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For rapid, unambiguous isotope identification, scintillator detectors providing high-resolution gamma ray spectra are required. We have fabricated Lutetium Aluminum Garnet (LuAG) using transparent ceramic processing, and report a 2-mm thick ceramic exhibiting 75% transmission and light yield comparable to single-crystal LuAG:Ce. The LuAG:Ce luminescence peaks at 550 nm, providing an excellent match for Silicon Photodiode readout. LuAG is dense (6.67 g/cm3) and impervious to water, exhibits good proportionality and a fast decay (∼40 ns), and we measure light yields in excess of 20,000 photons/MeV.

We report on the operation of the Mercury laser with fourteen 4 × 6 cm 2 Yb:S-FAP amplifier slabs pumped by eight 100 kW peak power diode arrays. The system was continuously run at 55 J and 10 Hz for several hours, (2 × 10 5 cumulative shots) with over 80% of the energy in a 6 times diffraction limited spot at 1.047 μm. Improved optical quality was achieved in Yb:S-FAP amplifiers with magneto-rheological finishing, a deterministic polishing method. In addition, average power frequency conversion employing YCOB was demonstrated at 50% conversion efficiency or 22.6 J at 10 Hz.

Yttrium calcium oxyborate YCa 4 O(BO 3 ) 3 (YCOB) is a novel non-linear optical crystal possessing good thermal, mechanical and non-linear optical properties. Large-aperture YCOB crystals with 75 mm diameter were grown for high-average power frequency conversion on the mercury laser system. The growth morphology (included facet and spiral growth), cracking and inclusions in the as-grown crystal boule were discussed as the critical problem for large-aperture YCOB crystal growth. This can be minimized through modification of the growth program, including pulling rate, separation procedure, and cooling program. High-average power frequency conversion of the mercury laser using YCOB has been demonstrated, and experimental validation of YCOB material yields 50% conversion at 10 Hz has been achieved.

Structural and optical properties of GaN/AlGaN multiple quantum wells (MQWs) grown by plasma-assisted molecular-beam epitaxy on ( 1 1 0 0 ) plane free-standing GaN substrates and (0 0 0 1) GaN quasi-substrates have been compared. Atomic force microscopy studies indicate that the films and MQW structures grown on both substrates replicate the surface morphology of the substrates. MQWs with AlGaN barriers grown in the presence of In flux have stronger photoluminescence (PL) intensity than those with AlGaN barriers without In. X-ray diffraction spectra of MQWs grown on the (0 0 0 1) GaN substrates show larger number of superlattices peaks than those grown on ( 1 1 0 0 ) substrates suggesting that the former have smoother interfaces. The PL spectra of MQWs deposited on (0 0 0 1) GaN substrates, where the growth is in a polar direction, exhibit a red-shift as well as a decrease in peak intensity with increase in well widths. Similar MQW structures on the ( 1 1 0 0 ) GaN, on which the growth is in a non-polar direction, do not exhibit this phenomenon, which we attribute to the absence of internal electric fields in these structures. PL intensity of MQWs with a well width of 75 A is 20 times stronger for those grown on the ( 1 1 0 0 ) plane than on the (0 0 0 1) plane GaN substrate.

Substituted gadolinium gallium garnet (SGGG) crystals with calcium, magnesium and zirconium as substituting ions have been used as substrates for bismuth-substituted iron garnet epitaxial films. Single crystals have been grown from the melt composition Gd 2.68 Ca 0.32 Ga 4.04 Mg 0.32 Zr 0.64 O 12 using the Czochralski pulling technique. Spiral growth is the most common problem in this system. With proper adjustment of the growth environment, colorless and optically transparent single crystals of SGGG up to 2 in diameter and 125 mm in length were produced. We will discuss the effect of different growth parameters on the crystal morphology, and propose an optimum growth condition for SGGG crystals.

Simultaneous cw laser operation at 1044 and 1120 nm in Yb 3+ :Sr 5 (VO 4 ) 3 F (SVAP) under diode pumping has been achieved for the first time. Output powers of 29.9 mW at 1044 nm and 26.5 mW at 1120 nm were obtained under simultaneous operation. A maximum output for each transition operating separately was 103 mW at 1044 nm and 157 mW at 1120 nm. In addition, we demonstrate over 27 nm of wavelength tunability of the 1120 nm band.

The mixed niobate–borate crystals KNbB2O6 and RbNbB2O6 are monoclinic with space group Pna21 and point group m. Some of their properties are favourable for applications in non-linear optics including exceptional resistance to laser damage, UV cut-off at wavelengths below 290 nm and large d-coefficients. Although they have refractive indices that are similar to KTiPO5 their birefringences are smaller than anticipated for structures containing planar borate rings and distorted NbO6 octahedra, leading to large acceptance angles for second harmonic generation (SHG). Type 1 phase matching has been observed at 532 nm in RbNbB2O6. The birefringence of KNbB2O6 is smaller than that of RbNbB2O6, and KNbB2O6 is not phase-matchable at this wavelength. Nevertheless, it is possible to tune the birefringence of KNbB2O6 by replacing some K+ ions by the larger Rb+ ions so that non-critical phase matching may be possible at room temperature.

