Flux growth and laser operation of highly Yb3+-doped cubic Gd2O3 laser crystals

M. Velázquez, Ph. Veber, G. Buşe, E. Véron, D. Rytz, F. Druon, Ph. Goldner, P. Aschehoug, S.

Janicot, S. Péchev, O. Viraphong, M. Abdou-Ahmed, Th. Graf and P. Georges

Scintillation materials Eyesafe telecommunications lasers (Y2O3:Tm3+) Yb3+:Lu2O3 and LuScO3-based high-power and/or sub-100 fs pulse lasers Visible phosphors and red solid state lasers (Y2O3:Eu3+) Upconverter materials for 3rd generation photovoltaïc cell, upconversion lasers (Y2O3:Er3+, Gd2O3:Tm3+)

Rare Earth SesquiOxides siNgle crystAl growTh by the flux mEthod

MRS Bulletin issue emphasizing the two main

challenges of today’s crystal growth :

production of well-established crystalline materials with

improved quality and larger size at a lower cost ;

bulk growth of new categories of materials with extreme

thermodynamic characteristics.

Yb3+:Lu2O3 and LuScO3-based high-power and/or sub-100 fs pulse lasers

Yb:Lu2O3 more promising than Yb:YAG for high-power systems diode-pumped at 976 nm (with M21.2)

Evolution of the average power of mode-locked thin-disk lasers based on Yb:YAG and Yb:Lu2O3 (Baer et al., Opt. Lett., 2010)

Yb3+:Lu2O3 and LuScO3-based high-power and/or sub-100 fs pulse lasers (2)

Average power of mode-locked TDLs versus pulse duration

(Saraceno et al., Appl. Phys. B, 2012 Ricaud et al., Opt. Lett., 2012)

CW multimode laser performance of a Lu2O3:Yb3+ (5 at.%) : 670 W, M220,

e=250 m/=7 mm (Weichelt et al., Laser Phys. Lett., 2011)

CALGO:Yb3+

Damaged Yb:LuScO3 disk (e=250 m/=6-7 mm) in the pumped area marked with the white circle Need for better quality and easily available Yb:Lu2O3 and Yb:LuScO3 single crystals

Yb3+:Lu2O3 and LuScO3-based high-power and/or sub-100 fs pulse lasers (3)

high thermal conductivity : ~10 to 13 W.m-1.K-1 at RT in Lu2O3:Yb3+ (3.6 W.m-

1.K-1 in LuScO3:Yb3+) low population inversion threshold dependency on temperature low fluence threshold emission bandwidths broader than that of YAG:Yb3+ crystals (12 nm in Lu2O3:Yb3+, 22 nm in LuScO3:Yb3+) absorption cross section of Lu2O3:Yb3+ crystals four times higher than that of YAG:Yb3+ crystals exp~1 ms, optimal for both energy storage and Q-switching avoidance

Extreme thermodynamic conditions for crystal growth of RESO

high melting point : ~2400 0C (Sm(Y2O3)=4.1 R) expensive and soluble Re crucibles, the stability of which requires an H2-containing gas flow at ~2400 0C rich polymorphism (Adachi et al., Chem. Rev., 98 (1998) 1479-1514)

HEX P32/m

MONO C2/m

CUB Ia-3

CUB Im3m

HEX P63/mmc

Hot topic in crystal growth

A new solvent for a simple flux growth furnace

crystallization temperature ~TM/2 cubic phase stability domain directly reached isotropic thermal, thermomechanical and thermooptical behaviours higher thermal conductivity, higher liquid surface tension, lower vapour pressure crystal growth in air Yb3+ stabilization without post-growth thermal treatment annealing neither heavy metal, hydroxyl groups nor corrosive species in the solvent

New flux in Gd2O3-B2O3-Li2O system : Li6Gd(BO3)3

A new solvent for a simple flux growth furnace (2)

Pt crucible and spatula Alumina rod, tube and screens Silica wool Kanthal wire Mechanical stirring (20 rpm) Low thermal gradients (1 K/cm)

"Method for preparing single-crystal cubic sesquioxides and uses thereof“, Veber et al., PCT Int. Appl. WO 2011/055075 A1, 12/05/2011, PCT n° FR 2010052355.

