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A full understanding of the complicated spectra of the “N lines” in ruby that result from exchange-coupled pairs of chromium ions has remained a challenging but daunting task, especially for the nearest neighbour pair. Various inventive techniques were employed by spectroscopists in their studies of the problem. A major advance was made in 1962 by Kaplyanskii and Przhevuskii where they showed how the piezospectroscopic technique (stressing the ruby crystal in various directions) could be employed to help associate specific N lines with specific types of pair centres. In this work, using the methods of selective excitation combined with phase-sensitive detection of luminescence, we have succeeded in identifying all four transitions between the lowest level of the excited state of the nearest neighbour pair centre and the ground state, thereby confirming the energy level structure in the ground state. The low-temperature emission from this centre is found to be dominated by an exchange-induced electric dipole process, but the emission is almost all contained in the vibrational sideband. A similar behaviour is found for second nearest neighbour pair centres.

We have employed two-photon excitation to study the higher energy levels of Gd 3+ ions in CaAl 12 O 19 and we compare the results with those obtained using conventional UV excitation techniques. Under two-photon excitation, the luminescence intensity exhibits an unusual temporal behavior, a very long build-up followed by a decrease by orders of magnitude, ascribed to a recombination-assisted luminescence excitation mechanism assuming photo-ionization of Gd 3+ ions and trapping of free electrons on deep traps. We also find that the two-photon excitation spectra contain an additional broadening contribution which can be attributed to homogeneous broadening of excitation levels caused by excited state absorption into the conduction band. We believe that this may be a general phenomenon whenever participating photons produce ionization of impurity ions from metastable excited states. The phenomenon can manifest itself also in two-photon ionization spectral hole burning and in up-conversion processes (in the latter case, the homogeneous broadening can be caused by an intra-ion excited-state absorption).

The light induced ionization of impurity centers and defects in insulating crystals results in many phenomena such as charge transfer optical transitions in absorption spectra, photoconductivity, the photovoltaic effect, optical bleaching, and photochemical persistent spectral hole burning. This paper considers specific effects in photoionization related phenomena which are closely connected with inversion symmetry of impurity centers. These recently observed effects include (a) the drastic dependence of the photoionization energy on impurity sites lacking inversion symmetry (Li2Ge7O15:Cr3+), (b) the direct dependence of the sign of the photovoltaic effect on the polarity of impurity sites in doped pyroelectric crystals (LiNbO3:Cr3+), and (c) the anomalous broadening of photochemical persistent spectral holes in zero-phonon spectra of inversionless impurity centers (CaS:Eu).

We present here a further study of unusual features observed in photoluminescence (PL) of Eu2+ in CaF2 layers of CaF2–CdF2 SLs (SLs) [N.S. Sokolov, S.V.Gastev, A.Yu. Khilko, S.M. Suturin, I.N. Yassievich, J.M. Langer, A. Kozanezcki, Phys.Rev B 59 (1999) R2525]. PL of Eu2+ in SLs, in contrast to bulk CaF2:Eu crystals, fades out considerably under continuous UV excitation. This is mainly due to tunneling-assisted photoionization of Eu2+. We have measured the PL time dependence in uniformly and selectively doped SLs with different periods and compared the results with the calculations in the tunneling model. A careful analysis of PL in selectively doped SLs revealed an additional ionization process suggested to occur in all SLs independent of the Eu ion position. With the account of this process we find that the experimental results are in good agreement with theoretical calculations. We have estimated the values of the main parameters in the tunneling model, which show the best fit with the experiment.

The optical properties of insulators activated with rare earth ions are determined by the energy levels of the impurity; hence, the study of intra-ion transitions has been the principal focus of numerous investigations in the past. This has occurred to the neglect of other properties, particularly those involving the most energetic states of the impurity. To understand the optical properties in regions of higher energy, i.e. UV, the intrinsic bands of the host materials and the charge transfer dynamics between impurity and host must be taken into account. In such cases, the exact positions of the impurity states relative to the host conduction and valence bands become important and need to be determined in order to establish the physical behavior of the activated material. A complete treatment of the impurity–host system leads to a donor–acceptor model, similar to the common approach in semiconductor physics. Here we apply this model to discuss recent experiments on rare earth doped materials, including the luminescence efficiency of rare earth doped scintillator and phosphor materials, and laser cooling of solids.

