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Nonstoichiometric Laser Materials;

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Presented at the International Conference on Luminescence - Lyon, France (July 7 - 11, 2008) …

Presented at the International Conference on Luminescence - Lyon, France (July 7 - 11, 2008)

Publication Reference: B.M. Walsh, N.P. Barnes, “Nonstoichiometric laser materials; designer wavelengths in neodymium doped garnets, “ J. Lumin., 129, 1401-1406 (2009).

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  • 1. Nonstoichiometric Laser Materials; Designer Wavelengths in Neodymium Doped Garnets Brian M. Walsh Norman P. Barnes NASA Langley Research Center Hampton, VA 23681 USA International Conference on Luminescence - Lyon, France (July 7 - 11, 2008) National Aeronautics and ICL08 Space Administration Lyon, France (July 2008)
  • 2. Prelude “Lanthanum has only one oxidation state, the +3 state. With few exceptions, this tells the whole boring story about the other 14 lanthanides.” G.C. Pimentel & R.D. Sprately, quot;Understanding Chemistryquot;, Holden-Day, 1971, p. 862 So much for ‘Understanding Chemistry’… Let’s do some physics! National Aeronautics and ICL08 Space Administration Lyon, France (July 2008)
  • 3. NASA - Laser Material Research Activity Input Results Quantum X-ray data, refractive Energy levels, transition Mechanics Index, crystal symmetry probabilities, ET parameters Materials meeting requirements Small spectroscopic Cross sections, lifetimes, Spectroscopy Samples - inexpensive energy levels, ET parameters Best Materials Only Laser quality samples Laser demonstration, Laser research (rods, discs, fibers modeling National Aeronautics and ICL08 Space Administration Lyon, France (July 2008)
  • 4. Remote Sensing Applications 4X DIAL: CO2 Backscatter Lidar: Aerosols/Clouds 2 Micrometer laser Coherent Winds: Lower Troposphere & clouds 3X Noncoherent Winds: Mid/Upper Atmosphere 1 Micrometer 2X Altimetry: laser Surface Mapping Oceanography 2X OPO DIAL: Ozone Backscatter Lidar: Aerosols/Clouds 0.94 Micrometer laser DIAL: H20 National Aeronautics and ICL08 Space Administration Lyon, France (July 2008)
  • 5. What is a Nonstoichiometric Material? Stoichiometry - Derived from the Greek words stoikheion, meaning element and metron, meaning measure. In Chemistry it is related to : Conservation of Mass Law of Definite Proportions Law of Multiple Proportions Stoichiometric Material - The elements composing the crystal appear as ratios of integers. Example: YAG (Y3Al5O12) Nonstoichiometric materials are crystals composed of elements that can’t be represented by a ratio of whole numbers. Correct valence state, site symmetry and atomic size constraints are important considerations. National Aeronautics and ICL08 Space Administration Lyon, France (July 2008)
  • 6. Compositional Tuning YAG -to- YGG YAG YGAG The arrangement of atoms in a crystal structure depends on: the ion charge, bonding type between atoms, and atom size. National Aeronautics and ICL08 Space Administration Lyon, France (July 2008)
  • 7. The Garnet Structure {Dodecahedral} {A3+}3[B3+]2 (C3+)3O12 Oxygen Rare Earth Al, Ga, Fe [Octahedral] (Tetrahedral) Rare Earth: Y, Sc, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu Lanthanides National Aeronautics and ICL08 Space Administration Lyon, France (July 2008)
  • 8. Compositional Tuning Approach • Stoichiometric Materials – Garnet = {A3+}3[B3+]2 (C3+)3O12 – YAG = Y3Al2Al3O12 • Nonstoichiometric Materials – Compositional tuned garnets – YGAG = Y3GaxAl(5-x)O12 (0 < x < 2) – charge neutrality (correct valence) – atomic size (coordination number) • Crystal dependence – LN3+ ion site symmetry (Group theory) – lattice constant variation (Crystal field) – chemistry&crystals (Pauling’s Rules) National Aeronautics and ICL08 Space Administration Lyon, France (July 2008)
