Effect of sensitizing and recording intensities

Two-center holographic

3.4 Theory

3.4.4 Effect of sensitizing and recording intensities

Figure 3.11 (a) shows the variation of theoretical saturation diffraction efficiency with the recording intensity (ho) when the ratio of the recording intensity to sensitizing intensity is fixed (hoi Iuvo

=

25). The two curves in Figure 3.11 (a) are calculated with and without neglecting the absorption of the sensitizing beam within the crys- tal. The neglection of this absorption is an acceptable assumption only for very thin crystals or for cases where we are interested in local hologram strength. However, it does not apply to thick crystals since UV absorption can not be neglected in such crystals. Typical absorption coefficients of the crystals we used are close to 9 mm-¹ at 365 nm. As Figure 3.11 (a) shows, the final diffraction efficiency is not a func- tion of the absolute intensities while the intensity ratio is constant. Figure 3.11 (b)

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Figure 3.10: Comparison of theory and experiment: (a) sensitization by a 20 m W /cm² homogeneous UV beam at 365 nm monitored by a weak red beam (wavelength 633 nm), (b) bleaching of a sensitized crystal by a 300 mW/cm² red beam, and (c) holographic recording by simultaneous presence of a UV beam (wavelength 365 nm, intensity 20 m W /cm^2), and two red beams (wavelength 633 nm, intensity of each beam 250 m W /cm²⁾ with subsequent read-out performed by one of the red recording beams only.

shows the variation of the saturation diffraction efficiency with recording intensity (ho) with constant sensitizing intensity (Iuvo). Figures 3.11 (a) and (b) suggest that the final diffraction efficiency in two-center holographic recording is a function of the intensity ratio (IRolluvo) only and not a function of the absolute intensities. This can be intuitively understood from Figure 3.1 (b). While sensitizing light both populates Fe traps and partially erases the hologram, recording light records the hologram by redistributing electrons among traps via the conduction band. Effectively, sensitiz- ing light populates and recording light depopulates the Fe traps. The strengths of the processes caused by sensitizing and recording lights depend on sensitizing and recording intensities, respectively. Therefore, if we change sensitizing and recording intensities while keeping their ratio constant, we will not change the relative strength of the processes involved in recording the hologram, and we will obtain the same saturation diffraction efficiency. Note that holographic recording speed still depends on the absolute intensities as stronger beams result in faster processes.

The dependence of the final diffraction efficiency on the intensity ratio does not depend on the absorption of the sensitizing beam as evidenced by Figure 3.11 (a).

However, higher UV absorption results in a smaller maximum obtainable diffraction efficiency by reducing the effective thickness of the crystal. Figure 3.11 (b) shows that the peak in the theoretical variation of the final persistent diffraction efficiency with recording intensity is also broader for higher UV absorption. This is due to the fact that the ratio of the recording and UV intensities (hoi Iuvo) varies through the thickness of the crystal as the absorption of the recording light is much weaker than that of the UV light. Therefore, the best UV intensity corresponding to the approximately fixed recording intensity can not be provided for all points within the thickness of the crystal. If the UV intensity is high enough, there is a relatively narrow region within the crystal with optimum intensity ratio. By increasing the UV intensity, this narrow region moves away from the UV entrance edge. If we increase the UV intensity beyond some maximum value, there is no region within the crystal with optimum intensity ratio as the UV intensity remains too high at all points within the crystal thickness. For UV intensities above that maximum value, the final diffraction

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Figure 3.11: Variation of the final persistent diffraction efficiency with recording intensity while (a) the ratio of the recording to sensitizing intensity is fixed

(hoi

Iuvo

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efficiency decreases with increasing UV intensity. Therefore, we get a broad peak in the variation of the final diffraction efficiency with the UV intensity while recording intensity is fixed. Similar argument holds for the variation of the final diffraction efficiency with recording intensity while the UV intensity is fixed. The width of the peak in the variation of the final diffraction efficiency with recording intensity depends on the UV absorption coefficient: The larger the UV absorption coefficient, the broader the peak.

Finally, it is important to note that there is no intensity threshold for two-center holographic recording as shown in Figure 3.11. We can record holograms with very low recording and UV intensities and obtain large diffraction efficiencies if the intensity ratio is picked properly. This is a big advantage of two-center recording over two-step persistent holographic recording using small polarons in LiNb03:Fe crystals.