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This paper describes the growth of single crystals of Cr4+ -doped Y3 Ga5 O12 (YGG) and Y3 Al5 O12 (YAG). Control of melt composition and post-growth annealing yields material that contains optimal concentrations of Cr4+ ions in distorted tetrahedral sites normally occupied by Ga3+ ions in YGG and Al3+ ions in YAG. Both Cr4+ -doped garnets exhibit strong visible and near-infrared absorption bands with peak cross sections of order 10-18 -10-19 cm2 and emit into vibronically broadened but weakly allowed 3 B2 (3 T2 ) 3 B1 (3 A2 ) transitions. Nonradiative decay is more efficient in YGG than in YAG as a consequence of the slightly reduced energy gap against radiative decay in the Ga-based garnet.

Electron spin-resonance (ESR) spectra of Cr3+ ions in LiSrxCa1-xAlF6 crystals with x = 0, 0.5, 0.8, and 1 have been measured at X-band microwave frequencies in the temperature range 1.3-300 K. The intense ESR lines from Cr3+ ions in these crystals have been fitted to a trigonal spin Hamiltonian to determine values of the g-tensor (g,g) and zero-field splitting, 2D. The principal g-values (g g1.974) are almost the same in all cases whereas 2D increases sequentially for x = 0, 0.5, 0.8, and 1. The value of |2D| for Cr3+:LiCaAlF6 decreases gradually as the temperature is increased from 1.3 K to 300 K. Over this same temperature range the value of |2D| for Cr3+:LiSrAlF6 increases. The ESR spectra for Cr3+ in LiSr0.5Ca0.5AlF6 and LiSr0.8Ca0.2AlF6 are inhomogeneously broadened by discrete centres with pseudo-trigonal symmetry, different configurations of which correspond to the random occupation of the second-nearest-neighbour cation sites by Ca2+ and Sr2+ ions.

Electron spin-resonance (ESR) spectra of -doped (LiCAF) crystals measured at low temperature reveal three distinct centres in the crystal. The most intense spectrum is fitted to a spin Hamiltonian with trigonal symmetry and is associated with ions that substitute for ions in the LiCAF structure. Two other spectra have orthorhombic symmetry: they are assigned to ions with charge-compensating -ion vacancies near to the impurity ions. The and components of the polarized optical absorption and luminescence spectra of in LiCAF show there to be at least two centres. The intensities of optical transitions of are calculated using the eigenfunctions of the ground and excited states of in trigonal and orthorhombic symmetry and are compared with the observed polarizations of the absorption and luminescence.

Single crystals of - and -doped have been grown by top-seeded solution growth. Large concentrations of rare-earth substituents on the site are possible in this crystal on account of the orientation and packing of adjacent coordination polyhedra. Odd-parity distortions of the rare-earth site result in large absorption and emission cross sections for both and . The absorption, emission and lifetime spectroscopy of : are reported. Applications of : and : as gain media in GaAlAs diode-pumped laser devices are discussed in terms of the spectroscopy of these materials. Preliminary experiments connected with such applications are reported. Whilst pumping the materials close to 800 nm, low-threshold, high slope efficiency continuous wave laser action was achieved at in : and strong upconversion photoluminescence close to 550 nm was observed in :.

The configuration ions and have been incorporated in strontium fluorovanadate (SVAP) crystals grown using the Czochralski technique. Such crystals provide fourfold-coordinated sites for occupancy by the and ions, which are located near the centres of their respective tetrahedrally bonded molecular ions and . The optical absorption spectra of these ions are in the range 500 nm to 1100 nm and feature a number of overlapping bands related to the splittings of the and excited states by the distorted tetrahedral crystal field. In terms of point-ion crystal field theory strong- and weak-field descriptions are appropriate to and molecular ions respectively. This is confirmed for by photoluminescence spectra which reveal sharp zero-phonon lines attended by vibronic structure characteristic of coupling to molecular vibrations of both and the host ion. Luminescence from the ion is quenched by efficient nonradiative decay at all temperatures between 10 and 300 K. The energy level structure of these ions has been calculated using group theory assuming reductions in symmetry from to to . A comparison of the calculated level structure with the absorption and luminescence spectra shows that and occupy distorted tetrahedral sites in SVAP. Selection rules derived for all transitions assuming symmetry account for all the features in the absorption spectra of and and in the photoluminescence spectrum of .

