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Interaction of light with impurities in lithium niobate crystals [Elektronische Ressource] / Judith Renate Marie-Luise Schwesyg. Mathematisch-Naturwissenschaftliche Fakultät

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130 pages
Interaction oflight with impurities inlithium niobate crystalsDissertationzurErlangung des Doktorgrades (Dr. rer. nat.)derMathematisch-Naturwissenschaftlichen FakultätderRheinischen Friedrich-Wilhelms-Universität Bonnvorgelegt vonJudith Renate Marie-Luise SchwesygausRheinfeldenBonn 2011Angefertigt mit Genehmigung der Mathematisch-Naturwissenschaftlichen Fakultätder Rheinischen Friedrich-Wilhelms-Universität Bonn1. Gutachter: Prof. Dr. Karsten Buse2.hter: Prof. Dr. Karl MaierTag der Promotion: 06.06.2011Erscheinungsjahr: 2011ContentsAbstract v1 Introduction 12 Fundamentals 32.1 Interaction of light with matter . . . . . . . . . . . . . . . . . . . . . . . . . 32.1.1 Optical absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.1.2 Nonlinear-optical processes . . . . . . . . . . . . . . . . . . . . . . . 52.2 Lithium niobate crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.2.1 Crystal growth and stoichiometry . . . . . . . . . . . . . . . . . . . . 92.2.2 Intrinsic defects - lattice defects and polarons . . . . . . . . . . . . . 102.2.3 Extrinsic defects – impurities and dopants . . . . . . . . . . . . . . . 122.2.4 Overview – Optical transitions . . . . . . . . . . . . . . . . . . . . . 152.3 Implications of optical absorption – Light-induced refractive index changes . 172.3.1 Photorefractive effect in bulk-photovoltaic media . . . . . . . . . . . 172.3.2 Thermo-optic effect and thermal lensing . . . . .
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Interaction of
light with impurities in
lithium niobate crystals
Dissertation
zur
Erlangung des Doktorgrades (Dr. rer. nat.)
der
Mathematisch-Naturwissenschaftlichen Fakultät
der
Rheinischen Friedrich-Wilhelms-Universität Bonn
vorgelegt von
Judith Renate Marie-Luise Schwesyg
aus
Rheinfelden
Bonn 2011Angefertigt mit Genehmigung der Mathematisch-Naturwissenschaftlichen Fakultät
der Rheinischen Friedrich-Wilhelms-Universität Bonn
1. Gutachter: Prof. Dr. Karsten Buse
2.hter: Prof. Dr. Karl Maier
Tag der Promotion: 06.06.2011
Erscheinungsjahr: 2011Contents
Abstract v
1 Introduction 1
2 Fundamentals 3
2.1 Interaction of light with matter . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1.1 Optical absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.2 Nonlinear-optical processes . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 Lithium niobate crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2.1 Crystal growth and stoichiometry . . . . . . . . . . . . . . . . . . . . 9
2.2.2 Intrinsic defects - lattice defects and polarons . . . . . . . . . . . . . 10
2.2.3 Extrinsic defects – impurities and dopants . . . . . . . . . . . . . . . 12
2.2.4 Overview – Optical transitions . . . . . . . . . . . . . . . . . . . . . 15
2.3 Implications of optical absorption – Light-induced refractive index changes . 17
2.3.1 Photorefractive effect in bulk-photovoltaic media . . . . . . . . . . . 17
2.3.2 Thermo-optic effect and thermal lensing . . . . . . . . . . . . . . . . 20
2.4 Thesis overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3 Absorption in congruent LiNbO and LiNbO :MgO between 350 and3 3
2000 nm 23
3.1 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.2 Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.2.1 Undoped congruent LiNbO crystals . . . . . . . . . . . . . . . . . . 263
iContents
3.2.2 MgO-doped LiNbO crystals . . . . . . . . . . . . . . . . . . . . . . 283
3.2.3 LiNbO :MgO crystals codoped with a transition metal . . . . . . . . 293
3.3 Results and discussion: Congruent LiNbO crystals . . . . . . . . . . . . . . 313
3.3.1 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.3.2 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.4 Results and discussion: MgO-doped LiNbO crystals . . . . . . . . . . . . . 393
3.4.1 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.4.2 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
3.5 Comparison: Congruent LiNbO vs. MgO-doped LiNbO . . . . . . . . . . . 493 3