From 2010 to 2013 : useful V40

Rare Earth SesquiOxides siNgle crystAl growTh by the flux mEthod

Section=2.654.35 mm2

Thickness=1.2 mm

Gd2O3:Yb3+

Clear crystals Section > 5 5 mm2

Thickness > 1. 5 mm

Section=2.654.35 mm2

Thickness=1.2 mm

Powder X-ray diffraction (XRD) characterization

Two-phase Le Bail full pattern matching of Yb3+:Gd2O3 (Ia-3, a=10.7847 Å, Rwp=27.7, 2=2.085) shows the presence of Li6Gd(BO3)3 inclusions

Yb3+:Y2O3

low OH- content, ~2,2.1021 cm-3

Yb3+:Lu2O3 and Yb3+:Gd2O3

virtually OH--free

FT IR spectra

S. Hraïech, PhD thesis, University Claude Bernard Lyon 1 (2007)

Veber et al. Crystengcomm, 13 (16) (2011) 5220-5225

Absorption and stimulated emission cross section spectra

simple energy level diagram of Yb3+ ions (=1 ; no ESA, no down-conversion cross-relaxation, no up-conversion) large crystal field splitting of the ground state manifold (900 cm-1 ; quasi-four level laser operation) large absorption cross section at ZPL (use of thinner disks) and low absorption cross section at L highest emission peak cross section on the 53 transition

e(Gd2O3)=670 µm e(Lu2O3)=590 µm

Non radiative relaxation mechanisms

...

111

?

1111111

1exp

'

nsdislocatioYb

TmYbErYbcoop

YbYbVYbOHphmrad

iem

OYb

n

self-trapping ~14.4%

m-ph min ~104 s

~0 (growth in air between 1250 and 1100 °C)

AR WNR E=p·ħM

Energy-gap law at fixed T

Mphm pCEW exp

2S’+1L’J’

2S+1LJ

S. Hraïech, PhD thesis, University Claude Bernard Lyon 1 (2007)

OH-~15 ms

C. K. Jørgensen, J. Phys.,

C7, S n° 12, t 48, (1987), 447-450

Transmission measurements performed over Lu2O3:Yb3+ and Gd2O3:Yb3+ crystals

No Yb2+ ions absorption bands at 480, 520 and 600 nm (V. Peters, PhD thesis, University of Hamburg (2001), Germany)

Absorption coefficient measurements performed over Sc2O3:Yb3+ and Lu2O3:Yb3+ crystals

Yb2+ ions absorption bands at 480, 520 and 600 nm

(V. Peters, PhD thesis, University of Hamburg (2001), Germany)

Cooperative emission

=exp/2

Non radiative relaxation mechanisms (2)

...11?

11111exp

nsdislocatioYbTmYbErYbcoop

YbYbrad

Yb-Er and Yb-Tm ETUs

[Er3+],[Tm3+] ~ 1016 cm-3

1

1

'''

dIII ememcoop

YbYb

480 nm

~660 nm

~550 nm

~800 nm

500 nm

[Nd3+] ~ 1016 cm-3

[Dy3+],[Fe3+] ~ 1017 cm-3

Anti-Stokes emission spectra in Gd2O3:Yb3+

Typical AS emissions from Er3+ (4S3/2,4F9/2) and Tm3+ (1G4) ions in the visible range

and Yb3+-pairs cooperative emission

RT AS fluorescence transients from levels 4S3/2 of Er3+ and 1G4 of Tm3+ ions

tWtW risedec eetI

Buisson and Vial, J. Phys., L42 (1981) 115-118

In Gd2O3:Yb3+ single crystals, rise and average decay times on the order of a few hundreds of s ;

At high excitation powers, such ETU processes are likely to deplete the laser emitting level and to increase the head load in the crystals under laser operation.

Examples of cavities in V and I shape, diode pumped and qcw Ti:Sa pumped and associated profiles, uncoated and uncooled Gd1.72Yb0.28O3 crystal

Laser tests

P=977 nm, abs98 %, P=60 µm, L=1069-1083 nm, TOC=4 % ; In qcw regime : Pthr,dp=729 mW, dp=33 %, Pthr,Ti:Sa=464 mW, Ti:Sa=45 % ; In cw regime : dp=26 %

Laser performances

P=975 nm, TOC=3.2 %, Pthr,dp=274 mW, dp=40 % ; t=260 m, TOC=1.6 %, Pthr,Ti:Sa=500 mW, Ti:Sa=76 % (with HR coatings and water cooling) ;

Conclusions and perspectives

we have disclosed a possibility to develop a cheap, simple, reliable and safe crystal

growth process of Yb3+-doped Y2O3, Gd2O3 and Lu2O3 by the widely spread flux

method, using an original and non toxic solvent and growth setup design operative

in air and at half the melting temperature of these compounds ;

high Yb3+-concentrations can be achieved in Gd2O3, Y2O3 and Lu2O3 crystals ;

the cubic phase crystallization is favored by the LGB solvent ;

laser tests on thin samples (e~200-300 m) ;

Yb3+ emission (and possibly stress-induced birefringence) mapping ;

dislocation analysis and density measurements ;