Persistent spectral-hole burning was performed in the gamma line of V(4+) in the wide-gap semiconductor 6HSiC. Spectral holes burned at 11 K were stable to temperatures of at least 320 K for several days. The hole-burning mechanism consists of two-step photoionization of V(4+) (self-gated spectral-hole burning). The spectral holes could be erased optically, either by pumping of electrons back from stable traps or, presumably, by a charge-transfer transition from the valence band to the V(5+) ions.
Persistent spectral-hole burning was performed in the gamma line of V(4+) in the wide-gap semiconductor 6HSiC. Spectral holes burned at 11 K were stable to temperatures of at least 320 K for several days. The hole-burning mechanism consists of two-step photoionization of V(4+) (self-gated spectral-hole burning). The spectral holes could be erased optically, either by pumping of electrons back from stable traps or, presumably, by a charge-transfer transition from the valence band to the V(5+) ions.
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.
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.
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.
We report on investigations of the dependence of ionization spectral hole burning efficiencies on the location of impurity ion energy levels relative to both the host valence band and the conduction band. An ideal system is Cr-doped Li2Ge7O15, where the Cr3+ ions occupy three distinct noncentrosymmetric Ge4+ sites, which differ from one another by the location of their charge-compensating ions. Most important for our investigation is the large (\ensuremath{\sim}eV) and site-specific shift of the Cr3+ energy levels relative to the host valence and conduction bands. As a result, only one of the three sites shows persistent spectral hole burning. It is shown that this result can be understood from an analysis of the differences of the energy-level locations of the ground and excited states of the three Cr3+ centers relative to the host bands. Two of the centers have their energy levels sufficiently close to the host conduction band such that two-step ionization, and thus persistent spectral hole burning, can occur. However, for one of the centers the ground state lies so close to the valence band that an erasure process involving promotion of an electron from the valence band to the ionized center leads to the restoration of the initial charge state. This erasure process, clearly demonstrated in this paper, has to be considered when evaluating the hole burning potential of doped insulators, especially those with relatively narrow band gaps.
In the first experimental search for persistent spectral hole burning in an elemental semiconductor, we examined a variety of electronic defects in Si and in a Si:Ge alloy. No persistent spectral changes have been observed for Be-Li, Se0, or thermal donors in Si; however, spectral holes which persist for ~5 ms have been produced in the 1s-2p absorption lines of a sulfur-hydrogen complex in Si. The addition of 0.7% Ge to the Si host ensures that an inhomogeneously broadened system is being studied. Measurements combining a CO2 laser pump with a Fourier transform spectrometer probe reveal that the hole burning is due to migration of electrons from the S-H centers to shallow traps. The large observed hole width (4.6 cm−1) is attributed to spectral diffusion by resonant excitation transfer. The hole burning behavior of the 2p± line is temperature independent below 30 K, whereas the hole burning of the 2p0 line exhibits strong temperature dependence, suggesting that the ionization of the laser-excited centers in the 2p± hole burning process occurs via energy transfer between neighboring excited centers rather than by thermal ionization.
Summary form only given. Using persistent spectral hole burning, rare-earth ions doped in crystalline or glassy matrices may prove useful in optical data storage. In the case of Sm/sup 2+/, persistent spectral hole burning is due to photoionization: the Sm/sup 2+/ is oxidized to form Sm/sup 3+/, and the ejected electron is trapped in the matrix. For applications, it is important to understand the trapping mechanism, to identify the nature of the traps, and to see whether they are stable at room temperature.