firb activities on lithium niobate: characterization of bulk materials and photoinduced effects...

FIRB Activities on FIRB Activities on lithium niobatelithium niobate: :

characterization of bulk characterization of bulk materials and photoinduced materials and photoinduced effectseffects

Keypoints of our activities on LN

“crystalline quality” - characterization methods

Some examples

Microstructures in LN by fs laser irradiation

FIRB Project Microdevices in Lithium Niobate –Università di Pavia

Electro-optic coefficients measurements

Fe3+ EPR spectra

microRaman

Characterization of structural, optical and electronic properties of LiNbO3 crystals and substrates in connection with different growth processes and different doping

Study of the transport phenomena and charge localization due to optical irradiation of LiNbO3 (or other ABO3 compounds, eventually doped) and of the irradiation effects on the linear and nonlinear optical properties

Study of the feasibility of 1D, 2D and 3D periodical structures, waveguides and microstructures on LiNbO3 (or other ferroelectric oxides) crystalline substrates by means of femtosecond laser irradiation in the transparent spectral region

Keypoints


Crystalline quality1

2

3

How we study crystalline quality?How we study crystalline quality?

•Raman and micro-Raman spectroscopy

•Optical absorption, PL, TL, PC, TSC

•Hall, Photo-Hall and magneto-optical spectroscopy

•Ellipsometry

•Electron Paramagnetic Resonance (EPR) and Photo-EPR

•Static magnetization measurements

•Electro-optical characterization

•Femto-second laser sources *

Lattice of ideal, defect-free LN crystal

coupling and mutual influence of intrinsic and extrinsic defects

decrease of the intrinsic defect concentration


Due to the Li-deficiency the conventional congruent crystals have high concentration of intrinsic (non-stoichiometric) defects, which can easily compensate a high concentration of extrinsic defects (for instance, optically or acoustically active impurities)

Possibility to vary both the [Li]/[Nb] ratio and [O] contents (in addition to the modification by dopants!) is a very powerful tool for the optimisation of crystal parameters

•strong increase of the spectrum resolution due to line narrowing

•changes of some LN properties

•appearance of new impurity centers

EPR

Raman

EPR spectroscopy :•Control of the material quality:

check of purity of growth processesdetection of defects and/or unwanted EPR active magnetic

impurities information about structural disorder

•Evaluation of the oxidation state of the transition ions•Information about site symmetry from the EPR signal angular dependence


Fe3+ EPR lines (B//c) in CLN (LN:Fe 0.1%)

…in quasi-st LN(LN:Fe 0.1%) 500 1000 1500 2000 2500

Der

ivat

ive

EP

R S

ign

als

(arb

. un

.)

B (G)

Raman in LiNbO3In crystals, Raman spectrum depends on the direction and

polarization state of the incident and scattered light with respect to the cristallographic axes

Porto notation: ki(ei,ed)kd

The crystal structure of pure LiNbO3 has Rc3 space group symmetry and 4A1+ 9E Raman-active modes are predicted by factor-group analysis

b

a

a

zA

00

00

00

:)(1

00

00

0

:)(

d

c

dc

xE

00

0

00

:)(

d

dc

c

yE


0 200 400 600 800 1000 12000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

In

tens

ity (

arb.

units

)

Raman Shift (cm-1)

z(x,x)z x(z,z)x

RS is strongly sensitive to orientation

Elight | c

Elight // c

-Raman to check disorientation, multidomains…


RS is sensitive to the deformation of the lattice and to the presence of point defects, becoming a powerful tool to deal with the problem of stoichiometry


The mode at 880 cm-1 is the vibration, parallel to the c axis, of the oxygen ions which consists basically in the stretching of the Nb–O and Li–O bonds.

800 820 840 860 880 900 920 940

0

1

2

3

4

5

6

7

Inte

sity

(ar

b.un

its)

Raman Shift (cm-1)

When a Nb ion sits at a Li site its oxygen first neighbors increase their bonding forces respective to the perfect crystal situation because of the stronger electrostatic interaction.

