Pulsed laser deposited epitaxial garnet films for efficient low threshold waveguide lasers

Robert Eason, Tim May-Smith,

Christos Grivas, Dave Shepherd,

Optoelectronics Research Centre, University of Southampton, Highfield, SO17 1BJ, UK. E-mail: rwe@orc.soton.ac.uk

Contents

1. Pulsed laser deposition (PLD)2. Motivations for thick (designer) films3. Garnet crystals4. Physical properties of films grown5. Lasing from an unclad thick film6. Self-imaging in a clad thick film7. Conclusions

Pulsed laser deposition (PLD) 1

The target material is ablated by a KrF excimer laser.

The ejected material forms a plasma plume and expands across to the substrate where it is deposited as a film.

The substrate is heated by a raster-scanned CO2 laser.

A background gas is used to control the plume stoichiometry and dynamics.

Pulsed laser deposition (PLD) 2

The process of ablation preserves the stoichiometry of the target material well, meaning many different target materials can be used. Bulk off-cuts make very suitable targets.

Substrate heating allows epitaxial growth and hence films with high crystalline quality. CO2 laser heating can heat substrates to temperatures up to 2000 C.

The plume and film stoichiometry can be altered by a background gas. In the case of garnet crystal the use of oxygen is essential.

Relatively high growth rates up to 10 m per hour can be achieved.

PLD is well suited to experimental work because it is relatively simple to change the setup and deposition conditions.

Important experimental parameters 3

The target (crystal, sintered ceramic..) The substrate temperature Background gas used and pressure Target-substrate distance Energy density (laser fluence) on target Repetition rate of the laser Stability of all these with time (hours..)

Motivations for thick films 1

Thick films reduce the effect of particulates. Thick films have lower propagation losses than thin

films. Pump light can be coupled more easily into thicker

films because there is a larger area to launch light into. Cladding layers can be used to change the optical

dynamics of thick waveguides and produce structures capable of self-imaging an input beam mode at the output face, or large-mode-area cladding-pumped devices for single-mode operation.

Motivations for thick films 2

Self-imaging

n Thickness2

.Length =

n = refractive index, = wavelength

A single-mode laser beam focussed into the centre of a thick multi-mode waveguide will propagate multi-mode inside the guide and periodically self-image back to single-mode, obeying the formula:

A Nd:GGG waveguide with an optimal length and thickness can be used as an amplifier that doesn’t change the mode of the signal beam. A device with a core thickness ~60 m could be efficiently pumped by diode bars.

Desired Structures: Designer films 3

Thick multimode films Need film thicknesses of ≥ 50 m to pump efficiently with high power diode laser arrays.

MultilayersCan be used for cladding pumping and to change the NA, making single-mode output possible.

Motivations for thick films 3

Cladding layers increase the numerical aperture of a waveguide allowing more pump light to be launched.

Light guided by the cladding can pump the core as it passes through.

Index-matched cladding can be used to produce large-mode-area waveguides for high power single-mode operation.

Cladding-pumping

Pump light

confinement

Pump

light

Laser light

confinement

Versatile Garnet Crystals 1

Excellent laser host– Optically isotropic– Wide transparency range– High mechanical strength– Good thermal conductivity

Easy to growHigh quality films can be grown by PLD.

Many different garnetsDifferent refractive indices but similar thermal expansions and lattice sizes.

Useful material systemIdeal for multilayer structures.

Material

Refractive index

(at 1.06 m)

Thermal expansion (×10‑6 K‑1)

Lattice constant

(Å)

YAG 1.82 6.9 12.006

YbAG 1.83 8.6 11.939

YSAG 1.86 ‑ 12.271

GSAG 1.89 7.7 12.389

GAG ‑ ‑ 12.113

YGG 1.91 ‑ 12.273

YSGG 1.93 8.1 12.446

GSGG 1.94 8.0 12.544

GGG 1.95 8.3 12.383

We have grown all of the garnets highlighted in blue

Nd:GGG General Properties

‘Water clear’The first test of quality is if films look clear to the naked eye.

Highly textured crystalXRD peaks approaching bulk quality.

Gallium deficientGallium is preferentially lost in the deposition process.

Broadened SpectroscopyDue to the slightly deficient stoichiometry and crystal structure.

0

25000

2 (degrees)10 20 30 40 50 60 70 80

YAG (400)

Nd:GGG (400) Nd:GGG (800)

YAG (800)Intensity (counts)

EDX

&

RBS

Gd:Ga – 3:4

O – 60%

Physical properties of films grown 1

Nd:GGG films with thickness up to 135 m have been grown on YAG (100) substrates, the thickest reported to date by PLD.

X-ray diffraction shows that the films are epitaxial and of high crystalline quality.

Rutherford back scattering analysis shows that the films have a stoichiometry close to that of the bulk material.

Physical properties of films grown 2

The films have fluorescence and absorption properties close to that of bulk, but slightly broadened.