Consideration of the ternary phase relationships between components of the XYB 2 O 6 borates, where X = Na, K, Rb, or Cs and Y = Nb or Ta, suggest that the conditions for their optimal single-crystal growth can be determined from predicted pseudo-binary phase diagrams. Several compounds have been synthesised on the basis of the predicted Nb 2 O 5 : Rb(K) 2 B 4 O 7 pseudo-binary systems and single crystals of KNbB 2 O 6 (KNB) and RbNbB 2 O 6 (RNB) grown for the first time by spontaneous nucleation from their self-fluxes. Larger crystals of RNB were grown and these also had a better optical quality. Small crystals of alkali-substituted KNB (Cs 0.3 K 0.7 NB) and RNB (Na 0.07 Rb 0.93 NB and K 0.44 Rb 0.56 NB) were also grown by spontaneous nucleation to provide additional information on the structure-property relationships of the XYB 2 O 6 compounds. Single crystals of both KNB and RNB measuring ∼ 20 x 10 x 10 mm 3 were grown by top-seeded solution growth (TSSG) using the same melt compositions as used in spontaneous nucleation. These crystals contained large densities of bubbles and solvent inclusions. However, small (5 x 5 x 3 mm 3 ) defect-free samples could be cleaved from these crystals. Even larger single crystals ( 38 x 33 x 6 cm 3 ) of RNB were grown by TSSG from an Rb 2 MoO 4 flux and these were free of inclusions. The potential of these single crystals for application in nonlinear optical devices was assessed by X-ray diffraction, optical absorption, laser damage threshold and efficiency of second harmonic generation at 532 nm.

Tunable laser operation in Yb:YCOB from 1018 to 1087 nm pumped by 1.4 W from a Ti:Sapphire laser tuned to 900 nm is demonstrated. The combination of the ability to grow large crystals with high optical quality, broad infrared emission, diode pumpable absorption bands, and a significant non-linear coefficient makes this material very promising for use in tunable and ultrafast laser systems.

As a new self-frequency doubling crystal, Nd:YCOB demonstrates a great potential for diode-pumped cw visible solid state lasers in a simple hemispherical cavity. We previously reported 62 mW cw green output with less than 1 W diode pump power absorbed in the crystal. Now we present more than 16 mW red cw laser at 666 nm from 5% Nd:YCOB crystal. As far as we know, it is the first time that diode-pumped cw red lasing is achieved by self-frequency doubling. Our effort on blue laser is also addressed.

The growth of YCa 4 O(BO 3 ) 3 (YCOB), one of the rare-earth calcium oxyborate family of crystals that melts nearly congruently is reported. Typical crystals produced in our laboratory are about 40 mm in diameter and 150 mm long with good optical quality. The crystal has good potential to be used for self-frequency doubling due to its excellent combination of nonlinear and laser properties. The single crystal growth of YCOB with different rare-earth substitutions is presented. We find that the substitution limit is reduced with either an increase or decrease in the ionic size of the substituted rare-earth element. The melt also moves further away from congruently melting with these substitutions. YCOB belongs to monoclinic crystal system. The orientation relationship between the crystallographic axes ( a , b , c ) and optical indicatrix axes ( X , Y , Z ) is discussed.

We describe experiments characterizing a new nonlinear optical crystal, YCa/sub 4/O(BO/sub 3/)/sub 3/ (YCOB). This crystal has a number of advantages over other commonly available nonlinear optical crystals. It has a higher nonlinear coefficient than KDP, can be fabricated to large sizes (/spl sim/3-in diameter, 8-in length), and has a high damage threshold. Moreover, this new nonlinear optical crystal is nonhygroscopic, has good optical quality and mechanical properties, allowing easy optical polishing. This crystal, YCa/sub 4/O(BO/sub 3/)/sub 3/, commonly termed YCOB, is one of a family of new nonlinear crystals, the oxyborates, that include RECa/sub 4/O(BO/sub 3/)/sub 3/ (RE=La/sup 3+/, La/sup 3+/, Y/sup 3+/, Sm/sup 3+/, Gd/sup 3+/, Er/sup 3+/, and Nd/sup 3+/). In this paper, we also successfully demonstrate a technique for improving the nonlinear optical properties of this crystal. This technique, ion substitution, has previously had limited success with other crystal hosts. However, the inclusion of yttrium in YCOB provides the opportunity to exploit this technique. Yb/sup 3+/, which has larger mass, but approximately the same atomic size as Y/sup 3+/ can be substituted into the crystal structure without introducing stress and nonuniformities. A systematic investigation of the linear and nonlinear characteristics of several crystals doped with various levels of Yb demonstrate that selective substitution of Yb in YCa/sub 4/O(BO/sub 3/)/sub 3/ improves the second-harmonic conversion efficiency by increasing the optical nonlinearity.

