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V Yb%BaCaBO,E A Potential New S elf-Frequency-Doubling Laser Material

IC. I. Schaffers L. D. DeLoach C. A. Ebbers S. A. Payne

This paper was prepared for submittal to the Advanced Solid State Lasers 10th Topical Meeting

Memphis, Tennessee January 30-February 2,1995

March 9,1995

DISCLAIMER

This document wasprepared as an account of work sponsored by an agency of the United States Govemment. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclceed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commeraal product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Govemment or the University of California, and shall not be used for advertising or product endorsement purposes.

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Yb&:BaCaBO,F: A Potential New Self-Frequency-Doubling Laser Material

Kathleen I. Schaffers, Laura D. DeLoach, Christopher A. Ebbers, and Stephen A. Payne

Lawrence Livennore National faborazory, University of California, PO Box 5508 Livennore, California 945.50

Abstract

Yb:BaCaBwF (Yb:BCBF) has been investigated as a new laser crystal with potential for self-frequency- doubling. An YIxBCBF laser has been pumped at 912 nm, and a measured slope efficiency of 38% has been obtained for the fundamental laser output at 1034 nm. Single crystal powders of BCBF have been compared kth KD*P for a relative measure of the second harmonic generating potential, yielding deffIBCBF]= (0.66)d&KD*P]. The growth, spectroscopy, laser performance, and linear and nonlinear optical properties of YIxBCBF are reported here.

Introduction The recent surge of interest in solid-state lasers based on Yb3+d0ped crystalline hosts is centered around the attractive feature of pumping the laser medium with high-power InGaAs diode laser^,^*^ a reduced quantum defect resulting in less heating of the gain medium, and a substantially longer radiative lifetime than their ~d3+40ped counterparts (-34 times 1onger).3*4 The longer energy storage times permit greater pumping efficiency for systems employing diode lasers.

Here, we report on a new infrared laser, Yb:BaCaB03F or Yb:BCBF, with some characteristics similar to Yb:YAG, but offering properties which uniquely suggest its potential for multi-functional use. BaCaB03F has been recently introduced by Keszler, Akella, and coworkers as a new solid-state material that melts congruently and crystallizes in a noncentrosymmetric, hexagonal structure which is predicted to be a phasematchable frequency conversion crystal5 Our studies have focused on the 1.034 pm laser properties of Yby as well as its potential for

intracavity doubling. The potential in Yb:BCBF for both lasing at 1.034 p and selfdoubling the laser light to generate its 0.517 pm harmonic is particularly attractive, since the ~ b 3 + laser ions, in contrast to Nd3+, do not exhibit an absorption loss at the frequency of the doubled light.

Crystal Growth Two synthesis methods were employed for preparing a melt from which a crystal could be grown. The first method attempted was to melt together stoichiometric ratios of the starting materials (BaCO3 (Johnson- Matthey, puratronic), BaF2 (Advanced Technologies, Inc., optical grade), CaCO3 (Johnson-Matthey, grade l), B2O3 (Alfa, 99.99%) and YbF3 (Alfa, 99.999%)] in a platinum crucible. The resulting melt was brownish in color and exhibited an inhomogeneous nature as evidenced by insoluble solids at the bottom. This is likely due to the formation of Ba and/or Ca peroxides upon decomposition of the starting carbonates, which then attack the crucible, as suggested by Keszler6. To avoid these problems, a second method, proposed by Keszler? was employed whereby a two-step process would provide for the presynthesis of BaCa2(BO3)2 powder followed by addition of BaF2 and YbF3 and subsequent melting in the Pt crucible in an argon atmosphere, as described by the reaction:

BaCa2-2x(B03)2 + BaF2 + 2xYbF3 + 2BaCal -xYbxB03F1 -xOx (1)

A powder of BaCa2(B03)2 is prepared by grinding together the appropriate ratios of BaC03, CaCO3 and B2O3 and heating at 960°C for -18 hours to fully decompose the carbonates and form the product. This

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4. c QlSTRlBUTlON OF THIS DOCUMENT IS UNttMmD

method produced clear, nonviscous melts suitable for crystal growth experiments.