3.6 Outlook and conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
4 Suppression of mid-IR absorption in congruent LiNbO and3
LiNbO :MgO 533
4.1 Crystals and measurement method . . . . . . . . . . . . . . . . . . . . . . . 55
4.1.1 Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.1.2 Measurement method . . . . . . . . . . . . . . . . . . . . . . . . . . 56
4.2 Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.2.1 Infrared absorption spectra of MgO-doped LiNbO crystals . . . . . 573
4.2.2 spectra of congruent LiNbO crystals . . . . . . 603
4.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.4 First OPO experiments – Operation of a 1550-nm-pumped singly-resonant
continuous-wave OPO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
4.5 Outlook and conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
5 Pyroelectrically-induced photorefractive damage in LiNbO :MgO 693
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
5.1.1 Photorefractive damage due to an externally applied electric field . . 70
5.1.2 Pyroelectrically-induced photorefractive damage . . . . . . . . . . . 71
5.2 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
5.2.1 Steady-state photorefractive effect – one-dimensional case . . . . . . 72
5.2.2 Time-dependenceofpyroelectrically-inducedphotorefractivedamage
– one-dimensional case . . . . . . . . . . . . . . . . . . . . . . . . . . 73
iiContents
5.2.3 Time-dependenceofpyroelectrically-inducedphotorefractivedamage
– two-dimensional case . . . . . . . . . . . . . . . . . . . . . . . . . . 75
5.2.4 Impact on applications . . . . . . . . . . . . . . . . . . . . . . . . . . 77
5.3 Experimental setups and results . . . . . . . . . . . . . . . . . . . . . . . . . 86
5.3.1 Beam distortion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
5.3.2 Interferometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
5.3.3 Determination of the specific photoconductivity and the bulk-
photovoltaic coefficient . . . . . . . . . . . . . . . . . . . . . . . . . . 94
5.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
5.5 Outlook and conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
6 Summary 103
Bibliography 105
iiiivAbstract
Congruent lithium niobate (LiNbO ) and 5-mol% MgO-doped LiNbO (MgO:LN) crystals3 3
are widely used as nonlinear-optical crystals in frequency-conversion devices due to their
large nonlinear-optic coefficients. These devices usually require high optical pump powers,
but absorption of photons by impurities limits their usability due to heat accumulation
that leads to thermo-optic refractive index changes. These refractive index changes distort
the beam shape and disturb the phase-matching condition. Furthermore pyroelectric fields
can build up.
In this thesis the residual optical absorption in congruent LiNbO (CLN) and MgO:LN3
crystals is studied. Absorption spectra of CLN and MgO:LN crystals between 400−
−12000 nm reveal a residual absorption up to 0.04 cm . This absorption is mainly caused
by transition metal impurities. Between 2300−2800 nm unknown hydrogen absorption
−1bands in CLN and MgO:LN are revealed on the order of 0.001 cm . High-temperature
annealing is applied to the CLN and MgO:LN crystals, which decreases optical absorption
by up to one order of magnitude. As an application, the operation of a 1550-nm pumped
singly-resonant CW optical parametric oscillator, resonant around 2600 nm, using a low-
loss, periodically-poled, annealed CLN crystal is demonstrated.
Another issue that affects CLN is photorefractive damage (PRD), i.e. light-induced refrac-
tive index changes. In contrast, MgO:LN crystals do not suffer from PRD even at high
optical intensities. However, it is shown in this thesis that PRD can occur within seconds
2in MgO:LN, using green laser light at light intensity levels as low as 100 mW/cm , if the
crystal is heated by several degrees Celsius during or before illumination. Photorefrac-
tive damage does not occur in CLN crystals under the same conditions. We show that
the pyroelectric effect together with an elevated photoconductivity compared to that of
CLN causes this beam distortion and that this effect also influences frequency conversion
experiments in the infrared due to beam self-heating.
vABSTRACT
vi

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