ion in the crystals YVO4, GdVO4, and Sr5(PO4)3F. The measurements were performed in the spectral region of the main laser transitions 4F3/2→4I9/2, 4F3/2→4I11/2, and 4F3/2→4I13/2by a continuous wave pump and probe technique. The calibrated gain and ESA spectra are presented and possible implications of ESA on the laser performance are estimated. It is shown that ESA can be a small loss factor to the laser emission near 1060 nm but does considerably diminish the effective emission cross sections near 1340 nm especially in Nd:YVO4 and Nd:GdVO4.

We present evidence for finite and/or imperfect low-angle grain boundaries and associated long-range stress fields, formed during the hydrothermal growth of berlinite (α-AlPO 4 ) crystals on multiple-seed-plate arrays. These defective boundaries originate at angularly misaligned junctions between adjacent seed plates as a result of the failure of some component sets of dislocations to propagate epitaxially into the new growth. Lacking a full complement of dislocations, the newly grown boundary may be imperfect and generate long-range stresses, leading to fracture and/or plastic flow.

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 mixed niobate-borate compound RbNbB2O6 (RNB) and its isomorphs have been produced in single crystal form using the spontaneous nucleation and top seeded solution growth (TSSG) techniques. X-ray diffraction studies show that RNB, KNB and certain K+, Na+ and Cs+ substituted variations of the pure compounds have monoclinic symmetry with the P11n space group. These crystals are transparent in the wavelength range 275 nm to 3.0 μm and have exceptionally high resistance to optical damage by high intensity laser radiation. The three indices of refraction of RNB and KNB have been measured as a function of wavelength by a polarised interferometric technique and observed to be intermediate between those of such compounds as s-BaB2O4 and LiNbO3. However, the birefringences of both RNB and KNB are smaller than those of other pure borate and niobate crystals. These results are discussed in terms of the structures of these different materials, in particular the role of odd-parity distortions of the NbO6 octahedra and B2O5 molecular ions.

This paper reports growth and optical properties of PbB 4 O 7 , a new host for χ (2) -type nonlinear optical interactions. Large, inclusion-free single crystals (20 × 8 × 8 mm 3 ) of PbB 4 O 7 have been grown from self-flux and stoichiometric melts contained in Pt crucibles, using a resistance heated furnace. The crystal has an orthorhombic structure, with space group P2 1 nm. The optical absorption spectrum is essentially featureless from the band edge near 300 nm through the visible and near infrared regions. The principal indices of refraction have been measured with high accuracy using a modified angle of deviation technique. These crystals are optically biaxial, point symmetry mm2: their non-zero d -coefficients d 15 = d 31 , d 24 = d 32 and d 33 have been measured using the Maker fringe technique. The optical properties are compared with those of other nonlinear optical crystals and related to the differences in their respective structures.

Polarized fluorescence spectra produced by site-selective excitation, and conventional polarized absorption spectra were obtained for Tm3+ and Er3+ ions individually incorporated into single crystals of strontium fluorapatite, Sr5(PO4)3F, also known as SFAP. Substitution of the trivalent rare earth ion for divalent strontium was achieved by passive charge compensation during Czochralski growth of the fluorapatite crystals. Spectra were obtained between 1780 and 345 nm at temperatures from 4 K to room temperature on crystals having the hexagonal structure [P63/m(C6h2)]. The polarized fluorescence spectra due to transitions from multiplet manifolds of Tm3+(4f12), including 1D2, 1G4, and 3H4 to manifolds 3H6 (the ground-state manifold), 3F4, 3H5, 3H4, and 3F3 were analyzed for the details of the crystal-field splitting of the manifolds. Fluorescence lifetimes were measured for Tm3+ transitions from 1D2, 1G4, and 3H4 at room temperature and from 1G4 at 16 K. Results of the analysis indicate that the majority of Tm3+ ions occupy sites having Cs symmetry. A point-charge lattice-sum calculation was made in which the crystal-field components, Anm, were determined assuming that trivalent thulium replaces divalent strontium in the metal site having Cs symmetry. Results support the conclusion that the nearest-neighbor fluoride (F−) is replaced by divalent oxygen (O2−), thus preserving overall charge neutrality and local symmetry. Crystal-field splitting calculations predict energy levels in agreement with results obtained from an analysis of the experimental data. By varying the crystal-field parameters, Bnm, we obtained a rms difference of 7 cm−1 between 43 calculated and experimental Stark levels for Tm3+(4f12) in Tm:SFAP. Absorption and fluorescence spectra are also reported for Er3+ ions in Er:SFAP. Measurement of the temporal decay of the room temperature fluorescence from the 4I11/2 and 4I13/2 manifolds yielded fluorescence lifetimes of 230±20 μs and 8.9±0.1 ms, respectively. The experimental Stark levels obtained from an analysis of the spectroscopic data were compared with a crystal-field splitting calculation. The initial set of Bnm parameters for Er3+(4f11) was established from the three-parameter theory and the final set of Bnm parameters obtained for Tm3+(4f12) in Tm:SFAP. The best overall agreement between calculated and experimental Stark levels is 8 cm−1 for 48 Stark levels, representing 12 observed multiplet manifolds of Er3+(4f11) in Er:SFAP.