The linear Stark shift of optical frequencies in impurity centers lacking inversion site symmetry results in various external electric field effects observed in persistent spectral hole-burning (PSHB) phenomena. This paper reviews some new effects of inversionless symmetry of centers in internal electric fields observed in PSHB of doped insulators with photoionization hole burning. (1) In multisite R-line ( 4 A 2 - 2 E) spectra of Cr 3 + ions, substituting Ge 4 + in the inversionless position in the crystal lattice of Li 2 Ge 7 O 15 (LGO), the two-photon self-gated photoionization PSHB was observed in the R-spectra of one site only. This drastic site dependence of PSHB demonstrates extremely strong(∼ eV) relative shifts of the position within the host energy gap of the 4 A 2 , 2 E levels of Cr 3 + ions, located at complex centers. These centers differ in the nature and the position of the nearby charge-compensating defect, where the Coulomb field produces different large linear Stark shifts of the electronic states of Cr 3 + . (2) Photon-gated PSHB in f-d spectra of Eu 2 + multisites in CaS crystal was observed with unusually broad hole wings in the case of lines belonging to perturbed inversionless sites. This efficient off-resonant PSHB is due to strong linear Stark shifts of the f-d transition frequencies of Eu 2+ ions in time-varying Coulomb fields resulting from photoionization and recharging of Eu 2+ -Eu 3+ ions during the hole-burning process in optically excited crystals.

In this paper we present novel spectroscopic data clarifying the nature of the dominant Cr 3+ centers in LiNbO 3 . In the past, the Cr 3+ centers in LiNbO 3 have been the subject of intense debate, which was partly caused by the intrinsic Li + deficit present in the available congruent samples, which leads to highly complex emission spectra. Recently, stoichiometric samples have become available which show a greatly reduced number of emission lines of defect-related high-field sites in the so-called R-line region. We have carried out extensive absorption, emission, resonant photoexcitation, and time-resolved measurements between 2 K and room temperature to unravel the nature of the dominant Cr 3+ sites in LiNbO 3 . From these studies we conclude that the dominant luminescent center is a Cr 3+ ion at a low-field site in both stoichiometric and congruent samples.

To properly describe the optical properties of doped insulators, one must include the host valence and conduction bands in addition to the energy levels of the impurity ion. Thus, the impurity-host system can be treated in a donor–acceptor model, similar to the semiconductor approach. Here we report another parallel between doped semiconductors and insulators: the formation of an impurity band. In Y 2 (SiO 4 )O : Ce 3+ , the lowest 5d band of the Ce 3+ ion lies about 500 meV below the conduction band. We observe conductivity in heavily doped samples while exciting the lowest 4f–5d transition. This effect is attributed to the formation of an impurity sub-band. In contrast to the semiconductor case, where the impurity ground state levels form a sub-band due to the large Bohr radii of the impurities, the Ce 3+ ground state 4f wave functions are very compact and only for the more extended 5d states is an impurity band found.

The importance of donor- and acceptor-like charge transfer processes in the luminescence properties of rare earth doped crystalline solids have become increasingly evident in recent years. As an example, we present optical studies, including photoconductive spectroscopy, of cerium-doped hosts that exhibit drastically different luminescence efficiencies. The differences are demonstrated to be due to the location of the defect electronic levels with respect to the host bands. Small, but crucial, differences in the relative position of the lowest cerium 5-d level can result either in complete quenching of the emission or in luminescence with quantum efficiencies close to unity.

The donor and acceptor charge-transfer transitions in YAlO{sub 3}:Ti{sup 3+}/Ti{sup 4+} were investigated through absorption, emission, excitation, one-photon and two-photon photoconductivities, excited-state absorption, and thermoluminescence. The results are contrasted to similar studies previously made on Al{sub 2}O{sub 3}:Ti{sup 3+}/Ti{sup 4+}. The dominant Ti{sup 3+} population has a photoionization threshold at 37000 cm{sup {minus}1}. The lowest energy Ti{sup 4+} charge-transfer transition has its zero-phonon energy near to 33900 cm{sup {minus}1}. The sum of these two quantities agrees with the reported band-gap energy of YAlO{sub 3}. The position of the Ti{sup 3+}/Ti{sup 4+} level in the band gap of YAlO{sub 3} is determined by these measurements. Stimulated emission from the {sup 2}{ital E} state of Ti{sup 3+} is limited by photoionization from this state. {copyright} {ital 1996 The American Physical Society.}