RS can be used to check the stoichiometry (Li/Nb ratio)

monitoring the changes of linewidth of some Raman modes.

48.0 48.5 49.0 49.5 50.05

10

20

25

30

FWHM @ 152 cm-1

FWHM @ 870 cm-1

(cm

-1)

Li content (mol%)

The fact that the linewidth of some Raman modes scale with the composition

xc = [Li/([Li] + [Nb]) of LN crystals, together with the use of a confocal

microscope (microRaman spectroscopy), allow a three dimensional estimation of

the sample stoichiometry.



Non-destructive stuctural toolNon-destructive stuctural tool

Micron-scale spatial resolutionMicron-scale spatial resolution

Presence of a structurally disordered layerPresence of a structurally disordered layer

Effectiveness of damage removal methodEffectiveness of damage removal method

Control on optical surface finishingControl on optical surface finishing

Raman for surface quality analysis after wafering process:Raman for surface quality analysis after wafering process:

Important complete characterization of: stoichiometry, nature and content of impurities, degree of structural disorder before starting with investigation of charge trapping mechanisms and phenomena related to photo-induced defects


Study of the transport phenomena and charge localization due to optical irradiation of LiNbO3 (or other ABO3 compounds, eventually doped) and of the irradiation effects on the linear and nonlinear optical properties

2

• Photovoltaic current, photoconductivity,

• Photo-EPR

vs %, doping, T

Study of the feasibility of 1D, 2D and 3D periodical structures, waveguides and microstructures on LiNbO3 (or other ferroelectric oxides) crystalline substrates by means of femtosecond laser irradiation in the transparent spectral region

3

“MICROSTRUCTURAL MODIFICATION OF LINBO3 CRYSTALS INDUCED BY

FEMTOSECOND LASER IRRADIATION”Appl. Surf. Science in press


Activity of Pavia Unit in fs-laser writing

Ti:Sapphire oscillator (25 nJ-130 fs-82 MHz)

Laser system 2 (low energy, high repetition rate):

Amplified Ti:Sapphire (1 mJ-130 fs-1 kHz)

Laser system 1 (high energy, low repetition rate):

femtosecond irradiation of congruent LN as a function of pulse energy,

exposure time, exposure depth, crystal orientation, etc. .

characterisation via in situ optical microscopy and a posteriori micro-

Raman spectroscopy

energy deposition through multi-photon absorption

energy transfer strongly depends on pulse intensity


Microstructures in LiNbO3 crystals by fs laser irradiation (laser 1)

b) 50-m-diameter hole in a z-cut CLN plate (laser 1, 10 s, 50 J, 63x microscope objective lens)

c) same as a) imaged by a polarizing microscope

a) 2-m-diameter holes in a z-cut CLN plate (laser 1, 10 ms, ~1J, LWD 50X microscope objective lens)

a)

b)

c)


Microstructures in LiNbO3 crystals by fs laser irradiation (laser 2)

a) 125-m-diameter hole in a z-cut CLN plate (laser 2, 30s, 10 nJ, 63x microscope objective lens)

b) same as a) imaged by a polarizing microscope. A bright zone aside the hole is visible due to photo-induced birefringence.


Micro-Raman investigation of microstructures formed by laser 2

25

30

35

40

20 25 30 35

25

30

35

40

20 25 30 35

635 cm-1

880 cm-1

200 400 600 800 1000

Raman Shift (cm-1)

Ram

an I

nten

sity

(ar

b.un

its)

z(xx)z Raman spectra recorded at positions 1 to 4 as shown in the top left side image. A1 symmetry -forbidden Raman lines appear as approaching the edges of the microstructure indicating some orientation changes in the crystal structure