Electron spin-resonance (ESR) spectra of ions in (CYA) crystals have been measured at X-band microwave frequency. The spectra are only observed below 50 K: they are very broad and easily saturated by low microwave power at 4.2 K. The ESR lines, which vary in position with orientation of the magnetic field relative to the crystalline axes, have been fitted to a tetragonal spin Hamiltonian with the principal components observed to be close to those of ions in and . The associated microwave absorption is due to the lowest crystal-field level of the ground state of the ion in CYA. The measured g-tensor components and for in CYA can be explained in terms of the ground-state wavefunction composed of and components of mixed by the crystal-field terms and . The inhomogeneous broadening of the asymmetric ESR lines reflects a distribution of g-values, which is produced by tilting the principal z-axis of the effective spin away from the crystal c-axis and a distribution of the ground-state spin levels in of the ions through the random occupation of ions at the appropriate site in the disordered CYA lattice.

The electron spin resonance (ESR) of in was measured with an X-band frequency and low temperatures. The ions in this crystal show tetragonal symmetry with and . The analysis of the g-tensor indicates that the ground-state wavefunctions have dominant components of , resulting in a negative value of the crystal-field parameter . Three optical absorption bands with peaks at , 248 nm, and 298 nm were observed at room temperature. The 298 nm band is more strongly polarized along the a-axis than along the c-axis, whereas the 248 nm band shows the reverse polarization. The properties of the polarization can be explained by using wavefunctions of the lower excited state of , which is split into and in tetragonal symmetry, being lower in energy.

Single crystals of -doped aluminoborates (YAB) and (GAB) have been grown by the top-seeded solution growth technique, and the optical absorption, photoluminescence and electron spin resonance (ESR) properties of dopants measured at low temperature. The optical absorption spectrum of comprises two broad bands with peaks at 514 and 576 nm in YAB and at 520 and 586 nm in GAB. The energy separation of the two bands in each crystal is due to the static Jahn - Teller splitting of the excited state of the ions. Photoluminescence from excited ions occurs as a broad band in the near-infrared region with a peak at nm in YAB and 754 nm in GAB when measured at 14 K. Both absorption and photoluminescence spectra are strongly polarized. The spin Hamiltonian parameters for ions substituting at trigonally symmetric sites in YAB have been determined from the orientation dependence of the ESR spectra. The measured shifts in the components of the g-tensor from the free electron g-value of 2.0023 are interpreted in terms of the mixing of the higher component of the ground states and of the excited state into the lowest ground state by spin - orbit interaction.

The broadening of the lines of impurity ions in the substitutionally disordered crystals (x=0, 0.1, 0.5, 1) has been studied. Fluorescence line narrowing removes the inhomogeneous broadening created by variation in the crystal field and results in instrument-limited spectral features for x = 0 and 0.1. Exchange interactions between ions in octahedral positions and ions at second-nearest-neighbour sites cause significant broadening of the narrowed line when x = 0.5 and 1. Comparisons of experimental and simulated spectra have been used to estimate the magnitude of the exchange-coupling constant, - . Such simulations also show that the selection rule for the transition is broken by the spin in the - exchange system.

Electron spin resonance (ESR) spectra at X-band microwave frequencies are reported for dopant ions in (CGGO), (SGGO), (LGGO) and (LGS). The ESR measurements identify two distinct sites for in CGGO and SGGO and a single site in LGGO and LGS characterized by spin S = 3/2 for ions in the ground state. In CGGO a sharp line ESR spectrum, fitted to an axially symmetric spin Hamiltonian with , and , is assigned to substituents at trigonally distorted octahedral sites with a regular array of and ions in second-nearest-neighbour sites. A second spectrum of broader lines, due to ions in orthorhombically distorted sites associated with random occupation of the second-nearest-neighbour sites by and ions, has slightly different spin Hamiltonian parameters; , and . In this spectrum the principal axis is rotated by about from the c-axis towards the a-axis. In SGGO there are also ESR spectra from ions in trigonal and orthorhombic symmetry sites with almost identical spin Hamiltonian parameters. The ground state splittings of the trigonal centres measured optically by fluorescence line narrowing gives for both crystals. In contrast, only the trigonal centres are identified in LGGO and LGS, their ESR spectra being very similar to those of the trigonal centres in CGGO and SGGO. These results are discussed in terms of the substitutional disorder in these crystals and the nature of the site environment of substituents. The spectra correspond to ions in strong and weak crystal field sites, respectively, in agreement with earlier optical studies.

