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Physical review. B, Condensed matter

Off-resonant spectral hole burning in CaS:Eu by time-varying Coulomb fields

S. A. Basun, Madis Raukas, Uwe Happek, Alexander A. Kaplyanskii, Jean‐Claude Vial, John Rennie, William M. Yen, Richard S. Meltzer

Published Nov 15, 1997• 21 citations
Publisher

Abstract

Persistent spectral hole burning has been observed on the zero-phonon lines of the main site and several perturbed sites of ${\mathrm{Eu}}^{2+}$ in CaS:Eu single crystals. Hole burning occurs by two-step photoionization, can be strongly gated with IR irradiation, and takes place by excited-state absorption from the metastable ${4f}^{6}5d$ excited state to the conduction band followed by electron transport to ${\mathrm{Eu}}^{3+}$ centers which are the dominant traps. A complex hole structure consisting of a narrow feature (200 MHz), and broader features ($\ensuremath{\approx}5$ and $\ensuremath{\approx}100\mathrm{GHz}$) is observed. A mechanism is described for the occurrence of these unusually broad features. Time-varying internal electric fields which occur during the hole burning due to photoionization and trapping can lead to burning of holes at frequencies non-resonant with that of the laser. In addition, a mechanism for hole erasure, tunneling between Eu ions, is demonstrated. This mechanism is identified from the frequency dependence of the hole erasure which follows the ${\mathrm{Eu}}^{2+}$ absorption, and the linear dependence of the photoconductivity and hole erasure efficiency on irradiation power, both of which indicate erasure in a single-photon process.

DOI
10.1103/physrevb.56.12992

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Journal of the Optical Society of America B•Feb 1, 2001•3 citations
Persistent spectral hole burning has been observed on the zero-phonon lines of the main site and several perturbed sites of Eu2+{\mathrm{Eu}}^{2+}Eu2+ in CaS:Eu single crystals. Hole burning occurs by two-step photoionization, can be strongly gated with IR irradiation, and takes place by excited-state absorption from the metastable 4f65d{4f}^{6}5d4f65d excited state to the conduction band followed by electron transport to Eu3+{\mathrm{Eu}}^{3+}Eu3+ centers which are the dominant traps. A complex hole structure consisting of a narrow feature (200 MHz), and broader features (\ensuremath≈5\ensuremath{\approx}5\ensuremath≈5 and \ensuremath≈100GHz\ensuremath{\approx}100\mathrm{GHz}\ensuremath≈100GHz) is observed. A mechanism is described for the occurrence of these unusually broad features. Time-varying internal electric fields which occur during the hole burning due to photoionization and trapping can lead to burning of holes at frequencies non-resonant with that of the laser. In addition, a mechanism for hole erasure, tunneling between Eu ions, is demonstrated. This mechanism is identified from the frequency dependence of the hole erasure which follows the Eu2+{\mathrm{Eu}}^{2+}Eu2+ absorption, and the linear dependence of the photoconductivity and hole erasure efficiency on irradiation power, both of which indicate erasure in a single-photon process.
It is demonstrated that persistent hole burning in the zero-phonon line of Eu2+ in SLs of CaF2:Eu/CdF2 with uniformly doped CaF2 layers is mainly due to tunneling of electrons from the 4f65d excited state of Eu2 to the conduction band of adjacent CdF2 layer. However in selectively doped suoerlattices (SLs), the two-step photoionization process dominates in the formation of spectral holes. Two mechanisms of spectral hole broadening were discussed. These are the Stark-shifting of the Eu2 f-d transition energy induced by internal electric field and phonon assisted transitions.
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.
Spectral hole burning on the $4{f}^{7}(^{8}S_{\frac{7}{2}})\ensuremath{\rightarrow}4{f}^{6}5d$ transition of Eu2+{\mathrm{Eu}}^{2+}Eu2+ is observed to occur by two mechanisms in CaF2{\mathrm{F}}_{2}F2​:Eu2+{\mathrm{Eu}}^{2+}Eu2+. At zero magnetic field, persistent spectral hole burning occurs by two-step photoionization where the hole burning survives cycling to room temperature. The observed hole spectrum is compared with a calculation which considers the small Eu2+{\mathrm{Eu}}^{2+}Eu2+ ground-state splitting and both the ground- and excited-state hyperfine interactions. In a magnetic field above 1 T, transient spectral hole burning occurs by population redistribution among the ground-state hyperfine levels. Long-lived (minutes) holes are observed for a half integral spin system. The resulting hole spectrum consists of holes, antiholes, and a nuclear spin-flip sideband and can be explained from the hyperfine interactions in the ground and excited states. The dominant hole decay occurs by phonon-induced transitions among hyperfine levels of the ${M}_{S}=\ensuremath{-}\frac{7}{2}$ and ${M}_{S}=\ensuremath{-}\frac{5}{2}$ electron-spin sublevels. Central hole linewidths of 200 MHz are observed at zero magnetic field. Hole linewidths at high magnetic fields are as narrow as 40 MHz but these are inhomogenously broadened by superhyperfine interactions with the FFF nuclear spins. Estimates of hole-burning quantum efficiency are also obtained.
With a ring dye laser persistent spectral hole-burning was measured by fluorescence excitation spectroscopy in the zero-phonon lines of the 2llg ↔ 2llu transition in KI:S2 − and of the 3H4 → 3P0 transition of a defect center with three D− ions in SrF2:Pr3+:D−and also in the zero-phonon line at 462.5 nm of a color center in neutron-irradiated and annealed α-Al2O3. In all three lines holes with a width in the order of 1 GHz could be obtained at 1.5 K with single frequency laser. With broad-band laser in KI:S2 − and SrF2:Pr3+:D− spectral redistribution within the full range of inhomogeneous broadening could be observed which are explained by a photophysical model based on barrier crossing in asymmetric double-well-potentials. In the case of sapphire a photobleaching process is dominant which is effective also for nonselective excitation, and the holes are thermoresistant up to 300 K.
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.