1

2

3

4

20 m

2D mapping of Raman

intensity in the square

The brighter the colours the

larger the Raman line intensity

Image of a hole in z-cut CLN plate (laser 2,

0.01s, 5 nJ, 20x objective lens). The

maximum depth of the hole is 10 m


1

2

3

4

Micro-Raman investigation of microstructures formed by laser 1

200 400 600 800 10000,0

0,4

0,8

3

2

1

Ram

an I

nten

sity

(ar

b.un

its)

Raman Shift (cm-1)

1 2 3

10 m

Raman spectra recorded in zone 1 to 3. The main E-type peaks are strongly quenched while the A1 peak at 635 cm-1 increases. In spectrum 3 even Nb-O related vibrations at frequency larger than 500 cm-1 are absent, as it would happen in an amorphous layer

Image of a microstructure in a z-cut CLN plate (laser 1, 10s, 300J, 63x microscope objective lens)


Conclusion


femtosecond irradiation induces disorder in the crystal structure causing the appearance of Raman peaks of forbidden symmetry

niobium oxides are formed in the ablation process with laser system 2

amorphous surfaces are present in the region ablated by means of laser system 1

High-intensity ultra-short pulses from laser system 1 probably leads to the formation

of an electron plasma and localized optical breakdown, whereas charge

accumulation and photorefractive-like damage may be the mechanism excited in the

case of the high-repetition-rate-low-energy fs pulses from laser system 2.

Ablation edges of microstructures formed by laser system 2 are smooth and a strong induced birefringece is present all around.

In both cases multi-photon absorption is the path for energy transfer into the medium

EO coefficients of Lithium Niobate

12

1

jjij

i

Ern

CLN(xc=0.485)

SLN(xc=0.500)

rTc

17.5 1

20.5 7,10

(201)14,15

19.918,20

(181) 15

rT13

(111.0) 2,3,8,9

(10.49 0.07) 6

(6.280.07) 11

(9.25 0.07) 16

(10.40.8) 19

10.5 9

rT33

(34.02.5) 2

(31.51.4) 8,3,4,9

(31.4 0.2) 6

(29.4 0.2)16

(38.31.4) 19

37 9

Class 3m

r33, r13= r23, r22= -r12=-r61, r42=r51, rc=r33-(no/ne)3r13

r in pm/V at =633nm

Measuring techniques may rely on ellipsometry or interferometry with DC or AC applied electric field

Constant stress (rT) or constant strain (rS) EO coefficients are measured when the electric field frequency is below/above the acoustic resonance of the crystal (above 500KHz)


NbLi

Lixc

*

i =1,6 j =1,3

EO coefficients of Lithium Niobate

EO coefficients of Crystal Technology CLN (empty symbols) and SLN (full symbols) with xc=0.497 provided by the Crystal Growth laboratory- Universidad Autonoma de Madrid


r33

rc

r13

EO coefficients of commercial SLN

SLN wafer from OXIDE Co. Japan with nominal xc=0.50

(kindly provided by project partner AVANEX)


rTc rT

13 rT33 rT

c= rT33-(no/ne)3rT

13

19.4 9.6 30.7 19.8

r in pm/V ± 5%, =633nm

from AC field

ellipsometry

from AC field

interferometry

OXIDE data sheet: r33=38.3 r13= 10.4

Contradictory results from the literature

K. Chah et alii, APB 67 (1998) 65Y. Kondo et alii, J.JAP 39 (2000) 1477


Different trends were measured for LN EO coefficient as a function of 100 .xc

Femto-second laser writing and sculpturing

The extremely high power density (> TW/cm2) of focussed fs pulses easily excites

multi-photon absorption

avalanche ionization

optical breakdownhIR

Ev

Ec

leading to ablation or refractive index changes in transparent media

A lot of work in glass but still few examples in LN

Long penetration depth and low thermal damage open the way to microstructuring in bulk materials

Perspective of micro-channels and holes, 3D gratings, buried waveguides, 3D directional couplers etc


firb activities on lithium niobate: characterization of bulk materials and photoinduced effects...

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