2.90

3.10

3.30

3.50

3.70

3.90

4.10

0.50 1.50 2.50 3.50 4.50 5.50 6.50O2 background gas pressure (×10-2 mbar)

X in

Gd

xGa 8

-x

Composition vs. O2 pressure 3

2 experiments using different substrate-target distances suggest a pressure of about 5 ×10-2 mbar minimises the gallium deficiency.

Unstable pressure leads to poor films because the composition varies throughout depth.

target-substrate distance = 4 cmextrapolated minimum ~ 4.6 ×10-2 mbar

target-substrate distance = 6 cmminimum ~ 4.7 ×10-2 mbar

Composition vs. Lattice 4

XRD and EDX can be used to analyse crystal structure and composition respectively.

Lattice increases as gallium deficiency gets worse.

2.90

3.10

3.30

3.50

3.70

3.90

4.10

12.300 12.400 12.500 12.600 12.700

Lattice constant (A)

X i

n G

d xGa 8

-x

stoichiometricGGG

Ga Gd

Ionic radius (pm) 76 107.8

Ionic radii for the octahedral site:

Gadolinium ions are occupying gallium lattice sites, making the lattice bigger and producing strain in the film.

Thick films cannot tolerate the strain produced, and are liable to crack when polishing.

Where is the gallium going? 5

Pattern for vertical and horizontal spread

Rectangular spot! – Aspect ratio of spot is important.

2.50

2.70

2.90

3.10

3.30

3.50

3.70

3.90

4.10

5 10 15 20 25 30 35Distance from substrate centre (mm)

X in

Gd

xGa 8

-x vertical spread

horizontal spread

Composition vs. Substrate Temperature 6 The ideal substrate temperature is about 800 °C.

The composition only starts to be affected when the substrate temperature is too high.

Substrate temperature is actually more critical to the crystal structure.

2 (degrees)

2 (degrees)

2 (degrees)

27 28 29 30 31

27 28 29 30 31

27 28 29 30 31

Inte

nsity

(ar

b. u

nits

)

Inte

nsity

(ar

b. u

nits

)

Inte

nsity

(ar

b. u

nits

)

2.90

3.10

3.30

3.50

3.70

3.90

4.10

650 750 850 950 1050

Temperature (°C) [estimate]

X i

n G

d xG

a 8-x

Composition vs. substrate-target distance 7

This trend of recovering gallium inclusion suggests that gallium can’t simply be diverging faster than gadolinium in the plume.

Perhaps gallium is re-sputtered out of the film if it is too close to the target.

2.90

3.10

3.30

3.50

3.70

3.90

4.10

3.5 4.0 4.5 5.0 5.5 6.0 6.5

Target-substrate distance (cm)

X in

Gd

xGa 8

-x

Composition vs. fluence (constant spot size) 8

Range of acceptable fluences.

Spot aspect ratio is perhaps more important.

2.90

3.10

3.30

3.50

3.70

3.90

4.10

1.00 1.50 2.00 2.50 3.00 3.50 4.00

Excimer laser fluence (Jcm-2)

X in

Gd

xGa 8

-x

Problem 1 - Particulates

Particulates can form scattering centres and contribute significantly to the overall loss.

Solutions: Burial

– Thick films are less affected.– Capping layers allow burial without

increasing the core size. Target reconditioning

Restricting growth runs to < 2 hours prevents the target surface degrading by a significant amount.

Problems 2 – Film Stress

Gallium deficiency in GGG films causes film stress which can lead to cracking.

Varying growth conditions (in particular oxygen pressure) produces different compositions.

EDX and XRD used to determine compositions and lattice constants respectively.

End view of cracked waveguide

Solution:

Precise stoichiometry controlOptimise growth conditions to minimise the gallium deficiency.

Lasing from an unclad thick film 1Ti:sapphire pumping

Lasing has been achieved in a 40 m thick unclad Nd:GGG film.

An absorbed pump power threshold of 18 mW was observed using a high-reflectivity output coupling mirror.

Lasing has been observed at 1060.6 nm and at both 1059.0 and 1060.6 nm for pump powers approximately twice that of threshold.

Slope efficiencies of 17.5% and 12.3% have been obtained using two output coupling mirrors with different reflectivities.

Ti:sapphire Pumped Nd:GGG

17.8 mWLowest threshold with a 40 μm thick film.

32.0%Highest slope efficiency with a 50 μm thick film.

0.1 dBcm-1

Lowest loss with a 40 μm thick film.0

5

10

15

20

25

30

35

40

50 100 150 200 250

Absorbed pump power (mW)

Out

put p

ower

(mW

)

output coupling mirror = 21.8%Ts lope efficiency = 32.0%

Tim? How do the last two slides compare. The slope efficiencies quoted are different.

I’d like to show the ‘best’ slope efficiency onlt etc..

Lasing from an unclad thick film 2Ti:sapphire pumping

The threshold for lasing using output couplers with different reflectivity’s can be used to estimate propagation losses using the Findlay-Clay technique.

