nonstoichiometric laser materials;

National Aeronautics andSpace Administration

ICL08Lyon, France (July 2008)

Brian M. WalshNorman P. Barnes

NASA Langley Research CenterHampton, VA 23681 USA

Nonstoichiometric Laser Materials;Designer Wavelengths in

Neodymium Doped Garnets

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



“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,"Understanding Chemistry",Holden-Day, 1971, p. 862

So much for ‘Understanding Chemistry’…Let’s do some physics!

Prelude



NASA - Laser Material ResearchActivity Input Results

X-ray data, refractiveIndex, crystal symmetry

Energy levels, transitionprobabilities, ET parameters

QuantumMechanics

SpectroscopySmall spectroscopicSamples - inexpensive

Cross sections, lifetimes,energy levels, ET parameters

Laser research Laser quality samples(rods, discs, fibers

Laser demonstration,modeling

Materials meeting requirements

Best Materials Only



Remote Sensing Applications

2 Micrometerlaser

1 Micrometerlaser

Lower Troposphere & clouds

2X

DIAL: CO24XBackscatter Lidar: Aerosols/Clouds

2X OPO

Coherent Winds:

Altimetry:Surface Mapping

Oceanography

DIAL: OzoneBackscatter Lidar: Aerosols/Clouds

Noncoherent Winds:Mid/Upper Atmosphere

3X

0.94 Micrometerlaser 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 MassLaw of Definite ProportionsLaw 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 berepresented by a ratio of whole numbers. Correct valence state, site symmetryand atomic size constraints are important considerations.



YAG YGAG

Compositional Tuning

YAG

-to-

YGG

The arrangement of atoms in a crystal structure depends on: the ion charge, bonding type between atoms, and atom size.



{Dodecahedral}

(Tetrahedral)[Octahedral]

Oxygen

Rare Earth

Al, Ga, Fe

Rare Earth: Y, Sc, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu

Lanthanides

{A3+}3[B3+]2 (C3+)3O12

The Garnet Structure



• 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)

Compositional Tuning Approach



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

• Atoms (cations and anions) are charged particles and electrostatic forces hold structure together.

• Bond strength - ionic charge

• Atoms satisfy charge balance by sharing electrons with adjacent orbitals in hybrid or molecular orbitals.

• Bond strength - orbital overlap

Ionic

Covalent



Compositional Tuning Experiments

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

-



Compositional Tuning - YGAG

• Wavelength tuning is linear with Gallium concentration (x)• Wavelength can be predicted according to:

λYGAG = 1/5[(5-x)λYAG+xλYGG]

Measurement of Nd 4F3/2 → 4I9/2 transition wavelengths

860 870 880 890 900 910 920 930 940 950 960

-1

0

1

2

3

4

5

6

R2-Z1 AR2-Z1 BR1-Z1R2-Z2R2-Z3R1-Z2R1-Z3R2-Z4R1-Z4 AR1-Z4 BR2-Z5R1-Z5 AR1-Z5 B

Wavelength (nm)

Gal

lium

conce

ntr

atio

n



Compositional Tuning - Mixed



!(" ) =1

#

$L

" % "0( )2

+$L

2

!(" ) =1

#G

ln 2

$e% ln2( ) "%"0( )

2#G2

Lorentzian: (Homogeneous width)

Gaussian: (Inhomogeneous width)

Voigt: (Convolution - Lorentzian&Gaussian)

!(" ) =SV

#G

1

$ln 2

$e% t2

SV

2+ " % "

0( ) ln 2 % t&' ()2

%*

*

+ dt

SV=

1

ln 2

!L

!G

GaussianVoigtLorentzian

!V"!L

2+

!L

2

4+!

G

2#$%

&'(

1/2

Voigt width:

Line shape parameter

Spectral Lineshapes



Inhomogeneous Broadening - YGAG

YGAGY3GaxAl(5-x)O12

B.M. Walsh, N.P. Barnes, et al., J. Opt. Soc Am. B., 15, 2794 (1998)

YAGY3Al5O12

YGGY3Ga5O12



Inhomogeneous Broadening - YAG/YSAG

Nd:YAGR1→ Z5 line (Voigt shape = 1.74)More Lorentzian than Gaussian

Nd:(YAG)0.18(YSAG)0.82R1→ Z5 line (Voigt shape = 0.37)More Gaussian than Lorentzian

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

940 942 944 946 948 950

cross

sec

tion (

x10

-20 c

m2)

Wavelength(nm)

!EL

=8.09 cm-1

!EG=3.85 cm-1

!EV=10.02 cm -1

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

940 942 944 946 948 950

cross

sec

tion (

x10

-20 c

m2)

Wavelength(nm)

!EL= 7.73 cm -1

!EG= 17.28 cm -1

!EV= 21.58 cm -1



Voigt Fitting ParametersMaterial !

(nm)

"EL

(cm-1)

"EG

(cm-1)

"EV

(cm-1)

Voigt

shape

#R1$Z

5

(x10-20cm2)

%r

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

Wavelength (nm)Wavelength (nm)

YAGY3Al5O12

YSAGY3Sc2Al3O12

YAG/YSAG(YAG)0.18(YSAG)0.82

Wavelength tuningEmission cross sectionsFavorable cross section ratio is beneficialin limiting the deleterious effects of ASEfor Q-switched laser operation



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



• 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.

YAGxYSAG(1-x) Material Assessment



Laser Schematic

HR 0.94

HT 1.06

Laser

rod

Laser

rod

PFN

Output

mirror

Pickoff

Energy

meter

A-O

Q-SwitchHR 0.94

HT 1.06

PFN

Laser

rod

Oscillator

Amplifier

Energy

meter

• Flashlamp pumped oscillator• 5 x 55 mm laser rods• Acousto-optic Q-switch• Flashlamp pumped Amplifier

0.94 µm resonator

Nd operating on theR1 → Z5 transition.



Laser Performance

0.0

20.0

40.0

60.0

80.0

100.0

120.0

140.0

20 30 40 50 60 70 80 90 100

Nd:GYAG (NM)

Nd:GYAG (QS)

Nd:YAG/YSAG (NM)

Nd:YAG/YSAG (QS)

La

ser

ener

gy

(m

J)

Electrical energy (J)

Slope efficiency ~ 0.5%Threshold ~ 26 JλL = 0.946 µm

Slope efficiency ~ 0.2%Threshold = 41 JλL = 0.944 µm



• 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

Summary



Brian M. WalshLaser Remote Sensing BranchEmail: [email protected]: 757 864-7112

NASA LangleyResearch Center

nonstoichiometric laser materials;

Technology

crystal structure

crystal symmetryenergy

correct valence state

atom size

ion charge

c3 3o12 yag

oxidation state

co24xbackscatter lidar