  • 9. Chemistry and Crystallography • The nature of crystals - Chemistry dictates bonding character (ionic and covalent) - Crystallography dictates geometry and structure • The important role of charge - Pauling’s theory of electronegativity (effective charges) - Influences bond lengths (Pauling’s Rules) J. Am. Chem. Soc. 1929 • Size constraints - cations and anions - As cation size decreases, coordination number (CN) decreases - 1.000 = Rc/Ra (CN =12) Cubic - 1.000 > Rc/Ra > 0.732 (CN =8) Cubic - 0.732 > Rc/Ra > 0.414 (CN = 6) Octahedral - 0.414 > Rc/Ra > 0.225 (CN =4) Tetrahedral - Generally true, but many exceptions exist. National Aeronautics and ICL08 Space Administration Lyon, France (July 2008)
  • 10. Ionic and Covalent Bonds Ionic • Atoms (cations and anions) are charged particles and electrostatic forces hold structure together. • Bond strength - ionic charge Covalent • Atoms satisfy charge balance by sharing electrons with adjacent orbitals in hybrid or molecular orbitals. • Bond strength - orbital overlap National Aeronautics and ICL08 Space Administration Lyon, France (July 2008)
  • 11. Compositional Tuning Experiments • Wavelength tuning • Cross section ratios • Inhomogeneous broadening • Laser performance - National Aeronautics and ICL08 Space Administration Lyon, France (July 2008)
  • 12. Compositional Tuning - YGAG Measurement of Nd 4F3/2 → 4I9/2 transition wavelengths 6 5 R2-Z1 A R2-Z1 B Gallium concentration R1-Z1 4 R2-Z2 R2-Z3 3 R1-Z2 R1-Z3 2 R2-Z4 R1-Z4 A R1-Z4 B 1 R2-Z5 R1-Z5 A 0 R1-Z5 B -1 860 870 880 890 900 910 920 930 940 950 960 Wavelength (nm) • Wavelength tuning is linear with Gallium concentration (x) • Wavelength can be predicted according to: λYGAG = 1/5[(5-x)λYAG+xλYGG] National Aeronautics and ICL08 Space Administration Lyon, France (July 2008)
  • 13. Compositional Tuning - Mixed National Aeronautics and ICL08 Space Administration Lyon, France (July 2008)
  • 14. Spectral Lineshapes Lorentzian: (Homogeneous width) Line shape parameter 1 !L Gaussian 1 $L SV = Voigt ! (quot; ) = ln 2 ! G # (quot; % quot; 0 ) 2 + $ L 2 Lorentzian Gaussian: (Inhomogeneous width) 1 ln 2 %( ln 2 )(quot; %quot; 0 )2 #G 2 ! (quot; ) = e #G $ Voigt: (Convolution - Lorentzian&Gaussian) Voigt width: * e% t 2 +S 1/2 SV 1 ln 2 ! L # ! L2 & ! (quot; ) = dt !V quot; +% + ! G2 ( #G $ $ 2 + &(quot; % quot; 0 ) ln 2 % t ( 2 2 $ 4 ' %* V ' ) National Aeronautics and ICL08 Space Administration Lyon, France (July 2008)
  • 15. Inhomogeneous Broadening - YGAG B.M. Walsh, N.P. Barnes, et al., J. Opt. Soc Am. B., 15, 2794 (1998) YGAG Y3GaxAl(5-x)O12 YAG YGG Y3Al5O12 Y3Ga5O12 National Aeronautics and ICL08 Space Administration Lyon, France (July 2008)
  • 16. Inhomogeneous Broadening - YAG/YSAG Nd:YAG Nd:(YAG)0.18(YSAG)0.82 R1→ Z5 line (Voigt shape = 1.74) R1→ Z5 line (Voigt shape = 0.37) More Lorentzian than Gaussian More Gaussian than Lorentzian 4.0 4.0 cross section (x10 -20 cm 2) cross section (x10 -20 cm 2) 3.5 !EL =8.09 cm -1 3.5 !E L= 7.73 cm -1 3.0 !E G=3.85 cm -1 3.0 !E G= 17.28 cm -1 2.5 2.5 !EV=10.02 cm -1 !E V= 21.58 cm -1 2.0 2.0 1.5 1.5 1.0 1.0 0.5 0.5 0.0 0.0 940 942 944 946 948 950 940 942 944 946 948 950 Wavelength(nm) Wavelength(nm) National Aeronautics and ICL08 Space Administration Lyon, France (July 2008)