Electron spin-resonance (ESR) studies of in (CYA) reveal spectra consisting of fine-structure pairs of narrow symmetric lines and fairly broad asymmetric lines, corresponding to ions in sites with ordered and disordered configurations of the second-nearest-neighbour ions, respectively. The polarized intensities of the absorption band and the -line fluorescence, measured with the E-vector of the radiation parallel and perpendicular to the c-axis, are in the ratio . These polarization ratios suggest that the octahedra undergo an angular distortion such that the magnetic -axis of the spectrum is rotated away from the crystal c-axis. The microstructure of the surrounding is discussed in terms of the ESR and polarization results.

The optical absorption and photoluminescence properties of , and crystals grown by laser-heated pedestal growth (LHPG) have been measured at T = 14 K, 77 K and 300 K. These spectra show very high absorption coefficients for the three rare-earth (RE) ions and strong luminescence signals characteristic of ions at high concentration in distorted symmetry sites. The rare-earth ions in these compounds are shown to occupy single sites only. The strong luminescence output at 300 K makes these crystals potential hosts for diode-pumped laser gain media.

Results of optical absorption, photoluminescence, fluorescence lifetime and fluorescence-line-narrowing studies on several -doped crystals with the gallogermanate crystal structure are presented. Such crystals are unusual in that most fluoresce via both the broad-band transition and the sharper transition (R line), both of which are considerably broadened by the effects of substitutional disorder. An established adiabatic model is used to reproduce the selectively excited broad-band photoluminescence spectra and this shows that the substitutional disorder creates a distribution in the strength of both the octahedral crystal field and the electron - phonon coupling. The variations in electron - phonon coupling are attributed to variations in the strength of the non-octahedral crystal fields that determine the magnitudes of the coupling to non-symmetric phonon modes. This effect is shown to dominate the broadening of the band. The analysis of results of fluorescence-line-narrowing measurements on the R lines shows that these lines are also broadened by a distribution of both octahedral and non-octahedral crystal fields. In the case of crystals that contain a substitutionally disordered site in the close vicinity of the ion, the strong-field sites that give rise to R-line emission are shown to comprise an independent distribution and the broadening is dominated by the effects of the non-octahedral component of the crystal field. When the substitutionally disordered sites are further from the ion, the broad-band and R-line fluorescence derive from a single distribution of crystal fields. In this case the effect of the variation of the crystal field is again shown to be dominated by the non-octahedral contribution though the R-line fluorescence shows little evidence of this due to the strong-field sites being the extreme of the overall distribution.

A high-resolution study of the optical properties of -doped is presented. Excitation into the usual broad absorption bands produces fluorescence from a very broad line with a peak at at 15 K. The line is observed at an energy higher at temperatures above 100 K or by using excitation spectroscopy. The large broadening of the excited state is attributed to substitutional disorder at the and sites on the basis of site selective excitation and fluorescence line narrowing measurements. Estimates of the distributions of octahedral and non-octahedral contributions to the crystal field are made and it is shown that the non-octahedral crystal field is dominant.

The optical spectroscopic properties of Cr3+ in LiNbO3 have been investigated using optical absorption, fluorescence, fluorescence line narrowing (FLN), selective excitation and radiative lifetime measurements. The present results are compared with data obtained using other techniques including electron spin resonance (ESR) and electron nuclear double resonance (ENDOR), and shown to be consistent with there being five Cr3+ centres. The predominant centre is due to substitution at the Li+ site, which provides the weak crystal field in which Cr3+ ions emit into a broad 4T2 to 4A2 band. Using FLN, two centres experiencing a stronger crystal field are shown to be due to substitution at this same site, probably perturbed by the location of Nb5+ antisites and Nb5+ vacancies in the next-nearest-neighbour cation shell. The addition of Mg2+ ions to LiNbO3 appears not to create new Cr3+ complexes: rather it modifies the nature of the disorder, thereby changing the relative concentrations of the different centres. To this extent it becomes possible to identify two other lightly occupied Cr3+ centres where substitution is at the Nb5+ site. This total of five Cr3+ centres in LiNbO3 gives a consistent interpretation of all the available spectroscopic evidence, including ESR and ENDOR, while pointing to the difficulties of theoretical work in sites where the differences in the energy levels of Cr3+ ions are too small to be distinguished by the available models.