Crystals of BaCaBO3FYb were grown by the Czochralski technique in a Pt crucible using a R wire with a loop fabricated at one end as the nucleation source. Pull rates varied from 2-5 mm/day with rotation rates of 4-10 rpm. Typical cooling rates of the crystals after each growth experiment were 10-16 hours. The overall dimensions of the boules average 2.0 cm in diameter by 25 cm in length. Although the material was substantially fractured due to the unavailability of a single crystal grain for seeding the runs, volumes of single crystal material several cubic millimeters in diameter were available for spectroscopic and laser measurements.

Spectroscopy Results Room temperature absorption and emission spectra were acquired for the two principal polarizations of this optically uniaxial crystal, for light polarized parallel and normal to the c-axis. In addition, the emission lifetime of Yb3+ in the BCBF host has been determined.

The effective absorption cross section (Gabs) of the 2Rn-2F5n transition was determined from the spectral absorbance through a given pathlength and the

. known impurity concentration, while the Einstein relation? the normalized emission lineshape function, the refractive index, and the, emission lifetime are employed to calculate the emission cross section (oemI.3

The room temperature polarized absorption and emission spectral cross sections for Yb3+ in BCBF are presented in Figures 1 and 2, respectively. The spectroscopy robably arises from the substitution of n 3 + on a ag+ lattice site in the crystal structure since the size of an ~ b 3 + ion is most closely suited to the sevencoordinated Ca site; the local site symmetry of Ca has five borate group oxygens that are all coplanar and lie in a plane normal to the c-axis with F ions above and below this plane along the c-axis.6 Because Yb3+ substitutes for a Ca2+, charge balance is likely achieved by replacing one of the two bonding F ions by an 02- ion.

In Figures 1 and 2, Yb3+ may be seen to have large crystal field splittings, and a strong polarization dependence for both the absorption and emission transitions. Oscillator strengths observed for the transitions are highest in the direction of E l c and accordingly this is the preferred orientation for laser pumping and extraction.

I

Figure 1. Absorption spectra of Yb:BCBF with ELc and Elk. The pump band is at 912 nm.

la 1 I I I

wWrhnoth(nm1

Figure 2. Emission spectra of Yb:BCBF with Elc and Elk. Laser action occurs at 1034 nm.

c

Y

i

The pump and extraction characteristics for an Yb:BCBF laser have been considered based on these spectroscopic data, and the values of the important spectral features and derived laser parameters are summarized in Table I. For comparison, a tabulation of Yb:YAG data4 are also presented and found to be similar.

Table I. Spectroscopic and laser parameters of Yb3+ laser media

The data predict that mBaCaB03F will be a favorable diodepumped laser which operates with a fundamental wavelength of 1.034 pm. Notable differences occur in the Yb3+ upper laser level lifetime

+ which is somewhat longer, 1.17 ms for BCBF versus 1.08 ms for YAG. On the other hand, the emission cross section at b x t is less than for YbYAG (ae t

respectively). Even though the two materials have similar optical laser parameters, Yb:BCBF is not likely to compete for the same applications with Yb:YAG since the ‘ YAG host possesses superior thermomechanical properties. However, the actual laser wavelength, 5 = 1.034 pn for BCBF versus 1.031 pm for YAG, and the potential for tunability of the laser line (M = 24 nm) would possibly suggest complementary uses. Furthermore, the SHG properties of BCBF may be useful for some devices.

values are 1.3 x and 2.0 x cm 1 ,

Laser Performance The performance of an Yb:BCBF laser oscillator operating at its fundamental wavelength of 1.034 pn has been investigated with a Ti:sapphire laser (-912 nm), mechanically chopped for a 25% duty cycle, as a surrogate source for laser diode pumping. The crystal plate used in our laser measurements was doped with 4.8 x 1g0 Yb iondcm3. A crystal slice from the region of highest clarity was cut from a boule with visible scattering defects and parallel sides were polished down to a thickness of 1.0 mm. The crystal was oriented in the laser cavity such that the pump light

entering the crystal and the Yb laser output weze both polarized for light along the ordinary ray corresponding to the ELC absorption and emission spectra; in m B C B F this polarization coincides with the greater pump light absorption and higher laser gain. The crystal was centrally located in a IO-cm nearly concentric laser cavity.

