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Physical review. B, Condensed matter

Photoionization spectral hole burning and its erasure in Li2Ge7O15:Cr3+: Effects of site-dependent energy-level shifts within the band gap

S. A. Basun, S.P. Feofilov, Alexander A. Kaplyanskii, Uwe Happek, J. Choi, Kiwan Jang, Richard S. Meltzer

Published May 15, 2000• 14 citations
Publisher

Abstract

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 ${\mathrm{Li}}_{2}{\mathrm{Ge}}_{7}{\mathrm{O}}_{15},$ where the ${\mathrm{Cr}}^{3+}$ ions occupy three distinct noncentrosymmetric ${\mathrm{Ge}}^{4+}$ 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 ${\mathrm{Cr}}^{3+}$ 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 ${\mathrm{Cr}}^{3+}$ 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.

DOI
10.1103/physrevb.61.12848

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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,{\mathrm{Li}}_{2}{\mathrm{Ge}}_{7}{\mathrm{O}}_{15},Li2​Ge7​O15​, where the Cr3+{\mathrm{Cr}}^{3+}Cr3+ ions occupy three distinct noncentrosymmetric Ge4+{\mathrm{Ge}}^{4+}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+{\mathrm{Cr}}^{3+}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+{\mathrm{Cr}}^{3+}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.
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 ($\ensuremath{\approx}5$ 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 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.
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 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.
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.
Persistent spectral hole burning (PSHB) in the ${}^{7}{F}_{0}{\ensuremath{-}}^{5}{D}_{0}$ transition and the electron excitation in the ${}^{7}{F}_{0}\ensuremath{-}4f5d$ transition of Sm2+{\mathrm{Sm}}^{2+}Sm2+ doped in ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}{\ensuremath{-}\mathrm{S}\mathrm{i}\mathrm{O}}_{2}$ glasses were studied from the measurements of hole burning efficiency and the refilling of the burnt hole. The PSHB at low temperature is attributed to the optically activated rearrangement of OH bonds surrounding Sm2+{\mathrm{Sm}}^{2+}Sm2+ ions. On the other hand, the PSHB at high temperature is attributed to one-step electron tunneling in the excitation state. The barrier heights for hole filling corresponding to the two mechanisms were determined to be \ensuremath{\sim}0.27 and \ensuremath{\sim}0.90 eV, respectively. Thermal depth of the trap that captures electrons by two-step ionization via the 4f5d4f5d4f5d state was determined to be \ensuremath{\sim}0.35 eV below the conduction band. A model was proposed for describing the PSHB and electron excitation of Sm2+{\mathrm{Sm}}^{2+}Sm2+ doped in ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}{\ensuremath{-}\mathrm{S}\mathrm{i}\mathrm{O}}_{2}$ glasses. In addition, the dependence of hole burning efficiency on the Al2O3{\mathrm{Al}}_{2}{\mathrm{O}}_{3}Al2​O3​ concentration, temperature, and burning wavelength was also studied.
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.