Photoinduced changes of the spectral, temperature and time dependences of the conductivity, the Cr3+ R-luminescence, the electron paramagnetic resonance, the refractive index and of the spatial variation of the linear birefringence are measured in nominally pure SrTiO3 and ferroelectric Sr1-xCaxTiO3 (x=0.007 and 0.12). In both materials a large, long time persisting increase of the photoconductivity for light with energy less than 2.1 eV was found as a result of previous illumination with higher photon energies. This memory threshold coincides with the low energy edge of the strongest extrinsic band in the excitation spectra. Basic mechanisms of extrinsic photoelectronic processes in both materials are proposed. They involve a parallel action of (i) photoinduced charge transfer from the valence to the conduction band via deep energy levels within the gap, (ii) recombination due to Cr3+-Cr4+ centers, and (iii) trapping and light-induced emission processes via relatively shallow traps.

Spectral and kinetic measurements of 2 E → 4 A 2 photoluminescence (PL) in SrTiO3 crystals containing small amounts of Cr were performed and a strong effect of IR illumination on the PL was observed. The dominant photoionization mechanism of Cr3+ PL excitation was established. The locations in the SrTiO3 band gap of Cr3+ donor levels (1.85eV below the bottom of the conduction band) and of Cr4+ acceptor levels (1.4 eV above the top of the valence band) were found. A new mechanism of optical erasing of burnt spectral holes of the R lines in SrTiO3: Cr was determined.

In Li2Ge7O15(LGO) crystals doped with Cr3+ and Mn4+ ions the transformation of luminescence spectra in the region of R-lines at ferroelectric phase transition D2h → C2x in LGO was used to determine the point group symmetry and microstructure of 3d3-centers. In experiments on pseudo-Stark splitting of R-lines in external electric field, the phase transition induced polarization of 3d3-ions was observed, the induced dipole moments being directed perpendicular to ferroelectric axis of C2x phase.

The point of this article is to investigate under what conditions the sum of a donor transition of an impurity ion, Mn+ → M(n+1)+ + e(CB), and an acceptor transition M(n+1)+ + e(VB) → Mn+ is equal to the band gap of the host. Our recent work on Ti3+, Ti4+ in aluminate host crystals provides the best of the few examples where both a donor and an acceptor transition are known.

Photoconductivity of Ti 3+ in Al 2 O 3 , Y 3 Al 5 O 12 and MgAl 2 O 4 were studied by one-step and two-step ionization at room and liquid-nitrogen temperatures. By virtue of its selectivity, two-step photoconductivity can give more accurate ionization thresholds than the one-step process. The thresholds obtained agree with the predicted values based on a simple model.

In pseudo-Stark effect studies on the R, R' lines of LGO: Cr 3+ the C 1 point symmetry of both types of Cr 3+ centers in the low-temperature ferroelectric (C 2v ) phase as well as evidence for local compensation of Cr 3+ (Ge 4+ ) extra charge in LGO lattices were established. In direct photoelectric studies it was found that Cr 3+ photoionization takes place in a two-quantum process via the intermediate 2 E excited state. With the help of fluorescent phonon detection the diffusive regime of THz acoustic phonon propagation in LGO at T = 2 K was observed; the anharmonic phonon decay times were measured for frequencies 55 cm −1 (150 ns) and 63 cm −1 (75 ns).

In photoconductivity studies in Al 2 O 3 :Ti 3+ and Y 3 Al 5 O 12 :Ti 3+ (YAG:Ti 3+ ) a nearly quadratic dependence of photocurrent on excitation intensity is experimentally observed. Spectral and kinetics measurements show that the Ti 3+ photoionization under visible and UV excitation in the t → e transition region takes place as a result of a two-step process via the intermediate e state. The experimentally estimated energy between this e level of Ti 3+ and the conduction band bottom is in Al 2 O 3 larger and in YAG smaller than the zero-phonon e-t line (ZPL) energy. This determines the essential difference in persistent hole-burning in the ZPL of Al 2 O 3 :Ti 3+ and YAG:Ti 3+ [1] and in the efficiency of e level optical pumping.

We review recently discovered processes that can lead to the erasure of persistent spectral holes burnt via two-step ionization (photochemical hole burning) in doped insulating crystals. These hole erasure processes are based on photoinduced recharging of ions. Two mechanisms are described here: acceptor transitions, which occur in the system Li 2 Ge 7 O 15 : Cr 3+ , and electron tunneling between impurity ions in different charge states, as observed in CaS : Eu.