Spectroscopic and laser-pumped laser studies of Nd 3+ -doped LiKYF 5 and LiKGdF 5 are reported. The optical absorption and photoluminescence spectra are strongly polarised, the sense of polarisation of the 4 I 9 2 → 2 F 5 2 , 2 H 9 2 absorption near 800 nm being opposite to that of the 4 F 3 2 → 4 I 11 2 laser emission near 1.05 μ. At ambient temperature (∼ 300 K) the luminescence decay times of excited Nd 3+ ions are in both hosts of order 480–490 μs, and strongly concentration dependent above 2 at.% Nd 3+ . Efficient laser action (∼ 25%) is observed under excitation with a long pulse Cr 3+ : LiSAF laser tuned to 797 nm, with low threshold energies of ca. 125 μ J .

Site selective upconversion of Nd3+:KLiYF5 is reported in dilute (0.2% molar) and concentrated (3.4% molar) crystals. On the basis of site selective excitation spectroscopy, the mechanism for upconversion is determined as being due to the sequential absorption of two photons. The excitation spectrum is shown to be composed of features resonant with each of the two stages of excitation. The dilute sample shows site selective upconversion fluorescence throughout the excitation range. For the more concentrated sample, energy transfer amongst the lower states equalises the population of the sites when shelved in the metastable 4 F 3 2 state so that site selectivity only occurs during the second absorption stage.

Spectroscopic data is presented for Pr 3+ -doped calcium fluorapatite (Ca 5 (PO 4 ) 3 F) and strontium fluorapatite (Sr 5 (PO 4 ) 3 F) crystals which were studied using polarized absorption and laser-induced fluorescence as optical probes. Complete tabulation of experimental energy levels is provided for strontium fluorapatite, where only one Pr 3+ site, having C s symmetry, is measurable. In calcium fluorapatite, at least three Pr 3+ sites of appreciable concentration are evident, though the majority site is demonstrated to show similarity to the site in the strontium fluorapatite host. An unusually high crystal field splitting of Pr 3+ energy levels is exhibited in both hosts. Intersite energy transfer, upconversion and cross relaxation are discussed for Pr 3+ in the calcium fluorapatite crystal.

The performance of the first diode-pumped Yb/sup 3+/-doped Sr/sub 5/(PO/sub 4/)/sub 3/F (Yb:S-FAP) solid-state laser is discussed. An InGaAs diode array has been fabricated that has suitable specifications for pumping a 3/spl times/3/spl times/30 mm Yb:S-FAP rod. The saturation fluence for diode pumping was deduced to be 5.5 J/cm/sup 2/ for the particular 2.8 kW peak power diode array utilized in our studies. This is 2.5/spl times/ higher than the intrinsic 2.2 J/cm/sup 2/ saturation fluence as is attributed to the 6.5 nm bandwidth of our diode pump array. The small signal gain is consistent with the previously measured emission cross section of 6.0/spl times/10/sup -20/ cm/sup 2/, obtained from a narrowband-laser pumped gain experiment. Up to 1.7 J/cm/sup 3/ of stored energy density was achieved in a 6/spl times/6/spl times/44 mm Yb:S-FAP amplifier rod. In a free running configuration, diode-pumped slope efficiencies up to 43% (laser output energy/absorbed pump energy) were observed with output energies up to /spl sim/0.5 J per 1 ms pulse. When the rod was mounted in a copper block for cooling, 13 W of average power was produced with power supply limited operation at 70 Hz with 500 /spl mu/s pulses.