The laser efficiency has been evaluated by a determination of the output power of the Yb:BCBF laser versus the pump power absorbed by the crystal. These results are plotted in Figure 3 for two different runs performed on the same crystal using a 3% output coupler where only the position of the pump beam on the laser crystal has been changed by translating the sample in the beam. A slight shift in power levels is observed, but the slope efficiency is approximately 38% in both cases. The threshold is near -100 mW for both runs. We anticipate the possibility of improved laser performance with the availability of crystals having lower scattering loss.

0

Figure 3. Efficiency data for Yb:BCBF crystal. Representative data are given for two different NIIS performed on the same crystal and using a 3% output coupler.

Second Harmonic Generation The effective nonlinear coefficient of Yb:BCBF was determined fiom a powder test by irradiating the sample with 1.064 pm light and observing the intensity of the 0.532 pm light generated. A similarly prepared sample of KD*P was used as a standard. The experiment utilized samples which were crushed and sieved to sizes between 80 and 100 pm. A monolayer of particles was spread and evenly distributed in DUCO cement on a glass slide. Using a cw NdYAG laser with an output of 100 mW for the source of 1.064 p light, the samples were irradiated with a beam focused to -5 mm. Appropriate bandpass filters were positioned

c

Y

..-

before and after the sample to remove possible 0.532 pm light that might impinge on the sample and to prevent excess light of other wavelengths fiom reaching the detector. A photodiode detector interfaced to an oscilloscope was employed for the relative intensity determinations. The effective nonlinear coefficient, a, is obtained &om the measured second harmonic intensities, Iza, according to:

&fABCBF) = (rI2a(BCBF)fl2(0(KD*P)l - [N(KD*P)M(BCBFII) - deff (KD*P) (2)

By using &ff(KD*P) = 0.35 pm/V (Type II)? the relative count, N, of KD*P versus BCBF particles in -25 mm2 area of 1662/923, and the measured signal ratio of 0.25, the effective nonlinear coefficient for BaCaB03FYb becomes &ff= 0.23 f 0.1 pm/V. This value is similar to that predicted by K d e r and coworkers where a &ff value of 0.35pmN is calculated from the orientation and density of the BO3 groups in the crystalline 1attice.5

Linear Optical Properties and Predicted Phase- matching angles Measurements of the optical properties including birefringence (no - ne) determinations for sodium D

, right and dispersion measurements for the extraordinary ray rebctive index, ne, have been completed for an assessment of phasematchability.

A polished plate of BCBF doped with 1.46 x 1g0 Yb i0ns/cm3 was characterized with a high accuracy AbW refractometer to determine the values of n, and ne for light of wavelength 589 nm where the birefringence .is 0.0484. Also, a method for measuring the refractive indices of small single crystals was employed for determining the wavelength-dependence of ne. Here, the method of spindle stage rehctometry allowed the rehctive indices to be measured for light wavelengths between 365 and 1000 nm as described in reference 9. The predicted refractive indices along and perpendicular to the c-axis have been used to calculate PM angles in BCBF. 10

Results of the linear and nonlinear optical property determinations and the subsequent analyses are summarized for Yb:BCBF in Table I1 and a comparison has been made with the common SHG material, KD,PO, or KD*P. From the dispersion curves, Type I PM can be accommodated without any compromise of the o p t i m i i laser performance, regardless of the selected PM angle and position of maximum SHG coupling given that the high gain configuration for Yb:BCBF laser action requires both pump light and stimulated laser emission to be polarized normal to the c-axis.

Table II. Linear and nonlinear optical properties - of Yb:BCBF compared to KD*P.