The nature of Cr3+ center in LiNbO3 has been the subject of intense discussions in recent years. Partly, contradictory assignments of the centers resulted from intrinsic defects in the so-called congruent samples. More recently, stoichiometric samples, produced via the VTE process, have become available and show a largely reduced number of Cr sites. In this paper we present new spectroscopic data on a stoichiometric LiNbO3 : Cr3+ sample produced using the HTTSSG process. This sample shows an even further reduced number of lines in the so-called R-line region. In addition, the study of this sample sheds new light on the origin of near infrared emission lines first discovered in LiNbO3 : Cr : Mg, which might be due to Cr3+–Nb4+ pairs. All emission lines show an unprecedented small line width, which allows high-resolution studies on this fascinating system.

We measured the R-line fluorescence ( 2 E→ 4 A 2 ) under site-selective dye laser excitation via pseudo-Stark R-line components in a modulated external electric field, and hence the kinetics of 2 E-energy transfer among the Cr 3+ ions in mirror (C s ) sites in alexandrite at T=2K were studied. Fast (microsecond range) resonant nonradiative 2 E-energy transfer was observed. This shows that during the metastable 2 E-state lifetime τ R =2.4ms the electronic 2 E-excitations can perform many jumps over the Cr 3+ (C s ) ion ensemble. This result correlates directly with the results of four-wave mixing experiments [1,2], indicating the existence of long-range spatial diffusion of 2 E excitation among the Cr 3+ (C s ) ions

A new technique for the optical detection of nonequilibrium phonons in disordered solids is accomplished for the case of the strongly disordered crystal MgAl 2 O 4 :Cr 3+ and of Cr 3+ doped glasses with the help of site-selective laser excitation of dopant Cr 3+ ions within the inhomogeneously broadened 4 A 2 - 2 E transition (Δ v ≈150 cm -1 ) at liquid helium temperature. Nonequilibrium phonons, produced either by the heat pulse technique or in the course of electronic relaxation after optical excitation, are detected in the frequency range 5–100 cm -1 with both temporal and frequency resolution using phonon-induced anti-Stokes luminescence from the upper crystal field components of the 2 E-doublets of Cr 3+ ions excited resonantly into the lower 2 E-components at laser frequency v exc . The phonon-induced anti-Stokes luminescence kinetics at frequency v = v exc + ω reflect the kinetics of phonons with frequency ω in the excited volume.

Persistent spectral hole burning has been observed on the zero-phonon lines of the main and several perturbed sites of 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 Eu 3+ centers which are the dominant traps. Unusually broad ~ 100 GHz wings were observed in hole spectrum for perturbed Eu 2+ sites. This is explained by a new off-resonant hole-burning mechanism due to the linear Stark effect of inversionless Eu 2+ sites in time-varying internal Coulomb fields produced by dynamical recharging of Eu ions in the course of hole-burning process.

In LiNbO3:Fe, quasi-periodic instabilities are observed when recording photorefractive reflection gratings using a contra-directional two-beam coupling geometry. The mechanisms responsible for the noise and a technique used to eliminate these instabilities will be discussed

The energies of the $5d$ excited states of ${\mathrm{Ce}}^{3+}$ and ${\mathrm{Pr}}^{3+}$ impurities relative to the conduction band of the insulators ${\mathrm{Y}}_{2}\mathrm{Si}{\mathrm{O}}_{5}$ and ${\mathrm{Lu}}_{2}\mathrm{Si}{\mathrm{O}}_{5}$ were investigated through a temperature and spectrally resolved photoconductivity study. The effective ionization barrier of ${\mathrm{Pr}}^{3+}$ from the $5d$ state to the conduction band is found to be $0.15\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ smaller than that of ${\mathrm{Ce}}^{3+}$ in both ${\mathrm{Y}}_{2}\mathrm{Si}{\mathrm{O}}_{5}$ and ${\mathrm{Lu}}_{2}\mathrm{Si}{\mathrm{O}}_{5}$. The difference is explained by a model, represented by rate equations, that takes into account interconfigurational $4f5d\ensuremath{\rightarrow}4{f}^{2}$ relaxation for ${\mathrm{Pr}}^{3+}$, a process that is absent for ${\mathrm{Ce}}^{3+}$.

