Rare-earth Doped Crystals Research Articles

EPR and optical study of Yb3+-doped β-PbF2 single crystals and nanocrystals of glass-ceramics

β-PbF2 single crystalsdoped with YbF3 (0.2% and 2%) were studied by x-ray diffraction (XRD), electron paramagnetic resonance (EPR)and optical spectroscopy. EPR revealed the presence of only one kind of paramagnetic ionYb3+, in a cubic symmetry site. The optical absorption, emission andexcitation spectra enabled us to identify the transitions attributed toYb3+ in the cubic site and to determine its energy level diagram. Site-selectivelaser spectroscopy also evidenced the presence of another type ofYb3+ ions, undetectable by classical EPR. This second type, which dominates in the2%-doped crystal and exhibits cooperative luminescence, was attributed toYb3+ ions forming clusters.Transparent oxyfluoride glass-ceramics, containingβ-Pb1−yYbyF2+y nanocrystallites, were also synthesized and studied by XRD, EPR and optical spectroscopy. Two types ofYb3+ ions werefound, as in β-PbF2 singlecrystals. The optical properties of the oxyfluoride glass-ceramics turn out to be similar to those of ytterbiumactivated β-PbF2 single crystals. Moreover, the Yb environments found inPbF2 single crystals seem to already occur in the parent glass. Therefore, these materials areexpected to be good laser media, like the rare-earth doped fluorite crystals, either in bulkor fibre form.

Rare Earth Doped Crystals for Quantum Information: Quantum Computing and Quantum Storage

Quantum information uses special properties of quantum systems to manipulate or transmit data. This results in new processes, which are impossible to obtain with classical devices. For example, quantum computing and quantum storage, which are two important fields in quantum information research, aim respectively at performing very fast calculations and at storing quantum states of photons. These two applications could be obtained in solid-state systems using rare earth doped crystals. In this context, the most important property of these materials is the long coherence lifetimes of rare earth ion optical and hyperfine transitions. This allows one to create long-lived superposition states, which is a fundamental requirement for efficient quantum computing and storage. Promising results have already been demonstrated in rare earth doped crystals but it will be difficult to improve them with current materials. In this paper, we discuss the general and specific requirements for rare earth ions and crystals in order to perform quantum computing with a large number of quantum bits as well as all solid-state quantum storage. We also present the properties of a few recently studied crystals: Ho3+:YVO4, Ho3+:LuVO4 (quantum computing) and Tm3+:Y3Al5O12 (quantum storage).

Growth and concentration dependencies of rare-earth doped lithium niobate single crystals

A series of lithium niobate LiNbO 3 (LN) single crystals doped by rare-earth elements (Tm, Er, Dy, Tb, Gd, Pr) were grown under the same conditions by the Czochralski technique in a wide range of dopant concentrations. The distribution coefficients of rare-earth elements in the “crystal—melt” system of LN were determined at the beginning of the crystal growth. Their dependence on the dopant concentration in melt ranging from 0.1 to 3.5 wt% was investigated. Concentration dependences of physicochemical, ferroelectric and structural characteristics of doped LN single crystals were studied using X-ray diffraction, thermal analysis and Raman scattering measurements. A mechanism of rare-earth elements incorporation into the crystal lattice of LN was proposed for a wide range of dopant concentrations.

Spectral properties and quantum efficiency of optical emission in rare earth doped crystals

We have studied the spectral properties of Pr3+ -doped CaAl4O7 and SrAl4O7 crystals at 77 and 290 K. Emissions from the 4f5d band are detected using two-photon stepwise excitation. We demonstrated a new method to conveniently measure the quantum efficiency of deep UV emission using the two-step excitation and the difference of the integrated spectral intensities. The quantum efficiencies of the emission from the lowest 4f5d state in the two systems have been estimated and are less than 10% at room temperature. The effect of excited state absorption of the lowest 4f5d on the measurement of the quantum efficiency is analyzed. The actual efficiency may be higher when the effect is taken into account, especially under the condition of strong excitation.

Spectral emission of rare-earth doped Lu2SiO5 single crystals

Radioluminescence of float-zone grown single-crystal Lu 2 SiO 5 :RE (RE = Tb, Sm, Tm, Eu, Pr, Yb) has been measured and prominent electronic transitions have been identified. Tb, Sm and Tm are excellent host-lattice activators that yield strong emission relative to the luminescence from well-known Lu 2 SiO 5 :Ce. Eu- and Pr-doped Lu 2 SiO 5 exhibit weak luminescence whereas the Yb-doped sample is non-luminescent in the UV–vis portion of the spectrum. With the possible exception of Lu 2 SiO 5 :Pr, Tb 3+ is found to be an unintentional impurity in all samples investigated. Lu 2 SiO 5 :Sm is a candidate phosphor for radiation detection because it is dense and bright with orange–red emission that matches the spectral response of high-efficiency photodiodes.