Polarized absorption and fluorescence spectra were analyzed to establish individual energy (Stark) levels of Nd3+ ions in host crystals of Sr5(PO4)3F (SFAP) and Ca5(PO4)3F (FAP). Site‐selective excitation and fluorescence facilitated differentiation between Nd3+ ions in emitting sites associated with 1.06 μm stimulated emission, and nonemitting Nd3+ ions in other sites. Measurements were made on samples containing different concentrations of Nd3+ at 4 K and higher temperatures. Substitution of Nd3+ for Sr2+ or Ca2+ was accompanied by passive charge compensation during crystal growth. Crystal‐field splitting calculations were performed according to site for Stark levels of Nd3+ ions identified spectroscopically. We obtained a final set of crystal‐field parameters Bnm for Nd3+ ions in fluorescing sites with a rms. deviation of 7 cm−1 (52 levels in Nd:SFAP) and 8 cm−1 (59 levels in Nd:FAP). For one of the nonemitting sites in Nd:FAP we obtained a final set of Bnm parameters which gave a rms deviation of 6 cm...

Absorbance and site‐selective fluorescence studies of Eu3+:Sr5(PO4)3F (Eu:SFAP) provide a nearly complete characterization of the Eu3+ energy levels. These results are compared with calculated energies generated by lattice‐sum analysis. Eu3+ is found to occupy one site predominantly, but numerous minority sites are also evident; polarization studies of absorption and fluorescence lines provide evidence for the principle site having Cs symmetry. Vibronic transitions occur in spectra as well. Interpretation of the lattice location and charge compensation mechanism for the principal site is discussed in the context of obstacles arising from spectral interpretation, high crystal fields in SFAP, low site symmetry, low J‐numbers in Eu3+ energy manifolds, and lattice covalency. Eu2+ emission upon ultraviolet excitation is also demonstrated; valleys in the emission profile are attributed to intracrystal absorption by Eu3+.

The pumping and gain properties of Yb/sup 3+/-doped Sr/sub 5/(PO/sub 4/)/sub 3/F (Yb:S-FAP) are reported. Using a tunable, free running 900-nm Cr:LiSAF oscillator as a pump source for a Yb:S-FAP rod, the saturation fluence for pumping was measured to be 2.2 J/cm/sup 2/ based on either the spatial, temporal, or energy transmission properties of the Yb:S-FAP rod. The emission peak of Yb:S-FAP (1047.5 nm in air) is shown to overlap with that of Nd:YLiF/sub 4/ (Nd:YLF) to within 0.1 nm, rendering Yb:S-FAP suitable as an effective power amplifier for Nd:YLF oscillators. The small signal gain, under varying pumping conditions, was measured with a cw Nd:YLF probe laser. These measurements implied emission cross sections of 6.0/spl times/10/sup -20/ and 1.5/spl times/10/sup -20/ cm/sup 2/ for /spl pi/ and /spl sigma/ polarized light. Respectively, which fall within the error limits of the previously reported values of 7.3/spl times/10/sup -20/ and 1.4/spl times/10/sup -20/ cm/sup 2/ for /spl pi/ and /spl sigma/ polarized light, obtained from purely spectroscopic techniques. The effects of radiation trapping on the emission lifetime have been quantified and have been shown to lead to emission lifetimes as long as 1.7 ms, for large optically dense crystals. This is substantially larger than the measured intrinsic lifetime of 1.10 ms. Yb:S-FAP crystal boules up to 25/spl times/25/spl times/175 mm in size, which were grown for the above experiments and were found to have acceptable loss characteristics ( >

The performance of a Q -switched Nd:YAG laser using Cr 4+ :YAG as a passive Q -switched is reported. The Q -switched pulse contains two orders of magnitude more energy than the free running spike. An increase in the energy extraction efficiency of the Q -switched pulse with the appearance of multiple spikes, as well as shortening of the Q -switched pulsewidth were observed and analyzed.

The luminescence of Er3+ and Ho3+ ions was sensitised by Cr4+ ions in an yttrium orthosilicate Y2SiO5 (YSO) crystal. The luminescence spectra of YSO : Cr : Er and YSO : Cr : Ho crystals and the kinetics of the luminescence of the Cr donor ions were investigated at 77 K and 300 K and analysed. Radiative energy transfer from Cr4+ to Ho3+ was observed in a Y2SiO5: 0.2% Cr : 0.5% Ho crystal. The Cr4+—Er3+ energy transfer in a Y2SiO5 : 0.1% Cr : 1.3% Er crystal at 77 K involved a nonradiative dipole—dipole static mechanism in the interval 2.7–24 μs. The efficiency of nonradiative energy transfer from Cr4+ to Er3+ was estimated to be 14%—19% at 77 K.