Electron delocalization processes of optically excited states of Ce3+ impurities in Lu2SiO5 were investigated by means of a temperature and spectrally resolved photoconductivity study. By monitoring separately the strength of the photocurrent resulting from excitation into each of the Ce3+?5d absorption bands, over a broad temperature region, three different delocalization processes, namely direct photoionization, thermal ionization, and tunneling, have been identified. The relative probabilities and temperature dependencies of each of these processes are discussed. The observed exponential temperature increase in the photocurrent, which spans six orders of magnitude, allows for the exact placement of the lowest energy 5d levels of the Ce3+ ions within the band gap. For Lu2SiO5:Ce3+, the lowest 5d state is determined to be 0.45 eV below the conduction band edge.

In chromium doped Li2Ge7O15 (LGO) crystals the stationary photocurrent dependence on the visible optical pumping intensity and the effect of additonal IR illumination on the photocurrent drastically depend on the content of Cr ions in different charge states. A model of photoionization, trapping and recombination processes in optically excited LGO : Cr crystals is proposed.

The dynamical behavior of 29 cm−1 phonons in single crystal fibers of Al2O2:Cr3+ (ruby) at 2 K has been investigated. The fiber diameters are of the order of the phonon mean free path and transport is dominated by surface reflections and scattering and by the acoustic mismatch at the crystal interfaces.

A study of photoluminescence, photo-EPR, and of photoconductivity was carried out on SrTiO3 and Sr1-xCaxTiO3 (x equals 0.007) crystals containing a small amount of Cr. As a result, the location of Cr electronic levels in the band gap was found, Cr3+ being a donor located at 1.85 eV below the CB bottom and Cr4+ being an acceptor at 1.4 eV above the VB top. In accordance with this scheme, it was shown that: (1) Photons with energies exceeding 1.85 eV excite Cr3+ R luminescence in the course of the events; h(nu) exc + Cr3+ yields e + Cr4+ yields Cr3+(2E) + phonons yields Cr3+(4A2) + h(nu) R. (2) Excitation with photon energies above 1.4 eV converts Cr4+ to Cr$3+):h(nu) exc + Cr4+ yields h + Cr3+. (3) A band with low energy threshold at 2.1 eV is observed both in excitation spectra of R luminescence and of photoconductivity. The specific character of photoluminescence excitation manifests itself in a number of interesting observations: (i) Strong increase of steady-state R luminescence intensity under IR irradiation additional to excitation in the visible. (ii) IR induced transient R luminescence caused by previous irradiation in the visible. (iii) Very long tails on the time scale of 1000s in R luminescence decay with weak dependence on temperature from 2 K to 78 K.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

From the time-resolved emissions of the ${\mathrm{Ti}}^{4+}$:${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ charge-transfer transition, two types of ${\mathrm{Ti}}^{4+}$ are found. The temperature-dependent lifetimes of the excited ${\mathrm{Ti}}^{4+}$ are well explained by a three-level system with a lower triplet excited state and a higher singlet excited state. The trapping of charge carriers following the charge exchange between Ti and the host is shown by the thermal release of trapped electrons and holes. These are revealed by thermally stimulated conductivity and thermoluminescence. The possible identities of these traps are discussed.

Studies of the spectral response of photocurrent under excitation in R-lines have revealed the mechanism of photocurrent in ruby. It consists in a motion over the Cr 3+ ion system of an “excess” charge (EC) (i.e. the Cr 2+ or Cr 4+ states), the charge transfer occuring under excitation to the 2 E state of Cr 3+ ions which are close to EC. The sign of current is determined essentially by the polar asymmetry in the spatial excitation to 2 E of the Cr 3+ ions around EC and is reversed with the excitation frequency swept across the R-lineshape. When excited in the broad U-band, the asymmetry resulting in the photocurrent flowing against the applied field orginates from resonant nonradiative transfer to EC environment of the 2 E excitations from the bulk of the Cr 3+ ions. The kinetics of this negative photocurrent agrees with known kinetics of nonradiative resonant energy transfer between Cr 3+ ions.

Ruby, Al2O3:Cr3+, is clearly an insulator, its electrical conductivity becoming finite only under optical excitation. Recently, the unusual photoelectrical properties of ruby have become a subject of considerable interest. A study has been carried out1 at a low (~ 10 K) temperature on crystals of concentrated ruby of the effect of intense Ar laser irradiation in the region of the broad U and Y absorption bands. A strong uniform electric field Es ~ 106 V/cm directed along the trigonal C axis was found to appear gradually and reach saturation in the excited region of the crystal: the field persisted after the pumping was turned off and the sample warmed up to room temperature.