Spectral properties of Yb3+ ions in LiNbO3 single crystals: influences of other rare-earth ions, OH− ions, and γ-irradiation

Absorption spectra have been studied in 190–3100nm region at various temperatures from 16 to 292K for Yb3+-doped and Yb3+/Nd3+-, Yb3+/Er3+- and Yb3+/Pr3+-co-doped LiNbO3 single crystals before and after γ-irradiation with a dose of 105–107Gy. Intense 400 and 500nm absorption bands were observed after γ-irradiation, which are due to the creation of oxygen vacancy (F-type color center) and Nb4+ polaron, respectively. Different change was observed in the 2870nm OH− absorption band intensity among the various rare-earth doped crystals. These are interpreted by discrepancy of ionic radii between substituting rare earth dopant ion and Li+ or Nb5+ host ion. The observed temperature dependence of the hot bands is understood by electronic transition from the thermally populated 2F7/2 Stark levels to the excited 2F5/2 level. The position of the Yb3+2F7/2→2F5/2 first resonant line was observed to be slightly different among the co-doped crystals. This is due to the perturbation of Yb3+ by co-doped rare earth ion which is located at the neighborhood of theYb3+.

Laser-induced spectral broadening and hole-burning in rare earth doped crystals and coherent optical spectroscopy for frequency upconversion

The excitation spectra for linear fluorescence and for frequency upconversion are recorded in Er3+ doped crystal Er:YAG. An excitation induced spectral broadening and hole-burning are shown to appear at the resonant frequency of the linear fluorescence excitation spectrum, where the upconversion is simultaneously enhanced. Rate equation analysis is applied to model the optical pumping processes in the crystal. We present evidence that the induced resonance broadening and hole-burning are the result of excitation competition between the linear absorption and the excited state absorption. Femtosecond pulse pairs are applied to excite the linear and nonlinear frequency conversion, resulting in coherent controlled branching ratio between the linear and nonlinear frequency conversion. The experimental reasults are in good agreement with the numerical evaluation based on optical Bloch equation.

Photo-luminescent screens for optically written displays based on upconversion of near infrared light

The structure and fabrication of a photo-luminescent screen for an optically written display based on upconversion of near infrared light is presented. Some of the rare earth doped fluoride crystals upconversion photo-luminescent materials are given. A uniform screen with a densely packed layer for high-resolution projection display was developed using the settling screening method.

Growth and spectroscopic properties of rare-earth doped YAl3(BO3)4 single crystals

A recent trend for UV-visible lasers is based on rare-earth doped crystals with self-frequency doubling (SFD). According to its absorption, luminescence, and nonlinear optical properties, the Huntite structured YAB (YAl3(BO3)4) is a prospective representative of such materials. Efficient laser emission and SFD have already been demonstrated for Nd:YAB and Yb:YAB crystals. Er, Yb, and Dy doped YAB crystals were grown by the top-seeded flux technique. Narrow absorption lines at 9K in the Er:YAB spectra proved the good crystal quality and the uniform Er incorporation. Weak satellite lines indicated Er interactions either with other Er ions or crystal defects at high Er concentrations. The good crystal quality was confirmed by the long (1.11ms) luminescence lifetime at RT in the Yb:YAB crystals. The Yb-transitions were broadened by strong coupling with lattice vibrations. The first growth and spectroscopic investigation of Dy:YAB were reported. Concentration dependent lifetime of the Dy luminescence was observed.

Direct nanosecond Nd→Ce nonradiative energy transfer in cerium trifluoride laser crystals

Using third harmonics of LiF:F 2 + tunable color center laser excitation and selective fluorescence detection the temperature and concentration dependencies of fluorescence decay curves of the high-lying 4 D 3/2 manifold of the Nd 3+ ion were measured in CeF 3 crystals. As a result the temperature dependence of energy transfer kinetics from the 4 D 3/2 manifold of the Nd 3+ donor ions to the 2 F 7/2 manifold of the acceptor Ce 3+ ions in the ordered practically 100% filled CeF 3 :Nd 3+ (0.056 wt%) crystal lattice was determined for 13– 55 K . Based on the temperature dependence the mechanisms and the channels of the Nd→Ce nonradiative energy transfer have been recognized. The net growth of the resonance Nd→Ce energy transfer rate in the temperature range from 25 to 55 K is found to be almost 3 orders of magnitude from 9.0×10 4 to 2.84×10 7 s −1 . In a CeF 3 :Nd 3+ (0.63 wt%) crystal a significant contribution of the Nd→Nd resonance energy transfer to the 4 D 3/2 manifold quenching is found for 20– 40 K and its channel and mechanism are suggested. Discussion of the possibility of subpicosecond and picosecond nonradiative energy transfer in rare-earth doped laser crystals is provided.

