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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

High-density frequency domain storage in alkaline earth sulfides double doped with lanthanide

Mohamed F. Aly, Levent Biyikli, Sameh I. Dardona, Zameer U. Hasan

Published Nov 29, 2000• 0 citations
Publisher

Abstract

Previously we have reported the largest number of photon- gated spectral holes ever burned in a solid. This has interest in their applications in optical storage. However, multiple holeburning in MgS:Eu resulted in noticeable erasure of the previously burned holes. This was attributed to the mechanism of holeburning in this material where both Eu2+ and Eu3+ are stable ions. In gated holeburning, Eu2+ ions are ionized. Eu3+ that form deep traps, capture the electrons generated during the holeburning and are converted to Eu2+. The Zero Phonon Line (ZPL) of these newly formed Eu2+ ions are randomly distributed across the inhomogeneous line causing a partial erasure of the holes burned earlier. This reduces the efficiency of holeburning. Co-doping of MgS:Eu and CaS:Eu with different rare earth (RE) ions has been investigated to provide deep electron traps other than Eu3+. Furthermore, co-doping provides the opportunity to burn holes in multiple ZPLs belonging to different REs, thus increasing the storage density many folds.

DOI
10.1117/12.408564

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Nov 22, 1999•0 citations
Previously we have reported the largest number of photon- gated spectral holes ever burned in a solid. This has interest in their applications in optical storage. However, multiple holeburning in MgS:Eu resulted in noticeable erasure of the previously burned holes. This was attributed to the mechanism of holeburning in this material where both Eu2+ and Eu3+ are stable ions. In gated holeburning, Eu2+ ions are ionized. Eu3+ that form deep traps, capture the electrons generated during the holeburning and are converted to Eu2+. The Zero Phonon Line (ZPL) of these newly formed Eu2+ ions are randomly distributed across the inhomogeneous line causing a partial erasure of the holes burned earlier. This reduces the efficiency of holeburning. Co-doping of MgS:Eu and CaS:Eu with different rare earth (RE) ions has been investigated to provide deep electron traps other than Eu3+. Furthermore, co-doping provides the opportunity to burn holes in multiple ZPLs belonging to different REs, thus increasing the storage density many folds.
We present the case of photoionization-induced holeburning in rare-earth-doped II–VI compounds for high-density persistent holeburning. In this case, the photoproduct of holeburning is distributed across the entire zero-phonon line. This maximizes the total number of possible spectral holes that can be burned into an inhomogeneous line as well as produces holes that are photoerasable. Experimental data on photon-gated holeburning in MgS:Eu2+, Eu3+ are presented. With the proper choice of the host electronic band structure, the optically active rare-earth ion and its electronic transitions involved in the holeburning process, to the best of our knowledge we have observed the highest number of photon-gated holes ever burned in a single electronic transition. The features of these holes are that they suffer no detectable erasure after several thousands of read cycles, they survive thermal cycling to ∼150 K, and they are completely photoerasable. A special case of photon-gated holeburning, power-gated holeburning, was employed to demonstrate that, in such systems, a single laser can be used for burning, reading, and erasing of the spectral holes.
We present the case of photoionization-induced persistent spectral holeburning in rare earth doped II-VI compounds for high density memory storage. Experimental data on photon-gated holeburning has been presented for different sulfide hosts (MgS, CaS: RE2+ and RE3+). With the proper choice of the host electronic band structure, the optically active rare earth ion and its electronic transitions involved in the holeburning process, we have observed the highest number of persistent holes ever burned in a single electronic transition. Efficient photon-gated holeburning in the 4f7 (8S7/2) - 4f65d1 transition of Eu2+ is a result of photoionization of Eu2+ to Eu3+. These holes have a width of less than 5 GHz, have no detectable erasing effects after thousands of reading cycles, survive thermal cycling up to the room temperature and have infinite lifetime at low temperature (2 K). Although self- gated holeburning is observed with reading laser at higher powers, the photon budget for reading these holes is so small that thousands of reading cycles can be performed without significantly affecting the optical signal. We discuss the unique features of these systems that make them the most promising candidates to date for the holeburning based optical memories.
In the form of micro-particles europium doped alkaline earth sulfides have been shown to exhibit high density of permanent spectral holes. The photon-gated spectral holeburning (PGHB) in these systems provided the most promising characteristics of any material known to date. These spectral holes can be used as optical memory. However, for any optical storage device either large size single crystals or thin films are required. Thin films of these materials are grown by Pulsed Laser Deposition (PLD) technique. This fast and simple growth technique is superior the single crystal growth or the molecular beam epitaxy (MBE) as far as the holeburning properties are concerned. Transparent glassy MgS:Eu and CaS:Eu films have been grown and tested for the spectral holeburning properties. Critical parameters such as the relative concentration of Eu2+ and Eu3+, and optical quality of thin films have been investigated. Int his paper we report on the growth and the high-density optical holeburning in these films. The density of spectral holes has been further increased by burning in multiple Eu-centers in a material and by depositing multiple layers of thin films of different materials in a stack.
The mechanism for highly efficient photoionization spectral hole burning in the 4f7–4f65d1 transition of Eu2+ in MgS host is investigated in detail. The time and power dependencies of the hole depth and its photoerasure are analyzed assuming that a resonant two-photon ionization process initiates the hole burning. The near-room-temperature cycling shifts the hole to low energies, demonstrating the relaxation of an unstable lattice resulting from the hole burning. The characteristics of hole burning change significantly in samples codoped with Ce and Eu. All of these studies support that the mechanism of hole burning is the electron transfer from the Eu2+ ion to the Eu3+ deep trap, both of which are located at the substitutional octahedral sites.
Persistent spectral hole-burning has been reported for singly, Eu-doped, and doubly, Eu- and Sm-doped, CaS phosphors. Efficient photon gated holeburning in the 4f7 (8S7/2)−4f65d1 transition of Eu2+ is a result of photoionization of Eu2+ to Eu3+. These holes have a width of <5 GHz (2 K), survive thermal cycling of the phosphor up to the room temperature, 300 K, and have no detectable deterioration over more than a day of storage time at low temperature (2 K). Although self-gated holeburning is observed with the reading laser at higher powers, the photon budget for reading these holes is so small that in excess of 1000 reading cycles can be performed without destroying the optical signal. The nature of holes burned by photon-gating is found to be very different from the self-gated holes. The characteristics for the holeburning are the same in singly and doubly doped phosphors, suggesting that under the conditions of our experiments, Sm traps do not play any significant role in spectral holeburning. Possibilities of high density optical memory storage using photon-gated holeburning in this THz broad transition are discussed.
For frequency domain storage rare earth doped sulfide show the most promising characteristics of any spectral holeburning material known. They provide the desired density of storage with a remarkable stability of memory against storage, read- out, and thermal surges. Problems associated with the fabrication of a prototype device not only require a careful consideration of design parameters but also raise some interesting questions regarding the material characteristics and the holeburning mechanism. This paper gives the status of our efforts in the direction of making thin films of MgS and CaS doped with Eu suitable for spectral holeburning based optical storage using the Pulsed Laser Vapor Deposition (PLD) technique.