Mn:Sr5(VO4)3F has been investigated as a laser material for operation near 1200 nm. Absorption and emission spectra are consistent with Mn5+ substitution into the distorted tetrahedral V5+ sites of this apatite crystal. Emission is almost entirely from the 1E state, resulting in a room‐temperature fluorescence lifetime of 475 μs. Together with a peak stimulated emission cross section of about 1×10−19 cm2 and strong, broad absorption bands, this is attractive for lamp‐ or diode‐pumped laser operation. However, the 1E emission makes Mn:Sr5(VO4)3F a three‐level system. Laser operation has been demonstrated at room temperature, but the efficiency is lower than expected. Single‐pass gain measurements and saturable absorption and fluorescence data indicate that the primary difficulty is excited‐state absorption at the pump wavelength. This absorption is very broad, and is probably due to 1T1 and 1T2 excited states.

High slope efficiency and low threshold laser performance have been achieved for both long pulsed and cw operation at 1.065 μm in Nd3+‐doped strontium fluorovanadate crystal, Nd3+:Sr5(VO4)3F, when pumped by narrow band pulsed Cr:LiSAF and cw Ti:sapphire lasers. However, there are inequivalent Nd3+ sites in the crystal. The absorption of Nd3+ ions in secondary sites, sites other than the site which contributes to lasing, may reduce the pumping efficiency and, consequently, the lasing efficiency. Strong concentration quenching of the Nd3+ 4F3/2 state was also observed reducing the quantum efficiency of the laser transition from this state.

Site‐selective spectroscopy reveals that Nd3+ ions occupy more than 40 different crystal‐field environments in Sr5(VO4)3F. Preferential energy transfer to the site responsible for 1 μm lasing occurs but becomes less complete with increasing temperature. The 4I and 4F3/2 Stark levels of the lasing site have been determined and an analysis of the crystal field performed. From the crystal‐field fitting parameters Bkq, a calculated energy‐level spectrum is determined up to 17 500 cm−1 with a rms deviation from the available experimental levels of 6 cm−1.

Free-standing wafers (50 mm diameter) of GaN were grown by halide vapor phase epitaxy on lattice-matched γ-LiAlO2. We report a transmission electron microscopy study of defects and defect densities in these wafers. The growth direction is [101_0]. Stacking faults in the basal plane are seen when viewing the specimen in the [12_10] direction with an average spacing of less than 100 nm. Convergent beam electron diffraction measurements show no switch in the polarity and thus the faults are proposed to be ABABACAC changes in the stacking. Threading dislocations are found to have a correlated arrangement with a density of 3×108 cm−2 when viewing the [12_10] direction and widely varying (depending upon location) when viewing in the [0001] direction. These dislocations act as “seeds” for postgrowth surface features that directly exhibit the correlated nature of these threading dislocations.

Spectroscopic properties of relevance for laser applications have been studied for rare earths in CaYA1O4. Fitting of a theoretical model to observed energy levels yields crystal field parameters and unobserved levels. Structural disorder leads to broad linewidths,but cross sections are larger than in many common hosts. Tm-Tm cross relaxation rates are comparable to those in Tm: YAG, and Yb-Er transfer rates are comparable to those in Yb, Er:phosphate glass. Room temperature Yb-Er upconversion is stronger than in YAG, but modeling shows that neither unconversion nor population of Er4I 13 2 is very efficient upon pumping Yb. The host's yellow color intensifies upon annealing in air, a potential problem for visible laser operation. Tm or Ho 2 μm laser operation, or Er 1.5 μ operation upon direct excitation of Er4I 13 2 , are more promising.

The spectroscopic parameters of Er3+‐doped crystals were determined with regard to the upconversion laser parameters of the green transition 4S3/2→4I15/2. The influence of excited‐state absorption on this laser channel was determined. Furthermore, upconversion pump mechanisms using ground‐state and excited‐state absorption around 810 and 970 nm were investigated by direct measurements of excited‐state absorption. The spectroscopic results confirm the pulsed room‐temperature laser experiments on the 4S3/2→4I15/2 transition. The lasers based on Er:LiYF4, Er:Y3Al5O12, and Er:Lu3Al5O12 were directly excited into the upper laser level by an excimer laser pumped dye laser in the blue spectral range. In Er:LiYF4, Er:KYF4, and Er:Y3Al5O12, laser action was achieved with two‐step upconversion pumping by a Ti:sapphire laser and a krypton ion laser. In the case of the fluorides, the additional pumping with the krypton ion laser was not necessary. The laser emission wavelengths were 551 nm for Er:LiYF4, 561 nm for Er...

A new class of Yb‐lasers is summarized in this article. The apatite family of crystals, based on the hexagonal structure of the mineral fluorapatite, has been found to impose favorable spectroscopic and laser properties on the Yb3+ activator ion. Crystals of Yb‐doped Ca5(PO4)3F, Sr5(PO4)3F, CaxSr5−x(PO4)3F, and Sr5(VO4)3F have been grown and investigated. Several useful laser crystals have been identified which offer a variety of fundamental laser parameters for designing diode‐pumped systems. In general, this class of materials is characterized by high emission cross sections (3.6–13.1×10−20 cm2), useful emission lifetimes (0.59–1.26 ms), a strong pump band (σabs=2.0–10.0×10−20 cm2), and pump and extraction wavelengths near 900 and 1045 nm, respectively. Efficient lasing has been demonstrated for several of the members of this class of materials, and high optical quality crystals have been grown by the Czochralski method. A summary of the laser parameters and a discussion of the Yb:apatite class of laser...