Optical pulse shaping using optical coherent transients

Using multiple temporally-overlapped, frequency offset and phase-tuned, linear frequency chirps, a new method of multi-GHz optical coherent transient optical pulse shaping and processing in inhomogeneously broadened rare-earth doped crystals is proposed. Using this technique with properly chirped laser sources, multi-GHz processing can be controlled with conventional low-bandwidth electronics and optical modulators. Specifically, this technique enables pulse shaping in the MHz to THz frequency regime with time-bandwidth-products exceeding 100,000, filling the gap between the operating regimes of femtosecond pulse shaping and analog electronics. The low bandwidth (~20 MHz) proof-of-concept demonstrations presented in this paper include pulse train creation, selfconvolution, auto-correlation, and chirped pulse compression.

Range of validity of McCumber theory in relating absorption and emission cross sections

The relation between absorption and emission spectra in a rare-earth doped solid is explored analytically and through numerical simulations on a model system. It is found that the widely used theory of McCumber [D. E. McCumber, Phys. Rev. 136, A954 (1964)] is strictly valid only when the linewidth of each individual transition between Stark levels is ≪kBT. When this assumption is relaxed, deviations between actual simulated spectra and McCumber-calculated spectra are found for both homogeneous and inhomogeneous broadening. The McCumber transform introduces a characteristic distortion of each component transition, in which the low-energy side of the spectrum is increased, and the high-energy side is decreased. The average degree of distortion for the model system is calculated to be 5% for a homogeneous width of 50 cm−1 at room temperature, but only 1% for a purely inhomogeneous width of 50 cm−1. For both homogeneous and inhomogeneous spectra, the distortion varies with the linewidth Δν as (Δν)3/2. The dependence of the distortion on temperature is different for the two types of line shapes, however: ∼T2 for homogeneous and ∼T−1 for inhomogeneous broadening. For most rare-earth doped glasses and crystals at room temperature, the average degree of distortion is expected to be on the order of 5% or less, although the actual discrepancy in the spectral wings can be much larger. The effects described here are likely to be of importance when using McCumber’s theory to obtain accurate cross section values in the spectral wings of rare-earth transitions.

Modeling of self-frequency-conversion lasers in rare-earth doped optical superlattice crystal

Recently attention has been focused on quasi-phase-matched self-frequency-conversion (SFC) laser generation in rare-earth-doped optical superlattice crystals, mainly because of their ability to generate multicolor lasers simultaneously across the visible portion of the spectrum through a single crystal. We present a general model for quasi-phase-matched SFC lasers in rare-earth-doped optical superlattice crystals that we fabricated by combining quasi-phase-matching theory and self-sum-frequency mixing (or self-frequency doubling) laser patterns. This model takes into account the TEM00 distribution of Gaussian beams with loose focusing, absorption, and coupling of pump beams and the effects of imperfect periodic structures. We analyze two types of errors, random period errors and linearly tapered period errors, in the periodicity of these structures to determine their effects on SFC laser properties (e.g., on effective nonlinear coefficients and phase-matching curves). Finally the model is applied to simulate SFC laser generation in a Nd3+ doped aperiodically poled lithium niobate crystal. By choice of one set of parameters, the calculated results, especially for threshold, total visible laser output power, and spectrum of relative laser intensity in the visible, explain the experimental phenomena in detail and indicate the validity of this model. Most significantly, the model presented makes understandable the simultaneous laser generation of multiple visible wavelengths (686, 605, 542, 482, 441, and 372 nm) from a single crystal.

