Effect of annealing in two-center holographic recording

Abstract

Persistent holograms are recorded with red light in lithium niobate crystals doped with manganese and iron. We find that the oxidation/reduction state of the crystal has a profound impact on the recording and readout performance. The underlying physical processes are investigated and the recording and readout responses are explained and optimized. © 1999 American Institute of Physics. ͓S0003-6951͑99͒05325-5͔Photorefractive materials are the best reversible storage materials known so far and several holographic storage demonstrators using iron-doped photorefractive lithium niobate (LiNbO 3 :Fe) were presented in the last few years. 1-3 However, erasure of the holograms during readout has been one of the major problems in the practical realization of holographic read/write memories. We recently proposed a method to solve this problem by using doubly doped LiNbO 3 . 4 In this letter, we explain the effects of annealing the crystal on the performance of the recently proposed twocenter recording method.In the recent work, 4 LiNbO 3 doped with manganese ͑Mn͒ and iron ͑Fe͒ was used. The energy band diagram of such a crystal is shown in Fig. 1. Fe and Mn ions occur in the valence states Mn 2ϩ/3ϩ and Fe 2ϩ/3ϩ , 5 and thermal depletion plays no role in room temperature. Electrons can be excited by ultraviolet light either from Mn 2ϩ or from Fe 2ϩ into the conduction band while red light excites electrons only from the shallower Fe 2ϩ . The conduction-band electrons can recombine with both centers, and thus, ultraviolet illumination populates the Fe 2ϩ/3ϩ level partially while red light illumination empties the Fe sites. The filled Fe levels cause a broad-band absorption in the visible with a maximum at a 477 nm wavelength. 6 Thus, ultraviolet light sensitizes the material while red light bleaches it. The basic idea of twocenter holographic recording is to bring with the ultraviolet light electrons from Mn to Fe via the conduction band, use these electrons to record the hologram with red light, and eventually transfer the electrons from iron back to the manganese centers by red light. This results in a hologram stored in Mn centers that persists against further red illumination.One of the key material parameters in two-center holographic recording is the initial electron concentration in Mn and Fe traps. These concentrations can be varied by an annealing treatment. 7 Since Mn traps are deeper in the band gap than Fe traps, electrons would fill the Mn traps before Fe traps when the crystal is reduced. For persistent holographic recording, it is necessary that the final hologram be stored in Mn centers. Therefore, it is essential that at the end of the annealing process all Fe traps be empty, and only a portion of the Mn traps be filled. To investigate the effect of the oxidation/reduction state of the crystal we performed experiments with four x-cut congruent LiNbO 3 crystals doped with 0.075 wt % Fe 2 O 3 and with 0.01 wt % MnO. The crystals were all from the same boule. The samples were strongly oxidized ͑LN1͒, oxidized ͑...

Sensitivity improvement in two-center holographic recording

“…% MnO. Properly annealing the crystal 4 yielded fully empty Fe traps and partially (more than 90%) filled Mn traps before sensitization with UV light. A 100-W mercury lamp (wavelength, 404 nm, unpolarized; intensity, 4 mW͞cm 2 , homogeneous) was used as the UV source.…”

Investigation of nonvolatile holographic storage in LiNbO3:Mn:Ce:Fe crystal

“…From Fig. 2 we found that the largest nonvolatile holographic diffraction in the doubly doped LiNbO 3 :Mn:Fe exists in LM4, which means about 95% of the Mn traps are filled with electrons, as reported by Ali Adibi et al [12]. From Fig.…”

Improvement of sensitivity and refractive-index changes in LiNbO3:Ce:Cu crystals with UV light

“…Due to its advantage of all-optical processing, this technique has attracted considerable interest. The characteristics of doubly doped LN crystals have since been investigated extensively [3][4][5][6][7][8][9] and the application of such LN crystals to holographic memory has been reported [10]. However, there are a number of crucial practical disadvantages with this technique: insufficient levels of refractive-index change; low sensitivity [5,7,8,11]; the depth of the total grating was greatly reduced as a result of the 180 phase difference between shallow and deep gratings [2,12]; and the extra step of bringing electrons from deep traps to shallow traps and the small absorption cross-section of the shallow traps at large recording wavelength prolong response time [13].…”