Site-selective excitation and fluorescence spectra, and conventional polarized transverse and axial absorption spectra of trivalent erbium-doped single crystals of calcium fluorophosphate (Ca 5 (PO 4 ) 3 F), a fluorapatie mineral known as FAP, have been analyzed over the wavelength range 1.60 μm to 0.34 μm at temperatures between 4 K and room temperature. There was no spectroscopic evidence to indicate a phase transition from the hexagonal phase at room temperature to the monoclinic phase around 133 K as reported earlier. Most spectra are interpreted in terms of Er 3+ ions replacing Ca 2+ ions in Ca(I) sites and Ca(II) sites. A majority of the Er 3+ ions prefer the Ca(II) sites, where charge compensation is possible, by replacing F - with O 2- during the growth process. Measured room temperature fluorescence lifetimes are reported for Er:FAP, YbEr:FAB, and Yb:FAP. Absorption cross-sections are reported for Er:FAP. The absorption cross-sections for Er 3+ ( 4 I 15/2 → 4 I 13/2 ) are larger than the emission cross-sections for the lasing transition in Er:phosphate glass. Overlap of the two spectra makes Er:FAP a passive Q -switch for the Er:phosphate glass laser operating at 1.53 μm. A crystal-field splitting calculation for Er 3+ ions in both Ca 2+ sites is described. The initial crystal-field parameters were obtained from a lattice-sum calculation. By varying the initial set of crystal-field splitting parameters, B nm (cm -1 ), by only a small amount, we obtained good agreement between calculated and observed Stark levels.

Small-signal gain measurements of flashlamp-pumped Cr-doped LiSAF and LiCAF are summarized for a range of material parameters. Flashlamp pumping allows efficient energy storage for both materials in the order 1.6 to 2.6% of the electrical pump energy. Small-signal gain values of up to 0.16 cm/sup -1/ are measured for Cr:LiSAF and 0.09 cm/sup -1/ for Cr:LiCAF. The gain of both materials is found to be affected by excited state absorption and upconversion. In addition the implications of these results for the design of laser/amplifier systems are discussed. >

Temperature and Yb 3+ concentration dependences of the Ho 3+ to Yb 3+ energy transfer process in KYF 4 are reported. The energy transfer rate from Ho 3+ to Yb 3+ increases as T 4 at low temperatures and depends quadratically on Yb 3+ concentration. The results are explained by a one-phonon-assisted nonradiative energy transfer process with an electric dipole-dipole interaction mechanism.

Several fluoride and oxide Tm and (Tm, Yb) doped crystals such as YLF, GLF, YAG, and YAlO 3 were tested for pulsed lasing at the 1 D 2 → 3 F 4 transition (453 nm blue) at room temperature under two step pumping by a Ti:sapphire and a dye laser. Laser operation was obtained for the fluoride crystals. The input-output curves exhibit a saturation effect of emission with respect to both pump lasers. This can be explained by a depletion of the 3 H 6 ground state of Tm 3+ and the competition between the excited state absorption from the 3 H 4 intermediate level and the mechanisms depopulating this level

The effect on Nd 3+ Stark levels of substituting Gd for Y in YLiF 4 , KLiYF 5 , and YVO 4 laser crystals is slight, as demonstrated in our spectra and in our best-fit crystal-field parameters. Gd-based crystals maintain the spectral properties of Y-based crystals while allowing for larger Nd concentrations

High‐resolution Fourier‐transform absorption and photoluminescence spectroscopy with site‐selective excitation reveal two sites for Nd3+ ions in each of the two new crystals KLiYF5 and KLiGdF5. Energy levels for the 4I multiplets and the 4F3/2 multiplet have been determined separately for each site in each crystal. Comparison of spectra and levels indicates that the crystal fields in each of the two materials are nearly identical. This suggests that KLiGdF5 is isostructural to KLiYF5.

Site‐selective photoluminescence spectroscopy of Ho3+ ions in single‐crystal KYF4, measured with a Fourier‐transform spectrometer at 80 and 2 K sample temperatures, gives evidence for two dissimilar classes of crystal‐field sites. Three similar sites contribute to each class giving a total of six sites in agreement with recent crystallographic results. Analysis of crystal‐field splittings indicates that only one of the classes can contribute to Yb→Ho upconversion green lasing in (Yb,Ho):KYF4.