Property BCBF KD*P

Phasematching Type I Type 1

PM angle, 8, 37" 40.9" Angular 5.8 4.9 sensitivity, p(e)

[cm"lmrad] Rehctive 1.6548, 1.6064 1.4938, 1.4599

indices

d& 0 023 0.35 [Typelll

0

no,% @ 1034 nm Birefringence, 0.0484 0.034

An

Moreover, the fundamental wavelength of Yb:BCBF is 1.034 pm, so that the self-fiequency doubling process has been considered for producing 0.517 pm light. Also tabulated are the estimated angular sensitivities for each process, where smaller values correspond to less beam walk-off for a given pathlength. By comparison, new self-frequencydoubling crystals based on Yb, like Nd, may generally produce light in the green spectral region, but offer the advantage that the activator ions are not absorbing the generated light at the second harmonic.

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t

Summary We have reported on the spectroscopic parameters, laser performance, and the linear and nonlinear indices of Yb3+ in BaCaB03F. or BCBF. We have grown the first large single crystals and have judged the Yb laser parameters and the SHG features to be moderately favorable compared to other available crystals (e.g. Yb:YAG and KD*P, respectively). Perhaps it is more significant to note that Yb:BCBF is particularly unique in that both of these attributes are simultaneously available from a single material, since useful Yb-lasers and SHG crystals are both somewhat uncommon. The challenge now is to identify a suitable laser architecture that emphasizes the attributes of this novel laser crystal.

magnitudes of the SHG d-coefficients and the gain cross section. Perhaps with passive Q-switching or

compact green source.

The challenges entail compensating for the moderate . r

other means it will be possible to devise an efficient, 4

a

other means it will be possible to devise an efficient, compact green source.

Acknowledgments We would l i e to express our appreciation to several Lawrence Livermore National Laboratory employees, including J. Tassano, for supporting our efforts in the synthesis and crystal growth laboratories; L. Smith for providing assistance and advice during the laser- pumped laser experiments; P. Thelin and R. Vallene for providing the materials fabrication and polishing necessary for samples utilized in optical tests; and T. Duewer for conducting the Ybk concentration analyses. We also recognize the many insightful and helpful conversations with D. Keszler and A. Akella of Oregon State University, including the appropriate means for preparing the starting melt for crystal growth. This work was performed under the auspices of the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences, by Lawrence Livermore National Laboratory under Contract no. W- 7405-ENG-48.

References 1. H. K. Choi and C. A. Wang, "InGaAdAIGaAs strained single quantum well diode lasers with extremely low threshold current density and high efficiency," Appl. Phv -s. Lett. VOI 57, pp. 321-323, 1990. 2. D. P. Bour, G. A. Evans, and D. P. Gilbert, "High- power conversion efficiency in a strained

InGaAdAlGaAs quantum well laser," J. Aool; Phvs. vol. 65, pp. 3340-3343,1989. 3. L. D. DeLoach, S. A. Payne, L. L. Chase, L. K. Smith, W. L. Kway, and W. F. Krupke, "Evaluation of absorption and emission properties of Yb3+ doped crystals for laser applications," BEE J. Ouantum Electrona vol. 29, pp. 1179-1 191, 1993. 4. P. F. Moulton, "Paramagnetic ion lasers,'' in

m r s and Masers , M. Weber, Editor, Florida: CRC Press, Inc. 1982. 5. D. A. Keszler, A. Akella, K. I. Schaffers, and T. Alekel, "New borate structures for NLO applications," Proceedings from the 1994 MRS Conference, pp. 15- 22. 6. Data obtained via a private communication with Douglas A. Keszier and Annapoorna Akella at Oregon State University, Department of Chemistry, Corvallis, Oregon 97331-4003. 7. W. Koechner, Solid-state Lase r Engineering, New York: Springer-Verlag. 1992. 8. D. Eimerl, "Electro-optic, linear, and nonlinear optical properties of KDP and its isomorphs," Ferroelect. vol. 72, pp. 95-139, 1987. 9. L. D. DeLoach, "Microscopic single-crystal

Qf Am. B, vol. 11,pp. 1186-1 196,1994. 10. H. Ito, H. Naito, and H. Inaba, "Generalized study on angular dependence of induced second-order nonlinear optical polarizations and phase matching in biaxial crystals," J. Appl. PhvL vol. 46, pp. 3992-3998, 1975.

Randboo k of Laser Sc ience and Tech nolo?. Vol. I

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refractometry as a function of wavelength," J. OD t. SOC.

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