For pt.I see ibid., vol.4, p.7307 (1992). The R1 line of Cr3+ in an antiferromagnetic garnet crystal Gd3Sc2Al3O12 (GSAG) is broadened inhomogeneously and homogeneously. Site-selective techniques, such as fluorescence line narrowing (FLN), can remove the inhomogeneous broadening created by strain and compositional disorder. The FLN spectrum then reflects the broadening due to processes including spin-spin exchange interaction between Cr3+ ions in octahedral positions and Gd3+ ions in dodecahedral positions in the GSAG unit cell. A magnetic field can remove the homogeneous broadening due to antiferromagnetic exchange. The R1 line is associated with the spin-forbidden 2E to 4A2 transition. In general, the selection rule on the R1 line breaks down through spin-orbit interaction on the Cr3+ ion. Simulations of the FLN spectra of Cr3+:GSAG as a function of both temperature and magnetic field have been used to estimate the magnitude of the spin-spin exchange coupling constant, J approximately=0.25-0.33 cm-1. Such simulations also show that for the 2E to 4A2 transition to antiferromagnetic GSAG the Gd3+ spin in the Cr3+-Gd3+ spin exchange system is conserved in the presence of a magnetic field (B>2T), whereas the selection rule breaks down in the absence of a magnetic field.

This paper discusses the inhomogeneous broadening of the 2E to 4A2 emission (R line) in Cr3+-doped glasses. The impurity ion occupies approximately octahedral sites in silicate and borate glasses in which the R-line transition is broadened by site-to-site disorder associated with the glassy structure. The spectroscopic effects of disorder are to some extent removed by fluorescence line narrowing (FLN), which seeks to recover the homogeneous width of the R-line transitions. In the three glasses investigated, two silicate and a Li borate glass, the R-line width is approximately=190-200 cm-1. In magnetic fields up to 3.46 T and at low temperatures (T approximately=2 K) the narrowed R line splits into three components, one resonant with the excitation and two shifted to longer wavelengths.

The electron paramagnetic resonance (EPR), optical absorption and luminescence spectra of Ti3+ ions in YAlO3 have been measured at low temperatures. The optical absorption spectrum is composed of two broad bands with peak wavelengths of 434 and 492 nm, the energy separation of which is due to splitting of the excited 2E state. The broad-band luminescence with a peak at a wavelength of 605 nm at low temperatures is accompanied by a single sharp zero-phonon line at 540 nm. The absorption and luminescence spectra are strongly polarized. The spin-Hamiltonian parameters of Ti3+ ions substituting for Al3+ ions in sites with orthorhombic symmetry were determined from the orientation dependence of the EPR spectra to be gx=1.795(5), gy=1.850(5) and gz=1.950(5). The experimental g-tensor is compared with that calculated in terms of the mixing of the higher components of the 2T2 ground states and the 2E excited states into the lowest 2T2 ground state by spin-orbit interaction, the energies of these levels having been estimated from the broad-band absorption spectrum and the zero-phonon lines of the emission spectrum.

This paper discusses line broadening by compositional disorder and by spin-spin exchange interaction between the paramagnetic Cr3+ and Gd3+ ions in antiferromagnetic Gd3Sc2Al3O12 (GSAG) probed using fluorescence line narrowing (FLN) in the absence and presence of a magnetic field. The most intense R1 lines in Cr3+:GSAG, occurring at lambda =691.2 nm and 692.4 nm, have halfwidths of approximately=32 cm-1. The linewidths of the FLN spectra at 1.6 K are respectively 19 cm-1 and 13 cm-1, much larger than the width of the excitation laser beam (

The authors estimate, by reviewing the available photoluminescence data, the band alignments at heterojunctions of Zn(Cd)S(Se) semiconductors. The photoluminescence peak energies of ZnSe-ZnS, CdS-ZnS and CdSe-ZnSe strained layer superlattices (SLS), with a range of well and barrier widths, have been calculated using Kronig-Penney theory and assumed values of the band offsets which yield acceptable fits to both the experiments and data taken from the literature. For ZnSe-ZnS SLS, fair agreement is obtained between calculation and experiment by assuming, in line with previous work, a negligible conduction band offset. At the same time, good estimates of the photoluminescence peak energies of CdS-ZnS and CdSe-ZnSe superlattices are obtained by assuming a negligible valence band offset in each case. The authors construct a band alignment diagram which predicts that CdSe-CdS and CdS-ZnSe are type-II SLS with electron confinement in the CdS layers; these predictions are borne out by experiment.