Polarization effects have been studied in GaN/AlGaN multiple quantum wells (MQWs) with different c-axis orientation by means of excitation-dependent photoluminescence (PL) analysis. Quantum structures were grown on [0001]-oriented sapphire substrates (C plane) and single-crystalline [1100]-oriented freestanding GaN (M plane) using the metalorganic chemical vapor deposition technique. Strong PL spectrum line blueshifts (up to 140 meV) which are correlated with the excitation intensity have been obtained for C-plane MQWs, whereas no shift has been observed for M-plane MQWs. Theoretical calculations and comparison with the PL data confirm that the built-in electric field for C-plane structures is much stronger than the field present for M-plane MQWs. In the former case, the excitation-induced blueshift of the PL line is due to the screening of the built-in electric field by photoinjected carriers, which is consistent with the field strength of 1.23 MV/cm in the absence of excitation.

We report homoepitaxial GaN growth on freestanding (1100) oriented (M-plane GaN) substrates using low-pressure metalorganic chemical vapor deposition. Scanning electron microscopy, atomic-force microscopy, and photoluminescence were used to study the influence of growth conditions such as the V/III molar ratio and temperature on the surface morphology and optical properties of the epilayers. Optimized growth conditions led to high quality (1100) oriented GaN epilayers with a smooth surface morphology and strong band-edge emission. These layers also exhibited strong room temperature stimulated emission under high intensity pulsed optical pumping. Since for III-N materials the (1100) crystal orientation is free from piezoelectric or spontaneous polarization electric fields, our work forms the basis for developing high performance III-N optoelectronic devices.

A comprehensive investigation on the spectroscopic properties and laser performance of Nd3+ doped GdLiF4 (GLF), a new laser crystal, is reported. Our high resolution absorption and emission spectra for GLF are nearly identical to those of Nd:LiYF4 (YLF), a well known laser crystal, strongly suggesting that the two crystals are isostructural. The laser performance of Nd:GLF is very similar to that of Nd:YLF. A maximum laser‐pump‐laser slope efficiency of 68% and 67% was obtained for low (1.0 at. %) and high (4.0 at. %) Nd concentration GLF respectively. Concentration quenching of the fluorescence decay time was observed and appears to be due to the dipole‐dipole interaction between the isolated Nd3+ ions and Nd3+ ion pairs.

We report stable operation of a flashlamp-pumped LiSrAlF 6 : Cr 3+ regenerative amplifier for femtosecond pulses using chirped pulse amplification. We achieve Fourier-transform limited pulses of 100 fs duration with a time-bandwidth product of 0.44 and a pulse energy of 1.2 mJ.

We have performed several physical and optical measurements on the Cr:LiSAF (LiSrAlF(6)) laser material that are relevant to its laser performance, including thermal and mechanical properties, water durabilities, and Auger upconversion constants. The expansion coefficient, Young's modulus, fracture toughness, thermal conductivity, and heat capacity are all used to determine an overall thermomechanical figure of merit for the crystal. An investigation of the water durability suggests that the cooling solution should be maintained at pH = 7 to ameliorate problems associated with water dissolution. The Auger constant was found to become much more significant at higher Cr doping, in which excited-state migration leads to a substantial increase in the upconversion rate. We propose a design for a 50-W Cr:LiSAF laser system that is based on a detailed knowledge of all the relevant material parameters.

We report crystal growth, spectroscopic, and stimulated emission investigations of Cr4+ in the melilites Ca2Al2SiO7 (CAS) and Ca2Ga2SiO7 (CGS). The predominant absorption and emission features are similar to those of Cr:forsterite grown in an oxidizing atmosphere, consistent with the similarity of the tetrahedral cation sites in these crystals. Secondary emissiom and excitation bands of unknown origin are observed in both melilites. All emission is strongly quenched at room temperature, due primarily to intraionic processes in CAS and energy transfer in CGS. No stimulated emission has been observed in either material.

A boule of Cr:LiSGAF (Cr/sup 3+/:LiSrGaF/sub 6/), the Ga analog of Cr:LiSAF, was grown, a laser sample was fabricated, and its efficiency was measured using a laser-pumped laser configuration. The laser performance near 820 nm and related properties of Cr:LiSGAF are reported. The use of Sr instead of Ca results in a red shift of the absorption and emission band (along with an increase in the emission cross section). Insight into the effect of a change in the identity of the substitutional site on the properties of the laser is provided. >

We have observed true self-starting Kerr-lens mode locking in two distinct solid-state laser systems operating at room temperature by optimizing the cavity geometry to maximize the amplitude modulation with respect to the intracavity power variation. Pulses of 15 ps have been obtained from a Pr(3+):YLF laser system operating at 607 nm, and 53-fs pulses have been obtained from Cr(4+):YAG at 1.54 microm.