An estimate of 0.1 dBcm-1 has been obtained for the propagation loss, the lowest reported to date for a PLD grown waveguide.

This result emphasises the advantage of thicker films, and should be improved upon with the inclusion of capping layers.

R = output coupler reflectivity, L = cavity length.

Insert here the two figs side by side from your thesis (5.3.5 and 5.3.6)

Self-imaging in a clad thick film 1

Self-imaging of a single-mode Ti:sapphire laser beam has been observed in a 25-35 m thick (uneven) Nd:GGG film grown on a YAG (100) substrate with a 10 m YAG capping layer.

The modal output changed as the wavelength of the Ti:sapphire laser was tuned, and as the launch position was changed to different thicknesses of the sample.

Self-imaging in a clad thick film 2

For a setup with the wrong thickness and wavelength for self-imaging, the output was multi-mode.

Unguided direction

Gu

ided

dir

ecti

on

Self-imaging in a clad thick film 3

For a setup obeying the self-imaging formula, a single-mode beam with a 6 m diameter spot at the input face was reproduced at the output face.

Gu

ided

dir

ecti

on

Unguided direction

Diode-Pumping Launch Setup 1

Side-pumping not yet possible due to difficulties produced by film stress (as mentioned earlier). Side-pumping is preferable because it is more practical and scalable (make device longer and use more diodes).

Launch efficiency ~80% for a 50 m thick waveguide. Incident power limited to 100 W by mirror contact fluid boiling.

Output coupling mirrorHR @ 808 nm

12.86%T @1064 nm

3-bar diode stackEach bar = 60 W max @808 nm

50 m thick Nd:GGG waveguideon a YAG substrate

Fabricated using three deposition runs and polished

back to make a slab waveguide

Beam shaping

Primarycylindrical lens

Doubletlens

Waveguide couplingfinal cylindrical lens

Cavity mirrors

Input coupling mirrorHT @ 808 nmHR @1064 nm

Laser output

Diode-Pumping Results 2

The device operated with unstable highly multimode output.

Solution: Multilayers and

cladding pumping.

Diode-Pumping Results 3

7.44 W Threshold with a 2.00%T mirror.

11.2% Slope efficiency with a 12.86%T mirror. This could be improved better spatial overlap of pump and signal.

4.0 W Highest output for a film grown by PLD. Potential for more with a better film.

0.8 dBcm-1 Loss from the Findlay-Clay technique (uses thresholds from different output couplers to deduce loss).

Multilayer Trial Fabrication 1

SEM imaging further confirms the existence of the multilayers. A large index difference is required for the pump cladding to substrate and

superstrate steps. A small index difference is required for the core to pump cladding steps.

Multimode pump propagation

Single-mode laser output

Cladding pumpingillustration

Multilayer Trial Fabrication 2

A trial multilayer structure was fabricated by sequential deposition of YGG, Nd:GSGG, YGG and YAG.

Layers were kept thin to allow them to all be detected by XRD. An EDX linescan across the layers shows their different compositions.

YbAG Thin-Disk Laser Attempt

Crystallinity and stoichiometry close to bulkBest film overall in terms of XRD and EDX.

Absorption not the same as bulkWe suspect that some of the ytterbium is in the 2+ valence state.

0

10

20

30

40

50

60

70

80

90

100

850 900 950 1000 1050

Wavelength (nm)

Abs

orpt

ion

coef

fici

ent (

cm-1

)

Thick YbAG film

Bulk YbAG crystal

0

2

4

6

8

10

12

14

350 400 450 500 550 600 650 700 750

Wavelength (nm)

Abs

orpt

ion

coef

fici

ent (

cm-1

)

Thick YbAG film

Bulk YbAG crystal with ytterbium in the 2+ state

Bulk YbAG crystal with ytterbium in the 3+ state

Problems 3. The end of an era?

Growth of Alternative Garnets

7 different garnets depositedEpitaxial growth of highly textured YAG, YGG, YSAG, GGG, GSAG, GSGG & YbAG.

Best XRD: YSAGClosest match to the YAG substrates.

Best EDX: GSAGElements found to be least susceptible to loss in the deposition process.

Quaternary garnets better than tertiary garnetsScandium retained more easily than gallium and aluminium.

Scene is set for multilayer garnet crystal films

Summary 1

The fabrication and processing techniques have been proven successful for these two basic first steps towards more advanced structures.

Stoichiometry control needs to be improved to avoid cracking problems due to film stress.

With a carefully chosen optics setup, high power levels from diode stacks can be coupled efficiently into PLD waveguides.

Improved loss, output power and slope efficiency should come with improved crystal quality.

PLD films can tolerate a high level of pumping power without cracking.

Ideas for the future 1

Graded index Graded dopant concentration Blended layers Hybrid garnets Cross-beam……

Ideas for the future 2

Tim. Simple 3 quad laser cross beam pic?