Laser development of rare-earth doped crystals

Rare earth doped laser crystals present good optical properties providing most of the solid state lasers available today. In particular, some fluoride crystals are capable of forming solid solution with several rare earth fluorides, allowing one to take full advantage of the energy transfer mechanisms that might occur among them. LiREF 4 (RE=rare earth) crystals, for example, are so flexible that in some cases the doping concentration can go up to 100%. The Nd:LiLuF 4 (Nd:LuLF) system has a 1047-nm emission bandwidth 25% larger than Nd:YLF, which makes it very promising for laser mode-locked operation. Nevertheless, lutetium compounds are very difficult to obtain, therefore Nd-doped mixed crystals grown from LiF-Y 1− x Lu x F 3 (0< x<1) solid solutions were studied. A new laser medium was obtained for the Nd:LiLu 0.5Y 0.5F 4 crystal, which presents a Nd emission bandwidth close to the Nd:LuLF (1.82 nm). The mode-locked operation in a diode pumped laser system using the KLM technique was performed and pulses of 4.5 ps were readily obtained. It is also shown that the LiGdF 4 (GLF) is a promising host for diode pumped high power Nd lasers which require crystals with higher dopant concentrations. Another example is the Ho:LiYF 4 (Ho:YLF) laser operating at 2065 nm obtained as a result of concentration optimization of the sensitizers Er and Tm. The optimization was based on a model comprising the various energy transfer mechanisms that take place in these long lived metastable states, heavily dependent on the dopants concentration. As a quasi-four-level system, the Ho concentration must be kept very small (≤0.005 mol%). The laser operation was optimized by the dynamical coupling of pump and laser modes, and by the dopants optical cycle. These optimizations resulted in a CW Ho laser with 2 W output, in a diode pumped system operation.

Spectroscopic properties of Er in Bi2TeO5 crystals

The non-linear optical crystal Bi2TeO5 is a good host for several cations with segregation coefficients close to or larger than unity. Bi2TeO5: Er was the first rare earth doped crystal that was grown and studied. Eight Er3+ related transitions were identified in the absorption spectra, from the 4I15/2 ground state. All the expected Stark components of the observed transitions were detectable at 10 K. Further splitting of the lines related to the different Bi sites occupied by Er. Excitation from higher Stark levels of the ground state was detected at 200 K and above. Erbium related luminescence was observed in the doped crystals after photo-excitation of the 4F5/2 4F7/2 2H11/2 and 4S3/2 levels.

Magnetic circular dichroism of Nd3+ and Yb3+ ions in LiNbO3 crystals

Magnetic circular dichroism (MCD) of trivalent rare earth doped lithium niobate crystals is reported for the first time. Magneto-optical signals of Nd3+ and Yb3+ ions have been studied at 2 K as a function of the magnetic field strength up to 5 T. This study allows the identification of the Zeeman sublevels of these ions, which can be labeled by irreducible representations (Γ4, Γ5, or Γ6), so that the sign and allowance of transitions can be predicted. From the dependence of suitable MCD spectral lines on the magnetic field strength, the effective gyromagnetic factor of the ground state has been determined for both ions: (gNd)∥=1.4±0.1 and; (gYb)∥=4.7±0.1.

Progress in crystal growth and characterisation of rare-earth doped non-linear KTP crystals for laser applications

The incorporation of rare earth ions into KTiOPO 4 crystals and into other isomorphous phases is more efficient for impurities of small ionic radius. Some of the Nd 3+ and Er 3+ spectroscopic properties in KTiOPO 4 (KTP) and RbTiOPO 4 (RTP) are discussed to illustrate the contribution of several centres to the optical absorption and photoluminescence properties. The Nd 3+ centres observed in KTiOPO 4 crystals grown in a W-rich flux have larger optical absorption cross sections than those observed in KTP crystals grown in a self-flux.

Efficient phase conjugation via two-photon coherence in an optically dense crystal

We demonstrate efficient phase conjugation in an inhomogeneously broadened rare-earth doped crystal using electromagnetically induced transparency. The observed frequency conversion efficiency of the phase conjugate in energy over a backward pump beam is as high as 10% with \ensuremath{\sim}1/4 of maximal Raman coherence. This efficient phase conjugation has potential applications of nonlinear optical processes such as frequency up-conversion and Raman excited spin-echo optical memory.

Efficient electromagnetically induced transparency in a rare-earth doped crystal

We have observed up to 100% transmission of a probe field at line center due to electromagnetically induced transparency (EIT) in an optically dense rare-earth crystal of Pr3+ doped Y2SiO5 at 5.5 K. We also have examined both laser field intensity and temperature dependence of the EIT. Efficient EIT in this crystal opens potential applications such as efficient high-resolution image processing and signal processing, and optical data storage as well as lasers without population inversion in solids.

Luminescence properties of praseodymium- and erbium-doped silver bromide crystals

The luminescence of silver bromide crystals doped with praseodymium and erbium ions was investigated over the visible and near-infrared spectral regions. The emission, excitation, and absorption spectra, as well as kinetic parameters, were measured over a broad temperature range. The Judd-Ofelt analysis was used to calculate transition rates, branching ratios, and luminescence quantum efficiencies of rare-earth doped crystals. Good agreement between theoretical and experimental results was obtained.