Recent research has demonstrated the feasibility of an all-optical method of recording and readout of holograms in photorefractive crystals, with capabilities for both nonvolatile storage and erasure on demand. Heretofore, volatility has been the primary remaining obstacle to the full implementation of holography with these capabilities: In a typical previously developed holographic system of this type, the readout process erases the stored information and amplifies scattered light.
The present method involves a crystal of the photorefractive material lithium niobate doped with iron and manganese, which are present in the form of Fe2+ and Fe3+ ions and Mn2+ and Mn3+ ions, respectively. These ions act as deep electron traps, with energy levels between the conduction and valence bands of LiNbO3. The Mn traps are deeper than the Fe traps; this feature renders the doped LiNbO3 crystal photochromic in addition to photorefractive. The origin and nature of this photochromism are as follows:
Initially, the electrons tend to be in the deeper Mn traps and consequently the crystal is transparent at photon energies for excitation from the Fe2+/3+ level to the conduction band (these photon energies correspond to wavelengths centered at 477 nm). If the crystal is illuminated with light at higher photon energies (e.g., ultraviolet light) that can ionize the deeper Mn traps, then the Fe traps become populated and the crystal becomes absorptive over a wide range of visible wavelengths. One can make the crystal revert to transparency by illuminating it with visible light that transfers the electrons from the Fe traps back to the Mn traps. This photochromism is exploited in the present method.
In experiments, a crystal of LiNbO3 doped with Fe and Mn was illuminated by various combinations of (1) unpolarized ultraviolet light (wavelength of 365 nm) from a mercury lamp and (2) interfering beams generated by splitting the 633-nm-wavelength (red) beam from a He/Ne laser. From time to time, one of the He/Ne beams was blocked and the efficiency of diffraction of the remaining beam from the holographic grating formed by the interfering beams was measured.
The figure presents some of the results of the experiments and helps to illustrate the reasoning that led to the conception of the present method. The lower curve shows the evolution of the diffraction efficiency when a holographic grating was recorded with the He/Ne beams only, following a two-hour preexposure to ultraviolet light. The diffraction efficiency increased rapidly, reached a maximum, and thereafter decreased almost to zero. This curve is interpreted as follows:
- The ultraviolet preexposure excited electrons from the Mn traps and populated the Fe traps homogeneously.
- Because the Fe2+ ions could absorb red light, the interfering He/Ne laser beams recorded a hologram: interference maxima yielded large photovoltaic currents, which built up space-charge fields, which, in turn, induced changes in the index of refraction.
- However the Fe2+ sites became bleached in the high-intensity regions and hence the currents there decreased. Ultimately, the darker regions also became bleached and all electrons became trapped by the Mn3+ ions.
- The final Mn2+ concentration was almost completely spatially homogeneous because (a) the experiment began with a homogeneous concentration of Fe2+ and (b) each excited charge carrier was moved in the same direction by approximately the same distance before it became retrapped by Mn3+ ions, so that (c) the final space-charge field was very small. Thus, the exposure schedule described above was found not suitable for efficient nonvolatile storage.
The key to nonvolatile storage according to the present method is a different exposure schedule in which one illuminates the crystal with ultraviolet and red light simultaneously and waits until saturation is reached, then switches the ultraviolet light off. The upper curve in the figure shows the result obtained in an experiment in which this exposure schedule was used. The diffraction efficiency during recording by this schedule was much larger than in the previous experiment. In addition, readout by use of red light erased only a fraction of the hologram because even after complete bleaching of the Fe2+ sites, the hologram remained recorded in the Mn traps. Thus, the crystal was rendered insensitive to red light and readout by red light was thereby rendered non-erasing; that is, storage became nonvolatile.
Thus, in the present method, a nonvolatile hologram is recorded by exposing the crystal simultaneously to incoherent ultraviolet light and coherent interfering beams of red light. The hologram can be erased by exposing the crystal to the ultraviolet light only.
This work was done by Karsten Buse, Ali Adibi, and Demetri Psaltis of Caltech for NASA's Jet Propulsion Laboratory.
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