Nonstoichiometric Laser Materials;

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)

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!


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


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


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.


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.

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

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)


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.


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


Compositional Tuning Experiments

• Wavelength tuning
• Cross section ratios
• Inhomogeneous broadening
• Laser performance
-


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]

Compositional Tuning - Mixed


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 ' )

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


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)


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


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)


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


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.


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


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)


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


NASA Langley Brian M. Walsh
Research Center Laser Remote Sensing Branch
Space Administration Email: brian.m.walsh@nasa.gov
Lyon, France (July 2008)
Phone: 757 864-7112

Nonstoichiometric Laser Materials;

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