  • 17. Voigt Fitting Parameters Material ! quot;EL quot;EG quot;EV Voigt #R $Z %r 1 5 (nm) (cm-1) (cm-1) (cm-1) shape (x10-20cm2) YSGG 937.75 8.33 13.15 21.02 0.6666 1.7073 5.57 YAG1/2YSGG1/2 942.59 6.76 28.50 32.89 0.1975 1.5050 6.01 YAG3/5YSGG2/5 943.43 7.63 27.61 31.65 0.2303 1.4177 6.25 YAG2/3YSGG1/3 944.24 8.35 26.03 28.83 0.2671 1.5780 6.30 YAG3/4YSGG1/4 944.84 6.76 25.56 26.66 0.3432 1.4026 6.50 YAG 945.87 8.09 3.85 10.02 1.7400 3.7670 7.36 GGG 937.30 10.19 3.94 11.94 2.1491 1.7500 7.66 YSAG2/3GGG1/3 940.96 7.83 28.24 32.42 0.2310 1.4761 6.08 YSAG 943.63 8.15 15.52 20.13 0.43744 2.5825 4.64 YSAG 943.63 8.15 15.52 20.13 0.43744 2.5825 4.64 YAG0.18YSAG0.82 943.93 7.73 17.28 21.58 0.3721 2.3277 5.07 YAG 945.87 8.09 3.85 10.02 1.7400 3.7670 7.36 GSAG - - - - - - - YAG0.45GSAG0.55 944.23 7.37 22.45 26.43 0.2733 1.9673 4.44 YAG0.3GSAG0.050 944.51 7.59 22.55 26.66 0.2803 1.8017 4.75 YAG 945.87 8.09 3.85 10.02 1.7400 3.7670 7.36 YAG0.03(YSAG0.98GGG0.02)0.97 943.66 8.68 17.17 22.06 0.4208 2.1152 4.75 YAG0.20(YSAG0.98GGG0.02)0.80 944.16 7.97 19.26 23.65 0.3447 2.0205 4.96 YAG0.30(YSAG0.90GGG0.10)0.70 944.10 8.12 22.23 26.76 0.3043 1.8765 5.04 YAG0.40(YSAG0.90GGG0.10)0.60 944.81 8.36 22.19 26.65 0.3138 1.7903 5.27 National Aeronautics and ICL08 Space Administration Lyon, France (July 2008)
  • 18. YAG / YSAG Garnets Emission cross sections Wavelength tuning Favorable cross section ratio is beneficial in limiting the deleterious effects of ASE YAG for Q-switched laser operation Y3Al5O12 YAG/YSAG (YAG)0.18(YSAG)0.82 YSAG Y3Sc2Al3O12 Wavelength (nm) Wavelength (nm) National Aeronautics and ICL08 Space Administration Lyon, France (July 2008)
  • 19. YGAG Material Assessment • Continuous compositional tuning available - YAG (x = 0) through YGG (x = 5) - tuning is linear with Ga concentration (x) • Emission cross section (gain issues) - Some lines are split (A and B sites) - Lines are inhomogeneously broadening - 1.06 to 0.94 µm cross section ratio > 20 • Laser performance issues - Slope efficiency (< 0.1%) - Optical quality problems - ASE (amplified spontaneous emission) problems National Aeronautics and ICL08 Space Administration Lyon, France (July 2008)
  • 20. YAGxYSAG(1-x) Material Assessment • Continuous compositional tuning available - YAG (x = 1) to YSAG (x = 0) - Tuning is linear with x • Emission cross section (gain issues) - No line splitting observed - Lines are inhomogeneously broadened - 1.06 to 0.94 µm cross section ratio ~ 5 • Laser performance issues - Slope efficiencies > 0.2% - Optical quality good - ASE (amplified spontaneous emission) somewhat mitigated. National Aeronautics and ICL08 Space Administration Lyon, France (July 2008)
  • 21. Laser Schematic • Flashlamp pumped oscillator Nd operating on the • 5 x 55 mm laser rods R1 → Z5 transition. • Acousto-optic Q-switch • Flashlamp pumped Amplifier 0.94 µm resonator A-O HR 0.94 Q-Switch HT 1.06 Laser Oscillator rod Energy Amplifier meter PFN PFN Energy meter Laser Output Pickoff Laser HR 0.94 rod mirror rod HT 1.06 National Aeronautics and ICL08 Space Administration Lyon, France (July 2008)
  • 22. Laser Performance 140.0 Slope efficiency ~ 0.5% Slope efficiency ~ 0.2% Threshold ~ 26 J Threshold = 41 J 120.0 λL = 0.946 µm λL = 0.944 µm 100.0 Laser energy (mJ) 80.0 60.0 40.0 Nd:GYAG (NM) Nd:GYAG (QS) 20.0 Nd:YAG/YSAG (NM) Nd:YAG/YSAG (QS) 0.0 20 30 40 50 60 70 80 90 100 Electrical energy (J) National Aeronautics and ICL08 Space Administration Lyon, France (July 2008)
  • 23. Summary • Chemistry and crystallography - Chemistry describes bonding - Crystallography describes geometry • Spectroscopy of materials - Wavelength, cross section ratio, linewidth - YAG/YSAG is material of choice. • Laser demonstration - Compositional tuning to 944 nm - Over 100 mJ Q-switched energy National Aeronautics and ICL08 Space Administration Lyon, France (July 2008)
  • 24. NASA Langley Brian M. Walsh Research Center Laser Remote Sensing Branch National Aeronautics and ICL08 Space Administration Email: brian.m.walsh@nasa.gov Lyon, France (July 2008) Phone: 757 864-7112

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