crystals during growth.
Certain polymeric photorefractive materials have been described by Ducharme et al., U.S. Pat. No. 5,064,264, and Schildkraut et al., U.S. Pat. No. 4,999,809. These polymeric materials can be fabricated into thin-layered devices such as optical waveguides and multilayer stacks. Further, they can be readily doped with materials to improve a photorefractive effect.
Schildkraut describes a photorefractive device having a layer of material comprising a sensitizer, a charge transporting layer, a binder, and an organic molecular dipole, which has been poled in an electric field at elevated temperatures so that the alignment of the molecular dipoles remains for long times at ambient temperatures. Although the material is shown to have light-induced changes in measured properties, Schildkraut does not show the formation of a photorefractive grating to demonstrate a photorefractive device.
Ducharme et al. describe photorefractive materials comprising a polymer, a nonlinear optical chromophore, a charge transport agent, and optionally a charge generator. Although these materials are useful in certain applications, there still is a desire in the industry for a photorefractive process having longer characteristic decay time of diffraction efficiency.
It is therefore an object of the present invention to provide an improved process for photorefractive index grating formation. Other objects and advantages will be apparent from the following disclosure.
SUMMARY OF THE INVENTION
The present invention relates to a process for forming photorefractive index grating in a polymeric optical article comprising the steps of: (i) exposing the polymeric optical article to electromagnetic radiation having an intensity of at least 0.05 watts/cm
2
for a short period of time to achieve an absorbed energy/unit volume of at least 1×10
3
Joules/cm
3
to activate the article, and (ii) exposing the polymeric optical article to an electric field and electromagnetic radiation to form an index grating. The first exposure step of the process of the present invention surprisingly increases the diffraction efficiency of the article and also decreases the decay rate of the diffraction efficiency of the optical article. The polymeric optical article generally comprises a polymer, a charge transporting agent, a sensitizer, and a nonlinear optical chromophore (NLO chromophore). The charge transporting agent and/or the NLO chromophore may be covalently attached to the polymer or alternatively are dispersed in the polymer as guest/host.
The present invention also relates to holographic storage of information using the process of the present invention.
A more thorough disclosure of the present invention is presented in the detailed description which follows.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a process for producing a photorefractive index grating in a polymeric optical article comprising the steps of: (i) exposing a polymeric optical article to electromagnetic radiation having an intensity of at least 0.05 W/cm
2
for a short period of time to achieve an absorbed energy/unit volume of at least 1×10
3
J/cm
3
to activate the article, and (ii) exposing the polymeric optical article to an electric field and electromagnetic radiation to form an index grating.
In a first embodiment of the process of the present invention, the process for producing a photorefractive index grating in a polymeric optical article comprises the steps of: (i) flood exposing a polymeric optical article to a beam of electromagnetic radiation having an intensity of at least 0.05 W/cm
2
for a short period of time to achieve an absorbed energy/unit volume of at least 1×10
3
J/cm
3
to activate the article, and (ii) exposing the polymeric optical article to an external electric field and to two intersecting beams of coherent electromagnetic radiation of the same polarization. The activation of the article in step (i) of the process is enhanced by additionally exposing the article to an electric field during step (i).
In a second embodiment of the process of the present invention, the process for producing a photorefractive index grating in a polymeric optical article comprises the steps of: (i) patterned exposure of a polymeric optical article to electromagnetic radiation having an intensity of at least 0.05 W/cm
2
for a short period of time to achieve an absorbed energy/unit volume of at least 1×10
3
J/cm
3
to activate the article, and (ii) exposing the polymeric optical article to an electric field and electromagnetic radiation to form the index grating. The patterned exposure of step (i) can be achieved by two intersecting beams of coherent electromagnetic radiation of the same polarization to achieve spatially varying intensity. Alternatively, the article can be spatially patterned with one beam of electromagnetic radiation with, for example, a transverse intensity distribution that is modulated binary or gray scale. In step (ii) of the second embodiment, the article can be flood exposed to electromagnetic radiation to form the index grating or, alternatively, can be exposed to two intersecting beams of coherent electromagnetic radiation of the same polarization to form the index grating such as a reference and signal beam.
As used herein, index grating shall mean a sinusoidal spatial modulation of the optical index of refraction of the article. The spatial modulation has a spatial wavelength and a spatial frequency, which is defined as the number of peaks (or valleys) per unit length. A perfect index grating with a single spatial frequency component is produced by using two intersecting beams, each of which is a perfect plane wave, optionally with a Gaussian transverse intensity distribution. In that case, the spatial frequency of the index grating produced by the process of this invention is equal to 2n sin &thgr;/&lgr;, where &lgr; is the laser wavelengt

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