Zameer U. Hasan

PERSISTENT HIGH DENSITY SPECTRAL HOLEBURNING IN CAS:EU AND CAS:EU, SM PHOSPHORS

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. Possibil...

Power-gated spectral holeburning in MgS:Eu2+, Eu3+: A case for high-density persistent spectral holeburning

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 holebur...

Mechanism of high-density power-gated hole burning in Eu 2+ -doped sulfides

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.

High density photon-gated hole burning in sulfides

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.

Material challenges for spectral hole-burning memories

It was envisaged that spectral holeburning based optical memory will provide orders of magnitude improvement in storage densities. The progress has basically been hampered by the lack of materials with proper performance characteristics. In the past two decades, novel atomic scale processes involved in holeburning have been investigated in detail in order to improve the performance of existing materials and to engineer new ones. On the other hand, significant advances have been made in optical storage techniques to use the materials available. And now, new materials that can approach the specifications of a memory device using different techniques for the storage are either available or are just over the horizon. This paper reviews the status of different classes of spectral holeburning materials suitable for memory devices.

Photoluminescence and spectral holeburning in europium-doped MgS nanoparticles.

Luminescence and power-gated spectral holeburning studies have been performed on Eu-doped MgS nanoparticles. These particles are atomically tailored to produce and control the relative concentration of Eu(2+) and Eu(3+), which is necessary for power-gated holeburning. The spectral holes are permanent at low temperatures. Optical studies show that the electron-phonon coupling is stronger in nanoparticles than in thin films or microparticles of the same material. This is the reason for inherently broader spectral holes in nanoparticles as compared to microparticle or thin-film samples. Temperature broadening of spectral holes in nanoparticles follows a T(2.4) behavior, a faster rate than thin films or microparticles. This behavior can be attributed to the glassy nature of the particles produced.

Thin films of sulfides for high-density optical storage by photon-gated hole burning

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.

One and two photon absorption spectrum of Mn4+ in trigonal Cs2TiF6

Abstract One and two photon spectra of Mn 4+ in Cs 2 TiF 6 have been reported. A detailed analysis of these spectra testifies the presence of a strong electron-vibration coupling in this system. A novel consequence of this interaction, a reduction of the spin-orbit and trigonal field splittings in the vibronic spectra as compared to the electronic origins, has been observed for the first time.

Fabrication and plasma spectroscopy of PLD thin film deposition for power-gated holeburning

The fabrication of spectral storage materials presents a great challenge of tailoring the optical properties of a solid at the atomic scale. We show that by a careful choice of a host material and the luminescent rare earth centers created in it, thin film structures can be fabricated using pulsed laser deposition. It is shown that these structures can be tailored to satisfy the material requirements for the power-gated spectral holeburning, one of the most successful techniques for the spectral storage. In the form of multilayer thin film structures, these tailored materials can provide surface storage densities exceeding a terabit per square inch. Possibilities of tailoring new rare earth centers that can further multiply the storage densities have been discussed. Experimental data has been presented for the fabrication and characterization of such impurity based europium centers in MgS and CaS by chemically controlled pulsed laser deposition.

Atomic tailoring for ultra-dense multi-layer spectral memories

Spectral Storage using optical holeburning has the potential of providing ultrahigh densities approaching terabits per square inch. The progress on multilayer spectral storage in Eu-doped sulfides has been presented. It is shown that atomic scale tailoring of these structures is possible in order to design several different europium optical centers. In the spectrum of these centers, ultrahigh density storage can be achieved with the simultaneous optimization of other performance parameters. Results are also presented for tailoring the barrier and the capping layers.