ZnSe was first grown by MOCVD some years ago. However, it is only in recent years that the material quality has improved sufficiently for potential devices to be evaluated. This paper discusses the growth by MOCVD of high-purity ZnSe epitaxial layers, wide-bandgap II-VI ternary alloys, multilayers and low-dimensional structures with potential for blue or blue-green light emission. The growth conditions for high-purity ZnSe layers (ND-NA

The extent of interdiffusion in wide-bandgap II-VI superlattices of Zn-ZnSe, ZnSe-CdSe and ZnS-CdS grown at low temperature is assessed by measuring the photoluminescence spectra and X-ray diffraction profiles of samples annealed in the temperature range from 300 to 550 degrees C. It is shown that although Zn-ZnSe superlattices are stable against anion interdiffusion over the entire temperature range, the common-anion systems ZnSe-CdSe and ZnS-CdS show evidence of disordering at anneal temperatures greater than 400 and 450 degrees C respectively. These results confirm that the common-anion binary superlattices are stable against interdiffusion at low growth temperature.

For pt. I see ibid., vol.3, p.1915 (1991). The paper reports spectroscopic studies of Cr3+ ions in fluorozirconate. fluoride, borate and telluride glasses in which the average strength of the octahedral crystal field Dq/B varies from 1.6 to 2.4. The emission spectra are shown to be inhomogeneously broadened by disorder. Non-exponential decay of the emission intensity following pulsed excitation is another signature of the extensive disorder in these glasses. The large density of states of two-level systems is shown to account for the larger-than-expected homogeneous width of the R line in lithium borate and potassium borate glasses.

This paper reports optical absorption and luminescence studies of Cr3+-doped silicate glasses with compositions chosen to vary the octahedral crystal-field splitting at the Cr3+ site. The extensive site-to-site disorder in the glasses is signalled by the inhomogeneous broadening of radiative transitions and by non-exponential fluorescence decay patterns. Laser-excited luminescence shows that the octahedral crystal-field splitting Dq varies between the different sites occupied by Cr3+ ions. Both R-line and 4T2 to 4A2 emissions are inhomogeneously broadened by site-to-site disorder. Fluorescence line narrowing (FLN) of the R line gives a particularly dramatic demonstration of the site multiplicity. Measurement of the width of the FLN component of the R line as a function of temperature has been used to probe the density of two-level vibrational systems coupled to the electronic levels of the Cr3+ ion.

The luminescence of (F2+)H centres in additively doped KCl:CO32- crystals excited using light in the F-band is reported. The emission spectrum consists of two bands peaking at 610 and 750 nm which are strongly dependent on prior photo- and thermal-treatments. The two bands are associated with different configurations of the (F2+)H centres. The temperature dependences of the luminescence spectra indicate the existence of several stable configurations of the (F2+)H centres which are distinguished through the zero phonon lines observed in the absorption spectrum measured at low temperature.

CdS-CdSe multilayers form the only known example of a wurtzite, type II superlattice in which lattice strain generates giant piezoelectric fields which modify the band structure. The exciton luminescence band, ascribed to recombination of electrons in CdS layers with holes in CdSe layers, broadens and shifts to lower energy with increasing delay after excitation of the superlattice with a short pulse of light. These effects are interpreted in terms of dynamic band restructuring due to screening of the piezoelectric fields by photogenerated carriers.

A model of 2E to 4T2 tunnelling induced by zero-point vibrations is developed to account for the observed temperature dependence in the photoluminescence spectrum of Cr3+ ions in crystals with small energy separation Delta E=E(2E)-E(4T2). The model gives good agreement with experimental results for Cr3+ -doped Gd3Sc2Ga3O12, where Delta E=70 cm-1 is smaller than the mean phonon energy h(cross) omega approximately=180 cm-1.

The temperature dependence of Cr3+ photoluminescence has been measured in the garnets Y3Ga5O12, Gd3Sc2Al3O12 and Gd3Sc2Ga3O12. The observed intensities in the 2Eg to 4A2g and 4T2g to 4A2g emission transitions are interpreted using a model based on the effects of small changes in unit-cell size on energy level shifts due to variations in the strength of the octahedral crystal field. Such changes in unit-cell length are produced by either crystal non-stoichiometry or thermal expansion. The effect of thermal expansion is to reduce the energy level splitting between 2Eg and 4T2g excited states with increasing temperature. In garnets where this splitting is small, spin-orbit admixture of these levels produces measurable changes in the emission band shape and radiative decay time at low temperatures.